Fringe 2013

Sometimes an understanding of one imaging modality can inform us about properties of another quite different imaging modality. We illustrate this thought with an example. First we explore the principles of the plenoptic camera, which deals with so-called light fields used in computer graphics. We replace the concept of light field with the closely related quantity, the Wigner distribution, for the case of coherent optics, and show how the effects of limited detector MTF in holography can be predicted using the Wigner distribution. Next we consider the effects of detector nonlinearities on holographic images, again using a Wigner distribution approach.

Optical phenomena have a great advantage in that they can be directly observed by our most used sense - eyesight. Some of them could seem like magic when performed practically, for example the effects of invisibility or perfect imaging. We show some ideas of how the principles behind these phenomena can be explained and demonstrated to the general public.

The Fourier Modal Method (FMM) is perhaps the most popular numerical technique for rigorous analysis of diffraction gratings and other diffractive structures. The method has its roots in late 1960’s, in the work of Burckhardt on sinusoidally modulated volume gratings [1], and it is similar in nature as the so-called Rigorous Coupled-Wave Approach [2]. The method is applicable to dielectric, metallic, and semiconductor grating profiles of quite arbitrary shape, and the materials can be anisotropic. The convergence problems that long persisted for metallic gratings in TM polarized illumination were solved in mid-1990’s by introduction of correct Fourier factorization rules to deal with abrupt discontinuities in permittivity [3]. The FMM is also applicable to two-dimensionally periodic (crossed) gratings with complex permittivity variations in the nominal propagation direction of light [4]. Further, non-periodic structures can be analyzed by introducing so-called perfectly matched layers and nonlinear coordinate transformations. A comprehensive coverage of FMM can be found in Ref. [5].

Dimensional metrology based on optical vision systems is commonly used at the industrial level, for example in optical coordinate measurement machines (CMMs). Measurement uncertainties of well below 1

μ

m are more and more required. Thus for calibration samples a measurement uncertainty of the order of 100 nm is required. To enable accurate and traceable measurements both unidirectional (e. g. distance) measurements and bidirectional measurements (e. g. width or diameter) have to be performed for system calibrations and the reverification of the performance [1]. However, up to now the national metrology institutes (NMIs) are not able to offer internationally recognized bidirectional calibrations, which are covered by the Mutual Recognition agreement (MRA [2]) and documented in the key comparison database [3]. Currently only one NMI offers optical widths calibrations on suitable user-supplied samples, including line scales, as well.

Angular Fourier Scatterometry techniques to solve inverse-grating problems have been widely used in semiconductor metrology since the initial work by den Boef et al. [1] because it is simple, fast and robust and yet yield accurate measurements. These systems use incoherent light as a source and are commonly referred to as Fourier Scatterometry (FS). Coherent Fourier Scatterometry (CFS) was introduced a few years ago [2] as an alternative technique to the existing tools. This new method differs from Incoherent Fourier Scatterometry by the aspect of using coherent light as a source to illuminate the grating to be characterized. It is shown [3] that the use of the information about the phase difference of successive orders generates significant gain in sensitivity. This phase difference is implicitly obtained by laterally scanning the grating with respect to the optical axis of the set-up within one grating period and obtaining a few intensity measurements. This lateral scan affects phases of only orders higher orders than zeroth. Using this property, besides common parameters of grating reconstruction without making any additional measurement, the parameter

bias

[3] (lateral misalignment of the grating in experiment and its numerical model) could also available in CFS enabling one to align the target with respect to optical axis of the system to an accuracy of few nanometers. However, this works only when higher orders are available.

Gold nanorods (NRs) are typical noble metal nanoparticles (NPs) that are widely used nowadays in various applications, such as catalysis, medical diagnosis and therapy, and bio-sensing [1]. Reliable, fast, and accurate measurement methods and the related metrology standards are highly demanded for the production, characterization, and commercial use of gold NRs.

Diffraction gratings produce a number of diffraction orders, which relative intensity is directly related to the grating profile. Continuous blazed phase-only gratings have the greatest efficiency onto a single first order [1], but they are difficult to fabricate. In more common binary amplitude or binary phase gratings, numerous diffracted harmonic orders are produced [2]. Dammann gratings are of interest because they produce equal intensities in a given number of selected orders [3]. They are designed by subdividing each grating period into regions with phases 0 or

π

radians. In order to increase their efficiency, different studies were conducted in the field of the so called fan-out arrays, extensively developed in the late 80’s [4]. More recently, Romero and Dickey further extended and provided a general framework for the design of optimal fan-out phase elements [5,6], by extending a previous study on the optimal triplicator [7]. The method provides an analytical optimal solution for designing a grating with a set of target diffraction orders. The calculation involves optimization of two numerical variables for each target order, and results in a single function that generates a periodic diffraction grating with continuous phase modulation levels.

Over the last two decades or so optical methods have become increasingly popular as a means to measure surface topology. Compared to traditional contacting methods, optical instruments such as Coherence Scanning Interferometers (CSI) and Confocal Microscopes (CM) have the capability to collect large, high-resolution data sets without risk of surface damage and consequently are extensively used in research laboratories to calculate areal surface parameters [1]. Despite the significant advantages of optical metrology, however, unexplained errors are frequently observed especially when sloped artefacts are measured and consequently their use as a traceable measurement tool has been brought into question [2].

Phase reconstruction from intensity measurements in interferometry is classically solved by phase-shifting or phase-stepping techniques. At each pixel a sequence (set) of intensity measurements is taken, and between those measurements the bias phase is changed in a most precise manner (i.e. "phase-stepping"). High accuracy of the computed phases in each pixel is achieved by knowing the individual intensities but especially also the global bias-phase of every interferogram in the set. The obvious problem, that the bias-phase is a delicate quantity, highly error prone due to vibrations, air-turbulence and wavelength-instability of the laser has conventionally been tried to address by special phase-shifting formulas [1-6]. These enable correct phase reconstruction even with linear or quadratic phase-stepping errors as well as with non-linearity of the characteristic detector curve and also mitigate the effect of multiple reflections within the Fizeau-cavity.

In the area of high performance optics aspherical surfaces have become the solution of choice over the past years [1] [2]. The advantage of aspherical, compared to spherical elements, is the highly increased degree of freedom for the optics design. This allows better correction of aberrations, by simultaneously reducing the number of elements needed to fulfill a given design target, enabling the construction of more compact optical systems, with higher optical performance at the same time. As a result of these convincing advantages aspherical optics are widely used in state of the art optical systems, starting from mass products like imaging systems for micro cameras in smartphones, reaching to high end optical systems used in lithography or space applications. Even more degrees of freedom in the design can be reached, if the rotational symmetry of the aspheric surface is broken. Such free-form surfaces that do not have to show any symmetry at all can be used to further improve the performance of an optical system. One possibility is the construction of systems where the elements are no longer arranged along a straight line, but where the optical axis is folded. By taking advantage of this design option it is possible to develop very compact systems, which also are less sensitive to mechanical and thermal influences. Another advantage of off-axis systems that can be realized with free-form elements is the avoidance of reflexes that often occur at the center of the lens in a classical system. This is especially important for applications with coherent sources. Further, certain wavelengths demand the usage of mirror optics instead of lenses, if there aren’t any optical refractive materials with tolerable absorption available. To avoid central obscurations, here again the easiest way is to use free-from mirrors. One example for this kind of optical systems is the EUV lithography that will be used in the next generation of semiconductor fabrication.

Polarization based interferometers for single snap-shot measurements allow single frame, quantitative phase acquisition for vibration insensitive measurements of optical surfaces and have been successfully used on a variety of interferometer types. This technique generally involves the simultaneous acquisition of three or more images on the same camera or multiple cameras, which are phase shifted by polarization. Examples of these types of systems would include the multiple camera system of Smythe and Moore [1], and more recently, systems utilizing a micro-polarizer phase mask [2] on a single camera.

The aim of this work is to evaluate the shape of a free form object using single shot digital holography. The digital holography results in a gradient field and wrapped phase maps representing the shape of the object. The task is then to find a surface representation from this data which is an inverse problem. To solve this inverse problem we are using regularization with additional shape information from the CAD-model of the measured object.

The development of visible and infrared imaging systems continues to be an extremely active area of physics and engineering research despite the already considerable history of its development and accomplishments. This is due to the significant breakthroughs in the mathematics of image processing and the substantial advances in device performance over the last decade. Recently, many efforts in imaging that combine the relatively inexpensive and readily available computational power of microprocessors with new optical schemes have been undertaken to increase image resolution while decreasing acquisition time. One such approach that we have been investigating in our lab is based on compressive sensing mathematics coupled with micro-optoelectronic modulators. In essence, imaging via compressive sensing is a new strategy for imaging with a single detector in place of an imaging array. Additionally, the single detector can be replaced with a spectrometer to perform compressive hyperspectral imaging.

During the last couple of years light field imaging has become an intensively studied field of research. Unlike a conventional camera, which simply records a two-dimensional spatial intensity distribution, a plenoptic camera additionally measures directional information by incorporating a micro lens array in the optical setup. This allows the image manipulation after the capture to a much greater extent than with conventional images. Typical examples include the refocusing operation, the extension of the depth of field, the calculation of depth information and the reconstruction of parts of the initial object geometry.

Recovering the complex amplitude of a coherent wave field has many important applications in modern optics from practical metrology problems to basic diffraction research. Although several techniques exist, in this manuscript, we will examine the iterative Phase Retrieval (PR) [1-6] exclusively. To recover the phase information with this approach several intensity distributions, diffracted from the object of interest, are recorded at different optical planes. A significant advantage of the PR is the relative simplicity of the optical setup, compared with an interferometric approach. With this approach, noise sources such as an imperfect reference wave can be avoided. PR techniques are an ill-posed inverse problem and hence initial conditions play an important and relatively poorly understood role.

Single beam phase retrieval techniques [1-8] allow the reconstruction of the object phase information of a general volume speckle field for single beam devices such as optical microscopes. These techniques are well-suited for simple optical setups and recover the phase using the intensity recorded at a single or multiple planes. Deterministic Phase Retrieval techniques, as e.g. approaches based on the Transport of Intensity Equation [4-6] or the Contrast Transfer Function [7,8] allow recovering directly the phase of a wavefront, but are limited either to the paraxial case or impose restrictions on the phase to be estimated. Available Iterative Phase Retrieval (IPR) techniques as the Single-Beam-Multiple-Intensity-Reconstruction (SBMIR) algorithm [1-3] overcome this problem and are applicable to the non-paraxial case without imposing restrictions on the object to be investigated. However, this family of IPR techniques are subject to varying levels of performance or stagnate after few numbers of iterations. The stagnation is a result of the slow elimination of the remaining low frequency error components usually in form of a phase tilt, a bell-shaped error, or other low frequency artefacts. This problem results in a poor reconstruction for the case of slow-varying objects or the measurement of aberrations.

Extracting quantitative phase information has received increased interest in many fields where either phase imaging or structure retrieval is an issue, such as optical testing, bio-medical imaging and materials science. In the past couple of decades, digital holography (DH) has emerged as a front-runner for phase imaging by providing quantitative phase measurements of the wave field with high accuracy and in near real-time [1]. However, DH systems need a highly coherent light source, suffer phase aberration, ambiguity and unwrapping problems, and cannot offer the highest spatial resolution. Recently, however, direct phase retrieval from intensity measurements using the Transport-of-Intensity Equation (TIE) [2, 3] has gained increasing attention. A minimum of two measurements of the spatial intensity of the optical wave in closely spaced planes perpendicular to the direction of propagation are needed to reconstruct the spatial phase of the wave by solving a second-order differential equation, i.e., with a non-iterative deterministic algorithm. In this paper, these two quantitative phase imaging methods: DH and TIE are introduced and compared. Two samples: a regular array of micro-bumps fabricated on Si substrate based on laser induced non-ablative texturing and a refractive quartz microlens array from SUSS MicroOptics were tested by DH and TIE. The results were compared and the merits and limitations of each method are discussed.

The concept of the ensemble average plays a key role in coherence theory and statistical optics in general. Since the ensemble average is a conceptual quantity defined mathematically in terms of the probability and the ensemble, it needs be associated with a physical quantity that is observable by experiment. It is common practice to replace the ensemble average with the time average, assuming that the statistical field is stationary and ergodic in time [1]. This assumption is justifiable in many cases of practical interest, such as partially coherent illumination in microscopy and thermal light analyzed by Fourier spectroscopy. Such a system of traditional statistical optics based on temporal stationarity and temporal ergodicity may be called

temporal statistical optics

to distinguish it from

spatial statistical optics

to be discussed in this paper.

The resolution and the contrast of optical scanning microscopes can be enhanced by image field microscopy [1, 2, 3, 4]. The image of the sample is imaged by an image inverting interferometer onto the detectors positioned at both exits of that interferometer. There, the image is superposed with its spatially inverted copy (rotation by 180°). Image points distant from the inversion axis are superposed by points that are equally distant, but from the opposite site of the image. If the light coming from the probe is spatially incoherent, those image points far from the inversion axis are not correlated to each other. Thus the intensity values are added incoherently. In contrast, images of points close to the inversion axis are superposed (in part or fully) with their own copy, resulting in an interference structure. Integrating the full interference pattern gives a signal, which can be allocated with the point of the object on the inversion axis. The integration can be done numerically, if the detector does have spatial resolution. By using this method, it is possible to transfer small structures up to the cut-off frequency of the used microscope objective with a contrast of one. So far, an increased two point resolution of 26% could be realized experimentally [5].

Phase unwrapping still plays an important role in optical metrology field. The phase unwrapping process has direct influence on the accuracy of final results. This is why many algorithms have been proposed for phase unwrapping. In spite of this, there is no agreement between the current phase unwrapping algorithms for different applications. This is due to the existence of disturbance in the measured phase data. In the case that there is no disturbance in the phase data, the unwrapped phase can be obtained by integrating the phase gradients over the whole data set which is independent from the integration path. However, there are several sources of errors. Firstly, phase aliasing occurs when the true phase changes by more than one cycle (2

π

radian) between samples, which was caused by long baselines, objects discontinuities or high deformation. The second source is noise, which may be caused by speckle noise, electronic noise and/or fringe breaks. In the presence of these errors, it is hard to unwrap a phase map correctly.

Inspection of parts for manufacturing defects or in-service damage is often carried out by full-field optical techniques (e.g., digital speckle pattern interferometry, digital holography) where the high sensitivity allows small anomalies in a load-induced deformation field to be measured. Standard phase shifting and phase unwrapping algorithms provide full-field displacement and hence strain data over the surface of the sample. The problem remains however of how to quantify the spatial variations in modulus due, for example, to porosity or damage-induced micro-cracking. Finite element model updating (FEMU) is one method to solve problems of this type, by adjusting an approximate finite element model until the responses it produces are as close to those acquired from experiments as possible.

Many methods of phase shifting have been discussed for phase analysis in interferometry and digital holography[1]. High speed interference fringe analysis is strongly desired in practical industrial measurement.

The established approach for the characterization of sound beams is the acquisition of experimental data by measuring the acoustic field pressure distribution in a fluid medium, which is usually done with point detectors (hydrophones, microphones) or arrays of detectors. From these point measurements, important characteristics of the sound field emitted by the transducer (such as the axial and transversal beam profiles, focal length or beam spread) can be derived. When the propagation medium is a solid, these characteristics are normally obtained from the measurement of pulse-echo signals that arise from the interaction of the sound beam with targets placed in the material, such as metal balls embedded in plastics, or flat-bottom or side holes drilled in metallic blocks [1]. These measurements are more difficult to perform and provide a sparser set of data compared to the immersion techniques.

In this paper, we use sparse modeling for processing phase-shifting interferometry measurements. The proposed approach takes into full consideration the Poissonian (photon counting) measurements. In this way we are targeting at optimal sparse reconstruction both phase and magnitude taking into consideration all details of the observation formation. Many images (and signals) admit sparse representations in the sense that they are well approximated by linear combinations of a small number of functions taken from a know set. The topic of sparse and redundant representations has attracted tremendous interest from the research community in the last ten years. This interest stems from the role that the low dimensional models play in many signal and image areas such as compression, restoration, classification, and design of priors and regularizers, just to name a few [1].

Optical fringe pattern processing and analysis [1] plays crucial role in metrological applications (e.g., interferometry, moiré and structured illumination methods). It might be often a troublesome task because of fringe pattern defects such as noise, uneven background, low modulation and generally complex fringe shapes in a wide spatial frequency range. In this paper we present adaptive optical fringe pattern processing (filtering and normalization) techniques, robust to mentioned pattern imperfections, based on the empirical mode decomposition (EMD).

There are several engineering applications for interferometry where the measurement must be done in-situ in presence of mechanical vibrations or other disturbances. In those cases, optical techniques based in a single image acquisition are the best choices. Approaches like pixelated phase-mask interferometers, instantaneous or simultaneous phase measuring interferometry have been successfully reported in the literature. However, some of them are quite complex or difficult to be miniaturized or made robust enough to be transported and operated in quite hostile environments.

The use of projected fringes to measure object shapes and deformations is part of the general field of metrology using structured light, where any of a wide variety of patterns is projected onto an object in order to measure its shape and deformation. A very comprehensive review of this field has been provided by Geng.[1] The purpose of this paper is to present a variation of this technique that grew out of the development of a real-time, image-plane, digital holography system in the late 1980s.[2] The initial system was called PCHolo32 and it has been upgraded in the last decade to a newer system called HoloFringe300K. Digital holography allows measurement of static and vibratory displacements of objects, but it is limited to small displacements, typically only a few micrometers. The phase stepping routine used by PCHolo32 and HoloFringe300K is applicable, however, to any sinusoidally varying irradiation pattern so it is possible to illuminate an object with a sinusoidal interference pattern from a Michelson interferometer and step the phase of that pattern. If the illumination is at an angle to the object surface (typically 45 deg) and the observing camera essentially normal to the surface, out of plane movement of the surface creates a lateral shift of the observed fringe pattern that is proportional to displacement. Because the observed pattern does not involve the laser speckles, this technique can be used to compare the shape of one surface to another. Furthermore, the pseudo phase-step vibration measurement technique of the holographic processor can be used with fringe projection to measure large scale vibrations.

In the fight against marine pollution, the knowledge of the mixing dynamics between different liquids is of first priority. When shipwreck occurs with chemical payload, the chemicals can follow different behaviors through the water column (floating, dissolving, sinking, evaporating …). Floating and dissolving products are the main categories interfering with the local in-situ responders. Moreover these chemicals can be contact corrosive for rescue divers, generate toxic cloud and even explosive atmosphere for rescue aircraft and crew. This study mainly focuses on this liquid-liquid interaction. First tests with shadowscopy have shown trailing edge dissolution smears and also characteristic droplet shapes throughout the water column droplet rise [1]. Digital holographic set-up coping with high speed imaging has been designed to enhance the quality of the droplet contours and to measure the critical sizes. First results about n-butanol dissolution in seawater and the analysis procedure are presented.

Phase analysis of fringe pattern technique has been widely used for three-dimensional shape and deformation measurement by fringe projection method, precise inspection of optical components by various interferometry, etc. To analyze the phase distribution of fringe pattern, more than 80 phase analysis methods have been proposed by many researchers in this research field [1]. For instance, a Fourier transform [2], a wavelet transform, a windowed Fourier transform, and a sampling Moiré method [3] have been developed from a single-shot image by using the intensity information in the

spatial

domain. In addition, the phase-shifting method (PSM) [4], which uses multi-frame phase-shifted fringe patterns is mostly used to analyze the phase distribution by using the intensity information in the

temporal

domain. However, these techniques only use one-dimensional intensity information in the

spatial

- or

temporal

-domains. For this reason, the analyzed phase distribution for noisy fringe pattern remains large measurement error due to the low reflectance of the object and the random noise of the camera.

Coherent optical systems are widely used in modern non-contact metrology [1]. To understand and design these systems appropriately, it is of utmost importance to be able to relate the optical field at the input of the system to that at the output of the system. Many different models exist for achieving this goal. Here however, we assume that the paraxial approximation is valid for the analysis. The paraxial model generally strikes a reasonable balance between calculating an accurate solution while maintaining a relatively simple description of the physical process. For general speckle metrology systems, consisting of many lenses, Gaussian apertures, and sections of free space, the ABCD-matrix-diffraction approach by Yura et. al. [2] provides an excellent design framework [3].

Digital shearography [1] is a practical inspection tool for industrial applications. Traditionally, the output of shearography is in the form of shearogram depicting the phase change caused by deformation. To determine the phase change quantitatively, it is necessary to capture at least three shearograms with different values of phase-shift at each deformed state. This phase-shifting techniqueworks well for static deformation measurement, but it is unsuitable for measuring dynamic deformation. Therefore, phase demodulation from a single shearogram is of great interest in practical applications. Previous work by other researchers has demonstrated that the Fourier transform (FT) method [2]and the spiral phase transform (SPT) method [3-4] can be used to extract the phase map from a single fringe pattern. However, the FT method requires that a large spatial carrier frequency should present in the fringe pattern. The reported SPT method estimates the fringe orientation from intensity derivatives, which will cause errors when there are more than two extremes in the phase map. However, the phase of a shearogram usually has two or more extremes in digital shearography. To deal with this problem, in this paper we describe a new procedure to extract phase from shearogram. We calculate the fringe orientation by optical flow from two successive shearograms and then determine the phase of the shearogram by spiral phase transform. We describe the principle of the method and then demonstrate it through both simulation and experimental results.

Optical measuring technologies become more and more prominent in Metrology, Quality Control and other fields in which fast and accurate recordings of e.g. geo-metrical surfaces are crucial. Due to their inherent potential to record lots of measurements at once (e.g. 2D images), optical methods often generate a whole lot of data – a stream of images or video – which has to be evaluated by software tools. Paired with the common need for real-time measuring, this leads to a demand for high performance in the analysis of the recorded data. Nowadays performance gain on consumer hardware is achieved by leveraging parallel processing methods [4]. This especially applies for problems in Optics – e.g. Ray-Tracing or Pixel-Wise Filtering – which are inherently parallelizable.

For last several years the continuous wavelet transform (CWT) has been widely employed for signal processing purposes. Its popularity is due to noise resistance and exceptional capability for the non-stationary signal analysis. Lately, CWT has gained substantial interest in the optical fringe pattern analysis (FPA), most frequently to extract phase distribution and enhance fringes for further processing. The scope of possible CWT applications in FPA is much broader. In this paper we present recently proposed CWT algorithms developed by our team. They convert CWT into an extremely versatile FPA tool.

Fringe projection is one of the established background procedure for contactless optical 3D surface measurement based on geometrical triangulation of corres-ponding image points. It is used to generate phase values which identify corresponding points. Fringe generation may be realized by conventional digital (DMD, LCoS) or by classical slide projection. However, recent developments provide the chances to use particular equipment to obtain certain advantages. The use of a tailored free-form mirror (FFM) for phase shifted fringe generation is such an unusual hardware for a 3D scanning system which was developed at our institute [1].

For the measurement of wave fronts there is a variety of methods, which can be divided into interferometric techniques, gradient-based methods, iterative phase retrieval and phase space methods. Each of these methods has its advantages and disadvantages. The proposed technique is a combination of iterative phase retrieval and a diffractive overlapping micro lens array. Unlike interferometric or holographic methods, phase retrieval methods require no reference wave. Thus no laser is required and the measurement set-up is very simple.

The aeronautic industry makes now largely use of composite material in its products. Therefore huge efforts have to be done on their experimental validation of these materials. More specifically these composite materials introduce new type of defects (such as delamination, glue defects…) which need to be detected during the manufacturing and maintenance to prevent failures. Moreover the widespread use of these materials imposes to the future detection instruments to be cheap, robust and easy to manipulate by non-specialists. Shearography is an emerging technique for the detection of defects in the composite material [1,2]. It is cheap, and relatively robust and flexible, but requires special qualification to interpret the defect signatures in the interference pattern. Therefore efforts need to be carried out to ease the interpretation of shearographic results for non-specialist of the technique [3]. More specifically, we are looking for techniques to automatically identify defects with shearography by the development of specific algorithms.

Fringe projection is a method based on triangulation and is commonly used in form and coordinate measuring technology. For 3D object reconstruction, light signals are encoded using sequential projection processes. A distinct assignment of object and surface points is thus made possible with imaging measurements. Various methods have taken root as common projection sequences. A prevalent method is binary Gray encoding, often used in conjunction with phase-shifting ([1]) in practical applications to increase lateral and vertical resolution.

Various techniques in interferometry require lasers of short coherence length, taking advantage of the fact that light interferes only in the areas where the optical path difference (

OPD

) of two beams is shorter than its coherence length. Examples of these techniques are light-in-flight recording by holography [1], optical coherence tomography [2], 3D shape measurement [3-4], and short coherence microscopy [5]. These techniques use

a priori

knowledge of the laser’s coherence length in order to extract valuable information about the test object (e.g., its position, shape, or refractive index distribution).

Electronic speckle pattern interferometry (ESPI) is a well-known, nondestructive, whole-field optical technique for measurement. It has been extensively investigated and widely used for deformation measurements in numerous fields. The deformation to be measured is coded in a fringe pattern. Computer-aided fringe analysis can be used for quantitatively evaluating deformation information. However ESPI fringe pattern is contaminated by heavy speckle noise, which makes the fringe analysis difficult.

Complex object recovery from a digital hologram without the presence of dc or twin image terms is an extensively studied topic [1-2]. Recently we have demonstrated a novel computational method for high resolution image recovery from a single digital hologram of an amplitude object [8]. The complex object field is recovered from the hologram by solving a constrained optimization problem. This procedure is seen to overcome some of the current limitations on single shot digital holography. It can be shown that high quality image recovery is possible even when the dc and the cross terms in a hologram overlap in the Fourier domain. In this paper, through simulation, we intend to extend the work to the recovery of complex object information of pure phase objects.

This paper describes roles of speckle in measurements of shape and deformation of rough surfaces by digital holography and speckle correlation. Speckles are distinct natural markings of rough surfaces appearing in 3-d space because they show displacement and decorrelation due to surface deformation and/or changes of illumination. Digital recording and processing for coherent metrology started from speckle interferometry and speckle photography and fully bloomed by digital holography. These techniques offer perfectly noncontacting and quick measurement applicable for real objects.

An increasing number of important applications rely today on the possibilities of using digital color holography to record and reconstruct colored objects at high precisions using a simple optical setup. However, using multiple wavelengths in the same optical system can lead to severe chromatic aberrations. Several actions have been taken in the last ten years in order to correct them, especially in the field of digital color holography [1]. Different wavelengths can be used for recording colored objects, but due to the chromatic aberrations, a classical optical system, not only will image the same object at different planes, but these images will have different sizes regarding the wavelength. Furthermore, in a simultaneous color detection scheme [2], the reference beams cannot be perfectly aligned, resulting in the occurrence of a lateral shift between the different images. Finally, in a usual Fresnel configuration, the recording distance only can be increased so far, ruling out the study of large objects due to the Shannon theorem, which reduces considerably the area of possibilities. In 1996, Schnars proposes to use a negative lens so as to reduce the spatial frequency spectrum of the object, as well as the recording distance [3]. Unfortunately, the use of a negative lens introduces aberrations that modify both the sensor-to-object distance and the virtual object size along each wavelength. The opportunity provided by digital color holography is that chromatic aberrations can be corrected by a pure and simple numerical compensation. Figure 1 describes the basic scheme for a digital color holographic set-up. Three laser wavelengths in the red, green and blue domains [2] are used to illuminate the object. The three color beams are combined before the reference and object beams are separated. Then, the object wave illuminates the useful area with a unique or several dissociated illumination directions. The reference beam is extended and spatially filtered to produce an inclined reference plane wave (off-axis holography) that is combined with the diffracted object wave, using a beam splitter. The sensor records simultaneously the three colors, providing real-time capabilities to the holographic setup. For a large object, the negative lens is placed just in front of the beam splitter, in the object optical path. Thus, a virtual object is produced in front of the sensor at a smaller distance than the initial one, and this enables the Shannon conditions to be fulfilled.

Against the background of the activities of the National Metrology Institutes for the redefinition of the SI unit "kilogram" the sphere interferometer of PTB has been developed for the measurement of the volume of spherical material measures [1,2]. Provided that the shape deviation is small the mean diameter is sufficient for this purpose as long as only the volume is of interest [2]. Four years ago we have presented a stitching approach which has extended the applicability of the sphere interferometer beyond the determination of diameters [3]. The described method allows the reconstruction of radii based on the measurement results from the sphere interferometer and yields the real shape of the sphere under test. By a comparison with independent results from tactile roundness measurements the accuracy of the approach was estimated to be lower than 5 nm [4].

Creating and controlling complex laser polarization patterns has become essential for many areas of optics. In particular, radially and azimuthally polarized doughnut beams (also known as cylindrical vector beams, CVBs) were used widely, for example, to study the shape and orientation of individual nano-objects such as molecules or metal nano-particles (NPs), for particle trapping, for high-resolution microscopy and spectroscopy, and for the excitation of radially symmetric plasmonic structures, like in tip-enhanced Raman spectroscopy.

Electronic speckle pattern interferometry (ESPI) is widely used for displacement, stress and strain measurement [1]. Recently the introduction of Spatial Light Modulators (SLM), has allowed versatile optical systems that can overcome some of the limitations of the typical ESPI setups making them more robust and flexible [2]. For this kind of applications, the usual phase stepping techniques that require PZT can be replaced by carefully manipulating the modulator phase. Here a technique is proposed that introduces a multi-level spiral phase filter (MLSPF) allowing the extraction displacement in an out-of-plane ESPI setup. This MLSPF produce a specific speckle pattern denominated vortex-filtered speckle pattern (VF-SP), which results from the convolution of the standard speckle pattern and the MLSPF point spread function, similar to the phase contrast microscopy techniques [3-4]. This VF-SP contains within the speckle size, the phase terms of the filter, which can be controlled independently by the rotation of the MLSPF. Therefore by rotating the MLSPF, the standard phase stepping algorithms can be applied for phase extraction procedures, and for displacement measurements.

One of the most desired tools of image processing is spatial filtering. The Fourier filters in a 4

f

system can be used to detect the edges, enhance the contrast and to remove the noise so as to provide a better output image. Optical vortex has been used as a spatial filter to perform isotropic edge enhancement [1], the fractional and shifted vortex filters are used for selective edge enhancement [2,3] and anisotropic as well as shifted anisotropic vortex filters are used to get controlled orientation selective edge enhancement [4,5].

Phase retrieval (PR) [1] is a method implemented to determine the phase of a wave field. The most established PR techniques recover the phase from a number of intensity measurements corresponding to light diffracted at consecutive planes [2,3]. PR has comparably low demands regarding the coherence of light, since no reference wave is required. This should offer the opportunity to use partially coherent light for illumination, e.g. light provided by LEDs, instead of fully coherent light originated from a laser source for example. Thus, typical problems arising from the use of a laser source, such as parasitic interference fringes generated due to reflections from glass surfaces, high cost and high demands regarding laser safety are avoided.

In this manuscript we investigate the space-time intensity correlation function of doubly scattered speckle, that is, the observed speckle generated by the passage of a laser light through two sequential random phase screens. It has been shown that the amplitude and intensity of the doubly scattered speckle in this case obeys a K distribution [1]. Unlike circular Gaussian speckle field, Reed’s theorem [2, 3] is not applicable for the K distributed speckle. Thus a direct derivation of the fourth-order moment is required in order to obtain a space-time intensity correla-tion function. Unfortunately, the derivation of fourth-order moment is cumbersome [4], meaning that the analytic space-time intensity correlation function for doubly scattered speckle is rarely seen. Our main goal here is to demonstrate an approach that can solve this problem.

Low coherence interferometry (LCI), including white light interferometry (WLI), is a widely known, non-invasive, optical sensing technique which provide height/depth information with high resolution without wavelength depending range limitations . However, the majority of LCI setups have very high resolution thanks to implemented, complicated and time consuming image processing algorithms [1-6], which are based on wavelet, Stockwell as well as Fourier transforms etc. Those combined with the amount of data to process results in rather long times gaps between the exact measurement and the result given. From the other hand, the rapid recent development of technology, esp. in manufacturing processes, requires very fast full – field shape measurements together with large fields of view, even with the cost of sacrificing very high resolution.

Optical profile and surface evaluation constitute an important branch of optical metrology. On one hand, interferometric systems provide direct surface information. On the other, systems such as deflectometry or Hartmann-Shack based ones provide the slope of desired results instead. In this case, numerical integration methods need to be applied to retrieve desired results. A variety of integration methods have been previously studied [1]. All of them can be analyzed as the convolution with a given integration kernel. Fourier transform method was found to be the best solution in terms of resolution, frequency response and average error [2]. Nevertheless, when using Discrete Fourier Transforms (DFT) [3], Fourier method presents an undesirable ripple close to the stiff borders of the integrated function. Moreover, it requires making the target function anti-symmetrical or even more complex treatments in order to provide correct results. In this work we analyze the reasons for ripple and symmetry requirement with 1D DFT integration methods and we propose a different codification of the integration kernel which permits avoiding both the resulting ripple and the symmetrisation of target functions.

Calculating the propagation, refraction, and reflection of wavefronts does not only allow for the simulation of the behaviour of a given system of optical components, objects, and lights sources but also for the analysis of an actual optical system or component by measuring wavefronts of known origin. Thus the methods of wavefront tracing are used widely in a large range of application ranging from system design to quality inspection.

Optical imaging of microscopic objects in transparent media is well assessed and deeply exploited [1]. However, if the object under analysis is dipped into a turbid medium, its imaging through conventional optics is hindered due to the strong scattering of the colloidal particles, thus severely degrading image quality. Even if initially the fluid is transparent, but becomes turbid by effect of a whatever chemical reaction or mixing, the processes cannot be observed. Here we show that Digital Holography is a reliable technique allowing a clear coherent imaging of the specimens through turbid fluids at Lab on a Chip scale, which is not achievable with any other imaging methods.

Optical components manipulating both polarization and phase of wave fields find a lot of applications in today’s optical systems. It is possible to fabricate optical elements with nanostructured surfaces for the generation of any orientation of locally linearly polarized light [1]. The wave front of a radial polarizer made from a subwavelength aluminium radial grating on a glass substrate for the wavelength

λ

=633nm was measured. This optical element generates an additional phase term together with the desired polarization distribution [2].

Spectral-domain interferometry techniques have been widely in optical pro-filometry (see,

e.g.

, [1, 2]) and optical coherence tomography (OCT) [3, 4]. Conventional methods to extract useful information involve Fourier trans-formation (FT) of spectral fringes. Due to availability of light sources with controllable variation of wavelength in wide spectral range it becomes possible to register time-variable spectral fringes appearing at different wavelengths, and it is needed to provide high-speed processing of spectral data in time domain.

The presence of the multiplicative speckle noise, intrinsic to Electronic Speckle Pattern Interferometry (ESPI), limits the quality of ESPI measurements. Of many algorithms proposed to reduce the speckle noise in the image, only few recognize the statistical properties of the speckle noise and the multiplicative relation between the underlying cosinusoidal pattern and the speckle interferogram. Local averaging is a limited approach because of the possibly very high differences between the adjacent pixels in relation to the mean signal value, which corresponds to particularly low signal to noise ratio. In this paper we test an approach based on the non-local means algorithm, which is a global weighted averaging scheme based on the pixels neighborhoods (patches) similarity criterion.

It is known that intensity-dependent phase changes (by temporal self-action effects) and intensity changes (by spatial self-action effects) distort the wavefront of light as it propagates through the medium [1], therefore one can not propagate images directly through a nonlinear medium [2] (indirect methods can be used, but they do not allow to measure internal light wave mixing or its dynamics).

Finite Element Method Updating and the Virtual Fields Method are powerful techniques for elastic modulus and stiffness identification from full field measurements of displacement and strain [1]. They work at their best when provided with dense and multicomponent experimental displacements (or strain) data, i.e. when all orthogonal components of displacements (or all components of the strain tensor) are known at points closely spaced within the volume of the material under study. This reduces the chances of finding multiple solutions to the inverse problem of identification, a common limitation encountered when only surface deformations are measured to characterize damage or infer sub-surface characteristics in a material [2]. Digital speckle pattern interferometry (DSPI), a mature technique for surface deformation measurements, has been effectively extended into 3D by two fundamentally different approaches: 1) by using multiple wavelengths so that depth information can be encoded in the temporal frequency of the intensity signal, and 2) by using a monochromatic light source with multiple illumination directions by scanning the angle of the illumination beam. The former approach is known as Wavelengths Scanning Interferometry (WSI) [3], and the latter as Tilt Scanning Interferometry (TSI) [4].

Conventional correlation-based tracking methods are limited by the correlation window size and a lack in ability to track individual speckles or particles. Wang et al.[1] in their definition of Optical Vortex Metrology (OVM) propose that phase singularities are optimal encoders for position marking and can be localized with accuracy. OVM is widely used to process real-life and pseudo optical vortices [1, 2, 3], but so far the accuracy of the method has not been determined. Optical Vortex Metrology (OVM) uses phase singularities, produced in the complex signal generated by a Laguerre-Gauss filter operation applied to an image, for displacement measurements. Each phase singularity is essentially a pseudo-phase vortex, which can be uniquely identified from its structure, enabling it to be tracked between images. OVM is of interest here as not only the technique has potential to provide accurate position tracking but also pointwise rotation measurement.

Digital realization of holography (Digital Holographic Interferometry) and speckle interferometry (ESPI) are excellent tools for industrial application if the task is to measure deformation, shape or refractive index with high resolution and non contact way. Thanks to the rapid development of active optical elements (digital cameras, SLMs, computer controllable mirrors) automated measuring systems can be built. They can continuously adapt themselves to the change of measuring conditions for example to the varying measuring range.

Both amplitude and phase of an optical wavefront are necessary in many applications. However, the conventional detector can only measure the intensity of the wavefront. The phase should be measured by interferometry [1] or phase retrieval techniques [2, 3]. Compared to the interferometry, the phase retrieval methods just need diffracted intensity images without a reference beam, which makes the experimental setup simple. There are mainly two kinds of phase retrieval techniques. One is quantization phase retrieval which is based on the transport of intensity equation [2]. The other one is based on the iterative algorithm [3-5].

White-light scanning interferometer (WLSI) is a powerful method to measure shape of an object in larger range with higher resolution. It has been widely used in the micro-profile measurement of optical elements and micro-electro-mechanical components. With board light source bandwidth, WLSI is able to overcome 2

π

phase ambiguity problem, which is often occurred in traditional laser inter-ferometer. Typically, phase-shifting method is used to retrieve the corresponding fringe phases and extract the WLSI signal peak position. For the phase-shifting method, vertical scanning is required but thanks to the improvement of hardware performance, the scanning time is shorter than the traditional zero-order interference fringe identification method. Traditionally, locally linearized five- or seven-step phase-shifting algorithms could be used to retrieve the WLSI signal coherence function and determine the position of zero optical path difference (ZOPD).

The optimization of the lithographic process in semiconductor industry is continuously conducting towards smaller devices and structures but at the same time highly increasing metrology exigencies.

The efficient and precise description of surfaces of optical elements has been a long-standing challenge. The most widely used form of such a representation is given by the Zernike polynomials [1]. A more recent idea has been to use a set of polynomials characterized by the orthogonality of its gradients [2,3].

The requirements for diffractive elements (DOE) used in interferometric measurement techniques are increasing. To achieve the targeted accuracy of a few nanometers, the generated phase of the DOE has to be controlled or known down to values of

λ

/100. Additionally, the structures used in DOE design are getting smaller, so that a scalar description of the element is no longer sufficient. We studied the influence of fabrication errors on the generated phase for binary gratings using rigorous coupled wave analysis (RCWA). Binary gratings were chosen, as they are one of the most commonly used structures in interferometry. Our studies show that all geometry parameters have a non-neglect able effect on the generated phase and have to be controlled carefully. Also the effect of the polarisation state of the incident light, as well as the incident angle have to be taken into account. In order to measure the phase of these structures, an inverse problem has to be solved, as the absolute phase can not be measured directly with the targeted accuracy. Hence, accessible measurands like intensities have to be used. Ellipsometry is an established technique for the characterization of micro- and nanostructures. It allows to efficiently measure the optical response of the structures as a function of different parameters such as wavelength and angle of incidence. To solve the problem ways to link the ellipsometric measured intensity to the generated phase have been analysed.

The scattering of electromagnetic waves from rough surfaces has been actively studied for more than a century. This is mainly caused by its involvement in vast application areas, including remote sensing of the environment, medical imaging, sonar, optics, and astronomy [1-3]. In the last two decades, great advances have been made by incorporating multiple scattering effects into analytical approaches [4-6]. However, analytical models are valid only in some specific application ranges [7]. In order to gain a fundamental understanding of how light interacts with a wide range of rough surfaces, we choose to develop a rigorous numerical approach for light scattering from rough surfaces of general media. For this aim the full Maxwell equations were solved using surface integral equations combined with a boundary element method [8-9].

Although modern day microscopy offers a wide variety of methods to examine all kinds of specimen, the swap of methods can at times be lengthy and quite complex, despite optimized exchange elements, such as filter-wheels, objective-revolvers or blind-sliders.

Phase imaging is highly desirable for industrial research and biomedical investigations, since the phase contains information about the 3D shape of the object or the inner structure for the transparent specimen. Several approaches have been proposed to obtain the phase of a wavefront. The most common is based on interferometry [1-3], where the object wave interferes with an additional reference wave, and the phase is reconstructed from the formed hologram. This approach has high accuracy for phase measurement, but the need for the additional reference wave makes it complex, and sometime inapplicable. The beam-propagation-based approaches (without reference wave) [4-8] are alternative where the diffraction patterns are recorded at different plane [4, 5], with different wavelength [6] or different modulations of the object wave [7, 8], and the phase is recovered by iteration between these diffraction patterns.

Optical phenomena can be investigated without extensive ray-tracing within the phase space picture, where it is also possible to include interference and diffraction effects. The phase space concept provides an entire picture of the angular and spatial distribution of light. It is also possible to illustrate important radiometric quantities like irradiance and intensity of the incident light. Especially in illumination optics the transport of radiance L(x, u) is relevant and the phase space offers a different perspective onto illumination problems [1].

Is there a way to see an object obscured by a strong diffuser such as a transmissive ground glass or a reflective opaque surface? Such a question has long been addressed in the context of inverse scattering problems [1], and a technique has been known that can detect a 2-D periodic grating structure hidden by a diffuser [2]. Recently a technique of ultrafast time-of-flight 3-D imaging that can look around a corner using diffusely reflected light was demonstrated [3]. Also a SLM-based technique that compensates the random phase and permits imaging a 3-D object through a diffuser has been reported [4].

Extreme ultraviolet (EUV,

λ

=13.5 nm) lithography is expected as the next generation lithography to the ArF immersion lithograph (

λ

=193 nm ). Projection optics is composed of several aspheric mirrors and its aberration should be about 1/30

λ

rms (0.45 nm rms) or smaller. In order to fabricate such accurate optics, very high precision metrology tool with the sensitivity of 0.1 nm rms or higher is required. In addition, after completion, aberration may change because of the environmental change or the aged deterioration. So the aberration must be monitored at intervals on the exposure apparatus. As an accurate metrology tool using EUV source, point diffraction interferometer (PDI) or other several tools have been proposed [1]-[4]. In these interferometers, an approximately spherical wavefront emerging from the pinhole is used as a reference surface. Because the pinhole size is usually about 100 nm or less, the radiant intensity drastically decreased through the pinhole, so a very high brightness source like a synchrotron radiation source (SR) is required. However, such a large-scale equipment is not suitable for on-machine sensing. Plasma-induced EUV sources are widely used for various purposes as the compact source. This source has low brightness that is 10

− 6

to 10

− 7

lower than that of SR, and the real-time monitoring is impossible. We developed a new wavefront metrology tool using such a low brightness compact EUV source.

Homogeneity tests for glass samples require the separation of the refractive index variations – which are to be measured – from the unwanted contributions of the surface deviations of the sample. The influence of the surface deviations can either be reduced by physical means, i. e. by immersing the sample with an index matching fluid, or it has to be eliminated by suitably combining measurement results obtained in reflected as well as in transmitted light [1,2].

In recent years, the specifications and tolerances for aberration control in the lithographic industry have reached a level at which, apart from the static aberration, also the dynamic contributions, that are for example due to lens heating effects, have become significant. Although, most lithographic systems are already equipped with on-tool sensors to measure their aberrations, these sensors commonly operate in between wafer exposures. Consequently, these measurements are, in a sense, static and will never be able to pick-up dynamic contributions that can occur during the scanning motion of a full wafer exposure. As an alternative, the aberration estimation method proposed in this contribution relies on the analysis of aberration information encoded in printed resist features on the wafer. As a result, this method has at least the potential to pick up dynamic aberration effects that occur during wafer exposure. The downside of wafer based approaches is, however, that they are rather indirect and therefore prone to other complications. Nevertheless, we have been able to successfully devise a method that has proven to be feasible experimentally.

Current international standardisation effort in the field of areal surface texture measurement includes the development of a series of documents describing the nominal characteristics of, and calibration methods for, areal surface topography measuring instruments. Technical committee ISO/TC 213/WG 16, which is directly involved in developing such documents, identified, based on the fact that all areal surface topography measuring instruments produce 3D data sets of points [1], a set of metrological characteristics applicable to all these instruments regardless of their design or operation. Metrological characteristics are input quantities, in a measurement model, that can be measured directly [2], generally using calibrated material measures, and make an immediate contribution to the measurement uncertainty associated with the measured coordinates provided by the instrument. The set of metrological characteristics includes: measurement noise; residual flatness; amplification and linearity of the scales; squareness of the axes; and resolution. In two previous papers, methods to determine measurement noise and residual flatness [3] and amplification, linearity and squareness of the axes [4] were presented. A full paper on the determination of resolution is under review at the time of writing [5] and this work is also summarised here.

There are various optical methods for the measurement of the geometrical shape of objects. Some of them are suitable for the measurement of the shape of objects with optically smooth surfaces. They are, for example, classical interferometry and phase measuring deflectometry [1, 2].

Interferometers are widely used in industry for high precision measurements of optical surfaces. Precisely defined phase shifts (e.g. (/2) between reference and test surface are required to measure the topography of a surface by conventional phase shifting interferometers. Therefore, it is necessary to keep the phase shift stable during the measurement. The result is that the measurements are taken far away from the manufacturing process and the interferometers are mounted on vibration isolation tables to avoid disruptive vibrations. Vibration tolerant real-time interferometers are available but expensive. In [1] a vibration tolerant low budget interferometer for the measurement of plane surfaces is presented. The interferometer copes with vibrations and uses them to create phase shifts during the measurement. This paper describes the modified setup and evaluation for the measurement of spherical surfaces in the presence of vibrations.

Metrology is the science of measurement and its application. Whenever one performs a measurement one compares the measurand to an appropriate unit. The number of these units has changed since the metre convention in 1875 to 1960 when the 11

th

CGPM (

Conférence Générale des Poids et Mesures

) decided the actual SI (

Système International d’Unités

). Currently the SI is based on seven units – kilogram for mass, second for time, metre for length, ampere for electric current, Kelvin for thermodynamic temperature, candela for luminous intensity and – since 1971 – mol for amount of substance. Their definition also sometimes changes, due to demands on the unit or the advent of new methods for a possible better realisation. The metre, for instance, was incipiently defined as a part of a meridian of the earth. This definition was impractical to realise, so for the realisation the length between the ends of a platinum bar was used. To overcome the difficulties of the handling and measurement of this length standard, the definition was again revised to the distance between two marks on a new, x-shaped Pt-Ir rod. Although the measurement capabilities grew and even though interferometry was discovered early, the next redefinition took two ages. Eventually, in 1960 the metre was defined by a multiple of the wavelength of the radiation of

86

krypton. And since 1983, for the present finally, the metre is defined by a time-of-flight definition: one metre is the distance which light travels in 1/299792458 seconds where this special number is given by the fixed value of the speed of light.

Digital holography is a powerful and versatile full-field measuring technique that is able to measure nanometer scale displacements of surfaces. Despite some disadvantages, the technique has become increasingly useful with the rise of commercially available high-resolution, high-bit-depth cameras. Middle ear biomechanics is one example of a research field that can benefit from digital holography [1,2]. In the past, the motion of the eardrum or tympanic membrane (TM) has been measured using a wide variety of techniques, ranging from time-averaged classic holography [3] and single point laser vibrometry [4] to X-ray stereoscopy [5]. More recently, stroboscopic digital holography was introduced in this domain, since it allows measuring the full-field time-resolved motion of the TM in a wide frequency range with a nanometer resolution [1,2]. Therefore it allows us to study traveling waves on the TM. In this work, we present a new setup for digital stroboscopic holography, including a high-energy pulsed Nd:YAG laser and synchronization electronics which makes it possible to record holograms at well-defined time instants in the vibration period at high and low vibration frequencies. As a proof-of-concept, results on a circular piece of stretched rubber are shown, as well as results on rabbit and human TM’s, together with a discussion of the challenges that arise with these samples.

The state of polarization (SOP) is one of properties describing a wavefront, besides the amplitude, the phase, the coherence length and the wavelength. The beam whose state of polarization varying with the time and the location is also known as vector beam [1, 2]. In consideration of the complexity distribution of vector beams, full field and real-time detection methods are urgently needed.

Over the past decade there has been rising interest in the class of nondiffracting light beams and structured light fields. Structured light fields have increased relevance in optical trapping, manipulation and organization. Non-diffracting beams feature a significantly increased Rayleigh length and thus are suitable for optical potentials that are extended along the beam axis. Till now a variety of different optical non-diffractive beams and optical fields carrying orbital angular momentum have been investigated. This covers Laguerre–Gaussian beams, Bessel or Bessel-Gauss beams, Airy and Matthieu beams, which can carry an optical angular momentum of ℓħ per photon and have an azimuthal angular dependence of exp(in

ϕ

), where ℓ denotes the index for the unbounded azimuthal mode and

ϕ

is the azimuthal angle.

Particle Image Velocimetry PIV has been widely adopted in many areas of science and engineering for measuring the flow in fluids, and the pseudo flow, translational motion, in solids. In both applications, fluids and solids, the necessity for the presence of particles has been solved by the introduction of particles in the samples being monitored. The interference in the sample by the physical introduction of particles can be considered therefore a drawback of this methodology. In solids in particular, it is possible to overcome that drawback by providing a laser speckle pattern image on a solid surface, thereby mimicking the external particles needed in the correlation of the images during the PIV implementation. In fluids this possibility also arises, and in this work we present an approach to associate laser biospeckle to PIV analysis in fluids in the complete absence of external particles. Biospeckle, which is a term used to characterize the dynamic laser speckle, is the phenomenon that occurs when we illuminate a sample which shows dynamic change of its laser light scatterers. The difference in applications to fluids is related to the boiling effect of the speckle grains which in solids will behave in pure translational movements that will not occur with the dynamic speckle in the flow of fluids. The boiling effect in the flow of fluids has the translational movements but also the random boiling effect observed in the biospeckle. The challenge was then to isolate the translational contribution of the speckle grains and use them as a reference to PIV. The experiment was done using water and gel in a known flow, illuminated by a 632nm laser beam. The biospeckle images collected were processed in order to filter the signal and isolate the information related to the translational motion of the fluid. A collection of images at different times were filtered using the wavelet transform, with the PIV being carried on in the range of frequencies linked to the translation of the fluid. The results demonstrated the potential application of the proposed approach, which needs some thorough adjustments depending on the fluid under analysis. The methodology presents high sensitivity to the velocity of the flow, to the size of the grains, and to the density of the fluid proving itself to be better used in micro-flows.

While from a physical point of view the ray model of light propagation is outdated, it is still extremely useful for the design and modelling of optical instruments due to its simplicity and intuitiveness. It is therefore important to understand the connection between the ray model and the more physically correct wave model, in order to determine the former’s limitations and range of usefulness. Such understanding also sheds light into what a ray represents within the wave model: are rays normals to the wavefronts? plane waves? “local plane waves”? flux lines? Gaussian beams (or their centroids)?

Confocal gating is a very efficient tool to restrict light reaching the detector of an optical sensor to a certain volume. In fact, it was invented to facilitate imaging inside biological, strongly scattering media [1]. From there on a number of different sensor implementations relying on confocal gating were developed and so called confocal microscopes have been used extensively in many kinds of applications [2]. Meanwhile their popularity is so high, that confocal microscopy has been put on Natures list of milestones in light microscopy [3]. In classic confocal sensors, imaging is achieved by mechanically scanning the measurement object in axial direction relative to one or several optical measurement spots in parallel [4, 5]. This mechanical, axial scan has been replaced later by a spectral distribution of the measurement spot in axial direction both for point sensors [6] and areal sensor configurations [7]. More recently this chromatic confocal measurement principle has been combined with spectral interferometry in order to achieve a more constant lateral resolution over the measurement range while retaining an interferometric axial resolution [8]. Also it was realized recently that even in measurement systems, that utilize coherence gating [9], confocal gating is inherent and has to be taken into account [10]. Furthermore, it was shown that by properly aligning the properties of the coherence and the confocal gating, resolving power of coherence gating sensors can be improved [11].

Military grade MEMS gyroscopes are critical for navigation in guided munitions. In such applications the sensor must survive extremely harsh launch environments and maintain integrity, often requiring sensitivity to loads many orders of magnitude lower than those experienced during launch. Previous inertial measurement units (IMUs) have proven to remain functional at loads up to 20,000 g’s [1]. In other work, gyroscopes (as well as other MEMS) have proven to survive loading up to 60,000 g’s [2]. In these previous works, performance was examined exclusively by electrical means. The current study aims to monitor the mechanical effects that such loading will have on the die and structure through interferometric shape measurement of the sensors.

The advance of industrial manufacturing processes of high-precision components with fast production cycles poses great challenges for modern measurement techniques both in terms of accuracy and measurement speed. A promising candidate to satisfy both of those requirements is digital holography (DH) [1-9]. DH is a non-scanning optical measurement technique that intrinsically allows for resolution in the range of nanometers or even below on specular surfaces [2]. So far three major challenges have prevented DH from being applied in a wide variety of applications. The first is the limited axial measurement range given by half the wavelength that is used for measurement. The second is the fact that on rough surfaces speckles overlay the measured light field and prohibit direct access of the phase of the diffracted light field. The third challenge is posed by the time consuming numerical methods that have to be used to reconstruct digital holograms consisting of numerous sampling points, i. e. pixels.

Coherence scanning interferometry (CSI) [1][2] is a non-contact versatile and fast technology for precision measurements of samples with lateral dimensions of a few micrometers up to a few centimeters. CSI can even measure on rough surfaces and have a large vertical measurement range which is only limited by the maximum displacement of the employed mechanical scanning stage. The resolution and accuracy in vertical direction can be in the nanometer range but are strongly dependent on the positioning noise and accuracy of the scanning stage.

We addressed the question of how to assign confidence limits to the measured trajectory of a moving MEMS membrane. This was done by calibrating our SSWLI setup using a traceable piezoelectric transducer as a transfer standard. We calibrated our SSWLI setup for quasi-dynamic measurements of vertical movements up to 14

μ

m with standard uncertainty of 9.4 nm.

A simple method for measurement of temperature and temperature profile around heated wire is demonstrated by using digital holographic interferometry (DHI). A large numbers of experiments were conducted on different heating conditions of wire. Analysis of experimental results show that higher accuracy, better spatial resolution and higher sensitivity can be achieved by using digital holographic interferometry in comparison to other optical methods. Experimental results obtained by DHI and measured by Platinum-Platinum Rhodium thermocouple agree well within the experimental limits.

We present an absolute method of shift-rotation to calibrate the flatness of flats. Compared with the conventional three-flat test method, the method requires only two flat and no flips of the flats. And there will be no sag errors introduced by gravity deformations because of the flips of flats. The details of the method are given out in the paper. With multi-rotational measurements and translation measurements, the surface deviation of flats can be obtained with the method. Experimental results are presented to show the availability of the absolute method.

The primary function of the middle ear is to bridge the acoustic impedance difference between air in the ear canal and the fluid of the cochlea. At audible frequencies and at sound pressures below about 96 dB SPL, this system is thought to behave linearly. However, measurements in gerbils have indicated that at higher sound pressure levels the system begins to show small nonlinearities, which increase with the pressure level [1]. Given that they have been measured in gerbils, the same behaviour can be expected in other mammals, including humans.

Optical metrology for the characterization and inspection of objects has become increasingly popular and mature in recent years [1]. The principles of various optical metrology methods may be quite different. Among the large variety of optical measurement technologies, the phase-aided active stereoscopy (PAAS) based on sinusoidal fringe structured illumination becomes one of the most attractive approaches [2]. The PAAS can acquire the range images with high accuracy and dense point cloud thus it is widely used for surface profile measurement. Nevertheless, the inspection task with single three-dimensional (3D) optical sensor will be confronted with a serious of challenges, such as the limitation of the field of view (FOV), the contradiction between measurement accuracy and volume. To measure large and complex surface with single 3D optical sensor, it should be recognized that employing a relatively small FOV and multiple viewpoints to cover the entire surface area would be a simple and effective process to get a 3D surface model with high relative accuracy and spatial resolution. However, in the case of industrial inspection, if the objects cannot be moved and rotated freely, it would be a complex and cost solution to change the viewpoint automatically.

Application of the use of the structured light projection for a real solid digitalising and its recreation in 3D, full 360º vision, and replication of real pieces is a very well-known task. In spite of the well recognized of the technique, it is continued looking for new applications in different areas of the science and technology. In this communication, it is presented the possibility of measuring 3D surfaces that are immersed in a transparent media through the use of a fringe projection pattern. The importance of this application is that we find the necessity to make these measurements in hazardous media such as objects immersed in marine water, under fog, rain, or environmental contamination; also for targets surrounded by a medium transparent to the used light, for instance organic or biomedical media. The described technique is a near field one. This is, for objects that are submerged in transparent medium of small thickness or liquid depth (shallow). This measurement was made with a dummy submerged in turbid water. The wet medium was characterized optically for different light sparse and absorption conditions. As a consequence of those parameters, the visibility or the optical quality of the projected fringe pattern in function of the turbidity conditions was studied, giving fieldwork opportunities, such as measuring, collecting and recording data inside hazardous environments.

Digital shearography and fringes projection have been proven as value techniques for non-destructive testing of a wide range of materials. Booth methods are appropriate for remote measurement in “real” time operation mode and working conditions. This is essential for continuous monitoring, fault detection and failure prediction of objects subjected to risk, museum exhibits, materials and constructions with non linear mechanical response.

Analysis of butane torch burner flame is of great importance as it is used in glass blowing, domestic purposes and in various industries. [1]. A large number of optical methods that are full-field, non-contact and non-invasive have been investigated to measure the temperature and temperature profile of gaseous flames. These methods are discussed in [2-4]. DSPI uses the electronic devices such as CCD/CMOS for recording of interferograms. The resulting fringe pattern has similarities with conventional holographic interferometry (HI) and digital holographic interferometry (DHI). The image subtraction in DSPI is simpler than the numerical reconstruction in digital holography. DSPI can be used to study large size objects in comparison of digital holography. In this paper, the study of the butane torch burner flame (pre-mixed and diffusion) by using DSPI is presented.

Cardiovascular disease is one of the most important health problems worldwide and its prevalence is expected to rise [1]. A screening method to detect risk for cardiovascular events is much sought after. A major factor in cardiovascular problems is increased arterial stiffness, or arteriosclerosis [2]. Every heartbeat, a pulse wave emerges from the heart, propagating through the arteries at a velocity in the range of 5-13m/s in a healthy person: the pulse wave velocity (PWV). PWV is proportional to arterial stiffness, and PWV has a strong predictive value for cardiovascular disease and overall mortality. A possible approach to detect arteriosclerosis is to measure PWV locally, at the common carotid artery (CCA) in the neck. The CCA is easily accessible, and it has great physiological importance in onset and progress of cardiovascular health problems [3]. In the proposed work, PWV is assessed in the CCA using fringe pattern topography (FPT).

The project “Micro-Macro-Kinematics”, which is currently running at the Institute of Measurement and Automatic Control of the Leibniz Universität Hannover, deals with the development and analysis of a robotic system, which is able to manipulate micro-objects with an accuracy of 1-2 micrometers in a 3D-workspace of 10 cubic millimeters. For this purpose it is necessary to determine the position and orientation of the robot end-effector with high accuracy and in real time.

The current 50% market share of diesel engines in European passenger cars is attributable to impressive improvements in efficiency and power combined with progressive reductions in nitrous oxides, hydrocarbons, carbon monoxide, and particle emissions. Fuel injection systems are central to diesel engine design, and it is here that significant precision engineering advances have contributed to the success of these systems. In the latest common rail systems, a high-pressure pump stores a reservoir of fuel at 2000 times atmospheric pressure to optimize fuel atomization. These extraordinary pressures place high demands on the precision manufacture of injection pumps and valves. Common rail technology will be the principal means of complying with the Euro VI emission standard, which comes into effect in 2014 [1].

In ferroelectric materials, the piezoelectric, pyroelectric and electrocaloric effect and thermal expansion are caused by the coupling between the thermal, electrical and mechanical properties. Due to this multifunctional coupling, such materials are used in a wide range of technological applications like sensors, actuators, thermal detectors and novel solid-state coolers.

At present, polymer-dispersed liquid crystals (PDLC) are widely used as operating elements of optical devices such as microlenses and the elements, whose principle of operation is based on light scattering [1]. PDLC light scattering features depend on a shape and size of liquid crystal (LC) drops as well as on the director orientation inside of drops.

Investigation about the effect of magnetic field on the temperature and temperature distribution in diffusion and pre-mixed flame is presented.

The optical techniques are non-contact, non-destructive and highly efficient for diagnostics of rough surfaces. The optical techniques may be divided into profile interference and heterodyning techniques [1, 2], techniques based on measuring of the angular distribution of scattered radiation[3], and optical correlation techniques [4, 5]. However, when the height of surface inhomogeneities exceeds the wavelength of the probing beam the unambiguous connection between the statistical parameters of the roughness and of the scattered field is lost.

Digital holographic microscopy [1-8] is a powerful tool for the imaging of micro-objects contained into a three dimensional (3D) volume. Many techniques accomplish the tracking with good detection accuracy of particle, microorganism and cells [9-13]. The most popular approaches adopted in digital holography for the 3D tracking of micro-objects consist into estimate the 3D position dividing the computation process into two parts: the estimation of the coordinate along the optical axis of the imaging systems through automatic focusing on the amplitude reconstruction of the digital holograms, and the estimation of the transverse coordinates on the phase reconstruction of digital holograms computed at distance

d

equal to the estimated focal plane. About the estimation of the focal plane, several automatic focusing methods were developed in digital holography for pure phase objects. These techniques are based on a suitable image contrast parameters and perform a numerical scanning of the focal plane [14]. The estimated focus distance is typically achieved in their global maximum or minimum. Instead the localization in the plane can be computed through different image segmentation methods. The accuracy of each of these techniques is closely related to the kind of object to be tracked [11].

The ›NMM-1‹ nanomeasuring machine, developed at the Technical University of Ilmenau, Institute for Process Measurement and Sensor Technology and produced by SIOS Messtechnik GmbH, provides the opportunity for conducting many different types of measurements in nanometer range [1]. The NMM-1’s excellent metrological characteristics are being exploited for calibrating transfer standards, such as step-height standards, one-dimensional and two-dimensional lateral-displacement standards, flatness standards and roughness standards, at several government institutions around the world. The NNM-1 exhibits 0.1nm resolution in a measurement volume of 25x25x5mm

3

and can be equipped with various types of probe systems [2], [3].

Perishable products must be marked with labels which include the recommended sell-by date and batch number. This information can be included in paper labels or it can be directly written on the container, can or bottle. Usually the alphanumeric characters are formed by an ensemble of discrete points and a scan with a pulsed laser may be used to produce these dot marks. Another possibility is the use of a dynamic diffractive element which produces the date and batch number in a single shot. A phase only liquid crystal spatial light modulator could be used to display the diffractive element (DE).

Characterization of acoustic fields in the power ultrasound range in water is a common problem in diverse application areas like sonochemistry, biomedicine, or industrial cleaning. Different approaches exist for the visualization and mapping of such acoustic fields, being a classical solution the mechanical scanning with pressure sensors (typically, hydrophones) over a grid of points [1]. For high intensity ultrasound, the analysis of bubbles trajectory has also been employed [2]. Alternative optical techniques are the scanning of a pointwise sensor (PIV, LDV) [3, 4], and also full field techniques like deflectometry or schlieren [5], smooth wavefront interferometry [6], holographic interferometry [7], ESPI and similar interferometric speckle techniques [4] or light diffraction tomography [8].

The measurement of the entire 3-dimensional geometry of a transparent object like a lens is a challenging task for established measurement techniques. In addition to some process-specific drawbacks, like the long measurement time for scanning techniques or the small robustness for interferometric approaches, many optical measurement techniques suffer from the fact that they have to measure the front and rear surface of such a specimen in reflection, which implies some major drawbacks:

1

The measurement signal results from the residual reflectivity of the transparent object, which is an undesirable effect with small magnitude. Particularly in the presence of anti-reflective coatings the measurement is very sensible for disturbances caused by ambient light. Furthermore, additional reflexes from the rear surface are a common problem.

2

The geometric relation between the front and rear surface is lost if they are measured sequentially in reflection. The rotation around the optical axis can be determined by means of a marker but the measurement of tilt and distance of the surfaces is challenging and demands precise calibration of the specimen mount or additional measurements, with a coordinate measurement machine for instance.

3

In addition to the geometry the refractive index of the specimen determines the optical properties of a lens but its effect is not included in a reflective measurement. Thus the refractive index must be assumed as known or measured by other means.

The measurement of temperature field could not usually be done without the use contactless, non-invasive and very precise method. Several fundamental requirements which are generally in contradiction have to be fulfilled. The first one is measurement in large area exceeding 100 x 100 mm, need of high frame rate capture (kFPS), high sensitivity of the method and ideally multidirectional measurement for tomographic reconstruction.

White light interference (WLI) microscopy is a non-contact and high-resolution quantitative imaging technique which has been utilized extensively for 3D-surface profilometry of industrial objects. Another important area for the utility of WLI is the quantitative imaging of biological samples, such as, human red blood cells (RBCs) [1] for the measurement of refractive index (RI) of RBCs at different wavelengths. The RI is a vital parameter to find out the state of biological cell, i.e., disease diagnosis non-invasively [1]. There has been various phase microscopic techniques such as, Fourier phase microscopy [2], Hilbert phase microscopy [3], digital holographic microscopy [4], diffraction phase microscopy[5] etc. In all these methods the RI profile is reconstructed for a single wavelength of illumination. But the RI is a function of wavelength. Multiple wavelength interferometry, such as, spectroscopic phase microscopy [6] and quantitative dispersion microscopy [7] etc are used to measure the RI of RBC at different wavelengths. But these techniques use white light with color filters or diffraction gratings and multi-color lasers to measure the RI at multiple wavelengths. We use WLI in conjunction with color fringe analysis to determine RI of RBC’s at multiple wavelengths using a single chip color CCD camera. Our technique does not require multiple color filters and multiple laser sources.

Photoeleasticity has a long history of application to material testing. However, it often requires a complex coating of a suitable material to reveal the stresses experienced. The alternative is to create a transparent birefringent model. The disadvantages being that the measurements can be masked by the effect of the coating applied or in the case of a model, fail to mimic the properties of the original object. Further the coating or modelling process severely limits the type of application which can be applied. In this case the aluminium has a been anodised with a 8 micron phase sensitive coating.

Incoherent Optical Scatterometry (IOS) is a well-established technique in electronic industries for semiconductor metrology. Using this technique one is able to retrieve the properties of the scatterer using the diffracted field. In conventional optical scatterometry, light from an incoherent source is incident on the scatterer (grating) and after the interaction, the angular spectrum is recorded by a detector in the Fourier plane (called far field intensity data). The diffracted far field contains the information about the material composition and the geometrical profile (shape parameters) of the grating. Any change in either the material composition or the geometrical profile of the grating will result in a nonlinear change in the far field data. The technique has several advantages: it is fast, efficient, easy to implement and free from diffraction limit.

Many improvements have been done in 3D imaging of a micron-sized biological specimen using confocal scanning fluorescence techniques [1]. Other imaging approaches, like selective plane illumination microscopy [2] were aimed at imaging larger organisms. Despite high image quality, these methods have some limitations like being able to image only stained particles inside the organism. Label-free 3D microscopy methods have been also developed based on auto-fluorescence or scattering properties, like optical projection tomography [3]. Dark-field microscopy also showed to be an effective approach for contrast and resolution enhancement [4]. Lee et al. took the advantage of this approach and combined it with optical sectioning microscopy to achieve a high contrast 3D image of a biological specimen [5]. Being a scan-less and also a label-free technique, digital holographic microscopy (DHM) can be used to extract the object wave-front using a single image (digital hologram) [6]. It makes it possible to digitally focus on different layers of the specimen any time later and reconstruct a 3D profile of the optical thickness of the organism using the quantitative phase value [7]. Dubois et al. [8] and Verpillat et al. [9] both implemented dark-field approach in a digital holographic system to image and track nanoparticles. In dark-field imaging, because of having no bright background, the noise coming from the coherent light in the background is suppressed, leading to a significant contrast enhancement.

Hearing processes involve series of physical events where acoustic wavefronts are transduced into mechanical motions, chemo-electrical reactions, and into electrical signals that are interpreted by the brain [1]. At the very beginning of these series of events, when the acoustic wavefronts are coupled into the Tympanic Membrane (TM), shape and sound-induced displacements of this cone-shaped membrane play a primary role [2-4]. For a better exploration of the secrets of hearing, quantification of these two characteristics, shape and displacement, which are in millimeter and nanometer scale, respectively, is important [5]. Several research groups have been studying 1D sound-induced displacement of the TM [6,7], however, investigations of the 3D motion of the TM need further research. In this paper, shape and 3D sound-induced displacements of the TM are measured by a single lensless Dual-Wavelength Digital Holography System (DWDHS).

As we enter the age of quantum technology, solid-state defects become a leading contender for storage of quantum information, quantum information processing and quantum communication. Two of the most exciting prospects of quantum technology are the creation of computers that take advantage of quantum rather than classical laws to outperform current devices, and the realization of highly sensitive magnetometers limited only by quantum uncertainty. In pursuit of these two goals, many proposals and proof-of-principal experiments have been performed in the solid-state, which required location of defects very close to the host crystal’s surface. The contribution reviews some of the recent progress in the field.

Recently, there has been considerable work in studying the natural singularities of scalar optics, namely, optical vortices. The vortices are rotational phase structures, with a singularity at center where phase is undefined. Study on such structures has become a new topic in modern optics called

singular optics

[1]. The concept has been extended to vector optics as polarization singularities, where the geometric polarization parameters of coherent fields cannot be defined [2]. The two types of polarization singularities are the C-points where the major axis of the polarization ellipse is not defined and the L-lines where the polarization handedness is not defined. These structures have been observed in the transverse plane of different fields [3-5]. The polarization singularities have significant importance in understanding the fine details of beams with complex polarization structure.

In recent years optical 3D measurement techniques in the micro- and nano-range have become increasingly popular due to their non-destructing nature and their ability to obtain areal 3D measurements. However, most of these techniques, especially those that rely on vertical scanning, have not been fast enough for inline measurements, where measurement times of a few seconds are required. Here we present an optical measurement device based on the Focus Variation principle that is able to perform 3D measurements with 4 million measurement points within one second, making this device to one of the fasted optical 3D metrology devices in the world.

Due to both, 100% fill factor and the high shape gradient, the silicon molds are difficult to fabricate and are also demanding in characterization by optical methods. In the paper we present a method that overcomes some of measurement problems and can be used for recovering high numerical aperture (NA) shape of reflective microstructures, such as silicon molds. To achieve this practical goal we use the digital holography in microscope configuration with afocal imaging system [1] working in reflection mode. The standard method for topography reconstruction in optical full field metrology uses thin element approximation (TEA). In this paper we deal with the high NA optical field generated by an object. TEA would produce significant errors and cannot be applied. There are two algorithms that allow shape reconstruction with smaller error: the extended depth of focus (EDOF) [2] and the local ray approximation (LRA) [3,4]. The first one computes the shape from unwrapped measured phase. The phase is used in a refocusing algorithm to obtain the local object height from the optical field. The phase in this plane is reconstructed by TEA algorithm. Second algorithm is based on analysis of a local ray’s optical path differences in object. Both algorithms can be applied under condition that entire optical field is transferred by the imaging system. Also the algorithms require knowledge of the precise location of the plane from which the phase originates [5].

Study of the internal energy flows is a rapidly developing branch of physical optics (see, e.g., Refs. [1–8]). The internal flows (optical currents) not only constitute an “energy skeleton” of a light field, which reflects important physical characteristics of its spatial structure. They have proven to be valuable instruments for investigation of fundamental dynamical and geometrical aspects of the light fields’ evolution and transformations [1–5], provide a natural language for explaining the special features of singular fields [1, 2, 4-6], fields with angular momentum [3,6-8] and for interpreting the effects of spin-orbit interaction of light [5]. As physically meaningful and universal parameters of light fields, they permit to disclose physical mechanisms of the beam transformation upon free and restricted propagation and offer attractive possibilities for characterization of arbitrary light fields [5].

Mechanical vibration measurement has been an integral part of industry and research since its beginning. Therefore a wide variety of measuring techniques based on different approaches have been developed. The most used techniques utilize Doppler principle, correlation analysis, speckle ESPI (Electronic Speckle Pattern Interferometry), or holography. Holography provides a full-field and contactless measurement with great sensitivity, which makes it very suitable tool for very small amplitude measurement.

Taking into consideration the modern tendencies for developing nanotechnologies, much attention is paid to studying the behaviour of micro- and nanoparticles and the ways of controlling them [1-4]. To do this the micromanipulators and micromachines are needed. As a rule, beams of various nature, which reflect different forms of energy circulation [2], are used for these purposes. The formation of energy redistribution and energy circulation fields can occur in a simpler case as well. This research proposes the interaction scheme of two mutually orthogonal linearly polarized waves in the incidence plane, where the modulation of polarization and the modulation of the volume energy density occur simultaneously in the observation plane [5]. In this case the depth of the energy volume density modulation essentially depends on the degree of coherence of interacting waves. The way of monitoring particles of the Rayleigh light scattering mechanism in the energy inhomogeneous optical field is proposed for investigation. The task of defining the degree of coherence of mutually orthogonal linearly-polarized plane waves using the particle motion velocity in the created optical field is solved as well.

We present an interferometric technique based on differential interferometry setup for measurement in the subnanometer scale in atmospheric conditions. One of the important limiting factors in any optical measurement are fluctuations of the refractive index of air representing a source of uncertainty traditionally compensated when the index is evaluated indirectly from the physical parameters of the atmosphere. Our proposal is based on the concept of overdetermined interferometric setup where a reference length is derived from a mechanical frame made from a material with very low thermal coefficient on the 10

− 8

level. The technique allows to track the variations of the refractive index of air on-line directly in the line of the measuring beam and to compensate for the fluctuations.

Speckle is caused by the illumination of an optically rough surface by coherent light [1] which can cause problems in digital holography. Speckle in a reconstruction reduces visibility of details and may impede important measurements of an object. Speckle can be reduced by capturing multiple holograms and summing the reconstructed intensities [2-4]. Digital signal processing methods, which are applied after capture, have also been proposed [5–8].

A well-known problem with interferometry involves measuring an Optical Path Difference (OPD) larger than the source wavelength.

On-going hearing research efforts are focused on the tympanic membrane (TM) and its role in transforming the energy of the sound waves from the outer-ear into mechanical vibrations of the middle-ear bones. Quantification of acoustically-induced nanometer scale motions of the TM is important for the understanding of the human hearing process. Several research groups have been studying these displacements resulted from tone stimuli [1, 2]. However, transient response of the TM, e.g., the behaviour of the eardrum excited by a short pulse (as opposed to tone) needs further investigations.

Over the last decade, a number of Computational Imaging (CI) systems have been proposed for tasks such as motion deblurring, defocus deblurring and multispectral imaging. These techniques increase the amount of light reaching the sensor via multiplexing and then undo the deleterious effects of multiplexing by appropriate reconstruction algorithms. However, a detailed analysis of CI has proven to be a challenging problem because performance depends equally on three components: (1) the optical multiplexing, (2) the noise characteristics of the sensor, and (3) the reconstruction algorithm, which typically uses signal priors. In this paper, we utilize a recently proposed framework incorporating all three components [13]. We model signal priors using a Gaussian Mixture Model (GMM), which allows us to analytically compute Minimum Mean-Squared Error (MMSE). We analyze the specific problem of motion and defocus deblurring, showing how to find the optimal exposure time and aperture setting for defocus and motion deblurring cameras, respectively. This framework gives us the machinery to answer an open question in computational imaging: “To deblur or denoise?”

Digital holographic microscopy (DHM), when applied to semitransparent samples such as living cells, provides accurate measurements of phase shift resulting from a mean refractive index accumulated over the cellular thickness [1-5]. However a single shot holography is inadequate to provide true 3D reconstruction of a cell structure although such reconstruction is required to solve several hot biomedical topics including: label-free analysis of living cells and tissues, characterization of physical processes in cellular biophysics, extended studies in vascular and tumor biology, as well as recognition and monitoring of bacteria colonies. It has been proved by many groups that the solution can be provided by combining digital holography with tomographic techniques. The resultant method, often referred as optical diffraction tomography (ODT) [6-9] requires multiple complex object field captured for different illumination directions with respect to the sample and latter tomographic reconstructing of a three-dimensional distribution of refractive index. The projections are obtained through varying the illumination direction or rotating the specimen. Number of various approaches has been so far reported to deal with this problem including trapping a specimen with micropipette [10] or optical tweezers [11] or altering the angle of illumination using a galvanometer scanning mirror [12] and multiple fibre optics illumination [13]. In this study we present a cost-efficient, sample-rotational-based tomography module for 3D label-free quantitative live cell measurements based on a hollow optical fiber as the sample cuvette. Several problems connected with implementation of the full tomographic reconstruction process are discussed including tracking of a cell position, minimizing errors associated with a fluid field fiber capillary of a sample cuvette, determination of absolute values of refractive index and the possibility of reconstruction from a limited angle of projections. We also show the examples of exciting results which can be obtained with this tool.

3D-optical microscopes such as white-light interference microscopes, confocal microscopes, and structured illumination microscopes are well-established instruments in micro- and nanotechnology.

Since many years laser triangulation is a key technology for metrological measurements. Manifold sensors exist that measure single spot distances or distance profiles via triangulation between the illumination source (e.g. laser diode) and an imaging system incorporating typically a digital detector. The physical limits have been investigated and derived by several researchers [1]. It has been stated that the main limitation for laser triangulation is subjective laser speckle noise that occurs when the laser spot or line is imaged onto the detector.

Ptychography is a lensless coherent imaging technique. It therefore offers all advantages known from other lensless coherent imaging methods such as digital holography (high phase sensitivity, non tactile, non destructive, wide field imaging, diffraction limited optical resolution). Moreover, it offers new possibilities for radiations outside the visible spectrum where high quality numerical aperture (NA) lenses are difficult to be manufactured such as in X-ray or electron microscopy. In ptychography only diffraction patterns from the coherently illuminated object are recorded. Either the coherent illumination (which can be formed either by a simple aperture upstream or downstream of the object and/or by a lens) or the object is moved laterally to generate diffraction patterns at a series of known positions, which are recorded on a digital sensor. The illuminated areas of the object at each position overlap, which results in a large degree of data redundancy. This redundancy is used to solve the phase ambiguity problem. Hoppe in 1960 [1] originally postulated the ptychographic principle, but only recently with the advent of iterative algorithms [2-5] has ptychography become computationally practical. An inherent property of ptychography is that the phase of an image reconstructed from diffraction-pattern intensity has very high contrast and is quantitatively accurate over an adjustable field of view. Moreover, ptychography in combination with the ePIE algorithm, discussed in [5], results in the recovery of the complex object function and the illumination function. The separation of both terms leads to an increased signal to noise ratio for the reconstructed object function, reduction of disturbing artifacts resulting from the object illumination such as dust or diffraction rings and enables the application within the optical near field region (Fresnel approximation) since the parabolic phase term with respect to the object coordinates is accounted for in the illumination function. Unlike holography, it does not require a reference wave. Hence, it offers a more environmentally stable setup. Unlike standard metrology techniques, in reflection mode it can be used to measure step sizes of less than one nanometer while the apparatus is mounted on a conventional (not an optical) bench. Moreover, an aberration free reconstruction of the object transmission function is obtained (only true for in-line holograms in digital holography, as discussed in [6]). The temporal and spatial coherence requirement are reduced. This enables the application of short coherence length light sources, which results in a reduced influence of scattering effects and internal reflection effects when recording the diffraction patterns. In this manner an increased image quality is obtained.

According to the International Technology Roadmap for Semiconductors (ITRS) the semiconductor industry will start to apply Extreme Ultraviolet (EUV) Lithography for printing high-end electronic circuits within the next few years. For the critical dimensional structure parameters like CD, sidewall angle and line edge roughness (LER) besides SEM also atomic force microscopy (AFM) and scatterometry will be important metrology methods.

Tilted-wave interferometry (TWI) is a novel measurement technique for the highly accurate optical measurement of aspheres and freeform surfaces [1-7]. It combines interferometric measurements with ray tracing and mathematical reconstruction procedures. The first results of a sensitivity analysis were given in [8]. The results support the feasibility of the basic TWI principle, and good reconstruction results were obtained for surfaces whose deviations from the design topography are described by Zernike polynomials. The TWI reconstruction procedure is divided into two parts. The first part reconstructs the long-wave deviation of the specimen from its design form. The second part aims at the reconstruction of high spatial frequencies [7]. This paper investigates the robustness of the long-wave TWI reconstruction method. Therefore, we extend the sensitivity analysis to surfaces whose deviation from their design topography no longer corresponds to a Zernike polynomial. We utilize a simulation environment that was developed at PTB for assessing optical measuring systems [8, 9]. Reconstruction accuracies are then investigated in dependence on the variation of the specimen’s topography and the measurement noise. Different variations of the specimen’s topography with different spatial frequencies are studied. Furthermore, the influence of the deviation of the vertex radius of the design topography is investigated. Throughout all simulations no errors in the characterization of the TWI were assumed to be present which corresponds to a perfect calibration.

Displacement monitoring is a crucial stage in characterization of new materials, composits or microsystems. Depending on application, mentioned tests can be performed by means of point methods (strain gauges, optical fiber sensors) or full field methods (grating interferometry - GI, holography, digital speckle pattern interferometry - DSPI).

Considering modern manufacturing processes, there is an increasing demand for flexible, fast and precise inspection systems. An inspection can for instance consist of the verification of the geometrical form of the object or the detection of undesired surface features, like scratches, dents or bumps. All in all, an appropriate inspection system must comply with two main requirements: On the one hand, any part of the three-dimensional surface of the object must be examined with respect to a lot of different specifications, which often requires different, adapted measurement techniques and sensors. On the other hand, the inspection has to be done on large areas whereas the critical dimension of any feature may be much smaller.

Spatial and spectral information holds the key for characterizing incoherently-illuminated or self-luminous objects as well as for imaging fluorescence. By unifying the principles of incoherent holography [1-4] and Fourier spectroscopy based on van Cittert-Zernike theorem and Wiener-Khinchin theorem respectively, an interferometric method of multispectral imaging has been demonstrated [5].

Nowadays, the inspection of works of art and other artefacts is mostly subsumed under the academic field of

conservation science

.

Conservation science

is a general, by necessity an imprecise term that describes the work of scientists conducting research on works of art and their state of physical preservation. By common definition, work in this field falls into two major, extensively interacting areas:

technical art history

or

archaeometry

on the one hand and

science of preservation

on the other. Researchers in this applied science may come from very different backgrounds, transferring technologies into the field, or acting as interpreters of technical results.

We present a coherent optical measurement technique to determine the complex transmittance function of a transparent object. It is based on shear interferometry with the object in focus and subsequent numerical processing. In contrast to existing approaches [1,2] we illuminate the object by light with limited temporal coherence and periodic spatial coherence provided by an LCD monitor. The method has the potential to combine the large numerical aperture of an extended light source with the accuracy associated with interferometric techniques. Furthermore, the limited temporal coherence reduces systematic measurement errors arising from coherent amplification of parasitic reflections within the setup. As a proof of principle we show measurements of the complex transmittance of a flat with a defined dent and compare them with results obtained from a Fizeau interferometer.

Fringe projection offers a great variety of application fields in geometry measurements of free form elements ranging from large measuring areas down to geometry elements with sizes in the millimetre range. With advanced methods for deviation analysis, errors in fabrication lines can be found in almost real time. Therefore it is an excellent tool for quality control in production metrology.

In-line and off-axis digital holography are now used in a wide range of domains such as fluid mechanics [1,2] and microscopy imaging [3-6]. In the basic in-line holographic configuration (no phase shifting, no sequential recording), the reference and object waves are parallel and the path lengths of both waves are equal. For this reason, the in-line holography is only able to provide an amplitude image of the object. Therefore, it can not be used to measure the phase contrast of any transparent object. For a multiple object recovering, such as particles [1], this method suffers from the presence of the intrinsic speckle noise generated by the twin images during the reconstruction. In the off-axis holography (single-shot recording), the reference wave is shaped to provide a spatial separation in the reconstruction plane or in the Fourier plane of the hologram. In this case, the virtual and real images are well spatially separated during the reconstruction. The use of an independent reference wave induces a sensibility to external perturbations such as vibrations, temperature changes, etc., and leads to an increase in the setup complexity. So as to simplify the setup and to get a real immunity to external perturbations, the in-line configuration is well adapted, but the faculty for the phase contrast recovering has to be invented. In order to overcome both the problems related to the reference wave in off-axis holography and the impossibility of measuring the phase contrast in in-line holography, we propose in this paper a new technique having new features in digital holography: i) simplify the recording set-up by eliminating the reference wave and ii) measure of the phase contrast of the object. This new approach in digital holography is based on the use of a spatial phase modulation so as to produce multiple replicas of the incoming diffracted wave at a given distance.

In optical and automotive industry, there are many measurement applications with a need for high resolutions in the nanometer range under shop floor conditions. These are typical applications for interferometrical methods. Very often, the surface under test has a cylindrical or conical shape, so that the typical camera based flat wavefront interferometer has to be replaced by an optical stylus applied to a form measuring device.

A fringe pattern represents an encoding of spatial information. A means is developed to create the fringe pattern based on a method to encode the spatial variations. This paper considers fringe patterns created by the interference of optical wavefronts with spatial extent, thus representing the spatial variation of optical path difference between two interfering optical waves. How the fringe pattern is created, the selection of the interferometric methods, adds value to the quality of the fringe pattern (the quality of the information encoded in the fringes), but the results, the analysis of the fringe pattern is where the user (customer) derives value.

The application of mid-infrared optics in 3-5

μ

m is booming nowadays, but the tools for metrology still lack. For example, modulation transfer function is commonly used to evaluate the optical transmission system [1]; refractive index measurement requires sophisticated equipment or complex mathematical algorithms [2]. These prompt the demand for interferometer at this spectral band. In view of the cost-effectiveness, one detector should be shared for both alignment and view. Besides, 4.41

μ

m is selected here as the primary working wavelength, but the spectral band should be expanded to make the interferometer mainframe versatile. The working wavelength of commercial mid-infrared interferometer is 3.39

μ

m worldwide, while also considering the common long-infrared interferometer working at 10.6

μ

m, the interferometer should provide a broadband wavelength channel.

In recent years, interferometry-based techniques have been widely used to do non-contact measurement of vibrating or continuous deformation in industrial areas. In these measurements, Nyquist sampling theorem has to be satisfied along time axis so that various physical quantities can be obtained in any instant. Generally, there are two types of techniques: high-speed-imaging-based technique[1] and photodetector-based technique[2, 3]. The former technique is a high-speed camera combined with different optical interferometers, such as digital holography, moiré interferometry or shearography, etc. The latter technique is mainly for dynamic measurement (laser Doppler vibrometer & velocimeter) based on the high sampling rate of photodetector. However, it can only offer a measurement on one spatial point. Figure 1 shows the status of these two-techniques: the camera-based technique has higher spatial resolution but much lower temporal resolution than normal requirement of real applications. Even a 100k frame/sec capturing rate sometimes cannot satisfy the Nyquist sampling theorem, as for every measurement point, the phase change of any two adjacent images should be less than

π

. In addition, high power laser is need for illumination and the cost of high-speed camera also limits the applications in industrial areas. On the contrary, detector-based laser Doppler techniques have enough sampling rate along time axis, but only one measurement point in spatial domain. A scanning device is usually adopted for 2-D measurement. This method assumes that the measurement conditions remain invariant while sequential measurements are performed. Hence, it is only suitable to measure steady-state or well-characterized vibrations. However, most engineering applications do not satisfy these requirements. Transients, including impact or coupled vibrations, are commonly observed in real applications. This makes scanning LDVs impractical to generate a vibration image in these cases. In this paper, several techniques to increase the dynamic measurement capabilities of camera-based interferometry and detector-based interferometry will be discussed. Two methods are emphasized. One is a dual-wavelength image-plane digital holography using high-speed camera[4], and another is a spatially encoded multi-beam laser Doppler vibrometry using a single detector[5]. These two techniques show with a fully utilization of spectrum space, the capability of optical dynamic measurement will be tremendously improved.

Shape measurements of fast rotating rough objects are important for several applications such as in-situ monitoring of workpieces in lathes or roundness determination of shafts. For such tasks the laser Doppler distance sensor technique was invented. This technique is based on two mutually tilted interference fringe systems, where the distance from the sensor to the object surface is coded in the phase difference between the generated interference signals. Large tilting angles of the interference fringe systems are necessary for a high sensitivity of the measurement technique.

However, the correlation coefficient of the interference signals decreases with increasing tilting angles. The signals become more and more dissimilar due to the speckle effect, since different angles of the laser beams generate different speckle pattern. A trade-off between sensitivity and correlation coefficient occurs. It limits the tilting angle between the interference fringe systems to values below 1.5°, which on the other hand results in a low sensitivity of the measurement technique. In order to overcome this restriction a physical modelling and computer-aided simulation of the speckle patterns have been conducted. The outcome of these investigations is a new approach for the detection of the interference signal. The scattered light is received from two different directions matched to the illumination optics, i. e. the tilting angle between the fringe systems is equal to the sum of the main receiving angles of the two photo detectors. By this way, high correlation coefficients have been achieved also for high fringe tilting angles of 7.5°.

Based on the idea of matching of illumination and receiving optics, the measurement uncertainty can be reduced by more than one magnitude. For displacement measurements of a recurring rough surface a total standard deviation of 110 nm were attained at high lateral velocities of 5 m/s. The measurement uncertainty is independent of the surface velocity with a good approximation. Thus, the technique is particularly suitable for measuring fast moving rough objects for example at turning and grinding processes. Due to the simultaneous distance and velocity measurements the diameter, ellipticity, eccentricity and 3D-shape of rotating objects are determined by only one single sensor.

Nanometer traceability has been of great concern and no solution for long time [1-5]. The conventional laser interferometer is able to measure displacement very precisely when the displacement is more than half wavelength. The measurement result is n

λ

/2. The n is the sum of whole numbers of light interference fringes. The decimal fraction of light interference fringes is obtained by using electric subdivision method which may give rise to errors [6-7]. So, we are in doubt as we say how many nanometers displacement smaller than half wavelength due to the electrical division result not a nature benchmark in laser interferometers [8-9]. The X-ray interferometer provides the means for linear subdivision of light fringes in accordance with the lattice parameter of silicon [10-11]. However, the combined optical and X-ray interferometer (COXI) makes the system too complicated and expensive to use in practice widely [12-13].

The ancestral roots of the Fringe conference are in the automatic processing of fringe patterns. When we think of

patterns

, an image comes to mind of flowing lines beautifully wrapped around surface contours. But automatic processing of fringes is not limited to this kind of pattern: The fringes may be laid out along a line of sight as the time history of an object displacement, captured by a detector and processed to tell us something about how the object has moved, or more generally, where the object is with respect to a reference point in space.

Despite the popular employment of objective lenses as focusing devices, parabolic mirror (PM) is intensively used in quite different scopes of science. For instances, mirrors with a small numerical aperture (NA) are normally used in astronomic telescopes and in telecommunications. Some groups also use high NA PMs in single molecule optics as efficient light collecting element [1-5]. In principle PMs are the perfect focusing elements: a parallel incoming beam along the optical axis of the PM can be focused to one point without any aberration. Furthermore by choosing proper deflecting coating material, PM is free from chromatic aberrations and can be used over wide optical frequencies. Nevertheless they are seldom used in imaging applications because of their bad off-axis properties. A small deviation from parallelism of the incoming beam or from the optical axis leads to tremendous aberrations and results in a small field of view. In confocal microscopy the sample is imaged point by point and thus is the perfect optical arrangement for a PM. Especially when combined with a scanning stage, so that the mirror stays perfect aligned with respect to the incident beam. Based on our experiences in the last years, we will demonstrate the integration of a PM in confocal and super resolution optical imaging systems. Applications in single molecule and particle imaging, as well as Raman scattering mapping of molecular monolayer will be shown.

Light fields in the focal region of optical systems or in confined situations are of significant interest in many optical applications. It is known that the axial phase shift (i.e., the phase anomaly which occurs when light is focused) plays an important role in defining the optical response of optical systems, such as laser resonators and a focusing system. Therefore, an enormous attention led to profound investigations on the properties of focused and confined light field. The study of anomalous axial phase features is of cardinal importance, not only in conventional optical systems but also in micro- and nano-optical devices. In this paper, we present a powerful experimental method, which is named longitudinal-differential (LD) interferometry [1], to investigate anomalous axial phase behaviours for micro- and nano-optical problems.

Owing to the advancement of faster electronics and digital processing power, the past decade has seen an impressive re-emergence of digital holography [1]. As with conventional holography prominent speckle noise is a severe drawback for diffusely reflecting objects due to the usual coherent nature of the recording process. When compared with their coherent counterparts, incoherent systems have a much better signal-to-noise ratio. In addition, incoherent systems can capture fluorescence information, a big plus for biomedical fluorescence imaging. Currently, there are only two existing systems in incoherent digital holography. The first one is optical scanning holography (OSH) [2] and the other is Fresnel incoherent correlation holography (FINCH) [3]. Both of the techniques have been demonstrated to capture florescence information holographically for biomedical applications [4]. Recently a complex hologram of a diffusely reflecting object has been recorded without speckle noise using OSH [5]. In this talk, we review such technique and discuss that OSH can operate in three modes of operation: coherent mode, partial coherent mode as well as incoherent mode.

Fringe projection based portable 3D scanners with low measurement uncertainty are ideally suited for capturing the 3D shape of objects right in their natural environment. Battery powered, compact scanning devices without the need for other stationary equipment provide scanning opportunities in-situ at the customer, at the museum or directly at excavation sites. To fully measure an entire object, multiple views are usually required. However, elaborate manual post processing was typically necessary to build a complete 3D model from several overlapping scans (multiple views). Alternatively, expensive or complex additional hardware (like infrared or magnetic trackers etc.) was needed.

The temporal carrier beat frequencies in an optical heterodyne interferometer can be generated by ramping the wavelength in a laser diode (LD) based on the frequency-modulated continuous-wave (FMCW) techniques [1,2]. The beat frequency is proportional to the optical path difference (OPD) of each pair of interfering beams in an interferometer [3]. It enables us to construct an optical heterodyne interferometer on an unbalanced OPD without auxiliary frequency modulators. Multiple-beam interferometry can produce the different beat frequencies by using a wavelength-tunable LD [4]. An LD interferometer has been applied to 3-D imaging by using the optical frequency domain reflectometry [5]. A technique of a holographic radar has been applied to measure the range information with a frequency-tunable laser [6]. 3-D object can be holographically reconstructed by the electronic tuning with beat signals by a current-modulated LD [7]. A phase-shifting common-path interferometer has been demonstrated with a biprism beam splitter [8].

Automatic digitization of three-dimensional (3D) complex objects in a real scene using fringe projection profilometry (FPP) [1-3] has become an important issue in many application areas, especially for the cultural heritage protection. Because of the limited field of view (FOV) of 3D sensor, it is difficult for any 3D sensor to digitize the whole object automatically through a single acquisition, especially for the objects with complex geometric shape and topology. This issue could be addressed with a strategy of mobile digitization by single 3D optical sensor moving around the object. And the iterative closest points (ICP) algorithm, tracking the 3D sensor using other instruments, or other methods is utilized to register the multiple range images [4]. It is difficult for this strategy to achieve automation. Also a larger workspace is required for moving the 3D sensor. In our paper, we focus on studying a strategy of automatically controlled digitizing process and a complete digitization of 3D complex objects. A strategy that utilizes a one-dimensional (1D) array of 3D optical sensors combined with a rotation turntable to get a complete digitization of complex object automatically is proposed. With this approach, two important tasks should be concerned: (1) the calibration of multiple 3D optical sensors, and (2) the automatic registration of multiple range images taken from sensor array. In addition, for digitizing the cultural heritage, the color information is always necessary. Then a high-resolution digital camera is employed to take the photos of the object from different viewpoints. The key issue for correctly mapping of color photos onto the object surface is to accurately determine the mapping relationship between the color photo and the surface model. This can be done with help of the photogrammetry technique. Afterwards, we suggest a texture blending technique that utilizes a composite-weight strategy to blend the color images within the overlapped region, resulting in a photorealistic model.

A planar encoder is a device for measuring two-dimensional planar displacements. A crossed grating is used in the encoder to avoid the turbulence effects commonly encountered in laser interferometers or the Abbe errors associated with stacking two linear scales in orthogonal directions. It is important to make sure that the two periodic directions of the crossed grating be perpendicular to each other in order to reduce the cross error during a two-dimensional displacement measurement.

Fringe projection profilometry is a well-established method in optical metrology for capture of 3D objects [1]. The information is retrieved from the phase of the deformed fringe pattern formed by imaging the object onto a CCD camera after projection of a structured light pattern onto its surface. A variety of phase retrieval algorithms requires projection of sinusoidal fringes. Thin diffraction sinusoidal gratings under coherent illumination are a suitable choice for generation of good quality fringes focused in a large measurement volume [2]. The main drawback of coherent projection is the speckle noise. The lateral spacing of fringes produced by coherently illuminated sinusoidal grating does not depend on the wavelength. Thus, a light source with a wider spectrum can be used for reduction of the speckle noise in the fringe patterns recorded by the CCD camera provided the spatial coherence of the source is kept intact [3]. The aim of the present report is to prove speckle reduction in fringes projected with a sinusoidal phase grating (SPG) by using low coherent point light source with emphasis on LED illumination.

Three-dimensional (3D) measurement of object topographies has become an important challenge, e.g. in industrial quality management. Optical techniques are a well-established method for performing these measurements, as they provide benefits like operating contactless or offering the possibility of full-field measurements. Meanwhile, often not only high accuracy is demanded, but also high speed.

One of the most common absorption media for frequency stabilization of lasers is molecular iodine. It covers a wide spectral range from green to near IR and offers reach set of strong and narrow lines suitable for these purposes [1]. Thanks to this the molecular iodine is also recommended as an absorption media for realization of laser standards of length at several wavelengths by the CIPM committee [2]. Solid-state, frequency doubled Nd:YAG lasers stabilized by saturation subdoppler spectroscopy to hyperfine transitions in molecular iodine are used in metrological laboratories as reference stabilized lasers and represent the most stable conventional optical references for interferometry and other applications in fundamental metrology. They can achieve long term frequency stability better than 1*10

− 14

for an integration time of 100 s, which means more than two orders better result in comparison to traditionally used He-Ne-I

2

stabilized standards [3,4] and the short term frequency noise of these lasers is also lower [5-9]. Further, a linear spectroscopy of molecular iodine is also often used technique especially in case of interferometry applied at atmospheric conditions, where influence of refractive index of air plays a dominant role [10-12].

There are a number of approaches to measure absolute distances interferometrically without the need of mechanical guidance [1, 2]. Prospective use in coordinate machine measurements or laser trackers, for example, has driven this field for the last twenty years. Variable synthetic wavelength interferometry or frequency sweeping interferometry is one of the most promising approaches, offering an in principle indefinite range of non-ambiguity and being based on a single optical source.

High precision cylindrical parts have been used frequently in many fields with the development of manufacturing. It is very important to test cylindrical surface precisely. It’s well known that interferometry is an important technique to test the profiles of the aspheric surfaces [1, 2]. Though the cylindrical surface is a kind of aspheric surface, it is single-axial symmetrical and has some particularities, for example, in testing alignment and the removal of the errors of the alignment. In testing the cylindrical surfaces, it is hard to confirm the correct alignment status and there are some alignment errors remaining [3]. Usually the alignment aberrations are big enough and can’t be omitted in much high precision testing. The paper applies a Fizeau interferometer to get standard plane wave and a Computer-generated Hologram (CGH) cylinder Null to get an ideal cylindrical wave-front [4].Then it introduces the ways to align the measuring system and remove the alignment aberrations. The repeatability of the testing system is verified with experiments with a cylindrical lens.

A conventional analog hologram displays highly realistic 3D images of objects. A holographic printer can record such a hologram onto a holographic emulsion from digital contents. Various holographic printers for recording a holographic stereogram, a fringe pattern or a wave-front have been developed for the recent two decades [1-4]. Multiple perspectives of the scene are used as contents for holographic stereogram recording, and an observer perceives a 3D object by binocular vision. However, the acquired perspectives have longitudinal magnification distortion caused by camera geometry that leads to the distorted 3D space representation over the displayed hologram. Thus the holographic stereogram may not be suitable for specific applications which require actual size display. The fringe printer records computer generated fringe patterns onto a holographic emulsion without any reference wave, and the printed hologram is a thin diffractive optical element which provides non-distorted 3D reconstruction but without color selectivity. The wave-front printing technique overcomes these drawbacks - longitudinal magnification distortion and lack of color selectivity – and makes a wave-front printer a desirable choice of a holographic printer. Two wave-front printers have been recently independently designed [1,2] and the aim of this paper is to analyze and compare their properties.

The use of laser devices in the diagnosis phase-inhomogeneous layers biological revealed great promise for noninvasive imaging of polycrystalline layers receipt networks [1 - 3].

Gauge blocks still represent the most important standard for the transfer of the SI Unit meter[1]. The interferometrical calibration of these material measures is traditionally performed by using Twyman-Green or Kösters comparators [1, 2] which requires the wringing of the gauge block to a platen. In this approach, the interaction between the gauge block and the platen affects the measurement uncertainty. An alternative interferometer design, which is now being built at PTB, allows measuring both end faces at the same time which makes wringing redundant.

Fundamental for a further decrease of the measurement uncertainty of the Avogadro constant determination is to gain more information on how optical imperfections of the system influence the measurement results of the diameter determination of the Avogadro

28

Si-spheres with the sphere interferometer [1, 2]. Therefore a second sphere interferometer was set up with very different optical properties compared to the first sphere interferometer. Changes are amongst others the doubled etalon size, a decrease in the number of optical surfaces and the higher quality optical elements. A comparison of the measurement results of the first sphere interferometer and the new sphere interferometer can give an estimation of how much wave-front deviations influence the measurement result.

We present the first use of a widely tuneable, all-optical THz source (an intra-cavity parametric laser) for the thickness measurement of test objects. The optical thickness variation of a test target was measured in a Mach-Zehnder interferometer to within 0.5% of the THz wavelength, and wavelength tuning enabled the unambiguous measurement range to be extended to several wavelengths (half the synthetic wavelength). By detecting the phase change at different regions of an object, the system can also be used internal inspection of suitable materials that are opaque at visible wavelengths.

The subsidence of ground beds occurs frequently in large construction projects such as bridges, dams, high buildings and railways, which may cause damage and sometimes create disasters. Especially along with the establishment and the development of the high-speed railway, tiny subsidence and deformation of the railway beds will result in the decline in the service quality and safety. Therefore, automatic and long-duration subsidence surveillance is becoming increasingly important.

In conventional holography, it is difficult to gain access on the three-dimensional surfaces of objects under test [1]. In contrast, there are well established techniques like the stereophotogrammetry that reconstruct surfaces of objects in dense three-dimensional point-clouds by using at least two cameras and structured illumination [2]. Recent works demonstrated, that reconstructed digital holograms can be used as images in the stereophotogrammetrical determination of object surfaces resulting in a three-dimensional point-cloud of the object [3]. Additionally, it is possible to combine the point-cloud with holographic techniques, such as holographic interferometry or phase-shifting techniques [4]. Due to such a combination, full three-dimensional deformation vectors can be calculated, providing the user with a method of high precision deformation measurements [5]. To accomplish these calculations, four cameras are used to connect three point-clouds with each other, so that the deformations measured, depending on the sensitivity-vector of each camera, can be combined to a full three-dimensional vector.

We measured the coherence matrix of the spatially partially coherent light produced by a broad area laser diode (BALD) with Young’s double slit experiment using a spatial light modulator (SLM). With this data we modeled the behavior of the laser beam, and compared it with a simpler approximative shifted-elementary-mode method [1]. We show that it is often enough to use just simple intensity measurements rather than complex coherence measurements to have a rough but often good enough approximation of the laser coherence properties.

The phase-measuring profilometry with projection of pure sinusoidal fringes is known for its high accuracy of 3D object capture that is achieved by comparatively simple means [1]. It offers a number of key advantages when fringe projection is done by a spatial light modulator (SLM) due to software manipulation of fringes [2]. A lot of results have been reported for SLM fringe projection under incoherent illumination by using liquid crystal display or digital light processing technology. The main deficiencies of this type of projection are the discrete-like spatial intensity distribution and the gamma-distortion worsening the spectral content of the fringes [3]. The focus of this report is on the usage of SLMs for coherent projection of the sinusoidal fringes with phase encoding of the sinusoidal function.

Small angle deflectometry is commonly used to measure the topography of optical flats, mirrors or synchrotron optics with uncertainties down to the sub-nanometre range. Most of these deflectometric profilometers apply the slope measurement technique using commercially available autocollimators. These autocollimators can be calibrated, for example, at PTB and are then capable of measuring with uncertainties in the range of 0.01 arcsec. The lateral resolution of the measured topography is determined by the aperture of the autocollimator which is in the millimetre range for commercially available autocollimators.

The idea of stabilized wavelength within a certain defined measuring range leads quite directly to a cavity – based design. A passive Fabry-Perot cavity has been traditionally used as an etalon for laser frequency stabilization in a large number of configurations and applications. Linking a laser optical frequency to a resonance of the cavity means in fact a stabilization of wavelength within a cavity where the standing wave has been generated. We tried to use this “grid” of the standing wave as a reference for direct position sensing. The concept of measuring within a standing wave generated by a reflector has been reported previously [1], but we propose here the measurement within a cavity together with the effect of stabilization of wavelength.

The 3D acquisition of object shapes is of increasing importance. In the meantime, the market offers 3D-sensors for a wide spectrum of applications. Surprisingly, there are only a small number of approaches for a real-time 3D-camera. In this paper we will discuss, how the 3D measurement concept “Flying Triangulation” (FlyTri) [1] could be adapted (in the long term) to implement such a “3D movie camera”. By this term, we understand a 3D-sensor that provides dense data, calculated from one single camera frame without exploiting lateral context information. Principally, FlyTri could provide this features, however, it has to be pushed further to its (information-theoretical) limits. FlyTri allows for a motion-robust real-time 3D acquisition of object surfaces using a hand-guided sensor. The data acquisition process is based on a multi-line light-sectioning approach. As a “single-shot principle”, light sectioning enables the option to get 3D-data from one single camera exposure without exploiting neighbourhood information. Currently, our sensors project about 10 lines (each with 1000 pixels) per shot, reaching a significantly lower data efficiency than theoretically possible for a single-shot sensor. A high data density is then achieved by steadily moving the sensor around the object. We emphasize, that, due to fundamental information-theoretical reasons (incompressibility of space-time-bandwidth product), a single-shot sensor can never provide pixel-dense data if the object bandwidth is not severely limited. However, up to approximately 130 lines should be theoretically possible for a multi-line triangulation sensor with a 1000×1000 pixel camera (following from sampling theorem considerations) [2]. But there is a problem: if the line density is increased, severe indexing ambiguities occur. These ambiguities result in false 3D data which can harm or even disable the whole data acquisition process.

Optical technologies for measuring the human body shape without contact have gained popularity in the recent years. In particular, techniques based on fringe projection have demonstrated a good performance for generating three-dimensional (3D) topographies of the human body. For the 3D digitization of the human body, the technique has found various applications in different fields, including relevant cosmetic and medical applications such as 3D back shape detection in scoliosis [1], 3D shape measurement of pectus excavatum [2], 3D intra-oral dental measurements [3], or 3D measurement of the topography of human skin [4, 5, 6]. In the latter, optical measurement of the skin surface by means of fringe projection provides a less invasive, faster and more accurate result than the obtained with traditional methods established in the cosmetic industry based on skin replicas of silicone, which are applied along several minutes on the person, and therefore are more sensitive to errors associated with unintentional movements of the person due to breathing or muscle contractions.

Nowadays there are several approaches for optical form measurement of precision components. Optical single point sensors as well as optical matrix sensors are applied for this measurement task. Scanning a specimen with a point sensor requires long measuring times. Hence, drifts of the stages or variations of the air temperature can cause measurement errors. Using a matrix sensor yields to large amounts of data, needs high computing power and may lead to difficulties due to comparably low frame rates if the object under investigation is moving. Another approach for measuring highly curved surfaces are computer generated holograms (CGH) for a Fizeau null-test, but this is expensive and needs very accurate manufacturing of the CGH as well as a precise alignment to the object under test [1].

“Phase-Measuring Deflectometry” (PMD) has been developed at Erlangen to measure spatially resolved slope data of specular surfaces [1], [2] and [3]. The principle is simple: A camera observes the image of a sinusoidal grating via reflection at the surface under test. The local slope of the sample is encoded in the deformation of the observed pattern [4]. Meanwhile, PMD is well established to control the manufacturing of aspheric eyeglasses. PMD even has the potential to challenge interferometry. This specifically, because in contrast to interferometry, PMD does not display retrace errors. We will discuss the objectives and potentials of PMD on the road to sub-micrometer accuracy, as well as new applications including machine-integrated measurements.

To reveal what’s behind the flames is a key and challenging aim both in the industrial and, above all, in security field.

A wide variety of industries relies on furnaces and boilers for manufacturing processes. A failure of furnace and boiler equipment can cause quality problems and, in some case, they can also shut down an entire process line. Special thermal imaging cameras are often used to detect most of equipment problems during operation, so that failures can be prevented. This cameras uses a spectral waveband filter that only allows thermal radiation with those specific wavelengths through. Usually the filter is around 3.80 micron, where no hot gases are emitting. In fact, gases present in the flame have discrete absorption bands in the IR spectrum: the maximums for CO2 are at 2.7, 4.4 and 15

μ

m; for H

2

O they are at 1.4, 1.9, 2.7, 6 and 17

μ

m.

Since its demonstration in 2003 [1], the use of long-wave infrared (LWIR) CO

2

lasers in Digital Holography (DH) has been shown in an increasing number of applications. In particular it allows observing large objects and in metrology it allows measuring large displacements while rendering holography more immune to environmental perturbations, compared to holography in the visible. For metrology we demonstrated various holographic techniques in LWIR: speckle interferometry [2,4], DH interferometry [2-4] and shearography [4]. The LWIR holographic set-up generally makes use of microbolometer arrays based thermal imagers. The present paper discusses some result of the European project FANTOM in which another advantage was put forward when speckle interferometry is applied in LWIR: the thermal background is captured simultaneously to the specklegrams since it constitutes an offset in the signal. Therefore one can correlate uniquely the temperature and displacement information at the same time and in each pixel. We describe here the successful achievements of the project.

Digital Holography (DH) in the Long-Wave InfraRed (LWIR) range shows an increased interest since its first demonstration in 2003 [1]. In particular it allows observing large objects due to the fact that at such wavelengths the ratio between the wavelength and the pixel size allows reconstructing objects 5 to 10 times larger than with DH in visible light [2,3]. We already presented various configurations of LWIR DH interferometry and electronic speckle pattern interferometry for deformation metrology and non destructive testing [3,4]. In this paper we present the application of LWIR DH in interferometric testing of large deformation of large aspheric mirrors in the frame of a European Space Agency project. Here the study focuses on the case of parabolas and ellipses which are usually tested through interferometric wavefront error measurements which require expensive null-lenses matching each of the reflectors considered. In the case of monitoring deformation a holographic technique can be considered where the wavefront is compared with itself at different instants. Therefore the optical set-up can be quite simple and easily reconfigurable from one reflector to another. The advantage of using long wavelength is that large deformations can be measured at once, in addition to being more immune against environmental perturbations. In this paper we review different optical configurations of DH interferometer that led to test a parabolic mirror under thermal-vacuum test [5], as well as an off-axis ellipse tested in laboratory conditions, which is a new result.

Low coherence interferometry (LCI) is a well known measurement technique that allows shape measurements with a nanometer resolution. The range of LCI is theoretically unlimited but in practice it is determined by the length of a scanning motion along the measurement axis. The resolution of the measurement is determined by the scanning resolution, the spectral characteristics of the light source and the fringe analysis algorithm.

Real-time three-dimensional (3D) shape measurement is widely used in science and engineering fields. Many optical methods [1] have been developed to achieve the goal of 3D shape measurement. With the advantage of high accuracy and easy implementation, fringe projection profilometry is becoming more and more popular. Karpinsky N and Zhang S [2] proposed a real-time high-resolution method to obtain precise 3D shape information based on looping to project three fringe patterns and three-step phase-shifting algorithm. But the synchronization between the cameras for image capturing and the projector for fringe projecting pattern is hard to control.

Skin surface roughness is an important clinical property for aging process and pathological changes of the outmost organ of the body. Physicians use skin roughness as the key diagnostic feature to detect warts, actinic keratoses, and psoriases, and to differentiate benign seborrheic keratoses from malignant melanomas. Additionally, skin roughness has been used to monitor medical and cosmetic treatments for eczema, scars, stretch marks and wrinkles.

Environmental conditions dominate materials’ equilibrium processes with surrounding environment. The environmental fluctuations are accused for decay mechanisms of artworks even if the latter are kept in stable conditions, some for centuries inside controlled rooms and more recent in environmental-controlled galleries. Environmental control is considered the central issue in most modern museums and artwork display rooms to minimise equilibrium demanding processes. Conservators and conservation scientists concluded to keep the conditions stable and within short range of values in order to avoid the displacement of the materials in response to condition change. Hence if environmental change is kept within safety limits the environmental control is considered safe for the hosting artworks and the crucial parameter of fatigue and deterioration is accused only if these safety limits are not kept in uncontrolled environmental conditions.

During the past two decades, scientific journal publishing has undergone a veritable revolution, enabled by the emergence of the Internet and the World Wide Web. This revolution has contained two interconnected phases. The first, and to date the most visible, is the rapid shift from print-only journals to parallel print and electronic publishing. The second stage of this revolution has been occurring, most prevalently, in the past 20 years (i.e. since the 1990s) and is known as Open Access (OA)—a movement providing and indorsing unrestricted online access to peer-reviewed scholarly journal articles. OA has grown ever since – by only 2000, an increasing number of professional Open Access publishers had emerged (e.g., BioMedCentral, Public Library of Science, Hindawi, Bentham Open). Today, the number of OA peer-reviewed journals is around 5,000 and continues to grow.

There is an increasing interest in technologies enabling high resolution imaging for endoscopy of internal organs. For example, in many endoscopic applications it is essential to combine white light endoscopy with subcellular imaging. With the latter technique, the user can perform biopsy optically in real-time, which significantly improves diagnostic accuracy and also avoids extracting samples of tissue.

Cultural awareness is supported by temporary exhibitions in museums. The subsequent increase of museum loan services manifests in a large number of art transportation. Despite modern packaging technologies, vibrations and environmental climate change can add up and damage the transported objects [1]. This results in a conflict between wanted mobility of artwork and the preservation of our cultural heritage. In traditional conservation techniques the evaluation and quantification of changes and defects results mostly from an optical and subjective assessment and comparison of the previous and the subsequent condition. Due to fast construction of exhibition sites, also experienced experts are not always able to distinguish a new defect from an old one or detect a defect expansion in an appropriate and objective way. Especially it is difficult to distinguish between damages due to natural aging and mechanical stress. Therefore, in recent years the necessity for modern inspection technologies to deal with those challenges has extremely increased. For example, in the framework of the MutiEncode-Project different holographic measurement techniques were combined to monitor the condition of an artwork [2, 3]. Transport damages have been investigated in the Vasari-Project [4] and in Stuttgart [5].

Both the development of new optical sensors including overall measurement systems and the operation of such systems require a fast and flexible underlying software. This software has to be able to control and communicate with a wide range of different hardware systems, such as cameras or actuators, while providing as diversified and complete as possible a set of evaluation and data processing methods. Additionally, the rapid prototyping of such systems requires a software, where parameters or components can easily be changed at runtime while retaining the above focus on fast hardware access and high computational performance.