10. Distortion of Full-Field Surface Displacements from Heat Waves

One nearly ubiquitous, but often overlooked, source of measurement error in Digital Image Correlation (DIC) arises from imaging through heat waves. “Heat waves” is a colloquial term describing a heterogeneous refractive index field caused by temperature (and thus density) gradients in air. Many sources of heat waves exist in a typical DIC experiment, including hot lights, a heated sample, sunlight, or even a hot camera. This paper presents a detailed description of the error introduced to DIC measurements as a result of heat sources being present in the system. We present characteristic spatial and temporal frequencies of heat waves, and explore the relationships between the location of the heat source, the focal length of the lens, and the stand-off distance between the camera and the imaged object. Finally, we conclude with suggested methods of mitigating the effects of heat waves first by careful design of the experiment and second through data processing.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract No. DE-AC04-94AL85000.