A Compact System for Measurement of Absorbance of Light

In this work a compact device to measure absorbance based on light emitting diodes – LED - is proposed. Measurand is the concentration of specific chemical or biochemical components within the test fluid. The advantage of such a system lies in the use of low-cost standard optic-mechanical and electronic components that contribute to its compactness and robustness.

The proposed system measures absorbance (or transmittance) of monochromatic light inside a standardized cuvette of 10 mm light path within a temperature ranging from 25 to 40ºC (±0,2ºC). Some important features, such as stability, accuracy and precision of the measurement system are discussed. It is shown that the measurement stability is governed by the stability of the light source. An acceptable value is 0,002 units of absorbance per hour, which can be verified by measuring a dummy cuvette during a suitable long period. The accuracy and precision are estimated by means of traceable calibration standards throughout the desired spectrum of wavelengths. The wavelengths required for the target application of the proposed system are 340, 405, 505, 546, 600 and 700nm. Basic variables of influence are identified and a brief metrological analysis is presented in form of an uncertainty balance. First results obtained with the proposed system demonstrate uncertainties below 1,6 % relative to the measured value of absorbance.