Influence of the illumination spectrum and observation angle on temperature measurements using thermochromic liquid crystals

As measurements of velocity and temperature fields are of paramount importance for analyzing heat transfer problems, the development and characterization of measuring techniques is an ongoing challenge. In this respect, optical measurements have become a powerful tool, as both quantities can be measured noninvasively. For instance, combining particle image velocimetry (PIV) and particle image thermometry (PIT) using thermochromic liquid crystals (TLCs) as tracer particles allows for a simultaneous measurement of velocity and temperature fields with low uncertainty. However, the temperature dependency of the color appearance of TLCs, which is used for the temperature measurements, is affected by several experimental parameters. In particular, the spectrum of the white light source, necessary for the illumination of TLCs, shows a greater influence on the range of color play with temperature of TLCs. Therefore, two different spectral distributions of the white light illumination have been tested. The results clearly indicate that a spectrum with reduced intensities in the blue range and increased intensities in the red range leads to a higher sensitivity for temperature measurements, which decreases the measurement uncertainty. Furthermore, the influence of the angle between illumination and observation of TLCs has been studied in detail. It is shown that the temperature measurement range of TLCs drastically decreases with an increasing angle between illumination and observation. A high sensitivity is obtained for angles in between phi = 50 degrees and phi = 70 degrees, promising temperature measurements with a very low uncertainty within this range. Finally, a new calibration approach for temperature measurements via the color of TLCs is presented. Based on linear interpolation of the temperature dependent value of hue, uncertainties in the range of 0.1 K are possible, offering the possibility to measure very small temperature differences. The potential of the developed approach is shown at the example of simultaneous measurements of velocity and temperature fields in Rayleigh-Benard convection.