Thermochromic Liquid Crystal Thermography: Illumination Spectral Effects. Part 2: Theory

A theoretical model of a Thermochromic Liquid Crystal (TLC) imaging system was developed to aid in understanding the results of experiments on spectral effects and to investigate the various factors affecting the hue-temperature calibration of TLC’s. The factors in the model include the spectral distribution of the illumination source and UV filter, surface reflection of both the TLC and background, and the sensing device (camera) spectral characteristics and gain settings. It was found that typical hue-temperature calibration curves could not be entirely explained by a TLC reflectivity model with either a monochromatic spike or a narrow bandwidth reflectivity, which is often assumed. Experimental results could be explained, however, by a model that reflects over a relatively large band of wavelengths. The spectral characteristics of the five illumination sources (those for which experiments were performed) were considered. Background reflection, which commonly accounts for 30%–50% of the reflected light, was found to significantly attenuate the hue-temperature calibration curves toward the background hue value. The effect of the illumination source on the hue-temperature calibration curves is demonstrated and several experimentally observed phenomena are explained by the results of the theoretical calculations, specifically the spectral reflective properties of the liquid crystals and the transmissivity of the R, G, and B filters in the image capture camera.