Liquid-crystal thermography: Illumination spectral effects. Part 1 - Experiments

Experiments have been performed to examine the spectral effects of the illumination source on the hue-temperature characteristics of thermochromic liquid crystals (TLCs) used in a liquid-crystal thermography system. Five illumination sources were compared in this study. It was found that "full spectrum" sources, which have a relatively uniform radiant intensity across the visible spectrum, tend to have the lowest temperature uncertainties and the broadest useful ranges, which are desirable calibration attributes. Radiation in the infrared, which leads to (usually undesirable) heating of a test surface, and in the ultraviolet, which can damage TLCs, are discussed for the various light sources. Experimental observations of the effect that UV damage has on liquid crystal calibrations are also provided. The use of a new method called background subtraction and the use of white balancing are investigated as methods of improving the calibration characteristics of TLCs. The uncertainty in temperature associated with different illumination sources and both background subtraction and white balancing is determined and discussed. It is shown that these methods can reduce the uncertainty in some cases.