Thermochromic liquid crystal thermography: Hysteresis, illumination and imaging system effects, image processing and applications

Thermochromic Liquid Crystals (TLC's) reflect incident light iridescently depending on their temperature and are used extensively in thermography. Recent advances involve using the reflected color (e.g. hue) from digital images of surfaces coated with TLC's to produce full-field global surface temperature distributions. These advances have made it important to take a deeper look into factors influencing the hue-temperature behavior of TLC's and the effects of image processing.; The behavior of five different microencapsulated TLC's (both broad-band and narrow-band) has been investigated. Although TLC's have generally been thought to be reversible and repeatable, all five of these TLC's were found to exhibit a similar hysteresis behavior when cooled rather than heated. This hysteresis is characterized by a decrease in reflectivity and a shift in the temperature associated with the peak reflected intensity during cooling relative to heating. The combined effect is a shift in the hue-temperature calibration of the TLC. This shift results in temperature biases during cooling (relative to heating), of approximately 20% of the useful calibration range for broadband TLC's and as high as 40% to 60% of the useful calibration range for narrow-band TLC's. The magnitude of the hysteresis increases with an increase in the peak temperature prior to cooling. Repeatable heating (and cooling) calibrations are obtained when the TLC is initially cooled below (or heated above) an apparent reset temperature. These reset temperatures appear to be related to the red start temperature for heating and the blue stop temperature (about 20°C to 25°C above the top of the useful calibration range) for cooling. A permanent shift in the hue-temperature calibration (characterized by a decrease in reflectivity and a shift in the temperature associated with the peak reflected intensity) was observed during the tests. This shift appears to be due to high temperature exposure (60°C–80°C).; A theoretical model of a TLC imaging system was developed to investigate some of the factors affecting TLC hue-temperature behavior. These factors include the spectral distribution of the illumination source and UV filter, the surface reflection due to both the TLC and background, the TLC coating thickness and the sensing device (camera) spectral characteristics and gain settings. Results from the model are compared to experimental measurements. It is found that typical measurements cannot be explained by a TLC reflectivity model with a monochromatic spike or narrow bandwidth, the model that is often assumed. A model with TLC reflection over a relatively broad band of wavelengths results in good agreement between the model and measurements. The significance of background reflection, which commonly accounts for 30% to 50% of the reflected light, is examined. It is shown that the background reflection tends to attenuate the hue-temperature calibration curve toward the background hue value. Five illumination sources are compared to examine their effect on the hue-temperature behavior. It is found that “full spectrum” bulbs, which have a relatively uniform radiant intensity across the visible spectrum, tend to have the lowest temperature measurement uncertainties and the broadest useful ranges, which are desirable calibration attributes.; An interactive liquid crystal image processing toolbox was developed in MATLAB to assist in the analysis and processing of TLC images and in the subsequent calculation of heat transfer information. An overview of this toolbox is provided.