Hysteresis in Liquid Crystal Thermography

Hysteresis in five different Thermochromic Liquid Crystals (TLCs), both narrow-band and broad-band, has been investigated. All were found to exhibit a similar hysteresis behavior during cooling relative to heating. This hysteresis is characterized by a decrease in reflectivity and a shift in the temperature associated with the peak reflected intensity for each of the R, G, and B components during cooling. This causes a shift in the hue-temperature calibration of the TLC causing temperature biases, when cooled rather than heated, of 20–60% of the useful calibration range. The hysteresis effect increases as the peak temperature during a heating and cooling cycle is increased. Repeatable heating calibrations were obtained when the TLC was cooled below the red start temperature to an apparent low reset temperature. Somewhat repeatable cooling calibrations, different from the heating calibrations, were obtained when the TLC was heated somewhat above the blue stop temperature. A possible explanation of the hysteresis based on the texture of the liquid crystal helices is provided. In addition to hysteresis, a permanent decrease in reflectivity and a shift in the temperature associated with the peak reflected intensity was observed when the TLCs were exposed to extended higher temperatures (60°C–80°C).