Light Transmission Characteristics of Thermochromic Liquid Crystals

Abstract

This paper presents the results of a study to develop a novel thermochromic liquid crystal (TLC) temperature measurement system that uses light transmission instead of light reflection. We used Imagetherm 25C10W sprayable TLCs. The TLCs were painted on a clear surface and placed in a spectrometer. The amount of light transmitted at monochromatic wavelengths from 400 to 700 nm was measured for temperatures from 25–55 C under conditions of non-polarized, linearly polarized and cross polarized light. The transmission spectra exhibit a distinct “s-like” shape when the TLCs are in their active range. We detect significant changes in the transmission spectra for temperatures from 27 to 48 C (whereas in reflection the TLCs are useful over a significantly smaller range). We have analyzed the spectrometer data in a number of ways including (a) total amount of light transmitted (b) amount of red, green and blue light transmitted, and (c) spectral curve characteristics (peak transmission, inflection wavelength and wavelength for peak transmission) all as a function of temperature. There is a linear relationship between temperature and all of these variables which we believe can be exploited in the development of a CCD camera based light transmission system. We also investigated the effects of temperature overheat on the transmission spectra. Overheating the TLCs increases the amount of light transmission.