Thermography Using Liquid Crystals

Abstract

The dramatic variation of liquid crystalline properties with respect to temperature has resulted in the widespread use of cholesteric (chiral nematic) liquid crystals for thermography. The property that has been exploited most in liquid crystal thermography is the critical temperature dependence of the selective reflection from cholesteric liquid crystals, though other temperature dependent properties of mesophases have been utilized (e.g. the birefringence of nematic systems and selective reflection from other chiral phases). The helicoidal structure of cholesteric materials results in the selective reflection of visible light within a band of wavelengths of width Ahcentered at a wavelength ho, such that:Here, p is the pitch of the helical structure, An is the birefringence of the medium and ii is the average refractive index, (no + n J 2 .The temperature range over which the cholesteric material selectively reflects visible light is known as the colorplay of the material. The temperature dependence of the selective reflection from a cholesteric material is a consequence of the temperature variation of the pitch of the system, defined by the twist elastic constants. A critical divergence of the selective reflection occurs if a cholesteric material cools when an underlying SmA phase is approached. This happens because the twist deformation is not allowed within a SmA phase, and the twist elastic constants (and therefore pitch) show pretransitional divergence. The pitch of the system increases rapidly as the temperature reduces towards the SmA phase transition temperature, typically resulting in the reflection of red light at lower temperatures. In cholesteric materials without a SmA phase, the critical divergence of the selective reflection is less pronounced. Indeed, in some materials the selective reflection is at a constant wavelength over the entire temperature range. Thus materials that exhibit a low lying SmA phase possess the most useful thermal properties for most thermography applications, though temperature insensitive materials have found application as shear sensitive systems (see Sec. 3.3). At higher temperatures within the cholesteric phase (above any pretransitiona1 region) the pitch may either increase or decrease with increasing temperature, due