Cholesterics thermography

By far the most important commercial applications of nematic liquid crystals are in the a multi-billion dollar display industry. Cholesteric, or chiral nematic liquid crystals have been used in coloured guest-host displays and in thermography/ther-mochromic applications. 

Cholesteric liquid crystal films are applied by painting or spraying a solution of the liquid crystal onto a surface. Cholesteric liquid crystals have been used to measure microwave energy, infrared light (such as in surface thermography), visible... 

The reflective wavelength of cholesteric liquid crystals varies according to temperature. Such an effect has been made useful in thermography. It has been applied in the diagnosis of cancers by displaying the skin temperature distribution. It has also been applied to test faults in integrated circuits. The applications also include thermometers and temperature warning indicators and non-destructive detection. 

Dependence of pitch on temperature applications to thermography In most pure cholesteric materials, the pitch is a decreasing function of the temperature. An elementary picture of the temperature dependence of the pitch can be given in analogy with the theory of thermal expansion in crystals. " Assuming anharmonic angular oscillations of the molecules about the helical axis, the mean angle between successive layers... 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

When the pitch changes with variation of temperature the colours also change. This phenomenon is used in thermography, a sensitive technique for measurements of the distribution of temperature over various objects, for example, in medicine for making temperature maps of human skin. In technics, cholesterics are used for the estimation of the temperature distribution over plane electrOTiic circuits and other objects with a relatively flat surface. 

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 AX, centered at a wavelength Xq, such that ... 

It is clear that the optical properties of a cholesteric material used for thermography... 

The use of cholesteric liquid crystals for thermography was reviewed some years ago by Elser and Ennulat [4], and more recently in the commercial literature of Hallcrest [5]. Amongst the substances with the largest temperature coefficient are cholesteryl oleyl carbonate [6] and 5-cholesteryl esters... 

Other techniques of encapsulating liquid crystalline materials exist, and find application in areas apart from thermography. In particular, the methods used in the fabrication of polymer dispersed liquid crystal N and Sm display devices can be readily applied to cholesteric materials. Polymer dispersed displays are fabricated by dissolving the liquid crystal in the monomeric form of the continuous medium of the final device. 

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