Contd...

The properties of temperature sensitive liquid crystals are reported in terms of the event temperature and the clearing point temperature. The lowest temperature where liquid crystals scatter visible light is called the event temperature. At a temperature below the event temperature, liquid crystals will be in the solid state and will appear transparent. At a temperature above the clearing point temperature, it will enter the pure liquid state and will revert back to being transparent. Outside this range, the scattered light is negligible and, when viewed through a camera, the sheet would appear black. At the clearing point temperature, the helical pitch of the liquid crystals exceeds the wavelengths of visible light. The reflected color spectrum of liquid crystals will vary continuously from the longer wavelengths (red) corresponding to event temperature to shorter wavelengths (blue) corresponding to the clearing point temperature. At intermediate temperatures, the surface would take on a green color. Liquid crystals transmit a significant amount of the incident light with virtually no modification. Therefore, they are viewed against a non-reflecting (black) background. This precaution prevents the transmitted light from getting reflected without adversely affecting the interpretation of selectively scattered light from the liquid crystals.

The repeatability and thermal performance of liquid crystals degrade rapidly due to chemical contamination and exposure to ultra-violet radiation. Therefore, a manufacturing process known as micro-encapsulation is used to protect the raw liquid crystal material. Micro-encapsulation is a chemical process that takes raw liquid crystal material and encases it in protective capsules, 5-10 micron diameter. The micro-encapsulation process offers chemical contaminant resistance and radiation protection. Moreover, encapsulated TLCs have been found to be insensitive towards pressure while they were tested up to 133 bar. On the other hand, special care is required to avoid problems associated with over-attenuation of the reflected light coming from the liquid crystal and regeneration of the binder and microcapsules.