LCD Operating Temperatures – “Going Isotropic”

Liquid crystals (LCs) are a state of matter that has properties between those of a conventional liquid and those of a solid crystal. Most modern electronic displays are liquid crystal based (LCD).

LC devices usually work at different thermal regimes, within various temperature intervals and in different climatic conditions. LC displays (LCD) have a well-defined isotropic or operating temperature limit, above which the actual liquid crystal molecules will lose their orientation and will assume a random orientation instead of ‘twisting’ through the light valve. If the temperature rise is too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, thus forcing the material into a conventional isotropic liquid phase. In other words, the rod-like molecules will no longer lie in well-ordered planes stacked upon each other and will not be able to pass through the light valve. Isotropic conditions will cause positive image displays to become dark (see image below), while negative image LCD’s become transparent. This is the Nematic-to-Isotropic Transition Temperature or NI Transition.

LCD’s will recover from brief exposure to isotropic temperatures although temperatures above +100º C will damage the display’s internal coatings. The low-end of the Liquid Crystal operating temperature range is not nearly as well defined. At low temperatures, as the LC molecules’ movement slows down, the display’s response time decreases due to the fluid’s increased viscosity. At very low temperatures, the liquid crystal material assumes a solid or crystalline state referred to as the Theoretical Crystaline-to-Netetic (CN) Transition Temperature. However, LC material is very resilient, so it takes prolonged exposure to temperatures below the CN limit to crystallize LC material.

Low temp effects are usually reversible. LCD’s immersed in liquid nitrogen have been known to return to normal operation after a brief warm up. LCD’s configured with heaters can operate at temperatures as low as -55ºC. However, LCD heaters require a temperature compensated power source. LCD’s equipped with a thin film heater overlay can achieve response times equivalent to displays operating at 0°C (32°F) while operating at low temperatures. Increasing the power to the heater will decrease the warm up time, but will typically require 2 to 3 volts for every square inch of