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).
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.
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.
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 the display.