LIQUID CRYSTALS

12/11/99

Liquid Crystals are chemicals that can exhibit some of the properties of crystals even though they are liquids. When exposed to an electric field, the liquid crystals will align themselves into an orderly crystal like arrangement on one axis. When forced into order, the polarity of light perpendicular to the ordered axis will be rotated by an angle proportional to the electric field.

A liquid crystal display (LCD) consists of a light source, a polarizer that ensures the light entering the liquid crystals has a known polarity, circuits to create electric fields, a layer of liquid crystals and a second polarizer that will pass varying amounts of light depending on the amount of rotation in the liquid crystal.

LCDs do not generate light. They modulate light passing through the displays in proportion to the intensity of the electric field applied. Although use of reflected light is possible, In practice, computer displays are generally backlit or edgelit. Polarizers absorb much of the backlight, and "wires", electrodes, etc may absorb much of what is left. Fairly bright back lighting is needed to produce acceptable displays. This is a problem in portable equipment where battery drain must be traded off against screen visibility in bright light. As a result, LCD images may be somewhat dim and may appear washed out when viewed in bright light.

It is possible to design LCDs to be normally dark with images imposed as light areas, or normally white with images imposed as dark areas. Normally White LCDs are used almost universally.

Liquid Crystals degrade if a constant polarity voltage is continuously applied. To prevent this, polarity is reversed every display cycle reversing the orientation of the material.

In an ideal world, electric fields would be easily generated in tiny square boxes; liquid crystals would be colorless, liquid crystals would respond immediately to changes in voltage; liquid crystal rotation would be a perfectly linear function of voltage; There would be no internal optical affects in liquid crystals that are not voltage related; maximum optical rotation would be exactly 90 degrees; and the operation of the liquid crystals would be temperature independent. In practice, none of these things are true although improvements in materials technology and other techniques are improving things fairly rapidly. A number of liquid crystals materials exist with different advantages and problems.

With 1999 technology, response may be slow. Greater than 2 tenths of a second -- especially at low temperatures. At high temperatures, the material may "go isotropic" and become an opaque liquid rather than a light rotating liquid crystal. Light rotation off the viewing axis may not be the same as on-axis resulting in color and intensity changes, or even in reversal of colors when the display is not viewed straight on. Viewing properties may change when the screen is too warm. Otherwise desirable material may respond non-linearly or may have no practically usable linear response range. Pixels are not really square or rectangular and may interact with adjacent (or non-adjacent for that matter) pixels. Uneven illumination of the screen will be reflected in unequal brightness. A number of different liquid crystal materials are used with accompanying trade-offs.

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