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Before an electric field is applied, the orientation of the liquid-crystal
molecules is determined by the alignment at the surfaces of electrodes. In a
twisted nematic device (still the most common liquid-crystal device), the
surface alignment directions at the two electrodes are perpendicular to

each other, and so the molecules arrange themselves in a helical structure,
or twist. This induces the rotation of the polarization of the incident light,
and the device appears gray. If the applied voltage is large enough, the

liquid crystal molecules in the center of the layer are almost completely
untwisted and the polarization of the incident light is not rotated as it
passes through the liquid crystal layer. This light will then be mainly
polarized perpendicular to the second filter, and thus be blocked and the

pixel will appear black. By controlling the voltage applied across the liquid
crystal layer in each pixel, light can be allowed to pass through in varying
amounts thus constituting different levels of gray.

From the ubiquitous wrist watch and pocket calculator to an advanced
VGA computer screen, this type of display has evolved into an important
and versatile interface. This field has grown into a multi-billion dollar

industry, and many significant scientific and engineering discoveries have
been made.

Liquid Crystal Thermometers

As demonstrated earlier, chiral nematic (cholesteric) liquid crystals reflect

light with a wavelength equal to the pitch of helix. Because the pitch of
helix is dependent upon temperature, the color reflected also is dependent
upon temperature. Liquid crystals make it possible to accurately gauge
temperature just by looking at the color of the thermometer. By mixing

different compounds, a device for practically any temperature range can be
built.
The "mood ring", a popular novelty a few years ago, took advantage of the

unique ability of the chiral nematic liquid crystal. More important and
practical applications have been developed in such diverse areas as
medicine and electronics. Special liquid crystal devices can be attached to
the skin to show a "map" of temperatures. This is useful because often

physical problems, such as tumors, have a different temperature than the
surrounding tissue. Liquid crystal temperature sensors can also be used to
find bad connections on a circuit board by detecting the characteristic

higher temperature.

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