A liquid crystal display (LCD) is a thin, flat display device made up of
any number of color or monochrome pixels arrayed in front of a light source or reflector.
It is often utilized in battery-powered electronic
devices because it uses very small amounts of electric power.
Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent
shit electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases)
perpendicular to each other. With no liquid crystal between the polarizing filters, light passing
through the first filter would be blocked by the second (crossed) polarizer.
The surface of the electrodes that are in contact with the liquid crystal material
are treated so as to align the liquid crystal molecules in a particular direction.
This treatment typically consists of a thin polymer layer that is unidirectionally
rubbed using, for example, a cloth. The direction of the liquid crystal alignment
is then defined by the direction of rubbing.
Before applying an electric field, the orientation of the liquid crystal molecules
is determined by the alignment at the surfaces. 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. Because the liquid crystal material is birefringent,
light passing through one polarizing filter is rotated by the liquid crystal helix as it
passes through the liquid crystal layer, allowing it to pass through the second polarized
filter. Half of the incident light is absorbed by the first polarizing filter, but
otherwise the entire assembly is transparent.
When a voltage is applied across the electrodes, a torque acts to align the liquid crystal
molecules parallel to the electric field, distorting the helical structure (this is resisted
by elastic forces since the molecules are constrained at the surfaces). This reduces 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.
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