LCDs are non-emissive displays that use polarized light passed through a liquid crystal material. They offer advantages over CRTs by being thinner, lighter, and using less power. LCDs use nematic liquid crystals that are naturally twisted and keep molecules aligned. Applying a voltage untwists the molecules to varying degrees. Two glass plates with perpendicular polarizers sandwich the liquid crystal, with rows and columns of conductors defining pixel positions. Normally molecules are aligned to pass light through, but a voltage aligns them to block light. LCDs require no light of their own.

1. Liquid Crystal Display (LCD)
LCD’s are non-emissive devices produced by passing
polarized light from an light source through a liquid-
crystal material. LCD’s offer some real advantages
over other display technologies, as they are thinner
and lighter and draw much less power than Cathode
ray tubes.
Liquid crystal:have a crystalline arrangement of
molecules that are closer to a liquid state than a
solid. It takes a fair amount of heat to change a
suitable substance from a solid into a liquid crystal.

2. A particular sort of nematic liquid crystal,
called twisted nematics, is naturally twisted
& tends to keep the long axes of the rod-
shaped molecules aligned are used.
Applying an electric current to these liquid
crystals will untwist them to varying
degrees, depending on the current's
voltage.
Two glass plates, each containing a light
polarizer at right angles to the other plate
contains the liquid crystal material.

5. Rows of horizontal transparent conductor are
built into one glass plate, & columns of
vertical are put into other plate.
Intersection of two defines a pixel position.
Normally molecules are aligned in the “on
state”.
Polarized light passing to the material is
twisted so that it will pass through the
opposite polarizer.
The light is reflected back to the viewer.
Liquid crystal materials emit no light of their
own.

6. To turn off the pixel, we apply a voltage to
the
two intersecting conductors to align the
molecules so that the light is not twisted.
This type of devices are known as passive-
matrix LCD.
Colors can be displayed by using diff
materials at each screen position.

7. Another method for constructing LCD’s is to
place a transistor at each pixel position,
using thin –film transistor technology,
which are used to control the voltage at
pixel position locations & to prevent
charge from gradually leaking out of the
liquid-crystal cells. These device are called
active-matrix displays.