LCD

What is LCD

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 prized by engineers because it uses very small amounts of electric power, and is therefore suitable for use in battery-powered electronic devices.

Each pixel consists of a layer of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other.

How It Works
Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves with microscopic grooves on the electrodes. The grooves on the two electrodes are perpendicular, so the molecules arrange themselves in a helical structure, or twist (the "crystal"). Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. Half of the light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.

When an electrical charge is applied to the electrodes, the molecules of the liquid crystal are pulled parallel to the electric field, thus reducing the rotation of the entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel.

It is normal to align the polarizing filters so that pixels are transparent when relaxed and become opaque in the presence of an electric field, however the opposite is sometimes done for special effect.

The electric field necessary to align the liquid crystal molecules rapidly is also enough to pull them out of position, damaging the display. This is solved by using an alternating current to rapidly pull the molecules in alternate directions.

To save cost in the electronics, LCDs are often multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (say, in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (say, in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

Important factors to consider when evaluating an LCD monitor include resolution, viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, aspect ratio, and input ports (e.g. DVI or VGA).

Src Wikipedia