LCD Knowlege

LCD is the abbreviation for liquid crystal display. An LCD basically consists of two glass plates with a special liquid between them. The special attribute of this liquid is that it rotates or "twists" the plane of polarized light. This effect is influenced by the creation of an electrical field. The glass plates are thus each coated with a very thin metallic film. To obtain polarized light, you apply a polarization foil, the polarizer, to the bottom glass plate. Another foil must be applied to the bottom glass plate, but this time with a plane of polarization twisted by 90°. This is referred to as the analyzer.

In the idle state, the liquid twists the plane of polarization of the incoming light by 90° so that it can pass the analyzer unhindered. The LCD is thus transparent. If a specific voltage is applied to the metallic film coating, the crystals rotate in the liquid. This twists the plane of polarization of the light by another 90°, for example: The analyzer prevents the light getting through, and the LCD thus becomes opaque.

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. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).

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 reasonably transparent.LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are crossed.

LCD with top polarizer removed from               LCD with top polarizer removed from device
device and placed on top, such that the          and placed on top, such that the top
top and bottom polarizers are crossed.           and bottom polarizers are parallel.

TN, STN, FSTN, blue mode, yellow-green mode

Liquids that twist the plane of polarized light by 90° are referred to as TN (Twisted Nematic). STN (Super Twisted Nematic) liquids twist the plane of polarized light by at least 180°. This gives the display improved contrast. However, this technology does color the display to a certain extent. The most common colors are referred to as yellow-green and blue mode. There is also a gray mode, which in practice is more blue than gray, however.

In order to counteract the undesired color effect, the FSTN technology uses an additional foil on the outer side, but this causes a loss of light and means that this technology is only effective with lit displays.
However, the different colors occur only in displays that are either not lit or that are lit with white light. If there is any color in the lighting (e.g. yellow-green LED lighting), it overrides the color of the display. A blue-mode LCD with yellow-green LED lighting will always appear yellow-green.

Active matrix technologies

            A Casio 1.8" colour TFT liquid crystal display
                                                                               which equips the Sony Cyber-shot DSC-P93A
                                                                               digital compact cameras  Main article: TFT LCD,
                                                                               Active-matrix liquid crystal display Twisted nematic (TN)

Twisted nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved.

Passive-matrix and active-matrix addressed LCDs

A general purpose alphanumeric LCD, with two lines of 16 characters.

LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have individual electrical contacts for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.