LCD Displays

Liquid Crystal are organic, Molecules that have the properties of both solids and liquids. Lights passes through the crystals depending on low the molecules are aligned and applying an electric current to the crystals altars their molecular alignment. An LCD display consists of two polarizing filters arranged at 90 degree angle to which other with a layer of liquid crystals enclosed between them.

The crystals aligning themselves between the 2 grooved surfaces that encase them, twist the light passing through the 1st filter by 90 degree so that it can pass through the 2nd filter as wall, illuminating the screen. When a current is applied to crystal, it realigns itself and stops twisting the light that passes through the 1st filter. This enables 2nd filter to block the light and prevent it from illuminating the screen.

To form a display the liquid crystals are arranged on a grid representing the pixels that make up the screen.To supply power to individual crystals, the display contains an array of transistors. The number of transistor depends on type of display. There are 2 types of transistor configures called passive matrix and active matrix.

In passive matrix display, transistors are arranged along one horizontal edge and one vertical edge of the display . An 800*600 pixel display therefore has 1400 transistors. To supply current to an individual pixel, the 2 transistors at the appropriate points on the horizontal and vertical axis act in unison.

The most common type of passive matrix panel used today is called dual-scan twisted nematic (DSTN). Passive LCD panels are prone to shearing during rapid changes of screen content, ghost images (shadows) and streaking (lines follow an object as it moves across the screen). Passive LCD panels are relatively in expansive, used in low-to-mid range Laptop systems.

In active matrix display, each individual (crystal) pixel has dedicated transistors, arranged behind the display in a grid the same size as the server. A typical 800*600 active matrix display has 1440000 transistors instead of 1400,i.e., three transistors for every pixel, one each for red, blue and green. Monochrome display needs 480000 transistors, one each for every pixel.

The most common form of active matrix LCD used today is called this film transistor (TFT). In active matrix LCD panel, The transistors don't need to pulse the current because each pixel has transistors dedicated to servicing it. As a result, the amount of light passing through the filters is controlled more precisely allowing more light to get through and providing a brighter picture. In additional individual transistors eliminate the smearing, ghosting and streaking problems common to passive matrix displays.