Flat-panel display

The traditional computer display uses a cathode ray tube (CRT) like that in a television set (see monitor). The flat panel display is an alternative used in most laptop computers and some higher-end desktop systems. The most common type uses a liquid crystal display (LCD). The display consists of a grid of cells with one cell for each of the three colors (red, green, and blue) for each pixel. The LCD cells are sandwiched between two polarizing filter layers that consist of many fine parallel grooves. The two filters are set so that the grooves on the second are rotated 90 degrees with respect to the first. By default, the light is polarized by the first filter, twisted by the liquid crystals so it is parallel to the grooves of the second filter, and thus passes through to be seen by the viewer. (For color displays, the light is first passed through one of three color filters to make it red, green, or blue as set for that pixel.) However, if current is applied to a crystal cell, the crystals realign so that the light passes through them without twisting. This means that the second polarizing filter now blocks the light and the cell appears opaque (or dark) to the viewer. Color LCD displays can use two different mechanisms for  sending the current through the crystals. In passive matrix displays, the current is timed so that it briefly charges the correct crystal cells. The charges fade quickly, making the image look dim, and the display cannot be refreshed quickly because of the persistence of ghost images. This means that such displays do not work well with games or other programs with rapidly changing displays. In an active matrix display, each display cell is controlled by its own thin film transistor (TFT). These displays are sharper, brighter, and can be refreshed more frequently, allowing better displays for animations and games. However, fabrication costs for TFT displays are higher, and the displays are also vulnerable to having a few transistors fail, leading to permanent dark spots on the display. Active matrix displays also use more power, reducing battery life on laptop PCs. A general disadvantage of flat panel displays is that their pixel dimensions are fixed, so setting the display to a resolution smaller than its full dimensions usually results in an unsatisfactory image.