What is LCD (Liquid Crystal Display)?

LCD stands for liquid crystal display. Your digital watch uses an LCD to show you the time, and most portable computers use an LCD to display the screen. There is actually a liquid compound, liquid crystals, sandwiched between two grids of electrodes. The electrodes can selectively turn on the different cells or pixels in the grid to create the image you see.

An LCD consists of a layer of gooey material-the liquid crystals themselves-between two polarizing filters. These filters are sheets of plastic that let through only those light waves traveling parallel to a particular plane. Between the filters and the liquid crystal layer runs a thin grid of transparent electrodes.

The two polarizing filters are arranged so that their polarizing planes are at right angles. That setup would block light from passing through except for the fact that the liquid crystal molecules are “twisted.” They pivot the light coming through the first filter, aligning the light with the polarizing plane of the second filter. Since the light makes it all the way through both filters, the screen looks light in color. However, the liquid crystal molecules that are controlled by a particular electrode become untwisted when a current is applied. Light no longer passes through the second filter, and you see a black or colored dot on the screen. Most LCDs are passive matrix designs, in which each dot, or pixel, on the screen shares electrodes with other dots. Active matrix designs, which produce much brighter, more colorful images, have a separate transistor for each pixel, which allows greater control over the current for that pixel.

In “supertwist” LCDS, the liquid crystal molecules have a more pronounced twist than in the ordinary screens, improving contrast. The chemist’s term “nematic” refers to the molecular structure of the crystals-all LCDs use nematic crystals, so this term is used in ads just to impress you.

Although you can read an LCD screen in room light, the contrast is mediocre at best. Today, the LCD screens on most computers are illuminated by backlighting or edge lighting (fluorescent-type lights mounted behind the screen or along either side).