Liquid-Crystal Display

(L-C-D)

A flat panel display technology that forms images by passing polarized light through small cells which change their ability to transmit this light by varying electrical charges on the cells. Colored filters make a color display. A display is either a collection of individual cells or, more often, a single plate with a pattern of electrodes on the plate. Larger displays actually contain row and colums of electrodes with individual pixels activated at the intersections.

Initial computer displays were passive and their size limited by the fading times of the pixels. More current displays typically use an active element (i.e., a transistor) on top of each pixel. The active element remembers the state of the pixel and keeps it bright for a longer time, allowing much larger and brighter displays.

The elements of an LCD are shown below.

A background light, unpolarized, passes through a polarizer (vertical in the example above). The light passes through a glass plate topped with a layer of transistors. The transistors are used to activate portions of the liquid-crystal layer where light is to be on the output display. Those pixels where the transistor is idle allow the light straight through. Since the light is vertically polarized when it hits the horizontal polarization layer it is stopped and the display shows a dark dot.

Those pixels where the transistor is active change the orientation of the light from vertical to horizontal. It then is able to pass through the horizontal polarization filter and display a dot on the screen. The color of the dot will depend on the color of the filter the light passes through.

A typical LCD panel may take from 16 to 25 milliseconds to fully change from black to white and back to white again. Shades of grey take longer. This can cause problems when the display is showing moving objects which is why for moving objects an LCD display can have a fuzzy appearance. Faster-responding liquid-crystals are under development.

The other problem noted with LCD displays is that off-axis viewing becomes a problem. Because liquid-crystal molecules are shaped like rods the image only looks brightest when viewing the display straight on. Side views might look distorted and dim. Of course, if you are working on something sensitive on an airplane, this is not altogether a bad thing. However, microscopic structure changes to the cells in the liquid-crystals are under investigation as a method of solving the off-axis viewing problem.

More Information

• PC Magazine LCD Article Starting
• LCD Display Discussion