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(Liquid Crystal Display) A display technology developed in 1963 at the David Sarnoff Research Center in Princeton, NJ, that is quite amazing. Sandwiched between polarizing filters and glass panels, liquid crystals are rod-shaped molecules that flow like liquid and bend light like crystal. The orientation of the filters and panels determines how light passes through the crystals.

     Because it takes such little power to move crystal molecules, LCD-based digital wristwatches began to flourish in the 1970s along with myriad other monochrome displays. By the 1990s, color LCDs helped the sales of laptops to boom, and in 2003, LCD computer monitors outsold CRTs for the first time, making LCD the predominant electronic display technology.

Major Categories - Passive and Active
Passive displays are widely used for fixed-purpose, mostly monochrome readouts on printers, appliances and industrial products. In contrast, active displays, which are mostly color, are the graphics-based screens used in TVs, computers and numerous other devices. For more details about passive and active LCDs, see LCD types.




A Seven-Segment Digit
In this illustration (courtesy of LXD, Inc.), the center segment is used as the example. When it is unenergized (left), the crystals line up with the front and rear polarizers, and light travels down their spiral staircase and back up to the viewer. When energized (right), the "crossed" polarizers (front and rear at 90 degrees to each other) cause the center segment to appear dark. For more details about the process and modes in this illustration, see LCD example.

Twisting and Straightening
The example above is quite simple compared to an LCD TV set, where more than six million subpixel cells are constantly straightening and twisting hundreds of times per second to achieve the required color (more details in LCD example).

Reflective Vs. Backlit
Used in handheld calculators and low-cost readouts, a reflective mirror in the rear bounces ambient light back to the viewer (look at the bottom of the illustration above). However, in dimly lit rooms, reflective displays may be unreadable. With backlit screens, a light below the transflector (a translucent reflector) shines toward the viewer to provide a bright screen indoors. Passive displays may be reflective or backlit, but active matrix TV and computer screens are always backlit (see LCD types and LED TV). See flat panel display, OLED, transmissive LCD, plasma display, LCoS and indium.




Seven-Segment LCD Watch
This negative image display was one of the first seven-segment LCD watches on the market in the early 1970s. Because LCDs used less power, they quickly replaced the red LEDs in the first digital watches of that era. Notice how digits are made up from the seven segments. (Image courtesy of the private collection of Peter Wenzig.)








LCD Vs. Plasma
LCD and plasma screens display colors differently. LCD uses liquid crystals and color filters while plasma uses gas and phosphors (see plasma display). Whereas plasma displays always emit light, LCDs emit light when a backlight is used, or only ambient light if not (reflective).





LCD Vs. LED
LCDs and light emitting diodes (LEDs) are widely used in combination as in this printer control panel. Readouts are mostly LCDs, but the indicator lights found on billions of products such as this green Ready light are LEDs. Increasingly, LCD TVs use LEDs as backlights (see LED TV).