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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).

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