LCD

(Liquid Crystal Display) A display technology that uses rod-shaped molecules (liquid crystals) that flow like liquid and bend light. Unenergized, the crystals direct light through two polarizing filters, allowing a natural background color to show. When energized, they redirect the light to be absorbed in one of the polarizers, causing the dark appearance of crossed polarizers to show. The more the molecules are twisted, the better the contrast and viewing angle.

Because it takes less power to move molecules than to energize a light-emitting device, LCDs replaced the light-emitting diodes (LEDs) in digital watches in the 1970s. LCDs were then widely used for a myriad of monochrome displays and still are. In the 1990s, color LCD screens caused sales of laptop computers to explode, and in 2003, more LCD monitors were sold for desktop computers than CRTs.

The LCD was developed in 1963 at RCA's Sarnoff Research Center in Princeton, NJ. See flat panel display, LCoS and indium.

TYPES OF LCDs

Passive Display (TN, STN)
Called "passive matrix" when used for computer screens and "passive display" when used for small readouts, all the active electronics (transistors) are outside of the display screen. Passive displays have improved immensely, but do not provide a wide viewing angle, and submarining is generally noticeable. Following are the types of passive displays.

TN - Twisted Nematic - 90º twist
Low-cost displays for consumer products and instruments. Black on gray/silver background.

STN - Supertwisted Nematic- 180-270º twist
Used extensively on earlier laptops for mono and color displays. DSTN and FSTN provide improvements over straight STN (180º - green/blue on yellow background; 270º - blue on white/blue background).

Dual Scan STN
Improves STN display by dividing the screen into two halves and scanning each half simultaneously, doubling the number of lines refreshed. Not as sharp as active matrix.

Active Display (TFT)
Widely used for all LCD applications today (laptop and desktop computers, TVs, etc.). Known as "active matrix" displays, a transistor is used to control each subpixel on the screen. For example, a 1024x768 color screen requires 2,359,296 transistors; one for each red, green and blue subpixel (dot). Active matrix provides a sharp, clear image with good contrast and eliminates submarining. Fabrication costs were originally higher than passive matrix, which caused both types to be used in the early days of laptop flat panels. Active matrix displays use a 90º (TN) twist. Also called "thin film transistor LCD" (TFT LCD). See dead pixel.

Reflective Vs. Backlit
Reflective screens used in many consumer appliances and handheld devices require external light and only work well in a bright room or with a desk lamp. Backlit and sidelit screens have their own light source and work well in dim lighting. Note that the meaning of "reflective" in this case differs from light reflecting off the front of the screen into the viewer's eyes. See transmissive LCD for a comparison.

Twisted-Nematic Seven-Segment Display

Seven separate segments are used to make up the digits 0 through 9. In this illustration, the center segment is used as an example. The unenergized segment (left) causes the crystals to line up with the front and rear polarizers. The segment appears light gray, which is actually the light traveling down and reflected back up the crystals.

The energized segment (right) causes the crystals to become perpendicular to the polarizers. The segment appears dark, which is the effect of crossed polarizers. Most LCDs use a translucent reflector with light behind it, which makes the background brighter and the characters sharper. (Redrawn from illustration courtesy of LXD, Inc.)

Seven-Segment LCD Watch

This was one of the first seven-segment LCD watches on the market in the early 1970s. Notice how digits are made up from the seven different segments. (Image courtesy of the private collection of Peter Wenzig.)

LCD Vs. Plasma

LCD and plasma displays use entirely different means to create colors. LCD uses liquid crystals and color filters while plasma uses gas and phosphors (see plasma display).

LCD Vs. LED

LCDs and light emitting diodes (LEDs) are widely used in combination as in this printer control panel. Alphanumeric readouts can be LCD or LED, but the billions of on/off indicators found on countless products such as this green Ready light are LEDs.

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LCD - liquid crystal display