A common display found on digital watches and readouts that looks like a series of 8s. Each digit is formed by selective illumination of from one to seven separately addressable segments. For more details, see LCD types
. For more on color LCDs, see LCD subpixels
Using two seven-segment digits, this readout can display from 00 to 99. Each of the seven segments has its own contact to the outside electronics. (Image courtesy of LXD, Inc.)
Introduced in 1973, this was one of the first LCD watches on the market. Notice how the seven segments make up 0 through 9. LED watches, with their bright red digits, came out a couple years earlier. (Image courtesy of the private collection of Peter Wenzig.)
A Single Segment
The above illustration highlights one segment of the liquid crystal layer in a monochrome twisted nematic (TN) seven-segment display (Image courtesy of LXD, Inc.)
In their natural state, liquid crystals line up with the orientation "rubbed" into the front and rear glass panels. The rubbings are microscopic grooves.
In this "positive image" example, the background is lit, while the dark segments that make up the desired letters and digits are unlit. Notice that the front and rear planes are oriented 90 degrees from each other. The top left segment is the result of ambient light traveling from the viewer down the crystals and reflected back up the same twisted ladder. Light follows the liquid crystal path.
The top right segment gets no light, because the crystals reorient perpendicular to the rear polarizer, which absorbs the light.
Reflective Vs. Backlit
In low-cost calculators and readouts, a reflector bounces ambient light back to the viewer. However, reflective displays are hard to read in dim light. A backlight provides a source of light at all times (see transmissive LCD
). 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