Lightguide display
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A light guide display (also known as an edge-lit display) is an obsolete electronic mechanism which was used for displaying alphanumeric characters in electronic devices such as calculators, multimeters, laboratory measurement instruments, and entertainment machines such as pinball games. It contains a set of sandwiched acrylic or clear plastic panels, each of which is engraved with a numeral or character to be displayed. Light from independently controlled incandescent bulbs passing into the edge of these panels reflects off of the internal surfaces of the plastic. When the light encounters the engraved digits, it is scattered, rendering a brightly illuminated digit or character.
The technology was rendered obsolete by the development of LEDs in the 1970s, though lightguide tubes are still used in electronics manufacturing, in situations where it is difficult to place an LED in the appropriate physical location on a display or bezel. In such cases, LEDs mounted on printed circuit boards are fitted with lightguides to channel light to the appropriate position. This employs the same principle used in Optical Fibers.