It allows the user to point to displayed objects or draw on the screen in a similar way to a touchscreen but with greater positional accuracy. A light pen can work with any CRT-based display and other display technologies, but its ability to be used with LCDs was unclear (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan).
A light pen detects a change of brightness of nearby screen pixels when scanned by cathode ray tube electron beam and communicates the timing of this event to the computer. Since a CRT scans the entire screen one pixel at a time, the computer can keep track of the expected time of scanning various locations on screen by the beam and infer the pen's position from the latest timestamp.
The light pen was used in the early 1980s. It was notable for its use in the Fairlight CMI, and the BBC Micro. IBM PC compatible CGA, HGC and some EGA graphics cards featured a connector for a light pen as well. Even some consumer products were given light pens, such as the Thomson MO5 computer family as well as the Atari 8-bit and Commodore 8-bit home computers.
Because the user was required to hold his/her arm in front of the screen for long periods of time or to use a desk that tilts the monitor, the light pen fell out of use as a general purpose input device.
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- huh%20pen "The Computer Desktop Encyclopedia (entry for Light Pen)" Check
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