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Light pen
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{{Short description|Computer input device}} {{About|the computer input device|the pen-sized flashlight|penlight}} {{Use dmy dates|date=August 2021|cs1-dates=y}} {{Use list-defined references|date=December 2023}} [[File:HypertextEditingSystemConsoleBrownUniv1969.jpg|thumb|220px|Photo of the [[Hypertext Editing System]] (HES) console in use at [[Brown University]], circa October 1969. The photo shows HES on an IBM 2250 Mod 4 display station, including lightpen and programmed function keyboard, channel coupled to Brown's [[IBM 360]] mainframe.]] A '''light pen''' is a [[computer]] [[input device]] in the form of a light-sensitive wand used in conjunction with a computer's [[cathode-ray tube]] (CRT) display. 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, but its ability to be used with [[Liquid-crystal display|LCDs]] was unclear (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan<ref name="Slashphone_2006"/>). A light pen detects changes in 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 time stamps.
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