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Encyclopedic overview
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 cathode-ray tube (CRT) display.
Excerpted from Wikipedia’s “light pen” article, available under the CC BY-SA 4.0 licence.