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 cathode-ray tube (CRT) display.
Like a touchscreen, a light pen allows the user to point to displayed graphic elements such as menus or buttons, or even draw directly on the screen. However, while a touchscreen acts as a sensor to detect a fingertip or stylus, a light pen serves as the sensor itself, detecting the light emitted by the CRT's phosphors.
Light pens are compatible with both vector-based and raster-based CRT displays. As a CRT's electron beam traverses the screen, it excites a phosphor coating within the CRT, causing it to glow. If this occurs directly under the tip of the pen, a photosensitive component such as a photodiode causes a timing signal to be sent to the computer. Using either the known position of the beam at that precise moment (in a vector-based system), or the time since the last horizontal or vertical synchronization pulses (in a raster-based system), the exact position of the pen can be derived.
History
The first light pen, at this time still called a "light gun", was created around 1951–1955<!-- to be narrowed down better --> as part of the Whirlwind I project at MIT, where it was used to select discrete symbols on the screen. the Thomson MO5 computer family, the Amiga,
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