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A light pen is a light-sensitive computer input device, basically a stylus, that is used to select text, draw pictures and interact with user interface elements on a computer screen or monitor. The light pen works well with CRT monitors because of the way such monitors scan the screen, which is one pixel at a time, giving the computer a way to keep track of the expected scanning time by the electron beam and infer the pen's position based on the latest timestamp of the scanning.
The light pen can be considered as the predecessor to touchscreen technology and was first created in 1955 as part of the Massachusetts Institute of Technology's (MIT) Whirlwind Project, a Cold War vacuum tube military computer. The pen allowed users to precisely select individual pixels on the screen and to draw and interact with menu elements in much the same way as users interact with touchscreen displays. The light pen became common in the 1960s on graphics terminals such as the IBM 2250 and was also available for text-only terminals. It also became popular during the 1980s for home computers such as the Atari and Commodore 8-bit computers. However, the concept of operation of the light pen is not compatible with modern LCD screens and, as a result, the input device eventually died out.
The light pen works by detecting the change in brightness of nearby pixels, which indicates that the CRT's electron beam is scanning in that area; it then sends the timing of this event to the computer, which then compares this information to the timestamp of the last scan by the electron beam, allowing the computer to infer the precise location of the pen.