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Definition: light pen

A light-sensitive stylus wired to a video terminal used to draw pictures or select menu options. The user brings the pen to the desired point on screen and presses the pen button to make contact. Contrary to what it looks like, the pen does not shine light onto the screen; rather, the screen beams into the pen. Screen pixels are constantly being refreshed. When the user presses the button, the pen senses light, and the pixel being illuminated at that instant identifies the screen location.

Light Pen
Light pens provide a very precise pointing capability directly on the screen.

First Light Pen
This handmade prototype of a "light gun" in 1952 was part of the Whirlwind Project at MIT. Subsequent devices were used to hone in on suspicious blips on the CRT in the U.S. air defense system (see Whirlwind and SAGE). (Image courtesy of The MITRE Corporation Archives.)