Light pen

A light pen is a computer input device shaped like a pen that is used to select or modify text or graphics on a CRT (cathode ray tube) screen. It allows a user to point to displayed objects on the screen in a way similar to a touchscreen, but with greater positional accuracy.

The device works by sensing the light emitted by the CRT phosphor when the electron beam refreshes the screen. When the light pen is held against the screen, a photocell or photodiode inside the pen detects the light pulse from the refreshing pixel. This timing information is then sent back to the computer, which can determine the location of the pen on the screen based on the refresh cycle.

Light pens were commonly used in the early days of computer graphics and CAD/CAM applications because they provided a relatively inexpensive and precise method of interacting with the display. However, they have largely been replaced by other input devices such as mice, touchscreens, and graphics tablets due to limitations such as requiring the user to hold the pen directly against the screen for extended periods, which can cause fatigue, and incompatibility with LCD and other non-CRT display technologies. They were also difficult to use on the edges of the screen where the light was less intense.