light pen An obsolete penlike input device that was used with a cathode-ray tube display to point at items on the screen or to draw new items or modify existing ones. The light pen had a photosensor at the tip that responds to the peak illumination that occurs when the CRT scanning spot passes its point of focus. The display system correlated the timing of the pulse from the photosensor with the item being displayed to determine the position of the light pen.

The light pen was used to draw items with the aid of a tracking cross. As the light pen was moved across the screen, the part of the tracking cross sensed changes thus allowing the direction of movement of the light pen to be ascertained. The tracking cross can be redrawn to locate it at the expected new center of the light pen's position and thus appears to follow the light pen.

light pen • n. 1. Comput. a hand-held, penlike photosensitive device held to the display screen of a computer terminal for passing information to the computer.2. a hand-held, light-emitting device used for reading bar codes.