light pen

In computing, a device resembling an ordinary pen, used to indicate locations on a computer screen. With certain computer-aided design (CAD) programs, the light pen can be used to instruct the computer to change the shape, size, position, and colours of sections of a screen image.

The pen has a photoreceptor at its tip that emits signals when light from the screen passes beneath it. From the timing of this signal and a gridlike representation of the screen in the computer memory, a computer program can calculate the position of the light pen.