A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT TV set or monitor. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy.
A light pen is fairly simple to implement. Just like a light gun, a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun refreshes that spot. By noting exactly where the scanning has reached at that moment, the X,Y position of the pen can be resolved. This is usually achieved by the light pen causing an interrupt, at which point the scan position can be read from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen.
The light pen is a photosensitive pen. It is also a poinnting device, capable of sensing a position on the screen. When a light pen's tip is moved over the screen surface, its photocell sensing element detects light coming from the screen. Corresponding signals are sent to the processor that identifies the point (pixel) on the screen. A light pen is used to draw images onscreen.