A Light Pen is a input device, which is a light-sensitive pointing input device commonly used to select or otherwise modify text or data on a screen. It allows the user to point to displayed objects or draw on the screen in a similar way to a touchscreen but with greater positional accuracy. Used with a CRT monitor, these devices were an early form of manipulating and highlighting data on the screen.
The first Light Pen was invented by Ben Gurley in 1959. The basic idea for the computer mouse came to Douglas Engelbart in 1961, while he was attending a computer graphics conference.
A Light Pen detects a change of brightness of nearby screen pixels when scanned by cathode ray tube electron beam and communicates the timing of this event to the computer.