Light pen
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A light pen is a pointing device in the form of a wand connected to the computer via a cable. They were used with Cathode Ray Tube (CRT) displays.
The tip of the pen contains a light sensor and a pressure sensor. When the pen is pressed against the screen, it sends a pressure signal and a light signal back to the computer. The computer detects when the CRT's electron beam activates the light sensor, and is thus able to find the pen tip's screen position in pixel coordinates.
A light gun works similarly to a light pen, but it is shaped like a gun, works from a distance and the pressure sensor is a pistol trigger.
Some "light guns", such as the one for the Nintendo Entertainment System works differently though: When the trigger is pulled, the screen turns black except for a white field only within a valid target area.
