A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's cathode-ray tube display. 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. A light pen can work with any CRT-based display, but its ability to be used with LCDs was unclear. A light pen detects changes in brightness of nearby screen pixels when scanned by cathode-ray tube electron beam and communicates the timing of this event to the computer. Since a CRT scans the entire screen one pixel at a time, the computer can keep track of the expected time of scanning various locations on screen by the beam and infer the pen's position from the latest timestamp.
History
The first light pen was created around 1955 as part of the Whirlwind project at MIT. One of the first more widely deployed uses was in the Situation Display consoles of the AN/FSQ-7 for military airspacesurveillance. This is not very surprising, given its relationship with the Whirlwind projects. SeeSemi-Automatic Ground Environment for more details. During the 1960s, light pens were common on graphics terminals such as the IBM 2250 and were also available for the IBM 3270 text-only terminal. Light pen usage was expanded in the early 1980s to music workstations such as the Fairlight CMI and personal computers such as the BBC Micro. IBM PC compatibleCGA, HGC and some EGA graphics cards also featured a connector compatible with a light pen, as did early Tandy 1000 computers, the Thomson MO5 computer family, the Atari 8-bit, :Category:Commodore 8-bit family|Commodore 8-bit, some MSX computers and Amstrad PCWhome computers. For the MSX computers, Sanyo produced a light pen interface cartridge. Because the user was required to hold their arm in front of the screen for long periods of time or to use a desk that tilts the monitor, the light pen fell out of use as a general-purpose input device.
