This article is about the computer input device. For the pen-sized flashlight, see penlight.
Photo of the Hypertext Editing System (HES) console in use at Brown University, circa October 1969. The photo shows HES on an IBM 2250 Mod 4 display station, including lightpen and programmed function keyboard, channel coupled to Brown's IBM 360 mainframe.
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. It was long thought[according to whom?] that a light pen can work with any CRT-based display, but not with LCDs (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan) and other display technologies. However, in 2011 Fairlight Instruments released its Fairlight CMI-30A, which uses a 17" LCD monitor with light pen control.
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. 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.
The first light pen was created around 1955 as part of the Whirlwind project at MIT.
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.
The light pen found use during the early 1980s. It was notable for its use in the Fairlight CMI, and the BBC Micro. IBM PC compatible CGA, HGC and some EGA graphics cards featured a connector for a light pen as well. Even some consumer products were given light pens, such as the Thomson MO5 computer family as well as the Atari 8-bit and Commodore 8-bit home computers.