Light pen

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 (CRT) display.

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. A light pen can work with any CRT-based display, but its ability to be used with LCDs was unclear (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan[1]).

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.[2][3]

One of the first more widely deployed uses was in the Situation Display consoles of the AN/FSQ-7 for military airspace surveillance. This is not very surprising, given its relationship with the Whirlwind projects. See Semi-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 compatible CGA, HGC and some EGA graphics cards also featured a connector compatible with a light pen, as did early Tandy 1000 computers,[notes 1] the Thomson MO5 computer family, the Atari 8-bit, Commodore 8-bit, some MSX computers[4] and Amstrad PCW home computers. For the MSX computers, Sanyo produced a light pen interface cartridge.[5]

Because the user was required to hold their arm in front of the screen for long periods of time (potentially causing "gorilla arm") or to use a desk that tilts the monitor, the light pen fell out of use as a general-purpose input device.

gollark: > hey ___ can we have access to this thing that can quickly find the password to any kst wallet?It can find *a* password, though, not *the* actual password.
gollark: Sad.
gollark: Do you have potatOS-related ones?
gollark: What happened with that, anyway?
gollark: But killdan200!

See also

Notes
  1. For example, the Tandy 1000 SX has a DE-9 light pen conector on the rear panel; on the later-introduced Tandy 1000 TX, this light pen interface has been replaced with a serial port using the same connector in the same location.

References
  1. "Slashphone Article". 2006-04-20. Archived from the original on 2008-11-20. Retrieved 2008-11-12.
  2. "A Critical History of Computer Graphics and Animation". Archived from the original on 2009-05-05. Retrieved 2009-05-04.
  3. "The Computer Desktop Encyclopedia (entry for Light Pen)". Retrieved 2009-05-04.
  4. "MSX Wiki (entry for Light Pen)". Retrieved 2019-05-16.
  5. "Sanyo MLP-01 on Generation MSX".
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