Light pen
A light-sensitive wand that detects CRT phosphor glow for screen interaction.
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A light pen is a computer input device shaped like a light-sensitive wand, designed to work with cathode-ray tube (CRT) displays. It lets users point to on-screen elements like menus or buttons, or even draw directly on the screen, much like a touchscreen. The key difference is that a touchscreen detects a fingertip or stylus, while a light pen itself acts as the sensor, picking up the light given off by the CRT's phosphors.
These pens work with both vector-based and raster-based CRT displays. When the CRT's electron beam scans the screen, it excites a phosphor coating, making it glow. If this happens right under the pen's tip, a photosensitive component—such as a photodiode—sends a timing signal to the computer. The system then calculates the pen's exact position using either the beam's known location at that moment (in vector systems) or the time since the last horizontal or vertical sync pulse (in raster systems).
The first light pen, originally called a "light gun," was developed around 1951–1955 as part of MIT's Whirlwind I project, where it was used to pick discrete symbols on screen. It later became a component of the SAGE system for real-time control of radar-networked airspace. One of its earliest widespread uses was in the Situation Display consoles of the AN/FSQ-7 for military airspace surveillance, a natural extension of the Whirlwind work.
In the 1960s, light pens were common on graphics terminals like the IBM 2250 and were also available for the IBM 3270 text-only terminal. The first nonlinear video editor, the CMX 600, was controlled by a light pen; it used two black-and-white monitors, with the pen interacting with text overlaid on the preview video on the right screen.
Usage expanded in the early 1980s to music workstations like the Fairlight CMI and personal computers such as the BBC Micro and Holborn 9100. IBM PC-compatible graphics cards—including early MDA, CGA, HGC (plus HGC+ and InColor), and some EGA models—featured a light pen connector, as did early Tandy 1000 computers, the Thomson MO5 family, the Amiga, Atari 8-bit, Commodore 8-bit, some MSX computers, and Amstrad PCW home computers. For MSX systems, Sanyo produced a light pen interface cartridge.
Quick Facts
- First created
- around 1951–1955
- First project
- Whirlwind I at MIT
- Early military use
- SAGE system and AN/FSQ-7 Situation Display consoles
- Notable graphics terminal
- IBM 2250
- First nonlinear video editor
- CMX 600
Facts from the source article.
Lore & Background
The first light pen, at the time still called a 'light gun', was created around 1951–1955 as part of the Whirlwind I project at MIT, where it was used to select discrete symbols on the screen. Light pens were a component of the SAGE system, assisting in tactical real-time control of radar-networked airspace. One of the first more widely deployed uses was in the Situation Display consoles of the AN/FSQ-7 for military airspace surveillance. 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 first nonlinear video editor, the CMX 600, was controlled by a light pen; the system came with two black and white monitors, where the light pen was used to interact with text superimposed over the preview video on the right monitor. Light pen usage expanded in the early 1980s to music workstations such as the Fairlight CMI, and personal computers such as the BBC Micro and Holborn 9100. IBM PC-compatible MDA (only early versions), CGA, HGC (including HGC+ and InColor) 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 Amiga, Atari 8-bit, Commodore 8-bit, some MSX computers and Amstrad PCW home computers. For the MSX computers, Sanyo produced a light pen interface cartridge.
Reader's Guide
Light pens are compatible with both vector-based and raster-based CRT displays. As a CRT's electron beam traverses the screen, it excites a phosphor coating within the CRT, causing it to glow. If this occurs directly under the tip of the pen, a photosensitive component such as a photodiode causes a timing signal to be sent to the computer.
Using either the known position of the beam at that precise moment (in a vector-based system), or the time since the last horizontal or vertical synchronization pulses (in a raster-based system), the exact position of the pen can be derived. 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. The light pen was also perceived as working well only on displays with low persistence, which tend to flicker.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Light pen (CC BY-SA 4.0).
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