Light pen
Encyclopedia
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
input device
Input device
In computing, an input device is any peripheral used to provide data and control signals to an information processing system such as a computer or other information appliance...
in the form of a light-sensitive wand used in conjunction with a computer's CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
TV set or monitor
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy. It was long thought that a light pen can work with any CRT-based display, but not with LCD
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
screens (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan), projectors and other display devices. However, in 2011 Fairlight Instruments
Fairlight
Fairlight is a digital audio company based in Sydney, Australia. In 1979 they created the Fairlight CMI, the first digital audio sampler, quickly used by artists such as Peter Gabriel, Kate Bush, and Jean Michel Jarre. They are now a manufacturer of media solutions tools such as digital audio...
released its Fairlight CMI-30A
Fairlight CMI
The Fairlight CMI is a digital sampling synthesizer. It was designed in 1979 by the founders of Fairlight, Peter Vogel and Kim Ryrie, and based on a dual-6800 microprocessor computer designed by Tony Furse in Sydney, Australia...
, which uses a 17" LCD monitor with light pen control.
A light pen is fairly simple to implement. Just like a light gun
Light gun
A light gun is a pointing device for computers and a control device for arcade and video games.Modern screen-based light guns work by building a sensor into the gun itself, and the on-screen target emit light rather than the gun...
, a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
refreshes that spot. By noting exactly where the scanning has reached at that moment, the X,Y position of the pen can be resolved. This is usually achieved by the light pen causing an interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
, at which point the scan position can be read from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen.
The light pen became moderately popular during the early 1980s. It was notable for its use in the Fairlight CMI
Fairlight CMI
The Fairlight CMI is a digital sampling synthesizer. It was designed in 1979 by the founders of Fairlight, Peter Vogel and Kim Ryrie, and based on a dual-6800 microprocessor computer designed by Tony Furse in Sydney, Australia...
, and the BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
. IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
compatible CGA
Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
, HGC
Hercules Graphics Card
The Hercules Graphics Card was a computer graphics controller made by Hercules Computer Technology, Inc. which, through its popularity, became a widely supported display standard. It was common on IBM PC compatibles connected to a monochrome monitor . It supported one high resolution text mode and...
and some EGA
Enhanced Graphics Adapter
The Enhanced Graphics Adapter is the IBM PC computer display standard specification which is between CGA and VGA in terms of color and space resolution. Introduced in October 1984 by IBM shortly after its new PC/AT, EGA produces a display of 16 simultaneous colors from a palette of 64 at a...
graphics cards featured a connector for a light pen as well. Even some consumer products were given light pens, in particular the Thomson MO5
Thomson MO5
The Thomson MO5 was a 6809E-based computer introduced in France in 1984. It featured 32 KB of RAM, a 40×25 text display, and built-in Microsoft BASIC. The MO5 was replaced by the MO6 in 1986.-External links:...
computer family. Because the user was required to hold his or her 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.
The first light pen was created around 1952 as part of the Whirlwind
Whirlwind (computer)
The Whirlwind computer was developed at the Massachusetts Institute of Technology. It is the first computer that operated in real time, used video displays for output, and the first that was not simply an electronic replacement of older mechanical systems...
project at MIT.
Since the current version of the game show
Game show
A game show is a type of radio or television program in which members of the public, television personalities or celebrities, sometimes as part of a team, play a game which involves answering questions or solving puzzles usually for money and/or prizes...
Jeopardy!
Jeopardy!
Griffin's first conception of the game used a board comprising ten categories with ten clues each, but after finding that this board could not be shown on camera easily, he reduced it to two rounds of thirty clues each, with five clues in each of six categories...
began in 1984, contestants have used a light pen to write down their wagers and responses for the Final Jeopardy! round.
Since light pens operate by detecting light emitted by the screen phosphors, some nonzero intensity level must be present at the coordinate position to be selected, otherwise the pen won't be triggered.