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The history of the light pen begins in 1952 with a handmade "light gun" in the Whirlwind Project at MIT [pcmag.com. In 1963 at the MIT Lincoln Laboratory Ivan Sutherland oversaw the Sketchpad Project on Lincoln Laboratory's TX-0 computer. For his Ph.D. thesis, using a lightpen on a 9" CRT screen, he created engineering drawings. That was the first graphical user interface. [Source: Sun.com February 20, 2005]
Wikipedia posted this photo of a light pen in use at Brown University in 1969 using Hypertext Editing System on an IBM 2250 Mod 4 display station.
Light pens used to be available only on five digit priced graphics systems, but reached the home user with under $100 pricing in the early eighties. The Gibson Light Pen made by Steve Gibson for Koala Technologies works as an input device for a computer, by pointing right at the monitor screen. It does not produce light, but its tip has a light-sensitive cell to detect the presence or absence of light. Somehow it can also communicate its screen position to the computer.
The light pen has its own interface card which connects right to the motherboard. It requires graphic a application to run. Drawing directly on a monitor screen in graphic application is analagous to an artist with a canvas, but it could be tiring.
Steve Gibson subsequently programmed a light pen and graphics application for the Atari in 1984, which he found to be a joy to work with compared to the Apple with its 'Wozni-isms.'
What looks like similar technology is still in use -- I saw a wait person record my order using one. Or perhaps it was a stylus with a touch screen...