A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT display.
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 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.
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.
Famous quotes containing the words light and/or pen:
“Long enough I had heard of irrelevant things; now at length I was glad to make acquaintance with the light that dwells in rotten wood. Where is all your knowledge gone to? It evaporates completely, for it has no depth.”
—Henry David Thoreau (1817–1862)
“Every drop of ink in my pen ran cold.”
—Horace Walpole (1717–1797)