A light pen, also called a selector 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 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 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. A light pen is fairly simple to implement. Just like a light gun, a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun 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 ,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. 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.
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