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Light Pen, a pointing device in which the user holds a wand, which is attached to the computer, up to the screen and selects items or chooses commands on the screen (the equivalent of a mouse click) either by pressing a clip on the side of the light pen or by pressing the light pen against the surface of the screen.





The wand contains light sensors and sends a signal to the computer whenever it records a light, as during close contact with the screen when the non-black pixels beneath the wand's tip are refreshed by the display's electron beam. The computer's screen is not all lit at once—the electron beam that lights pixels on the screen traces across the screen row by row, all in the space of 1/60 of a second.



By noting exactly when the light pen detected the electron beam passing its tip, the computer can determine the light pen's location on the screen. The light pen doesn't require a special screen or screen coating, as does a touch screen, but its disadvantage is that holding the pen up for an extended length of time is tiring to the user.
 

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