light pen

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An obsolete penlike input device that was used with a cathode-ray tube display to point at items on the screen or to draw new items or modify existing ones. The light pen had a photosensor at the tip that responded to the peak illumination that occurred when the CRT scanning spot passed its point of focus. The display system correlated the timing of the pulse from the photosensor with the item being displayed to determine the position of the light pen.

Subjects: Computing — Chemistry.

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