Graphics Tablet and Light Pen

A graphics tablet is a specialist input device for technical drawings. It consists of a flat tablet and a stylus. The absolute position of the stylus on the tablet's surface can be detected. This provides an input device that is similar to the paper and pencil that the user may already be familiar with.

A similar effect is achieved with a light pen except in this case the flat surface is the VDU screen itself. The pen is triggered by the raster scan of the VDU - it detects when the electron beam building up the screen image has just passed the point where the pen is positioned. Knowing the instant at which the beam passed the pen, the software can calculate whereabouts on the screen it has been placed.

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