A light pen is a stylus containing a photodiode that allows a user to point directly at a cathode ray tube (CRT) screen. When the monitor's electron beam sweeps past the pen's location, the photodiode detects the sudden burst of light. The computer calculates the exact X and Y coordinates by comparing the detection time against the monitor's scan timing, enabling direct on-screen drawing or menu selection.
The radar operator used a light pen to tag the incoming target on the scope.
Related terms
Cathode ray tube
A vacuum tube containing one or more electron guns and a phosphorescent screen used to display images.
CRT
A cathode-ray tube is a vacuum envelope containing an electron gun and a phosphorescent screen.
Scan line
A single horizontal row of pixels or phosphors drawn across a display screen.
Touch screen
An electronic visual display that detects the presence and location of a touch within its viewing area.