A light pen detects a change of brightness of nearby screen pixels when scanned by cathode ray tube electron beam and communicates the timing of this event to the computer.
Another innovative feature for that time was the user interface combining a keyboard with a monitor and light pen for correcting texts and drawing on screen.
Sketchpad's light pen drew directly on a monitor, and the program locked in right angles, arcs and other shapes to help create detailed, accurate and replicable illustrations.