Editing Vectrex (section)

===Light pen===
The light pen allows the user to "draw", to create images and to indicate, on the screen. It has a photo-detector that can see the bright spot of the vector-drawing display monitor when it goes by under the light pen's position where it is being held to the screen. The photo-detector feeds internal pulse-catching circuits that tell the Vectrex and its software of the event. The prototype was made in the plastic casework of a Marks-A-Lot felt-tipped marker pen. The Vectrex draws a spider-web-like search-pattern to track the pen's location. The software changes the pattern size as the pen changes motions and velocity in an attempt keep a continuous lock on the pen's position. The Vectrex light pen was invented by John Ross.