work on the same principle. 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. Screen overlays In order to enhance the display visuals of the Vectrex, every commercially released game included its own unique translucent plastic screen overlay that accompanied the cartridge (a concept first seen with the Magnavox Odyssey, as well as some early arcade machines). Instead of physically touching the CRT screen, four tabs on the Vectrex console securely held them in place in front of it, with a small gap between the actual screen and the overlay. Made up of one to three colors for the play field area, these overlays provide the illusion of simple color graphics (on an otherwise black and white screen), helped reduced glare, flicker and gave the appearance of a flat screen. They also allowed changes in brightness intensity of vector graphics to be more visually distinctive. In some cases game designers created pseudo color cycling effects, for a sense of movement, by using alternating colored patterns. In addition to players' score areas, some overlays also contained additional artwork and patterns, to add to the game's play field. Across the bottom of each overlay are game-specific joystick and button functions as a guide for the player. Each overlay also displayed the title and logo of each game, along with a colored border or design, to add cosmetic flair to the Vectrex (much like an arcade machine with its marquee or side art). Overlays were not required, but added to the experience in terms of the visual look of game graphics and the overall display appearance of the console. Software Some of the Vectrex's library consisted of ports of arcade hits, most of them brought to the console through