Light Pen

Light pens offer a way for users to register a touch to a display that is even more precise than pixel perfect precision. They lost, or never gained, popularity in applications requiring the user to interact with the display for more than a few minutes at a time. Most computer systems during era of the light pen usually required the user to take a position other than seated to reach the screen. And the user would typically hold the pen near the screen for expediency during their entire use period. The resulting muscle fatigue and repetitive strain was called "gorilla arm" even though other parts of the human body could also be stressed. Arm rests, hand rests, tilted low rise monitor mounts, and various other apparatusum where employed in industrial settings to try an alleviate some of the issues but none were ever completely eliminated. Hand and wrist strain were ever-present. These ergonomic considerations were never available to home users. Since most games intend the user to play for more than a couple of minutes, light pens were even less popular for gaming. But, they were used.

Light pens work almost the same way as light guns and are often interchangeable depending on the physical portion of the interface (the plug).

An IBM CGA or EGA card, or a fully compliant CGA or EGA card, includes a light pen interface. IBM never manufactured a light pen for it. The first Tandy 1000 had a light pen port. CGA light pens have a reputation for being imprecise. There is also the problem of a light pen not reporting its position if it is pointing at a black area of the screen. I suspect this is due to the manufacturing standards of companies that made light pens, since the interface allows the pen to find locations much smaller than pixels and is supposed to sense even in black areas of a screen when the screen is powered. Lightpens were designed to read the raster scan position on a CRT monitor and generally required to physically connect to an analogue video signal. Other monitor types, such as LCD, have no such thing. This makes using a light pen with a non-CRT monitor quite problematic. However, software and hardware modification can be made to allow a light pen to find its position correctly on non-CRT monitors. Light guns have almost the same issue and the hardware solutions to use light guns with LCDs should also work with light pens.