A lightpen is a device similar to a touch screen, but is facilitated by use of a special light sensitive pen instead of the finger. The advantage of using a pen is that more accurate screen input is possible than with a touch screen. Also, a light pen can work with any CRT-based monitor, not just with a special touch screen. However, light pens cannot work with LCD screens, projectors et cetera.

A lightpen is fairly simple to implement, but is rather dependent on the video hardware which drives the display. This is one reason it fell out of use - it would require a special port on every video display card, whereas the mouse can be implemented solely in software. In addition, ergonomic factors favour the mouse - it can be tiring to operate a computer using a lightpen over long periods. The lightpen works by sensing the sudden small change in brightness of a point on the screen when the electron gun refreshes that spot. By noting exactly where the scanning has reached at that moment resolves the X,Y position of the pen. This is usually achieved by making the lightpen cause an interrupt, at which point the scan position can be read off from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen. Lightpens operate best with relatively slow-scanning displays of low resolution, such as a television screen.

The light pen became moderately popular during the early 1980s It was notable for its use in the Fairlight CMI, and the BBC Micro. However, it had the fatal flaw in that the user was required to hold his or her arm in front of the screen for long periods of time. Doing so quickly caused the user's arm to be sore and tired creating an effect known as gorilla arm . Because of this, the lightpen's usage greatly declined later in the decade with the adoption of mouse-based WIMP interfaces as well as changes in monitor technology.

The first light pen was used around 1957 on the Lincoln TX-0 computer at the MIT Lincoln Laboratory.

See also

• Light gun
• Graphics tablet