A lightpen is a device similar to a touch screen, but is facilitated by use of a special pen instead of the finger. The advantage of using a pen is accurate screen input.
It was notable for its use in the Fairlight CMI, and was a popular accessory for the BBC Micro. However the lightpen's usage greatly declined with the general adoption of the mouse.
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.