Stack Lightpen+ was a product of Stack Computer Services Ltd from Bootle, Lancashire, UK for the BBC personal computer.

Light pen is an input device that is used with a cathode-ray tube display to
point at items on the screen or to draw new items or modify existing ones. The light pen had a photo sensor at the tip that responds to the peak illumination that occurs when the CRT scanning spot passes its point of focus. The display system correlated the timing of the pulse from the photo sensor with the item being displayed to determine the position of the light pen. A light pen can work with any CRT-based display, but not with LCD screen (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan), projectors and other display devices.

The light pen was used to draw items with the aid of a tracking cross. As the light pen was moved across the screen, the part of the tracking cross sensed changes thus allowing the direction of movement of the light pen to be ascertained. The tracking cross can be redrawn to locate it at the expected new center of the light pen's position and thus appears to follow the light pen. The user brings the pen to the desired point on screen and presses the pen button to make contact. Contrary to what it looks like, the pen does not shine light onto the screen; rather, the screen beams into the pen.

Our model was kindly donated by Heather Hawthorn.