Light Pens:

Figure illustrated the light pen utilized to select screen positions by detecting the light coming from points on the CRT screen. These are sensitive to the short burst of light emitted from the phosphor coating at the instant the electron beam strikes a specific point. Other light sources, such as the background light in the room, are generally not detected by a light pen. An activated light pen, pointed on a spot on the screen as the electron beam lights up that spot, produced an electrical pulse that causes the coordinate position of the electron beam to be recorded. As along cursor-positioning devices, recorded light-pen coordinates may be utilized to position an object or to choose a processing option.

Although light pens still exist, but due to several disadvantages they are not as popular as other input devices that have been developed. For instance, when a light pen is pointed at the screen, part of the screen image is hidden by the hand and pen. And prolonged use of the light pen may cause arm fatigue. Also, light pens need special implementations for some applications because they may not detect positions within back areas. To be able to select positions in any screen area with a light pen, we should have some non-zero intensity assigned to each screen pixel. Additionally, light pens sometimes give false readings because of background lighting in a room.