A light pen
A light pen is a computer input device that looks like a long stick. It has photo-sensitive features that are used with the computer's CRT display. It helps the user direct or draw objects that appear in the display as it is done on the touchscreen, but it can be done with the ability to do this. It works with other display technologies, including CRT display, but the use of light pen with LCD monitor is unclear (although Toshiba and HiTachi have shown this type of concept in the exhibition "Display 2006" held in Japan).
A light pen display can capture pixel brightness when the electron ray of the CRT display is scanned and sends this change with the computer. Since the CRT demonstration scans a pixel of the whole spectrum on a single scan, the computer can calculate the precise scanning time of the photocell pen, and can calculate the amount of ray space and the location of the photocopier in different places of the exhibit.