light pen

noun Computers.

1. a handheld light-sensitive device used for pointing at characters or objects on a CRT in order to make or modify drawings or to indicate functions to be performed.

noun

1. computing
   1. a rodlike device which, when applied to the screen of a cathode-ray tube, can detect the time of passage of the illuminated spot across that point thus enabling a computer to determine the position on the screen being pointed at
   2. a penlike device, used to read bar codes, that emits light and determines the intensity of that light as reflected from a small area of an adjacent surface