Real Dictionary

light pen in American English
a hand-held, pen-shaped electronic device used to scan images, draw lines, designate points, etc. as on a computer video screen
noun: a hand-held 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

light pen in British English
noun: 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