Abstract: A light pen detection system used in a data display system utilizes a cathode ray tube (CRT) in which characters displayed on the screen of the cathode ray tube are detected by the light pen, the detection of the character by the light pen resulting in the background inversion of the character sensed thereby providing a visual indication to the operator the character sensed by the light pen. A locking circuit is provided which, upon the sensing of a displayed character by the light pen, conditions the sensing circuits connected to the light pen to continually output the character sensed until the light pen is moved completely away from the locked character.