Abstract: An apparatus for the detection of laser beam perforation of a target. A pality of fiber optic threads are disposed in a grid arrangement behind the target along the anticpated beam sensor line opposite the exposed surface of the target. Upon beam perforation of the target, the laser energy impinges upon the fiber optic threads causing them to produce optical signals and allowing them to behave as light pipes. Groups of these fiber optic threads are gathered together and optically coupled to photo-electrical transducers for generating electrical signals in response to the optical signals generated within the fibers. These electrical signals are detected and recorded to provide accurate information regarding the time and position of laser beam burn through of the target.