A uniform optical attenuation system for high power detection comprises a detector unit (11), a plurality of diffusing elements (14, 15, 16, 17) for receiving and diffusing input light and an apertured plate (18, 19) for limiting light received by the detection unit to a portion only of the light diffused by the diffusing arrangement. The plurality of diffusing elements comprise a first diffusing element (12) for receiving and diffusing an input light beam to provide a distributed light source for a subsequent one of the diffusing elements, and final one of the diffusing elements which directs diffused light towards the detection unit. The light detected by the detection unit (11, 18, 19) is substantially independent of variations in numerical aperture of the input light. Each diffusing element may be a diffusing surface, such as a ground glass or opal diffuser, which renders the system substantially independent of variations in optical properties due to heating effects caused by the high energy light.