Device and Method for Subaperture Stray Light Detection and Diagnosis

A device and method for subaperture stray light detection and diagnosis in an optical system. A test light beam is generated with a width whose cross sectional area is less than the cross sectional area of a system aperture. Stray light is detected at a system detection surface. Based on the stray light and the location and direction angles of the light beam, potential paths that light may have taken to arrive at the detection surface are determined so as to identify physical features of the optical system that may have produced the stray light. A testing device comprises a test light beam source, preferably including a beam width magnifier, whereby the cross sectional area of the test light beam is made less than the cross sectional area of the system aperture. A relative lateral positioning stage and an angular beam directing stage launch the test light beam into the aperture at a selected position and selected directional angles. A detector and a data processing system produce a data set relating the stray light to the location and directional angles of the test light beam. A light trap and complementary test light beam delivery system are also provided.