Abstract: An apparatus for detection and analysis of optical systems. Prior art systems exploit the phenomenon of photon scattering in order to range the distance of objects from the ranging apparatus. However, no information other than the distance of the target object is obtained from such systems. There is therefore provided an apparatus that exploits the technique of single photon counting and the phenomenon of retro reflection to provide information about a target optical system. Such information can be analysed and compared against known optical systems to provide a means of identification. Alternatively such information can be used as a method of quality control when constructing precision optical instruments such as telescopes or microscopes.