Abstract: An apparatus for mapping an optical element, the apparatus including a light source arranged to transmit a light beam toward the optical element, a beam separator including a plurality of beam separating elements, less than all of which are identical, which are operative to separate the light beam into a corresponding plurality of light beam portions including at least first and second light beam portions which differ from one another, an optical sensing device operative to generate a light spot map including a plurality of light spots corresponding to the plurality of beam separating elements and an optical element characteristic computation device operative to derive at least one characteristic of the optical element from the light spot map and including apparatus for identifying the beam separating element corresponding to an individual spot based at least partly on differences between the at least first and second light beam portions.