An inspection apparatus may determine precise OV measurements of a target on a substrate using an optical pupil symmetrizer to reduce the inspection apparatus's sensitivity to asymmetry and non-uniformity of the illumination beam in the pupil plane. The inspection apparatus includes an illumination system that forms a symmetrical illumination pupil by (1) splitting an illumination beam into sub-beams, (2) directing the sub-beams along different optical branches, (3) inverting or rotating at least one of the sub-beams in two dimensions, and recombining the sub-beams along the illumination path to symmetrize the intensity distribution. The illumination system is further configured such that the first and second sub-beams have an optical path difference that is greater than a temporal coherence length of the at least one light source and less than a depth of focus in the pupil plane of the objective optical system.