Abstract: An optical apparatus measures characteristics of a measurement target including an illumination portion, detection portion and processing portion. The illumination portion produces at least one pair of spatially separated areas of illumination for illuminating a measurement target to produce an associated light field. The light field produced by illumination of the measurement target includes a component corresponding to interference between the areas of illumination, illuminates a first site on the measurement target and illuminates a second site on the measurement target. The detection portion receives light from the measurement target, directs the received light onto a detector, and outputs signals from the detector dependent on the intensity of the detected light. The processing portion analyses the signals output by the detector to measure the characteristics of the measurement target.