1. An angle-resolved reflectometer comprising: a coherent light source; an optical system arranged to scan a test pattern using a spot of coherent light from the light source to yield a plurality of realizations of a light distribution in a collected pupil, wherein the spot covers a part of the test pattern and the scanning is carried out according to a scanning pattern; and, a processing unit arranged to generate a composite image of the collected pupil distribution by combining the plurality of realizations of the light distribution in the collected pupil.