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 pupil images 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 pupil images of the light distribution in the collected pupil, the processing unit further arranged to generate the composite image by applying an average to the plurality of pupil images of the light distribution in the collected pupil.