Apparatus for and a method of determining surface characteristics

A coherence scanning interferometer carries out: a coherence scanning measurement operation on a surface area carrying a structure using a low numeric aperture objective so that the pitch of the surface structure elements is much less that the spread of the point spread function at the surface to obtain structure surface intensity data; and a coherence scanning measurement operation on a non-structure surface area, which may be part of the same sample or a different sample, to obtain non-structure surface intensity data. A frequency transform ratio determiner determines a frequency transform ratio (the HCF function) related to the ratio between the structure surface intensity data and the non-structure surface intensity data. A structure provider sets that frequency transform ratio equal to an expression which represents the electric field at the image plane of the coherence scanning interferometer in terms of surface structure element size (height or depth) and width-to-pitch ratio and derives the surface structure element size and width-to-pitch ratio using the frequency transform ratio. The structure provider may also extract the surface structure element width, if the pitch is independently known.