Abstract: A beam is reflected and/or transmitted by a sample with a coating. The path of the beam has a unitary path part where the beam is affected by the coating but is split into parallel sub-paths in a fixed (sample independent) part of the apparatus, components of the beam with different directions of polarization being passed through the sub-paths. A spectrum is determined of ratio's between intensities of the components after they have been affected by the sample, and from the spectrum of the ratio's coating properties are determined. Typically a polarizing splitter is used to split the beam after it has been affected by the sample and the beam is chopped in the sub-paths. After chopping the beam may be recombined before being passed to a detector. The detector may be shared between different arrangements of splitters-choppers-combiners. The beam may be passed to and from the sample through the same splitter.