The difference in the transmission for left and right circularly polarised light though thin films on substrates in a magnetic field is used to obtain the magnetic circular dichroism of the film. However there are reflections at all the interfaces and these are also different for the two polarisations and generate the polar Kerr signal. In this paper the contribution to the differences to the total transmission from the transmission across interfaces as well as the differences in absorption in the film and the substrate are calculated. This gives a guide to when it is necessary to evaluate these corrections in order to obtain the real MCD from a measure of the differential transmission due to differential absorption in the film.
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