Scanning white light interferometry (SWLI) is now an established technique for the measurement of surface topography. It has the capability of combining sub-nanometre interferometric resolution with a range limited only by the z-traverse, typically at least 100μm. A very useful extension to its capability is the ability to measure thin films on a local scale. For films with thicknesses in excess of ~2μm (depending on refractive index), the SWLI interaction with the film leads simply the formation of two localised fringe bunches, each corresponding to a surface interface. It is evidently relatively trivial to locate the positions of these two envelope maxima and therefore determine the film thickness, assuming the refractive index is known. For thin films (with thicknesses ~20nm to
~2μm, again depending on the index), the SWLI interaction leads to the formation of a single interference maxima. In this context, it is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is addressed through the introduction of a new function, the 'helical conjugate field' (HCF) function. This function may be considered as providing a 'signature' of the multilayer measured so that through optimization, the thin film multilayer may be determined on a local scale. Following the derivation of the HCF function, examples of extracted multilayer structures are presented. This is followed by a discussion of the limits of the approach.