Optical measurements of surface topography require the translation of measured phase shifts, Φ , into distances, Z , using a physical model for the air-surface interface. The usual expression used for this purpose = etp is based on the simple-interface model: air or vacuum above and a homogeneous half space of material below, with a sharp dividing line in between. This paper discusses the corrections to this expression which appear when one considers more complicated models. These appear as added pseudoheight contributions which arise from spatial variations of the parameters appearing in the more camplicated models, in addition to the geametrical height variations included in the simple-interface model. Formulas are given and results illustrated for several cases of practical interest.