with rough surfaces (5). This is important because the optical transmission of an ab-
sorbing film of uniform thickness is lower than the transmission of a corresponding
inhomogeneous film of the same average thickness. This effect can be quite dramatic and
has been formulated mathematically by O'Neill using a step film model (9). In this
model the step film structure is defined by two parameters that characterize the fraction
of the thin and thick parts of the film and their difference in thickness