  A method is presented for modelling the optical properties of a photonic
crystal structure mounted on a substrate which is thick enough that the light
reflected from the back is incoherent with reflections from the front.
Transmission and reflection spectra are presented for the cases where the
structure is a multilayer planar microcavity and an opal photonic crystal. The
contributions from the incoherent reflections can be very large, and in the
photonic crystal case, change the positions of the spectral peaks.
