  Reflection and transmission for an artificial opal are described through a
model of stratified medium based upon a one-dimensional variation of an
effective index. The model is notably applicable to a Langmuir-Blodgett type
disordered opal. Light scattering is accounted for by a phenomenological
absorption. The interface region between the opal and the substrate -or the
vacuum- induces a periodicity break in the photonic crystal arrangement, which
exhibits a prominent influence on the reflection, notably away from the Bragg
reflection peak. Experimental results are compared to our model. The model is
extendable to inverse opals, stacked cylinders, or irradiation by evanescent
waves
