  We present an investigation of coherent backscattering of light that is
multiple scattered by a photonic crystal by using a broad-band technique. The
results significantly extend on previous backscattering measurements on
photonic crystals by simultaneously accessing a large frequency and angular
range. Backscatter cones around the stop gap are successfully modelled with
diffusion theory for a random medium. Strong variations of the apparent mean
free path and the cone enhancement are observed around the stop band. The
variations of the mean free path are described by a semi-empirical three-gap
model including band structure effects on the internal reflection and
penetration depth. A good match between theory and experiment is obtained
without the need of additional contributions of group velocity or density of
states. We argue that the cone enhancement reveals additional information on
directional transport properties that are otherwise averaged out in diffuse
multiple scattering.
