Microfacet projected area-based correction for unified model of Geant4 for rough surfaces

A modification of the optical model for rough surfaces, implemented in Geant4 as a part of the unified model, is suggested. The modified model takes into account the variation of the interaction probability of the photon with the microfacet based on the relative orientation of the photon and the sampled microfacet’s normal. The implementation is using a rejection algorithm and assumes the interaction probability to be proportional to the projection of the microfacet area on the plane perpendicular to the photon direction. A comparison of the results obtained with the original and the modified models, as well as obtained in direct Monte Carlo simulations are presented for several test surfaces constructed using a pattern of elementary geometrical shapes.