Thermal and optical engineering applications of electromagnetic wave reflectance from rough surfaces include temperature measurement, radiation heating process, etc. Most of the surfaces are random roughness and often with coating material different from the substrate. This paper presented a novel hybrid partial coherence and geometry optics (HPCGO) model to improve the generic geometry optics (GO) method by incorporating a previously developed partial coherence reflectance equation. In this way, HPCGO expands the applicable region of GO model, and largely reduces the computation time of integrating different wavelength results in the regular hybrid model that considers coherence effect only. First, the HPCGO model is validated by more rigorous Maxwell equations solvers, for example, the finite-difference time-domain (FDTD) method and integral equation (IE) method. Then, the HPCGO model is applied to study the coherent effect of directional-hemispherical reflectance from coated rough surfaces. It is found the roughness of coated rough surface can cause partial or non-coherent scattered light even if the incident light source is coherent. It also shows the coherence effect reduces with increased incident wave-number bandwidth.

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