Gloss measurement is a nondestructive way to investigate the progress of coating degradation. An adequate model predicting the gloss is critical in relating the optical performance of the coating to the surface topology, subsurface composition, as well as chemical changes of the coatings. The bidirectional reflectance distribution functions (BRDF) are employed in obtaining the relative reflectance with respect to a perfectly-smooth surface. The specular, directional-diffuse and subsurface reflections are considered for a roughening surface. The results for a modified Kubelka-Munk model are adopted in deriving the BRDF of the subsurface reflection. Shadowing and masking effects of the rough surface are also embraced in our model. The behavior of the gloss measurements is predicted as a function of the coating surface degradation. The effects of each modeling components are identified. The influences of the surface roughness, coating thickness, and the scattering and extinction coefficients of the coating film are investigated.

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