Abstract : There are two different physical models connecting the micro-geometry of a surface and its physical reflectance properties (BRDF). The first, Cook-Torrance, assumes geometrical optics: light is reflected and masked by the micro-facets. In this model, the BRDF depends on the probability distribution of micro-facets normals. The second, Church-Takacs, assumes diffraction by the micro-geometry. In this model, the BRDF depends on the power spectral distribution of the surface height. Measured reflectance have been fitted to either model but results are not entirely satisfying. In this paper, we assume that both models are valid in BRDFs, but correspond to different areas in parametric space. We present a simple test to classify, locally, parts of the BRDF into the Cook-Torrance model or the diffraction model. The separation makes it easier to fit models to measured BRDFs.