In this paper we consider diffuse and specular reflectance from surfaces modeled as distributions of glossy microfacets. In contrast to previous work, we describe the relative contribution of both of these components in the same terms, namely with resource to Fresnel theory. This results in a more highly constrained model with a reduced number of parameters. Also, the need for ad hoc and physically meaningless specular and diffuse reflectance coefficients is removed. This ensures that the conservation of energy is obeyed and only physically plausible mixtures of the two components are allowed. In our model, both specular and diffuse reflectance are related to the roughness and refractive index of the surface. We show how physically meaningful parameters of a surface can be measured from uncalibrated imagery and that our model fits observed BRDF data more accurately than comparable existing models.