Inhomogeneous dielectric surfaces exhibit both diffuse and specular reflection components. Although various reflection models have been proposed for both of these components, the prediction of the relative strengths of these components in computer vision and computer graphics has so far not had a strong physical motivation. We propose a reflectance model for combined diffuse and specular reflection from dielectric materials that involves purely physical parameters (i.e., no ad hoc weighting of specular and diffuse components). This reflectance model is used to predict the relative strength of diffuse and specular reflection components ih terms of imaging geometry, dielectric surface parameters, and solid angular extent of incident light. We derive lower bounds on the contrast ratio between a specularity and surrounding diffuse reflecting regions. These can be used effectively to rule out highly contrasting diffuse reflecting regions being misidentified as pecularities under a number of conditions that can significantly aid intensity-based specularity detection methods, and in turn image understanding. The presented theoretical developments can be used to predict the photometric dynamic range of illuminated objects, which can be essential to inspection methods in machine vision. These developments can also be used in computer graphics for the physically precise rendering of the relative strengths of specular and diffuse reflection from inhomogeneous dielectrics.