Current infrared reflectance techniques for measurement of the thickness of silicon epitaxial layers are generally unreliable for thicknesses below 2.5 μm. In order to extend these methods to thinner layers, an increased understanding of the physics of infrared reflectance from thin silicon layers is required. A model has been developed to calculate the complex refractive index as a function of impurity concentration and wavelength. It has been coupled with a multiple-layer treatment of reflectivity, allowing the simulation of the optical properties of an epitaxial layer above a substrate with arbitrary impurity distribution. The model is used to evaluate the shortcomings of both dispersive and interferometric measurement techniques.