Highly accurate non-contact polycrystalline silicon (poly-Si) film thickness measurements are important for both real-time feedback control and run-to-run control. Both spectroscopic ellipsometry (SE) and normal-incidence spectral reflectometry (SR) are complicated by poly-Si surface effects and variable bulk poly-Si refractive indices. In this article we will describe an empirical modification of Beckmann/Kirchhoff scattering theory to account for the effects of rough layers in SR. We will present results from in situ monitoring of reactive ion etching of poly-Si/SiO2/Si and ex situ comparisons to TEM and AFM. Preliminary conclusions on the applications of this model to SE analysis of poly-Si will also be presented.