Refractive indices of transparent materials can be measured with high accuracy on small laboratory samples using null ellipsometry. Measurement precision in both n and k obtained ellipsometrically is ±0.0004 for semitransparent samples. Systematic errors in ellipsometric characterization of optical constants for transparent materials can result from back-surface reflection as well as from front-surface scattering caused by surface roughness. An analysis of the contribution of these errors and the methods of eliminating them are discussed. We conclude that careful ellipsometric characterization can give indices of refraction to three decimal places in the infrared for materials with low k.