A principal component analysis (PCA) is applied to several sets of ellipsometric spectra. The relation between the first coefficient obtained from PCA and the film thickness or the coverage of a substrate with colloids is explored. In many cases a linear relation is found, but in the case of gold colloids that have a large optical contrast with the ambient medium, there is a nonlinear dependence on the surface coverage. It is shown that calibration of the coefficients allows extraction of quantitative information from the ellipsometric spectra. When calibration is performed using least-squares fits to a small number of spectra, fitting of all other spectra is avoided. When another experimental technique such as AFM or SEM is used for calibration, PCA can extend the applicability of spectroscopic ellipsometry to areas where quantitative information is required but optical modeling is impossible or inconvenient.