An optical measurement system having a spectrophotometer and a ellipsometer is calibrated, the spectrophotometer and the ellipsometer first being calibrated independently of one another. A spectrophotometer layer thickness (d.sub.photo) of a specimen is then determined at an initial angle of incidence (.theta..sub.init) using the spectrophotometer. An ellipsometer layer thickness (d.sub.elli) of the specimen is then determined using the layer thickness determined with the ellipsometer. The spectrophotometer and the ellipsometer are matched to one another by varying the initial angle of incidence (.theta..sub.init) until the absolute value of the difference between the spectrophotometer layer thickness (d.sub.photo) and the ellipsometer layer thickness (d.sub.elli) is less than a predefined absolute value.