An apparatus (20) for measuring the thickness of a thin film layer (32) on substrate (28) includes a probe beam of radiation (24) focused substantially normal to the surface of the sample using a high numerical aperture lens (30). The high numerical aperture lens (30) provides a large spread of angles of incidence of the rays within the incident focused beam. A detector (50) measures the intensity across the reflected probe beam as a function of the angle of incidence with respect to the surface of the substrate (28) of various rays within the focused incident probe beam. A processor (52) functions to derive the thickness of the thin film layer based on these angular dependent intensity measurements. This result is achieved by using the angular dependent intensity measurements to solve the layer thickness using variations of the Fresnel equations. The invention is particularly suitable for measuring thin films, such as oxide layers, on silicon semiconductor samples.