The present invention refers to the simultaneous measurement of four separately polarized beams upon diffraction from a substrate in order to determine properties of the substrate. Circularly or elliptically polarized light sources are passed via up to three polarizing elements. This polarizes the light sources by 0, 45, 90 and 135.degree.. The plurality of polarizing beamsplitters replaces the use of a phase modulator, but enables the measurement of the intensity of all four beams and thus the measurement of the phase modulation and amplitude of the combined beams to give the features of the substrate.