Abstract: Optical metrology method and apparatus wherein three optical wavelengths of a fixed polarization are generated and separated into a reference beam (RB) and a measurement or object beam (OB) having, ideally, equal optical path lengths. After reflecting from surfaces being measured OB is combined with RB and provided to sensors which measure the intensity associated with each of the wavelengths. Any difference between the intensities is indicative of a difference in the optical path lengths of OB and RB and is a function of the polarization state of each of the three returned wavelengths. Differences in optical path length may be indicative of a difference between a reference surface and a test surface, or a difference in thickness or index of refraction across an object. Two multi-mode laser diodes (12, 14) are provided for generating the three optical wavelengths.