Abstract: Metrology method and apparatus (10) for determining a characteristic of an object's surface. The apparatus includes a multiple wavelength source (12) having an optical output. The source is comprised of a multimode laser diode (18) for simultaneously providing a plurality of wavelengths from which a synthetic wavelength is derived for metrology purposes. The apparatus further includes a beamsplitter (30), preferably a polarizing beam splitter, coupled to the optical output for providing a reference beam (32) and a measurement beam (34), the measurement beam impinging on the surface and the reference beam being phase modulated. The apparatus also includes optical elements and devices for detecting a phase difference between the reference beam and a reflected measurement beam for determining the characteristic of the surface.