A calibration method suitable for highly precise and highly accurate surface metrology measurements is described. In preferred embodiments, an optical inspection tool including a movable optics system is characterized in terms of position and wavelength dependent quantities over a range of motion. Once the position-dependant quantities are determined at various wavelengths and positions, they are stored and used to interpret data from test wafers having an unknown metrology. Free of position-dependent variations and other information pertaining to the measurement system, the accuracy of the resulting wafer measurement more closely matches the precision of the tool than existing techniques. In particular embodiments, a portion of the characterization of the optical system is accomplished by using tilted black glass to provide a non-reflective reference.