1. A method of calibrating a wafer inspection tool, said wafer inspection tool including a polychromatic light source for generating a probe beam which illuminates a sample and a detector for measuring the probe beam as a function of wavelength after reflection from the sample, said inspection tool including an optics system for directing the probe beam onto the sample, with the optics system being moved over the wafer to scan measurement points on the wafer, said method comprising the steps of: measuring a calibration wafer over a range of wavelengths at each of a plurality of positions of the optics system; determining and storing position and wavelength related calibration information; and using the calibration information to correct for position variations of the optics system when measuring a test wafer.