1. A method of locating defects on a test surface, wherein the test surface is contained within a test volume represented by a Cartesian coordinate system having x, y, and z axes describing a set of unique x-y-z coordinates, the method comprising the steps of: scanning the test surface in the test volume with a focussed beam so that the focal point of the focussed beam coincides, in turn, with each unique x-y-z coordinate within the test volume; determining, for each column of points specified by a unique x-y coordinate in the test volume, a maximum reflected intensity value of the focussed beam; storing all the maximum reflected intensity values to form an array of test data representing a two-dimensional image of the test surface; extracting a set of intensity test primitives from the intensity test data; and comparing the set of intensity test primitives with a set of intensity reference primitives to determine whether the set of intensity test primitives is different from the set of intensity reference primitives.