There is a significant quality control problem associated with a class of surface imperfections known as surface pits. This typically occurs, for example, on nickel-plated aluminum substrates to be used in the manufacture of thin-film magnetic media, but may be a problem with respect to any smooth surface. Such depressions or pits are but one class of f1aws that can be encountered. Other classes of flaws include defects large and small and contamination such as dirt, dust, oil, fingerprints and the like. Defects on the surface of rigid magnetic media are a result of an impingement onto the surface or a tearing of material away from the surface. These types of defects can be very large scratches or gouges on the surface or very small (5 .mu.m and smaller) tears or pricks on the surface. The pits are a local depression or bump with a diameter of from ten to several hundred micrometers with a depth of one-tenth to one micron, typically. The pits themselves may be smooth or contain breaks such as craters in their surface. Presently quality control inspection is effected by human operators who are subject to a number of shortcomings. They are slow and incompatible with automated equipment; they are unable to reliably, repeatedly detect very small pits and to distinguish them from other defects.