1. A method comprising: combining test light emerging from a test object with reference light, wherein the test and reference light are derived from a common source, and varying an optical path length difference from the common source to a detector between interfering portions of the test and reference light; generating a scanning interferometry signal for at least a first surface location of the test object, wherein the scanning interferometry signal corresponds to an interference intensity measured by the detector as the optical path length difference is varied; using a computer, comparing information derivable from the scanning interferometry signal for the first surface location of a test object to information corresponding to multiple models of the test object, wherein the multiple models are parametrized by a series of characteristics for the test object, wherein the derivable information being compared relates to a shape of the scanning interferometry signal for the first surface location of the test object and is expressed as a function of scan position; and using the computer, determining information about the test surface based on the comparison with each of the models.