A method comprising: providing one or more interferometry signals for a test object, wherein the interferometry signals correspond to a sequence of optical path difference values which are not all equally spaced from one another because of noise and each interferometry signal corresponds to interference intensity values measured when test light emerging from the test object is combined with reference light on a detector for each of the different optical path difference values, wherein the test and reference light are derived from a common low coherent source, and the optical path difference is the optical path length difference between the test light and the reference light from the common source to the detector; providing the measured unequal spacing of the sequence of optical path difference values; decomposing each of the interferometry signals into a contribution from a plurality of basis functions, each basis function corresponding to a different frequency and each basis function being sampled at the unequally spaced optical path difference values, wherein the multiple basis functions comprise non-orthogonal basis functions and decomposing each interferometry signal comprises simultaneously fitting the basis functions to the interferometry signal by solving a linear equation system; and using information about the contribution from each of the multiple basis functions to each of the interferometry signals to determine information about a surface of the test object, where the information about the contribution from each of the multiple basis functions to each of the interferometry signals comprises an amplitude and phase of each basis function to each interferometry signal. providing one or more interferometry signals for a test object, wherein the interferometry signals correspond to a sequence of optical path difference values which are not all equally spaced from one another because of noise and each interferometry signal corresponds to interference intensity values measured when test light emerging from the test object is combined with reference light on a detector for each of the different optical path difference values, wherein the test and reference light are derived from a common low coherent source, and the optical path difference is the optical path length difference between the test light and the reference light from the common source to the detector; providing the measured unequal spacing of the sequence of optical path difference values; decomposing each of the interferometry signals into a contribution from a plurality of basis functions, each basis function corresponding to a different frequency and each basis function being sampled at the unequally spaced optical path difference values, wherein the multiple basis functions comprise non-orthogonal basis functions and decomposing each interferometry signal comprises simultaneously fitting the basis functions to the interferometry signal by solving a linear equation system; and using information about the contribution from each of the multiple basis functions to each of the interferometry signals to determine information about a surface of the test object, where the information about the contribution from each of the multiple basis functions to each of the interferometry signals comprises an amplitude and phase of each basis function to each interferometry signal.