Abstract: Interferometric apparatus that combines white-light VSI and single-wavelength PSI capabilities to improve the accuracy of height measurements in steep regions and in areas with large inter-pixel steps on the test surface. The technique consists of performing VSI measurements to obtain a relatively coarse profile of the test surface and to identify regions separated by a large step. Then PSI measurements are carried out over the test surface to obtain a relatively fine profile. Offsets between VSI and PSI measurements are calculated to correct for misalignments and phase shifts that may have occurred between the two sets of measurements. Finally, the fine PSI data are integrated to within one quarter wavelength of the coarse VSI data. The resulting quality of the height data in each of the step regions is thus improved to within the resolution of the PSI measurements.