Design considerations are discussed for an optical profiler consisting of an interference microscope adapted for phase shifting interferometry. The influence of several errors on the accuracy of the profiler are estimated. Specific attention is paid to the case of low-reflectance surfaces, which have to be measured with extremely high precision (e.g., uncoated bowl-feed polished glass surfaces). The accuracy-limiting factor for the measurement of these low-reflectance ultrasmooth surfaces is shown to be the inaccuracy of the measured intensity. A significant increase in accuracy is obtained by using a mercury arc lamp, which has a very high brightness, yielding a larger intensity signal and thus reducing the signal-to-noise ratio. Extensive test results of such an optical profiler using a Linnik interference microscope are presented, including the determination of the estimated reference profile accuracy. A measurement accuracy of 0.015-nm rms was obtained for uncoated glass surfaces by averaging 64 profiles. The accuracy of the estimated reference profile using 32 measurements was determined as being about 0.03-nm rms.