An optical microscope has been developed based on the differential interference contrast method to evaluate the roughness of supersmooth surfaces. The instrument uses a Bragg cell and a translation stage driven by a DC motor to produce an image of an area of a sample. Its lateral resolution is 2 μm and its vertical resolution is subangstrom. It takes only 7 seconds to scan an area of 1 mm 2. Three different curve fits can be used to remove the tilt, the curvature, and the low spatial frequency features of the sample. A figure for surface roughness is produced that is repeatable to 0.01 Å. The instrument is described and the noise sources and repeatability are discussed. The results of measurements of ring laser gyroscope mirror substrates are shown.