A laser tracker is a commercial device that measures the position of a retroreflector in 3 dimensions by using a distance-measuring interferometer (DMI) and two angular encoders. It is sensitive to sub-micron displacements in the radial direction, and the encoder accuracy is on the order of 1 arcsecond. It is capable of measuring a mirror surface to sub-micron accuracy if the tracker is located at the center of curvature, so angular errors do not affect the surface measurement, and if rigid-body motion of the mirror and tracker can be controlled during the course of the measurement, typically several minutes to an hour.