A method for remote optical measurement of the geometry of nonimaging concentrators is presented. A concentrator installed in a solar tower was measured by observation of transmission patterns from the heliostat field, and comparison of the measured patterns to a ray tracing simulation. The actual geometry of the concentrator was derived from optimization of the match between real and simulated patterns. The measurement was sensitive and accurate enough to detect small errors in the concentrator geometry such as 1 millimeter in linear dimension and 0.1degrees in concentrator tilt angle. The measurement procedure is simple and can be easily adapted to a wide range of nonimaging optical systems.