We report about the indoor characterization of small point focusing mirrors at Fraunhofer ISE. The goal is to determine the mean slope error of the concentrator. This is achieved by measuring the concentration distribution in the focal plane of such a mirror. We modified and expanded a test site which is used for Fresnel lens characterization [1]. A modified version of the method presented in [2] is employed to measure the concentration distribution. By comparing ray tracing simulation results of the ideal mirror to the measurement, the mean slope error can be deduced