Investigations of thermal effects caused by absorption of laser radiation in Faraday isolators and Faraday mirrors are becoming more important. A most promising medium for Faraday devices in the wavelength range of 600-1200 nm is the terbium gallium garnet (TGG) crystal. In order to measure its thermo-optic constants (which are unknown) we used a scanning Hartmann sensor technique. This technique can easily and inexpensively measure wavefront distortions induced by optical elements to within 2nm.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.