The ultra-precise manufacturing of surfaces needs ultra- precise characterization. A novel optical scanning technique for testing flats, aspheres and complex surfaces is presented which offers ultra-precision for the measurement of slope and topography. The scanning technique needs no external references; it is traced back exclusively to the units of angle and length. The technique is based on a combination of two principles, namely to perform difference measurements for slopes with large shears and to use only a single pentagon prism for the difference measurements, keeping the angular position of the pentagon prism constant in space. The combination of these two principles eliminates the influences of all first- and second-order errors of the facility. Whole- body movement of the artefact under test and distortion of the facility itself do not result in errors.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.