Novel scanning technique for ultra-precise measurement of slope and topography of flats, aspheres and complex surfaces

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 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 scanning facility, as this scanning technique is traced back exclusively to the units of angle and length. Especially whole-body movement of the artefact under test and distortion of the facility itself do not result in errors. The evaluation of difference measurements with large shears has become possible very recently with high accuracy and high lateral resolution because an error-free algorithm was developed. It is further a special feature of this technique that it measures differences of slopes and, consequently, allows the measurement of aspheres and complex surfaces with large asphericities.