Using gradient based measurements improves the efficiency of the used channel capacity of a measurement. Also, the measurement can be performed relatively and has not to be performed absolutely. This leads to a higher accuracy of the measurements compared to direct height measurements. Additionally, most of the common measurement techniques are using lenses. These lenses may introduce deviations to the results that cannot be traced back to its source.
In this work, we propose a new measurement technique to measure freeform optics without any optical component between the device under test (DUT) and the measuring sensor. For this, we introduce a narrow laser beam under a certain angle into the measurement system. After being reflected from the freeform surface, the direction and the position of the ray on the sensor are detected. By moving the DUT, the whole surface is investigated. From knowledge about the direction of the beam introduced into the measurement system and the directions of the reflected beams for the known surface positions, the gradient field of the surface is determined. Common surface reconstruction methods can be applied to these data.