Object reconstruction by deflectometry

Phase-Measuring Deflectometry (PMD) is a meanwhile established method to measure specular freeforms. A camera observes a sinusoidal pattern reflected by the specular object under test. While having extremely high local sensitivity, this method suffers from a distinct weakness: It is difficult to achieve a global accuracy that can compete with interferometry or CMMs. The reason is that the relationship between the actually measured effect and the underlying object form is complicated: It depends on unknown ray deflections at unknown positions in space. The most common existing reconstruction method is an iteration which needs knowledge of one single point location. Unfortunately, even under the assumption of a perfect sensor calibration the global accuracy still depends strongly on the accuracy of this point. To overcome this problem we developed a new reconstruction method which no longer relies on this information. Instead, the new method minimizes the global error of all observed points. Measurements with one or more subsequently tilted cameras can be used. The method is quite simple and user friendly, the accuracy can be improved, and the angular dynamic range can be increased.