Coding strategies for static patterns suitable for UV deflectometry

We introduce a method based on the deflectometry principle for the inspection of transparent, rotation-symmetric objects with steep angles. Examples are intraocular or strongly curved, aspheric lenses. Usually, an additional reflection at the lens' back side occurs and disturbs the signal. The solution is to use ultraviolet light which does not penetrate the material. As a consequence, static masks have to be used to generate the fringe pattern. A key feature of our approach is the specification of a spiral pattern, that meets the requirements for ultraviolet deflectometry with static masks. The spiral patterns are decoded by a multi-frequency phase shifting algorithm. We explain the principles of pattern design and present first evaluation results. Furthermore, we introduce a straight forward approach to obtain the absolute coordinates of the screen directly without any unwrapping. This facilitates the reconstruction of the three-dimensional shape of the lens in a subsequent step.