Deflektometrie in Transmission

Subject of this thesis is the development of a new technique for the measurement of the geometry of aspheric refractive optics. These are widely used in industry due to their superior imaging characteristics. In a discussion of the state of the art it will be shown that there are some established measuring techniques available. None of these however is able to optically measure the geometry in transmission, even though for refractive optics this would be favorable, compared to the common technique to separately measure both surfaces in reflection, from a metrological point of view.

With the deflectometric technique “active reflection grating photogrammetry”, which has originally been invented for specular objects, a measurement method with high development potential will be identified. This technique allows for the unambiguous spatial determination of both incident and refracted light rays for refractive optics. A measurement of the geometry however is not possible at first, because the ambiguity of the light path inside the specimen prevents a successful reconstruction of the surfaces.

Nevertheless both surfaces of a refractive optic can be determined with a model-based iterative approach working on the collected measurement data. The geometry and position of the specimen are described by a surface model whose parameters are gradually adapted based on an optimization process. By means of simulations the potential of an advanced technique with multiple cameras could be demonstrated. An experimental validation however failed up to now due to convergence problems in the geometry reconstruction process caused by deviations in the measurement data.

To minimize these deviations, two approaches are pursued in this thesis. On the one hand it became apparent, that LCD-monitors aren’t understood sufficiently well enough as optical components in deflectometric measurement system. Therefor an extensive study of the operation of LCD-monitors from a metrological point of view will be conducted and the supremacy of the IPS-technology will be demonstrated. On the other hand there is no reliable method to predict the stochastic phase deviations up till now. However a thorough understanding of the underlying mechanisms is mandatory and key to a successful improvement of the systems resolution. Therefor a prediction method will be developed enabling the determination of stochastic phase deviations for each measurement point directly from the measured intensities.

Based on these results optimal components that are well suited for the system will be chosen. Furthermore different geometrical configurations of the measurement system are to be discussed and the optimal setup in terms of error propagation is used. Besides these improvements of the hardware, also the measurement software will be optimized. On the one hand a method enabling the automatic calculation of optimal sinus patterns with respect to phase evaluation is going to be implemented. On the other hand some algorithms regarding calibration and data evaluation of the model-based reconstruction will be revised. Finally the operability of the measurement systems is demonstrated with an example measurement of an aspheric spectacle lens blank.