A self-calibrating interferometric method for asphere and freeform testing

• Applicant: Professor Dr. Wolfgang Osten
• Subject Area: Measurement Systems
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 273678658

This project aims at the development of a flexible and self-calibrating method for the testing of aspheres and freeforms. The method should overcome existing limitations concerning stability, precision and speed of the measurement. Starting point is the Tilted-Wave-Interferometry. This method allows testing a large class of specimen without changes in the instrument. In particular, there is no limitation to rotationally symmetric surfaces. The recording of the measurement data can be performed in less than 30 seconds. A crucial part of the method is the calibration of the instrument. The calibration result is valid, as long as the environmental conditions change only negligibly. Otherwise, either a systematic error in the measurement occurs or a time-consuming re-calibration is necessary. In particular with respect to applications close to the production chain, robustness against environmental conditions is desirable. A self-calibrating method allows the reconstruction of the system parameters during measurement yielding a significantly reduced time for calibration.As a first aim of the project, a geometrical model of the interferometer is developed. This model is analyzed with respect to separation of static and dynamic parameters and dependencies between different quantities. A set of parameters which allows a well-conditioned reconstruction from measurement data is to be identified. Starting from this model, a method can be constructed which introduces systematic redundancies to the measurement and exploits this redundancies for a numerical reconstruction of system as well as specimen parameters. A critical quantity is the lateral resolution of the measurement result. For a single-shot interferometric measurement, a resolution in the range of 10 microns is only possible with a variable focal plane of the imaging optics. Currently, this changes the state of a system and requires a re-calibration. The self-calibrating method allows reconstructing the induced change in the parameters directly during the measurement, without the use of a reference object in different positions. This reduces the measurement time from 30 minutes to 1 minute. In this project, the method is experimentally validated at a lab demonstrator. One part of the project is the design of an imaging optics with a variable focal plane which is optimized in particular with respect to its influence on the calibration state of the complete system.

• DFG Programme: Research Grants