Interferometric Wavefront Correction for Optical Surface Measurement
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Solution Overview
Problem
Interferometric measurements of optical surfaces face challenges due to coherent disturbances from interferometer components, which are not effectively addressed by traditional methods that require limitations on light source design or use of large light sources, leading to reduced measurement accuracy.
Innovation Solution
A method that computationally corrects wavefronts by localizing and quantifying coherent disturbances from individual interferometer surfaces, allowing for their subtraction from the measurement results, using either a punctiform or extensive light source without additional apparatus, by performing calibration measurements with a small light source and applying corrections during regular measurements with an extensive light source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large light source is used to minimize coherent disturbances, then measurement accuracy is improved, but device complexity and space requirements increase
Solution Approach 1:
The patent creates a computational model (copy) of the interferometer system's optical transfer function, including the effects of coherent disturbances from interferometer components. This digital model allows simulation and analysis without requiring physical modification of the light source or interferometer hardware, thus improving measurement accuracy while avoiding increased device complexity
Solution Approach 2:
The patent replaces the mechanical approach of using a physically large light source with a computational method. Instead of modifying the physical light source to reduce coherent disturbances, the system uses computer processing to model, identify, and correct the disturbances algorithmically, achieving the same measurement accuracy improvement without the space and complexity constraints of large light sources
2Measurement precision
If traditional calibration with rotary imparting is used for freeform surfaces, then measurement accuracy is improved, but the method becomes inapplicable for asymmetric test objects
Solution Approach 1:
The patent applies local quality analysis by examining the optical transfer function at different spatial frequencies and positions. It identifies coherent disturbances from specific interferometer components (collimator, reference element, beam splitter) and corrects them locally in the measurement data, allowing accurate measurement of asymmetric freeform surfaces without requiring rotary imparting calibration
Solution Approach 2:
The patent creates a computational copy of the interferometer system's optical characteristics, including the specific distortions introduced by each component. This digital representation allows the system to characterize and correct asymmetric aberrations in freeform surfaces without requiring physical rotary imparting, thereby maintaining measurement accuracy while expanding adaptability to asymmetric test objects
3Measurement precision
If a punctiform light source is used to reduce coherent disturbances, then measurement accuracy is improved, but the light source size is constrained
Solution Approach 1:
The patent uses a computational copy of the optical transfer function to model and correct coherent disturbances. This allows the system to use a small punctiform light source while algorithmically compensating for the disturbances that would otherwise require a large light source, thus improving measurement accuracy without increasing light source area
Solution Approach 2:
The patent changes the approach from physical parameter modification (light source size) to computational parameter adjustment. Instead of physically expanding the light source to reduce coherent disturbances, the system adjusts computational parameters in the optical transfer function model to correct the disturbances post-measurement, achieving the same accuracy improvement with a smaller light source
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the signal-to-noise ratio of the wavefront, allowing for more accurate determination of the optical surface shape by eliminating coherent disturbances, thereby improving measurement accuracy without the need for extensive light sources or additional apparatus.
Implementation Method 1
The wavefronts of both portions interfere in the interferometer and are guided via the beam splitter onto the sensor surface of a detector
Implementation Method 2
Surface-related coherent wavefront disturbances are determined during the evaluation
Data Source
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AI summary
A measurement method for interferometrically determining a shape of an optical surface of a test object is carried out by means of a measurement system comprising an illumination module for producing an effective light source in a light source plane of the illumination module and a plurality of optical components including a collimator, a transparent reference element disposed downstream of the collimator, and a beam splitter. A detector detects a superimposition of a measurement wave and a reference wave. The measurement method comprises a measurement of the surface of the test object in order to ascertain a measured waveform present in a detector surface of the detector and an evaluation of the wavefront in order to determine the shape of the optical surface. The evaluation comprises the following steps: identifying interferometer surfaces, an interferometer surface being an optically effective surface of one of the optical components of the measurement system that is used at least once during a measurement; determining surface-related coherent wavefront disturbances and assigning each of the wavefront disturbances to disturbing structures on one of the interferometer surfaces of the measurement system; ascertaining a wavefront corrected in respect of coherent wavefront disturbances by surface-specific subtraction of the surface-related coherent wavefront disturbances from the measured wavefront; evaluating the corrected wavefront.