Interferometer Illumination Pupil Segmentation for Optical Surface Shape Measurement
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Solution Overview
Problem
Multi-fringe interferometry for determining optical surface shapes is susceptible to errors due to defects in the interferometer, leading to reduced contrast and signal-to-noise ratio, despite efforts to minimize these errors through extended light sources which compromise measurement accuracy.
Innovation Solution
A measurement apparatus and method using an illumination module that produces an illumination wave with a spatially isolated or multiple contiguous surface regions in the pupil plane, configured to achieve a high aspect ratio, allowing for a multi-fringe interference pattern with enhanced contrast and reduced error susceptibility, by tilting the reference wave relative to the test wave and using a Fizeau interferometer with a splitting element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an extended circular light source is used in the illumination pupil, then defects on optical surfaces are minimized and measurement errors are reduced, but the contrast of the multi-fringe interferogram is reduced leading to poorer signal-to-noise ratio
Solution Approach 1:
The illumination pupil is divided into multiple discrete regions (first region, second region, third region) with different illumination characteristics. Each region can be independently controlled to optimize both defect suppression and interferogram contrast, resolving the contradiction between measurement accuracy and signal quality.
Solution Approach 2:
Different regions of the illumination pupil are assigned different illumination properties: the first region provides extended illumination for defect suppression, while the second and third regions provide concentrated illumination for high contrast. This local differentiation allows simultaneous achievement of both measurement accuracy and interferogram quality.
2Illumination intensity
If a punctiform light source is used, then the interferogram contrast is high, but defects on optical surfaces cause significant measurement errors
Solution Approach 1:
The illumination is segmented into multiple regions with different characteristics. The first region uses extended illumination to suppress defect effects, while the second and third regions use concentrated illumination to maintain high contrast, thus resolving the contradiction between contrast and accuracy.
Solution Approach 2:
The illumination system uses a composite illumination approach, combining extended source characteristics (for defect suppression) and punctiform source characteristics (for high contrast) within the same pupil plane, achieving both measurement accuracy and interferogram quality simultaneously.
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
The solution enables high-contrast multi-fringe interference patterns with reduced error susceptibility to interferometer defects, facilitating accurate determination of optical surface deviations while maintaining high signal-to-noise ratios.
Implementation Method 1
the interfering partial beams extend with such a tilt in relation to one another that a multi-fringe interference pattern arises
Implementation Method 2
the paths of the interfering partial beams extend along the same optical path or are at least tightly adjacent to one another in space
Implementation Method 3
the reference wave with such a tilt in relation to one another that a multi-fringe interference pattern is produced
Data Source
AI summary
A measurement apparatus (10) for determining a shape of an optical surface. An illumination module (16) produces an illumination wave (34), an interferometer (18) splits the wave into a test wave (50), which is directed onto the optical surface, and a reference wave (52). The relative tilt between the waves produces a multi-fringe interference pattern (66) in a detection plane (62) of the interferometer when the waves are superposed. A pupil plane (28) of the illumination module is arranged in a Fourier plane of the detection plane and the illumination module is configured to produce the illumination wave so that the intensity distribution thereof in the pupil plane includes at least one spatially isolated and contiguous surface region (38) such that a rectangle (74) with the smallest possible area fitted to the surface region or the totality of surface regions has an aspect ratio of at least 1.5:1.