Tilt-Corrected Lateral Shear Interferometry for Optical Flat Testing
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Solution Overview
Problem
Current methods for testing optical flats, such as the 3-Flat test, are limited by the need for three nominally identical parts, generate incomplete surface information, and are sensitive to gravitational orientation changes, leading to errors in measurement, especially when the optical axis is vertical.
Innovation Solution
A tilt-corrected lateral plus rotational shear method using a Fizeau interferometer for high-resolution, in-situ calibration, which measures cavity data at multiple rotational positions to calculate rotationally varying and invariant surface errors, compensating for tilt-induced errors during lateral shearing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the 3-Flat test is used to test optical flats, then surface errors can be measured, but the orientation of one flat with respect to gravity must be changed causing gravitational deformation changes
Solution Approach 1:
The patent segments the measurement process into two independent components: rotational shear measurements taken at multiple orientations to capture rotationally varying surface errors, and lateral shear measurements to capture rotationally invariant errors. This segmentation allows each measurement type to be optimized independently, avoiding the need to change gravitational orientation while maintaining measurement completeness.
Solution Approach 2:
The patent creates a universal measurement system that can test optical flats in any fixed orientation without requiring repositioning or orientation changes. The combination of rotational and lateral shear interferometry provides a multi-functional approach that replaces the orientation-dependent 3-Flat test with an orientation-independent methodology.
2Loss of information
If lateral shearing is used to obtain complete surface information, then the entire surface can be mapped, but the method is sensitive to drift and noise
Solution Approach 1:
The patent merges rotational shear interferometry and lateral shear interferometry into a unified measurement system. Rotational shear provides robust measurements of rotationally varying errors through averaging over multiple orientations, while lateral shear captures rotationally invariant errors. The combination of both methods compensates for their individual weaknesses, providing complete surface information with improved reliability.
Solution Approach 2:
The patent implements feedback through iterative refinement of surface error maps. Multiple rotational measurements are averaged to reduce noise, and the resulting surface error information is used to correct and refine the lateral shear measurements. This feedback loop enhances the reliability of the complete surface map by systematically reducing the impact of drift and noise.
3Reliability
If rotational shear averaging is used to obtain rotationally varying terms, then robust measurements are achieved, but only rotationally varying components are captured
Solution Approach 1:
The patent adds a lateral shear dimension to the rotational shear measurements. While rotational shear operates in the angular domain to capture rotationally varying errors, lateral shear operates in the spatial domain to capture rotationally invariant errors. This dimensional addition ensures that all surface error components are captured while maintaining the robustness of rotational shear measurements.
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
Enables accurate, high-precision testing of optical flats in any orientation without changing the mount-induced deformation, providing complete surface maps and reducing errors associated with tilt changes and mechanical drift.
Implementation Method 1
The invention uses a Fizeau (or other) interferometer to provide high resolution, in-situ calibration
Data Source
AI summary
A Fizeau or other interferometer is used to provide high resolution, in-situ calibration of an external angle measurement system such as widely spaced high stability plane mirror interferometers (HSPMIs)). The calibrated measurement system then measures mechanical tilt during shearing. The tilt data is used to correct the sheared data, preferably before computation of the rotationally invariant (RI) terms. Alternatively, the data may be used to compute the spurious quadratic term and correct after integration.