In Situ Interferometer Calibration via Fringe Orientation
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Solution Overview
Problem
Interferometers face errors such as retrace errors due to system design and spatial imperfections, which affect the accuracy of surface profiling measurements, especially in single-frame data acquisition scenarios where environmental disturbances like vibration and air turbulence are minimized.
Innovation Solution
The method involves taking multiple phase measurements of a test part with the same absolute carrier fringe frequency but different orientations, allowing for automatic correction of retrace errors on a pixel level, thereby reducing aberrations and mid- to high-spatial imperfections in the interferometer's imaging train.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If single-frame data acquisition is used to minimize environmental disturbances, then measurement speed and reliability are improved, but retrace errors and spatial imperfections increase
Solution Approach 1:
The patent changes the orientation parameter of carrier fringes by rotating the test object or reference surface between multiple measurements. By taking at least three phase measurements at different fringe orientations (e.g., 0°, 60°, 120°, 180°), the system can mathematically separate and eliminate retrace errors from the surface profile data, resolving the contradiction between single-frame reliability and measurement precision.
Solution Approach 2:
The patent implements a feedback mechanism where the measured phase data at different fringe orientations is fed back into a computational algorithm that calculates and applies correction terms. These correction terms are then used to compensate for retrace errors in the final surface profile, enabling high-accuracy measurements even in single-frame acquisition mode.
2Measurement precision
If multiple phase measurements at different fringe orientations are taken, then retrace errors are reduced, but measurement time and complexity increase
Solution Approach 1:
The patent performs preliminary calibration measurements to determine the retrace error characteristics of the interferometer system. By pre-measuring the phase at multiple known fringe orientations and computing the corresponding error terms, the system builds a calibration database that can be applied to subsequent test measurements, reducing the need for real-time complex computations.
Solution Approach 2:
The patent uses parameter changes in fringe orientation to efficiently capture error characteristics. By measuring at least three different orientations (e.g., 0°, 60°, 120°, 180°), the system obtains sufficient data to compute correction terms without requiring excessive measurements, balancing accuracy with measurement time.
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 enables high-accuracy surface mapping with reduced errors, allowing for single-camera frame data acquisition and analysis with short integration times, eliminating the need for additional calibration steps for similar test parts.
Implementation Method 1
one can use an interferometer to combine a test wavefront reflected from the test surface with a reference wavefront reflected from a reference surface to form an optical interference pattern
Implementation Method 2
The phase-shifts in PSI can be produced by changing the optical path length from the measurement surface to the interferometer relative to the optical path length from the reference surface to the interferometer
Data Source
AI summary
In-situ calibration of an interferometer includes making a sequence of phase measurements of a test object using the interferometer, each of the measurements having a same carrier fringe frequency, where at least some of the measurements are made at three or more different orientations of carrier fringes, and determining information about the test object based on at least some of the phase measurements, in which determining the information includes reducing errors in the measurements arising from imperfections in the interferometer based on the measurements made at the three or more different orientations.