Interferometer Cyclic Error Reduction via Dynamic Path Adjustment
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Solution Overview
Problem
Measurement apparatuses face challenges in accurately determining the shape of test surfaces due to cyclic nonlinear errors caused by stray light, which complicates the detection of optical path length differences and affects the precision of measurement points.
Innovation Solution
The apparatus detects optical path length differences in multiple states with varying distances between the optical system and the test surface, allowing for the cancellation of cyclic errors by obtaining detection results at specific intervals of the cyclic error cycle, thereby accurately determining the position of measurement points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a mechanism for changing the optical path length difference at a defined rate is introduced to reduce cyclic errors, then cyclic error reduction is achieved, but the device complexity increases
Solution Approach 1:
The patent applies the dynamics principle by making the optical system movable rather than static. The optical system is moved to change the optical path length of the reference light, enabling dynamic adjustment to generate Doppler shift and reduce cyclic errors without requiring complex dedicated mechanisms for each measurement point
Solution Approach 2:
The patent applies the universality principle by making the optical system serve multiple functions: it both performs the interferometric measurement and simultaneously changes the optical path length to generate Doppler shift. This multi-functionality eliminates the need for separate mechanisms, reducing overall device complexity while maintaining cyclic error reduction capability
2Measurement precision
If the optical path length difference is changed at a defined rate to obtain Doppler shift, then cyclic errors are reduced, but the measurement process becomes more complex
Solution Approach 1:
The patent makes the optical system movable to dynamically change the optical path length during measurement. This dynamic approach allows the system to automatically generate the required Doppler shift while performing measurement, simplifying the operational process compared to static systems requiring external path length modulation
Solution Approach 2:
The patent merges the measurement function with the optical path length change function into a single operational process. By moving the optical system, both the measurement and the path length modulation occur simultaneously, eliminating separate control steps and simplifying the measurement process
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 method enables precise determination of measurement points by canceling cyclic errors, improving the accuracy of shape measurement on test surfaces by accounting for cyclic fluctuations in optical path length differences.
Implementation Method 1
an interference signal generated by interference between test light reflected by the test surface and reference light reflected by a reference surface
Implementation Method 2
a cyclic nonlinear error (so-called cyclic error) can occur in measurement results due to stray light generated by, for example, unintended reflection by an optical element included in the apparatus. U.S. Patent No. 6252668 has proposed a method of reducing cyclic errors based on a Doppler shift obtained by changing an optical path length difference between the test light and reference light
Data Source
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AI summary
A measurement apparatus for measuring a shape of a test surface (1), comprises an optical system (20) which irradiates a point on the test surface and on a reference surface (25), and causes test light and reference light reflected to interfere with each other, a detector (30) configured to detect an optical path length difference between the test light and the reference light by using interfering light, and a processor (40) configured to determine a position of the point based on a plurality of detection results. A measurement apparatus for measuring a shape of a test surface (1), comprising a probe (80) brought into contact with a measurement point on the test surface; a detector (72) provided on the probe and configured to irradiate a standard surface (71z) and a reference surface with light and detect interfering light of the test light reflected by the standard surface and the reference light reflected by the reference surface, thereby detecting an optical path length difference between test light and reference light; and a processor (87) configured to determine a position of the measurement point based on a plurality of detection results by the detector. In both cases the detection results are obtained in n states in which optical path lengths of the test light are different from each other by 1/n (n ≥ 2) of a cycle of the error and wherein a detection result includes an error which cyclically changes.