Interferometric Measuring Apparatus for Optical Surface Shape
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Solution Overview
Problem
Existing interferometric measuring apparatuses face challenges in accurately determining the shape of optical test surfaces due to measurement errors caused by air schlieren and grating structure defects, which require complex calibration methods and operation in high vacuum.
Innovation Solution
A measuring apparatus that generates two plane waves with parallel directions of propagation and an offset, which are then adapted to a target shape using a wavefront adaptation module. This configuration allows for accurate determination of the test surface shape with minimal influence from air schlieren and grating structure defects, enabling operation under normal pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diffractive optical element (CGH) is used to generate test wave and reference wave, then measurement accuracy is improved, but measurement errors occur due to air schlieren and grating structure defects
Solution Approach 1:
The patent extracts and eliminates the problematic diffractive optical element (CGH) from the measurement system. Instead of using a CGH to generate test and reference waves, the invention employs a reflective optical element that simply reflects the incident wave back along its path, thereby removing the source of grating structure defects while maintaining the interferometric measurement capability.
Solution Approach 2:
The patent creates an inert environment by operating the measurement system in high vacuum conditions. This eliminates air schlieren effects that would otherwise interfere with the measurement accuracy, providing a stable and predictable optical path for the test and reference waves.
2Measurement precision
If operation in high vacuum is implemented to minimize air schlieren errors, then measurement accuracy is improved, but device complexity and operational outlay increase
Solution Approach 1:
The patent employs self-service principles by using the test surface itself as the reference element. The reflective optical element is integrated directly into the test surface, eliminating the need for separate reference mirrors and complex calibration procedures. The system measures deviations from the target shape directly during normal operation without requiring additional reference measurements.
3Reliability
If calibration methods are used to avoid grating structure defects, then measurement reliability is improved, but device complexity and operational outlay increase
Solution Approach 1:
The patent removes the diffractive optical element (CGH) that causes grating structure defects from the measurement system. By replacing it with a simple reflective optical element, the need for calibration to compensate for grating defects is eliminated entirely, simplifying both the device and its operation.
Solution Approach 2:
The patent uses the test surface itself as its own reference through the reflective optical element. Instead of requiring a separate reference mirror that would need precise calibration, the system creates a reference wave by reflecting the test wave off the test surface, ensuring perfect matching of optical paths and eliminating calibration requirements.
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 apparatus achieves high accuracy in determining the shape of optical test surfaces while reducing operational complexity and eliminating the need for high vacuum conditions, thereby lowering the overall outlay for measurement.
Implementation Method 1
generating an input wave and a splitting module configured to generate, from the input wave, two plane waves with parallel directions of propagation and with an offset from one another across the directions of propagation
Implementation Method 2
a wavefront adaptation module, i.e., a wavefront adaptor, configured to generate two measurement waves by adapting the respective wavefront of the plane waves with an offset from one another to a target shape of the optical test surface
Implementation Method 3
a detector for capturing at least one interferogram generated by superposition of the measurement waves following their interaction with the test surface
Data Source
AI summary
A measuring apparatus (10; 110; 210; 310; 410; 510; 610; 710) for interferometric determination of a property (50; 52) of a shape (50) of a test surface (12) of an object under test (14) comprises an irradiation device (22) for generating an input wave (24), a splitting module (18; 118; 318; 418; 518) configured to generate, from the input wave, two plane waves (32, 34) with parallel directions of propagation and with an offset from one another across the directions of propagation, a wavefront adaptation module (20; 720) for generating two measurement waves (44, 46) by adapting the respective wavefront of the plane waves with an offset from one another to a target shape of the optical test surface, and a detector (56) for capturing at least one interferogram (64) generated by superposition of the measurement waves (44r, 46r) following their interaction with the test surface.