Partially Nulling Optical Metrology for Large Freeform Surfaces
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Solution Overview
Problem
Current optical surface metrology systems for freeform surfaces are time-consuming and prone to alignment errors, especially when measuring large, non-standard optics, as they require the optic to be removed from the vacuum chuck for separate measurement, limiting mass manufacturing possibilities.
Innovation Solution
A partially nulling optical metrology system that uses individually positioned subapertures with a rotational spindle and reconfigurable mirrors to decouple manufacturing and measuring processes, allowing for inline measurements and reducing metrology bias, with a digital twin for optimized lattice design and stitching of subaperture data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If conventional interferometry is used to measure large, non-standard optical surfaces, then measurement coverage is achieved, but fringe density becomes excessively high making the underlying surface profile impossible to measure
Solution Approach 1:
The patent divides the large optical surface into multiple smaller subapertures (SAs), each measured individually with reduced fringe density. The measurements from these segmented regions are then stitched together to reconstruct the full surface profile, solving the problem of excessive fringe density while maintaining comprehensive coverage.
2Measurement precision
If the optic is removed from the vacuum chuck for separate measurement, then measurement capability is improved, but alignment errors are introduced and manufacturing time increases
Solution Approach 1:
The patent merges the manufacturing vacuum chuck with the measurement system by integrating the interferometer and subaperture positioning mechanisms directly onto the chuck. This allows the optic to remain mounted during measurement, eliminating removal and realignment steps while maintaining measurement precision through the specialized subaperture stitching approach.
Solution Approach 2:
The patent introduces a digital twin as an intermediary that simulates the measurement process and calculates optimal subaperture positions and overlap requirements. This digital model enables precise measurement planning without physical trial-and-error, reducing alignment errors and measurement time while maintaining accuracy.
3Area of stationary object
If subaperture measurements are stitched together to cover the full surface, then complete surface coverage is achieved, but measurement and processing time increases
Solution Approach 1:
The patent performs preliminary actions by using the digital twin to pre-calculate the optimal lattice configuration of subapertures, determining the exact positions and overlap requirements before actual measurement. This pre-planning minimizes the number of measurements needed and optimizes the stitching process, increasing productivity while achieving complete surface coverage.
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 efficient, accurate measurement of large, non-standard optical surfaces by reducing fringe density, minimizing metrology bias, and allowing real-time feedback for improved manufacturing processes without the need to remove the optic from its mount, thus enhancing the precision and speed of freeform optical surface production.
Implementation Method 1
The optical metrology system which includes a reconfigurable relay mirror and a reconfigurable tip/tilt mirror... to capture measurement data of the optical surface... wherein measurement data is captured from each of a plurality individual subapertures
Data Source
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AI summary
An optical metrology system for measurements of large aperture and freeform optical surfaces the system (100) comprising: a first mirror, having a plurality of degrees of freedom of motion; one or more optical elements comprising reconfigurable optical elements; and a controller (200). The system (100) is configured: configured to be optically coupled to an externally located surface under test, SUT (125), and to capture measurement data from each of a plurality individual subapertures, SAs, across the area of the SUT, each SA having a surface area smaller than that of the SUT. The metrology system is further configured to provide partial nulling of each of the SA measurements across the SUT, based on controlled adjustment of one or more of the metrology system, and the elements thereof. The measurement data, comprising measurements relating to the plurality of individual subapertures, SAs, is stitched to describe the full surface of the SUT. Methods and a software system for use in the measurement of large aperture and freeform optical surfaces are also described.