Metrology System Transparent Surface Focus Detection
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Solution Overview
Problem
Existing precision metrology systems face challenges in accurately imaging and measuring workpiece surfaces, particularly transparent surfaces, due to limitations in focus detection and image processing techniques.
Innovation Solution
A metrology system is developed with a transparent workpiece surface mode, utilizing a light source, lens, camera, processors, and memory to acquire an image stack, determine multiple local focus peaks for each focus curve, and selectively display either the first or second surface based on user selection, allowing for extended depth of field and 3D imaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional autofocus methods are used to determine focus position, then the system can accurately measure opaque surfaces, but it fails to accurately detect multiple surfaces on transparent workpieces
Solution Approach 1:
The patent segments the focus detection process by identifying and analyzing multiple local peaks in the focus curve separately. Instead of treating the focus curve as a single entity with one global maximum, the system divides it into multiple segments (local peaks), each corresponding to a different surface on the transparent workpiece. This allows independent analysis of each surface's focus position, enabling accurate measurement of both the front and back surfaces of transparent materials.
Solution Approach 2:
The patent extends the traditional one-dimensional focus curve analysis by incorporating surface identification in an additional dimension. The system not only finds the maximum focus value but also identifies which surface (front or back) each local peak corresponds to, adding a surface-dimension to the focus analysis. This multi-dimensional approach allows the system to distinguish between multiple surfaces that would otherwise appear as overlapping signals.
2Device complexity
If a single focus position is used for imaging, then the optical system maintains simplicity, but the depth of field is insufficient to capture both front and back surfaces of transparent workpieces
Solution Approach 1:
The patent introduces dynamic focus adjustment by varying the focus position across multiple discrete positions to capture an image stack. The system dynamically moves the focus plane through different depths, capturing images at each position, and then uses computational processing to reconstruct surfaces at different depths. This dynamic approach maintains optical simplicity while enabling access to multiple surface depths that would be inaccessible at a single focus position.
3Loss of information
If the system captures images at multiple focus positions to increase depth of field, then both surfaces can be visualized, but the processing complexity and time increase significantly
Solution Approach 1:
The patent performs preliminary action by pre-calculating and storing the relationship between focus positions and surface identifications during the image acquisition phase. The system analyzes the focus curve and identifies local peaks corresponding to different surfaces before the actual measurement process begins. This preliminary analysis creates a lookup structure that enables rapid surface identification and image selection during operation, significantly reducing processing time for subsequent measurements.
Solution Approach 2:
The patent creates simplified copies of the complex multi-position image data by generating representative images for each identified surface. Instead of processing all raw images from the image stack, the system creates condensed representations (copies) of the front surface and back surface separately, each optimized for its specific depth. This copying approach reduces the data volume requiring detailed processing while preserving all essential surface information.
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 system effectively addresses the challenges of measuring transparent surfaces by enabling accurate determination of multiple surface features, improving image quality, and allowing for precise measurements and surface inspections.
Implementation Method 1
a lens (e.g., an objective lens), a camera, one or more processors, and a memory. The lens inputs image light arising from surfaces of a workpiece which are illuminated by the light source, and transmits the image light along an imaging optical path.
Data Source
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AI summary
A metrology system is provided with a transparent workpiece surface mode, for which the system is configured to vary a focus position over a plurality of positions along a Z height direction proximate to a workpiece. An image stack is acquired, wherein each image of the image stack includes a first surface (e.g., an upper surface of the workpiece) that is transparent or semi-transparent and at least a second surface that is at least partially viewable through the first surface. A plurality of focus curves are determined based on the image stack (e.g., with pattern projection utilized for improved contrast), from which first, second, etc. local focus peaks may be determined from each focus curve that correspond to the first, second, etc. surfaces, respectively. An image is displayed (e.g., extended depth of field, 3D) including a selected surface and for which features of the selected/displayed surface may be measured.