Workpiece Surface Depth Mapping Through Focal Image Stacks
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Solution Overview
Problem
Conventional optical measuring apparatuses in industrial metrology are limited by their inability to accurately and reproducibly measure depth information in the Z-direction, often requiring tactile sensors that increase complexity and cost, and existing Shape from Focus (SFF) techniques are not suitable for high-resolution industrial applications.
Innovation Solution
A method involving a focal image stack is used to generate a depth map by varying the focal plane position relative to the workpiece, determining focus values, fitting functions to these values, and correcting for optical aberrations to achieve high accuracy and reproducibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If tactile sensors are integrated with optical sensors to obtain depth information, then measurement accuracy in Z-direction is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces tactile (mechanical contact) sensors with a purely optical measurement system. By using optical defocusing changes to determine depth information through the Shape from Focus technique, the system eliminates the need for mechanical contact sensors while maintaining measurement capability in the Z-direction.
Solution Approach 2:
The patent introduces image sharpness/focus as an intermediary parameter to infer depth information. Instead of directly measuring Z-position with tactile sensors, the system uses the optical property of image sharpness at different focal planes as a mediator to calculate depth, enabling non-contact 3D measurement.
2Ease of manufacture
If conventional SFF technique is used for depth measurement, then implementation simplicity is improved, but measurement accuracy and reproducibility deteriorate
Solution Approach 1:
The patent changes the parameter used for depth determination from simple focus detection to a comprehensive analysis of image sharpness across multiple focal planes. By evaluating focus values at multiple depths and applying mathematical optimization (least squares fitting), the system transforms the basic SFF approach into a high-precision measurement method suitable for industrial metrology.
Solution Approach 2:
The patent employs excessive action by capturing more images than the minimum required (multiple images at different focal planes beyond a single focused image) and performing comprehensive mathematical analysis. This over-determined approach with multiple measurements and least squares optimization ensures high accuracy and reproducibility, compensating for the increased processing complexity.
3Measurement precision
If multiple images at different focal planes are captured, then depth information accuracy is improved, but measurement time increases
Solution Approach 1:
The patent maintains continuous useful action by systematically capturing images across a continuous range of focal planes without interruption. This continuous sampling approach ensures that depth information is obtained efficiently through methodical progression through the focal stack, maximizing information gain per unit time.
Solution Approach 2:
The patent performs preliminary action by capturing the complete focal image stack before any depth calculation begins. All necessary images at different focal planes are acquired in advance, allowing subsequent processing to proceed efficiently without requiring additional measurements during the analysis phase.
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 the generation of highly accurate and reproducible depth maps, allowing for high-resolution 3D measurements with extended depth of field, overcoming the limitations of existing methods.
Implementation Method 1
Image unsharpness caused by optical defocusing changes in a predictable way. The optical system of the optical sensor has a focal plane, which is a plane of highest sharpness.
Data Source
AI summary
A method for generating a depth map of a region of a surface of a workpiece includes receiving a stack of images. The images image the region of the surface of the workpiece with defined focal plane positions that are different in a depth direction and a focal plane position is assigned to each. Image points of the images are respectively assigned to a corresponding object point on the surface. The method includes determining a focus value of each image point of each image. The method includes fitting a function along the depth direction to the focus values of those image points that are assigned to the same object point. The method includes determining a depth value of each object point on the surface in the depth direction based on an extremum of the fitted function. The method includes generating the depth map based on the determined depth values.