Optical 2.5D Profiling Height Map Correction for Servo Errors
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Solution Overview
Problem
Existing optical scanning 2.5D profiling methods face errors in height map calculations due to non-equidistant scan steps caused by servo errors and vibrations, leading to inaccuracies and reduced repeatability in measurements.
Innovation Solution
The method combines long range sensing for low spatial frequency components and short range sensing for high spatial frequency components to accurately estimate the distance between the object surface and the focal plane, using displacement sensors like linear encoders and accelerometers to correct image stacks for equidistant interpolation and improve height map calculations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If standard FT methods are used to calculate height maps from image stacks, then the calculation process is simple and fast, but measurement precision deteriorates due to sensitivity to non-equidistant scan steps
Solution Approach 1:
The patent applies preliminary action by correcting the image stack data before performing the Fourier Transform. Specifically, the measured heights are used to resample the image stack to equidistant height positions, thereby eliminating the sensitivity to non-equidistant scan steps before the main calculation process
Solution Approach 2:
The patent implements feedback by using the initially calculated height map (from the interferogram analysis) to correct the image stack positions. This feedback loop refines the height measurements by iteratively improving the position accuracy based on the measured heights, thereby enhancing measurement precision
2Extent of automation
If servo systems are used to control focal plane scanning, then scanning automation is improved, but reliability deteriorates due to servo errors and vibrations causing non-equidistant scan steps
Solution Approach 1:
The patent applies self-service by using the measurement system itself to detect and correct its own errors. The actual focal plane positions are measured during scanning, and these measurements are used to correct the image stack, thereby making the system self-correcting without external intervention
Solution Approach 2:
The patent replaces reliance on mechanical servo control with optical measurement and digital correction. Instead of depending on the mechanical precision of the servo system, the invention uses optical interferometry to measure actual positions and applies digital resampling to correct deviations, substituting mechanical precision requirements with measurement and computation
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 enhances the accuracy and repeatability of height map measurements by accurately accounting for displacement errors, reducing waviness and scaling issues, and providing precise height maps even in environments with vibrations.
Implementation Method 1
scanning a focal plane at different height positions with respect to the object surface; capturing an image at each height position to form the image stack
Implementation Method 2
capturing an image at each height position to form the image stack
Data Source
AI summary
Method and system for calculating a height map of a surface of an object from an image stack in scanning optical 2.5D profiling of the surface by an optical system, a focal plane is scanned at different height positions with respect to the object surface. An image is captured at each height position of the focal plane to form the image stack. The scanning of the focal plane comprises long range sensing and short range sensing a displacement of the focal plane for sensing low and high spatial frequency components. The height position of the focal plane is estimated by combining the low and high spatial frequency components. A height position of each image in the image stack is calculated, based on the estimated height position of each respective focal plane. The images of the image stack are interpolated to equidistant height positions for obtaining a corrected image stack.