Shack-Hartmann Wavefront Sensor for Surface Height and Focus
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Traditional machine vision inspection systems face challenges in accurately determining surface height and focus, especially with specular and diffuse surfaces, and fail to address abrupt height steps and unpredictable surface properties, leading to inefficiencies in measurement and focus operations.
Innovation Solution
A surface height and focus sensing system utilizing a Shack-Hartmann type wavefront sensor with a collimation adjustment element, which adjusts the illumination focus height to match the workpiece surface height, providing a null output and allowing for fast and insensitive measurements to surface optical properties, thereby enabling precise surface height determination without altering sensing system or workpiece positions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional autofocus processes are used to determine surface height and focus, then the system can obtain focus information, but the process is time-consuming and reduces measurement speed
Solution Approach 1:
The patent replaces traditional mechanical autofocus methods (moving the stage or objective lens to search for focus) with an optical wavefront sensing system. The Shack-Hartmann wavefront sensor measures focus and surface height by analyzing light wavefronts reflected from the workpiece surface, eliminating the need for mechanical position changes and enabling instant focus determination.
Solution Approach 2:
The patent introduces a wavefront sensor as an intermediary device between the light source and the imaging system. This sensor acts as a mediator that directly measures focus and surface height parameters through wavefront analysis, providing rapid feedback without requiring mechanical movement of the imaging system components.
2Measurement precision
If auxiliary focus sensors are used to determine focus position, then focus measurement may be achieved, but the sensors fail to work reliably with both specular and diffuse surfaces and have undesirable range vs. resolution capabilities
Solution Approach 1:
The Shack-Hartmann wavefront sensor is designed to universally measure focus and surface height for both specular and diffuse surfaces. By analyzing the statistical properties of the wavefront sensor output (such as spot spread and centroid positions), the system adapts to different surface optical properties, making the measurement process reliable across diverse surface types without requiring sensor changes.
Solution Approach 2:
The patent changes the measurement parameters from direct intensity-based methods to wavefront-based statistical analysis. By examining parameters such as spot position deviations, spot size variations, and wavefront curvature in the Shack-Hartmann sensor output, the system can extract focus and surface height information that is independent of surface reflectivity characteristics.
3Measurement precision
If traditional vision systems are used for surface height measurement, then dimensional measurements can be obtained, but the systems lack lateral resolution and have complex optical or control systems
Solution Approach 1:
The patent extracts the focus and surface height measurement function from the main imaging system by using a separate, dedicated wavefront sensor. This allows the imaging system to remain simple while the wavefront sensor provides specialized high-precision focus and height measurements through wavefront analysis, separating the measurement functions for optimal performance.
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 achieves precise and fast surface height measurements and focus operations, independent of workpiece surface optical properties, enabling micron or sub-micron resolution over larger ranges, and supports autofocus processes in machine vision inspection systems.
Implementation Method 1
A surface height and focus sensing system utilizing a Shack-Hartmann type wavefront sensor
Implementation Method 2
a plurality of lenslets arranged in an array are used to sample the wavefront. Each lenslet provides a corresponding sub-aperture
Implementation Method 3
collimation adjustment element which drives the system such that the illumination focus height matches the workpiece surface height
Implementation Method 4
outputting the light from the workpiece illuminating beam from the objective lens such that it is focused at an illumination focus height proximate to the portion of the workpiece surface
Data Source
AI summary
A surface height and focus sensing system is provided. In one embodiment, an illumination focus sensor is used in combination with a collimation adjustment element which drives the system such that an illumination focus height matches the workpiece surface height, which produces a null output from the illumination focus sensor. Under the null condition, the amount of collimation adjustment is directly related to the workpiece surface height, and the resulting height determination is relatively insensitive to the workpiece surface optical properties. In one embodiment, the amount of collimation adjustment is determined according to the control signal for the collimation adjustment element. In another embodiment, a collimation adjustment sensor is utilized to measure the amount of collimation adjustment.