Variable Focal Length Lens Calibration Using Tilted Grating
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Solution Overview
Problem
Existing precision machine vision inspection systems with variable focal length (VFL) lenses face challenges in calibration accuracy, ease of use, and repeatability, particularly due to the high-speed modulation of focal lengths, which complicates the calibration process.
Innovation Solution
A method utilizing a focus state calibration object with a planar tilted pattern surface and focus state reference regions (FSRRs) is employed to determine calibration data for the VFL lens system. This involves acquiring camera images at different phase timings of the periodic modulation, analyzing these images to determine phase timings corresponding to effective focus positions, and using this data to calibrate the VFL lens system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high-speed focal length modulation is used in VFL lens systems, then productivity and measurement speed are improved, but calibration accuracy and repeatability deteriorate
Solution Approach 1:
The patent performs calibration operations before high-speed measurement operations. A calibration object with known geometric features is imaged first to establish reference data mapping focal lengths to measured distances. This preliminary calibration creates a lookup table that enables accurate measurements during subsequent high-speed operations without requiring recalibration.
Solution Approach 2:
The patent uses a calibration object that replicates the geometric features of actual workpieces. By imaging this known reference object multiple times at different focal lengths, the system creates copies of measurement data that can be compared against known dimensions. This copying approach allows the system to learn accurate distance-focal length relationships that can then be applied to unknown workpieces.
2Adaptability or versatility
If multiple images at multiple focal lengths are acquired, then measurement capability is improved, but device complexity and calibration difficulty increase
Solution Approach 1:
The patent divides the calibration process into separate, manageable steps: (1) imaging the calibration object at multiple predetermined focal lengths, (2) calculating distances between known geometric features in each image, (3) storing these measurements in a lookup table, and (4) using this table for subsequent measurements. This segmentation transforms a complex simultaneous multi-parameter calibration into sequential simple operations.
Solution Approach 2:
The patent systematically varies the focal length parameter across multiple discrete values during calibration. By changing this single parameter while keeping the calibration object stationary and using its known geometry as reference, the system establishes accurate correspondence between focal length values and physical distances without needing to adjust other system parameters.
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 method improves the calibration accuracy and repeatability of VFL lens systems by precisely determining phase timings and effective focus positions, enhancing the system's ability to maintain high-speed focus operations while simplifying the calibration process.
Implementation Method 1
The sound waves may be created by application of an electrical field at a resonant frequency to a piezoelectric tube surrounding the fluid medium to create a time-varying density and index of refraction profile in the lens's fluid
Implementation Method 2
creates a lensing effect using sound waves in a fluid medium... create a time-varying density and index of refraction profile in the lens's fluid, which modulates its optical power
Data Source
AI summary
A system and method are provided for utilizing a focus state calibration object for determining calibration data for a variable focal length (VFL) lens system which includes a VFL lens (e.g., a tunable acoustic gradient lens). The calibration object includes a planar tilted pattern surface on which a set of focus state reference regions (FSRRs) are distributed (e.g., a tilted grating). The FSRRs have known geometric relationships relative to the planar tilted pattern surface and have known region relationships relative to one another. A plurality of camera images is acquired at different phase timings of the periodic modulation, and calibration data is determined based at least in part on analyzing the plurality of camera images. The determined calibration data indicates respective phase timings of the periodic modulation that correspond to respective effective focus positions of the VFL lens system.