Autofocus Image Acquisition System for Iris Standoff Capture
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Solution Overview
Problem
Current biometric image acquisition systems require precise focusing, which can be user-unfriendly and costly due to the need for subject cooperation and skilled operation, especially when capturing high-security iris images with small depth of field and high spatial frequency requirements.
Innovation Solution
An image standoff acquisition system captures a rapid sequence of frames while sweeping through the focus range of the optics, allowing for selection of the best-focused iris image without requiring optical autofocusing or precise range finding, using a combination of lens, subject, camera, or sensor movement, and processing to identify in-focus frames.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If precise focusing is required to capture high-quality iris images, then image quality is improved, but user operation becomes more difficult and time-consuming
Solution Approach 1:
The system performs autofocus automatically without requiring user intervention. The processor analyzes captured frames to determine which frame has the iris in focus, selecting it automatically. This eliminates the need for users to manually adjust focus or understand focusing techniques, resolving the contradiction between precise focusing and ease of operation
Solution Approach 2:
The system captures multiple frames at different focus positions in advance before selection is needed. By pre-capturing a sequence of frames covering the focus range, the system ensures that at least one in-focus frame is available without requiring real-time manual adjustment, improving both image quality and operational ease
2Measurement precision
If optical autofocusing is implemented to achieve precise focus, then focusing accuracy is improved, but device complexity and cost increase
Solution Approach 1:
The system replaces complex mechanical autofocus mechanisms with a computational approach. Instead of using motorized lens adjustment or phase detection hardware, the system captures multiple frames and uses image processing algorithms to identify the in-focus frame. This substitution of mechanical systems with computational methods reduces device complexity while maintaining focusing accuracy
Solution Approach 2:
The system captures multiple copies (frames) of the same scene at different focus positions and selects the best one. Rather than relying on a single precise mechanical focus adjustment, the system creates multiple copies covering the focus range and chooses the optimal copy through image analysis, reducing the need for complex mechanical precision
3Measurement precision
If the depth of field is reduced to capture high spatial frequency iris details, then image detail quality is improved, but the range of acceptable focus distances decreases
Solution Approach 1:
The system dynamically adjusts the focus position by capturing frames at multiple focus distances rather than using a fixed focus setting. This dynamic approach allows the system to adapt to different subject distances while maintaining high spatial frequency capture, resolving the contradiction between shallow depth of field for detail and adaptability to various distances
Solution Approach 2:
The focus range is segmented into multiple discrete focus positions, with frames captured at each position. This segmentation allows the system to cover a broader effective focus range while maintaining the shallow depth of field benefits at each individual position, as the composite result provides both detail and adaptability
Data Source
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AI summary
A system (10) having a sensor (27) and variable focus lens (12) for iris image standoff acquisition. The sensor (27) may capture a sequence of images (29) at a high frame rate of a person (20) for getting an eye or an iris (28) in a window (31) within the images (29). Even if the eye (28) moves around in the image (29), the window (31) may stay on the eye. During this capture, the focus of the lens (12) may be changed, with a best focus situated somewhere in between the end focus positions of the lens. The sensor (27) may be an infrared (IR) sensor and an IR illuminator or flash (48) may provide light for the capture of images (29). An intensity variance indicator may be incorporated to select an in-focus image of the sequence. Processing of the images may be subsequent to the capture of images (29), thus not hindering the frame rate of the system.