Active Autofocus Using Fourier Pattern Matching in Low Light
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Solution Overview
Problem
Passive autofocus systems in imaging devices struggle under poor or non-uniform lighting conditions, leading to unreliable focus determination due to lack of transition gradients in acquired images.
Innovation Solution
An autofocus system utilizing a varifocal lens and a focus tunable lens, adjusted by a processor, to generate and analyze a spatial Fourier transform of a known pattern, correcting frequency distortion and achieving accurate focus through iterative adjustment of lens focal lengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If passive autofocus systems use image features to optimize focus, then focus adjustment can be performed using only captured images, but the system suffers serious limitations under poor or non-uniform lighting conditions
Solution Approach 1:
The patent introduces an active illumination source (laser) as an intermediary to provide controlled lighting for the autofocus process. This mediator ensures sufficient and uniform lighting conditions for capturing the frequency pattern, eliminating the reliability issues of passive systems under poor lighting. The illumination source projects light onto the object to create a visible frequency pattern that can be reliably detected regardless of ambient lighting conditions.
Solution Approach 2:
The system performs preliminary action by actively illuminating the scene and projecting a known frequency pattern onto the object before capturing the image. This preliminary projection of structured light ensures that the necessary frequency information is present in the captured image, enabling reliable focus determination without depending on natural lighting conditions or existing image features.
2Measurement precision
If active autofocus systems use dedicated optical systems to measure distance independently, then correct focus can be achieved, but the system complexity increases with additional components
Solution Approach 1:
The patent makes the imaging device multi-functional by enabling it to perform both image capture and distance measurement for autofocus using the same optical components. The camera lens and image sensor are used not only for capturing images but also for detecting the frequency pattern to determine focus distance. This eliminates the need for separate distance measurement hardware, reducing system complexity while maintaining measurement precision.
Solution Approach 2:
The imaging device serves itself by using its own imaging capabilities to perform autofocus measurement. The same camera that captures images is also used to detect the projected frequency pattern and determine the optimal focus distance. This self-service approach eliminates the need for dedicated autofocus components, simplifying the overall system while maintaining accurate focus control.
3Measurement precision
If conventional autofocus systems rely on image gradients and contrast, then focus can be determined from captured images, but the systems fail under poor lighting conditions lacking transition gradients
Solution Approach 1:
The system performs preliminary action by actively projecting a known frequency pattern onto the scene before capturing the image. This structured light projection ensures that high-contrast frequency information is present in the captured image regardless of ambient lighting conditions. The preliminary illumination and pattern projection create the necessary measurement features that would otherwise be absent in poor lighting scenarios.
Solution Approach 2:
The patent changes the lighting parameter from passive ambient light to active structured illumination. By controlling the illumination source to project a specific frequency pattern, the system ensures that the captured image contains sufficient contrast and gradient information for accurate focus measurement. This parameter change from uncontrolled to controlled lighting enables the system to adapt to various lighting conditions while maintaining measurement precision.
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 ensures reliable autofocus in varying lighting conditions by accurately detecting predefined frequency patterns, avoiding focus oscillation and improving focus accuracy in imaging devices.
Implementation Method 1
The varifocal lens is configured to receive the light wave after the light wave has passed through a semi-transparent pattern on a rear focal plane of the varifocal lens, and generate a Fourier transform of the semi-transparent pattern therefrom
Implementation Method 2
The processor is configured to adjust a focal length of the varifocal lens to cause the Fourier transform carried by the reflected light wave to form a predefined pattern detected by the image sensor. The processor is configured to adjust a control parameter of the focus tunable lens until one or more spatial frequencies of the predefined pattern detected at the image sensor match one or more predefined spatial frequencies
Implementation Method 3
An imaging system includes a light source for generating a light wave
Data Source
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AI summary
Systems and methods for autofocus includes a light source for generating a light wave and a varifocal lens arranged in front of the light source. The varifocal lens receives the light wave and generates a Fourier transform of a known semi-transparent pattern positioned on the rear focal plane (or input plane) of the varifocal lens therefrom. An image sensor receives the Fourier transform carried by the light wave after being reflected from an object. A focus tunable lens is arranged in front of the image sensor and through which the reflected light wave passes. A processor adjusts a focal length of the varifocal lens to cause the Fourier transform carried by the light wave to form a predefined (expected) pattern detected by the image sensor, and adjusts a control parameter of the focus tunable lens until one or more spatial frequencies of the predefined pattern detected at the image sensor match one or more predefined spatial frequencies.