Dual-Ported Photodiode Imaging Array for Real-Time Exposure Control
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Solution Overview
Problem
Inexpensive cameras with CMOS imaging arrays struggle to maintain accurate exposure control, especially in rapidly changing scenes, due to long readout times and limitations in capturing a wide range of light intensities, leading to overexposure or underexposure issues.
Innovation Solution
The implementation of a dual-ported photodiode system with a charge conversion circuit and a controller that measures current flowing out of second gates to determine average light intensity and terminate exposure if it exceeds a threshold, allowing for real-time adjustment of exposure time without full image readout.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the entire image is readout and processed at each exposure time to determine correct exposure, then exposure control accuracy is improved, but readout time becomes too long to capture rapidly changing scenes
Solution Approach 1:
The patent extracts only the necessary information (average light intensity) from the imaging array without reading out the entire image. The controller measures current flowing through second gates of dual-ported photodiodes to determine average light intensity, eliminating the need to process all pixel data while maintaining exposure control accuracy.
Solution Approach 2:
Instead of reading out every pixel (excessive action), the system uses a partial measurement approach by measuring current through selected second gates that represent the average light intensity across the array. This partial action provides sufficient information for exposure control without the time cost of full image readout.
2Measurement precision
If exposure time is extended to capture low light levels accurately, then measurement precision for dark scenes is improved, but the scene changes before exposure completes
Solution Approach 1:
The controller continuously monitors current flowing through second gates during exposure and uses this feedback to determine average light intensity in real-time. This enables dynamic adjustment of exposure time based on actual lighting conditions, allowing the system to extend exposure when needed for low light scenes while terminating it when sufficient information is gathered, preventing capture of changing scenes.
3Device complexity
If the imaging array uses standard photodiodes with single port, then device complexity is reduced, but the ability to measure average light intensity without full readout is lost
Solution Approach 1:
The photodiode is segmented into two separate ports with independent gates: a first port for charge transfer to the charge conversion circuit and a second port for measuring average light intensity through current flow. This segmentation allows simultaneous operation of both functions without increasing overall device complexity, as each port has its own simple gate control mechanism.
4Reliability
If the camera processes every pixel data for exposure determination, then exposure control reliability is improved, but the processor becomes a bottleneck for rapid scene changes
Solution Approach 1:
The system extracts only the essential information needed for exposure control (average light intensity) from the imaging array through current measurement at second gates, rather than processing all pixel data. This extraction approach maintains exposure control reliability by using a representative sample of the scene's lighting conditions while dramatically improving processing speed for dynamic scenes.
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 solution enables faster and more accurate exposure control, reducing the likelihood of overexposure or underexposure, even in dynamic scenes, by measuring current on overflow buses to estimate correct exposure times and terminate exposure prematurely if necessary, thus improving the usability of cameras in capturing changing scenes.
Implementation Method 1
Each pixel sensor in the array includes a dual-ported photodiode characterized by ports having first and second gates... generates a signal that is a function of a charge on the dual-ported photodiode
Data Source
AI summary
An imaging array and method for using the same to capture an image are disclosed. The imaging array includes an array of pixel sensors and a controller. Each pixel sensor includes a dual-ported photodiode characterized by ports having first and second gates, and a charge conversion circuit. The charge conversion circuit generates a signal that is a function of a charge on the dual-ported photodiode when the first gate in the dual-ported photodiode is activated to transfer a charge on the dual-ported photodiode to the charge conversion circuit. The controller applies a potential to the second gates and measures a current flowing out of the second gates, each second port passing charge stored in the photodiode connected to the second port when a potential in the photodiode exceeds the applied potential. The controller determines an average light intensity incident on the array of pixel sensors.