Dual Photodiode Image Sensor for High Dynamic Range and Flicker Mitigation
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving high dynamic range without motion artifacts and mitigating LED flickering, especially in automotive and security applications where capturing high contrast scenes is crucial.
Innovation Solution
The method involves using a pixel circuit with two photodiodes of different sizes, where the larger photodiode handles darker regions with the longest integration time, the smaller photodiode handles brighter regions with shorter integration time, and a third integration collects blooming charge to achieve 120 dB dynamic range, while also desensitizing the smaller photodiode to mitigate flickering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single pixel is split into two photodiodes with different integration times to extend dynamic range, then high dynamic range is achieved, but motion artifacts appear due to non-overlapping integration periods
Solution Approach 1:
The pixel is segmented into a first photodiode and a second photodiode with different areas and integration times. The first photodiode uses a longer integration time for dim scenes while the second photodiode uses a shorter integration time for bright scenes, allowing the system to capture high dynamic range without motion artifacts by selecting appropriate photodiode outputs based on scene brightness
Solution Approach 2:
Different regions of the pixel array can use different photodiode configurations optimized for local requirements. The first photodiode with larger area is optimized for low-light conditions while the second photodiode with smaller area is optimized for high-light conditions, allowing local adaptation to scene requirements
2Illumination intensity
If the integration time of a photodiode is shorter than the ON duty cycle of LED lights to capture bright scenes, then bright scenes are captured, but LED flickering occurs causing the pixel to miss LED signals
Solution Approach 1:
The system dynamically adjusts integration time based on scene brightness requirements. For bright scenes with LED lights, the second photodiode uses a shorter integration time that is synchronized with the LED modulation frequency to capture LED signals effectively, while for dim scenes, the first photodiode uses a longer integration time
3Measurement precision
If a larger photodiode area is used to capture dim scenes with long integration time, then sensitivity to dim light is improved, but saturation occurs in bright scenes
Solution Approach 1:
The pixel is divided into two photodiodes with different areas: the first photodiode has a larger area for capturing dim scenes with high sensitivity, while the second photodiode has a smaller area that prevents saturation in bright scenes. The system selects the appropriate photodiode output based on the brightness level of the captured scene
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 approach enables the capture of high contrast scenes without saturation and reduces LED flickering, achieving a high dynamic range of 120 dB by reading out multiple exposure values and effectively managing blooming charge, thereby eliminating motion artifacts.
Implementation Method 1
performing a first integration on a first photodiode of a pixel circuit to obtain at least one first image signal; performing a second integration on a second photodiode of the pixel circuit to obtain a second image signal
Implementation Method 2
performing a third integration by collecting blooming charge overflowing from the first photodiode to obtain an overflow image signal
Data Source
AI summary
The invention is directed to a method of operating an image sensor. A first integration is performed on a first photodiode of a pixel circuit to obtain at least one first image signal. A second integration is performed on a second photodiode of the pixel circuit to obtain a second image signal, the first photodiode having a photodiode area larger than the second photodiode. A third integration is performed by collecting blooming charge overflowing from the first photodiode to obtain an overflow image signal. The first integration has longest integration time and the third integration has shortest integration time among the first integration, the second integration and the third integration.