Global Shutter Pixel Charge Steering for Flicker Mitigation
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Solution Overview
Problem
Conventional imaging systems suffer from image artifacts due to moving objects, flickering lighting, and objects with changing illumination, leading to issues like object distortion, ghosting, and low dynamic range, especially in high light conditions where charge storage capacity is limited.
Innovation Solution
The implementation of global shutter image sensors with charge steering structures that route excess charge to additional storage nodes or the pixel voltage supply, allowing for separate readout of overflow and non-overflow charges, thereby increasing the dynamic range and minimizing artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional image sensors are used to capture bright scenery, then the exposure time must be shortened to avoid overexposure, but this results in insufficient charge accumulation and poor image quality in high light conditions
Solution Approach 1:
The pixel is segmented into multiple independent storage nodes (first storage node and second storage node) that can independently accumulate charge. This segmentation allows the sensor to capture bright scenery with short exposure times while having sufficient total charge storage capacity across multiple nodes, resolving the contradiction between illumination intensity handling and charge accumulation.
Solution Approach 2:
The patent implements a nested storage structure where overflow charge from the first storage node can be transferred to a second storage node within the same pixel. This nested arrangement effectively increases the total charge storage capacity without increasing the physical pixel size, allowing sufficient charge accumulation even when capturing bright scenes with reduced exposure time.
2Device complexity
If rolling shutter mode is used to reduce complexity, then device complexity is lowered, but image artifacts such as distortion and ghosting occur due to asynchronous light acquisition
Solution Approach 1:
The patent implements a global shutter mechanism where all pixels simultaneously complete their exposure and transfer charge to storage nodes before readout begins. This preliminary action of completing all exposures before readout eliminates the temporal distortion and ghosting artifacts associated with rolling shutter, while the multi-storage-node structure manages the increased complexity through systematic charge routing.
3Quantity of substance
If integration time is increased to improve charge accumulation, then signal strength is enhanced, but moving objects and flickering lighting cause artifacts such as distortion and ghosting
Solution Approach 1:
The patent dynamically routes charge between multiple storage nodes based on the amount of charge accumulated. When charge accumulation is sufficient, overflow is directed to secondary storage nodes or discarded, allowing the system to adapt to different lighting conditions and object motion speeds, thereby reducing artifacts while maintaining signal strength.
Solution Approach 2:
The patent changes the effective integration time parameter by controlling when charge is transferred from photodiodes to storage nodes. All pixels complete exposure simultaneously (global shutter), then charge is transferred at a controlled time, allowing sufficient charge accumulation without extending the vulnerable exposure period where moving objects would cause artifacts.
4Adaptability or versatility
If scenes with both bright and dark portions are captured, then comprehensive scene coverage is achieved, but overexposure in bright areas and underexposure in dark areas occur simultaneously
Solution Approach 1:
By segmenting each pixel into multiple storage nodes with different charge capacities, the system can capture a wider range of illumination levels within the same exposure. Bright areas fill the first storage node and overflow to the second, while dark areas accumulate charge in the first node, effectively increasing dynamic range and improving exposure accuracy across diverse 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 approach enhances the dynamic range of image sensors, reducing image artifacts and improving the capture of high light conditions by accurately accounting for and managing excess charge, resulting in clearer and more detailed images.
Implementation Method 1
Each pixel may include a photodiode that generates charge in response to incident light
Data Source
AI summary
An image sensor may include one or more pixels having a charge steering structure that may selectively route charge from a photodiode to increase the dynamic range of the pixel. The charge steering structure may be a coupled gate structure that routes overflow charge to a voltage supply and to one or more integrating storage structures during an exposure period. The charge steering structure may be two integrating storage structures directly connected to the photodiode that each integrate charge generated by the photodiode in an alternating fashion during an exposure period. Storage structures and transistors within the charge steering structure may receive control signals, which may be asserted in a mutually exclusive manner. Pixels may also include a dual-gain structure, which may provide additional charge storage capacity.