Image Sensor Overflow Node Dynamic Range
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Solution Overview
Problem
Conventional image sensor pixels face challenges with dynamic range due to excess photogenerated charges causing blooming, leading to loss of signal and reduced image quality, as they drain charges to a supply node rather than contributing to the output signal.
Innovation Solution
The introduction of an overflow transistor and a separate overflow sense node allows excess charges to be stored and read out separately from the main sense node, enabling improved dynamic range by capturing both signals from the overflow and sense nodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an anti-blooming transistor is used to prevent blooming by draining excess charges to a supply node, then blooming is prevented, but the dynamic range is reduced because the excess charges are lost and do not contribute to the output signal
Solution Approach 1:
The pixel circuit is segmented into two separate charge collection paths: a first charge collection node for collecting photogenerated charges within a first dynamic range, and a second charge collection node for collecting overflow charges within a second dynamic range. This segmentation allows the system to capture both dim and bright regions without losing information, resolving the contradiction between preventing blooming and maintaining signal integrity.
Solution Approach 2:
An overflow transistor is introduced as an intermediary element between the first charge collection node and the second charge collection node. This overflow transistor selectively redirects excess charges from the first node to the second node based on charge levels, preventing blooming while preserving the charge information for later readout, thus eliminating signal loss.
2Object-affected harmful factors
If the anti-blooming transistor control signal is set to slightly reduce the potential barrier to drain excess charges, then blooming is addressed, but the excess charges are lost to the supply node
Solution Approach 1:
The invention converts the harmful blooming effect into a beneficial feature by capturing overflow charges in the second charge collection node. The same mechanism that causes charge overflow is now used to populate a dedicated overflow node, transforming what was previously signal loss into useful information about the brightest regions of the image.
Solution Approach 2:
The system adds a second dimension to charge collection by introducing a second charge collection node that operates in parallel with the first node. This dimensional expansion allows the system to capture charges across two different dynamic ranges simultaneously, preventing charge loss while maintaining blooming prevention.
3Loss of time
If a single exposure is used to capture the image, then the integration time is reduced, but the dynamic range is limited by blooming in high-intensity light scenarios
Solution Approach 1:
The system dynamically adapts to varying light conditions by using two charge collection nodes with different dynamic ranges. During integration, charges are automatically routed to appropriate nodes based on their magnitude, allowing the system to maintain optimal performance across different illumination levels without requiring multiple exposures or extended integration times.
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 configuration enhances dynamic range by capturing excess charges without losing them, allowing for a single exposure and integration in high-intensity light scenarios, and is compatible with global shutter operation, improving image sensor performance.
Implementation Method 1
a photodiode configured to produce photogenerated charges in response to exposure to light
Data Source
AI summary
A photodiode produces photogenerated charges in response to exposure to light. An integration period collects the photogenerated charges. Collected photogenerated charges in excess of an overflow threshold are passed to an overflow sense node. Remaining collected photogenerated charges are passed to a sense node. A first signal representing the overflow photogenerated charges is read from the overflow sense node. A second signal representing the remaining photogenerated charges is read from the sense node.