Dual-Pixel Light Sensor for Full-Well Saturation Avoidance
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Solution Overview
Problem
Existing CMOS image sensors face challenges in maximizing Full-Well Capacity (FWC) due to miniaturization demands, leading to limited dynamic range and signal-to-noise ratio, and the prediction of optimal integration time is difficult due to temperature and light level dependencies.
Innovation Solution
A light sensor with a first pixel acting as a FWC detector and two second pixels, where the second pixels are configured to avoid saturation by measuring charge before reaching their threshold, using a pinned photodiode structure and different reset voltages, allowing for long integration times without losing light information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the area of photodiode is increased to achieve larger FWC, then dynamic range and signal-to-noise ratio are improved, but pixel size increases which conflicts with miniaturization demands for higher resolution
Solution Approach 1:
The pixel is divided into two distinct photodiodes: a first photodiode configured to generate charges during exposure, and a second photodiode configured to detect when the first photodiode reaches FWC. This segmentation allows the first photodiode to be optimized for charge storage capacity while the second photodiode monitors saturation conditions, enabling larger effective FWC without proportionally increasing overall pixel area.
Solution Approach 2:
The first photodiode serves multiple functions: it acts as both the primary light-sensing element and the reference for determining optimal exposure termination. By using the first photodiode's saturation state as the trigger for the second photodiode, the system achieves multi-functionality that maximizes charge storage capacity while maintaining compact pixel dimensions through intelligent charge management.
2Loss of information
If integration time is extended to capture more light information, then signal-to-noise ratio improves, but pixels may reach FWC and saturate causing loss of light information
Solution Approach 1:
The second photodiode provides real-time feedback about the charge accumulation state of the first photodiode. When the first photodiode approaches FWC saturation, the second photodiode detects this condition and generates a signal to terminate the exposure period. This feedback mechanism enables the system to extend integration time as long as possible to capture maximum light information while automatically preventing saturation-induced information loss.
Solution Approach 2:
The second photodiode is pre-configured to monitor the charge state of the first photodiode throughout the exposure period. By having the second photodiode ready and waiting to detect saturation conditions, the system can immediately terminate exposure at the optimal moment, maximizing integration time without risking overflow. This preliminary preparation ensures continuous operation at maximum efficiency.
3Quantity of substance
If FWC is maximized by increasing photodiode area, then charge storage capacity improves, but prediction of optimal integration time becomes more difficult due to temperature and light level dependencies
Solution Approach 1:
The system uses itself to determine the optimal integration time by having the second photodiode monitor the first photodiode's charge accumulation in real-time. Rather than relying on external predictions or pre-calculated integration times that must account for temperature and light variations, the system self-regulates by directly observing when saturation is approaching and automatically terminating exposure. This eliminates the need for complex prediction algorithms while maintaining maximized charge storage capacity.
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 solution prevents saturation of the second pixels, enabling long integration times and power savings, particularly in low light conditions, by ensuring the second pixels never reach their FWC, thus maintaining high dynamic range and signal-to-noise ratio.
Implementation Method 1
a first pixel (205) configured to operate as a Full-Well Capacity (FWC) detector in response to incident light
Data Source
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AI summary
A light sensor is disclosed. The light sensor comprises a first pixel and a second pixel. The light sensor comprises measurement circuitry. The first pixel is configured to accumulate a first charge and the second pixel is configured to accumulate a second charge when the light sensor is exposed to light. The first pixel is configured to trigger the measurement circuitry to measure the second charge when the first charge reaches a threshold capacity of the first pixel. Also disclosed is an active pixel sensor comprising the light sensor, an image sensor and a device incorporating the light sensor.