Global Shutter CMOS Noise Compensation via Leakage Measurement
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Solution Overview
Problem
CMOS image sensors operating in global shutter mode face challenges with image degradation due to variations in charge hold time among rows, leading to noise and loss of contrast, especially at the bottom of the image, and require methods to compensate for leakage current to improve signal-to-noise ratio.
Innovation Solution
The method involves dispersing noise introduced by leakage currents among rows, accounting for time-dependent changes in dark pixel output, and using weighting techniques to locally average pixel values based on their sequential location during row sensing, as well as measuring and compensating for wait-time-dependent background signals to generate noise-compensated pixel values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If global shutter method is used to capture high speed subjects without image artifacts, then image quality for dynamic subjects is improved, but charge leakage from floating diffusion node degrades signal integrity
Solution Approach 1:
The patent applies preliminary action by performing a first read of the floating diffusion node immediately after charge transfer, before the charge can leak significantly. This early read captures the signal before degradation occurs, and the node is then reset to prepare for the second read, effectively isolating the charge leakage issue to a specific time window that can be compensated.
Solution Approach 2:
The patent uses feedback by measuring the output of the floating diffusion node at two different times (first read and second read) and using the difference between these readings to calculate and compensate for charge leakage. This feedback mechanism allows the system to quantify and correct the harmful effect of charge leakage on signal integrity.
2Duration of action of moving object
If signal is held at floating diffusion node for extended period in global shutter mode, then full frame capture is achieved, but charge leakage increases and degrades signal-to-noise ratio
Solution Approach 1:
The patent performs a preliminary first read of the floating diffusion node immediately after charge transfer, capturing the signal before significant charge leakage occurs during the extended hold period. This allows the system to establish a baseline measurement that can be used to compensate for leakage effects in the second read taken after the full hold period.
Solution Approach 2:
The system uses feedback by comparing the first read (taken immediately after charge transfer) with the second read (taken after the full hold period). The difference between these two readings provides a measure of charge leakage, which is then used to compensate for the degradation in signal-to-noise ratio, effectively correcting for the extended charge hold time.
3Productivity
If row-by-row read out is used to read the imager array, then data is read sequentially, but noise from charge leakage accumulates at the bottom of the image
Solution Approach 1:
The patent performs a preliminary first read of all rows immediately after charge transfer, before charge leakage can accumulate significantly. This first read establishes a baseline for each row that captures the signal before degradation. The subsequent second read is then compensated by comparing against this preliminary measurement, preventing localized noise accumulation at the bottom of the image.
Solution Approach 2:
The system applies feedback by measuring the output of each row at two different times and using the difference to compensate for charge leakage-specific to each row. This row-by-row feedback approach prevents noise accumulation by actively correcting for leakage effects in each row based on its specific leakage characteristics, rather than allowing noise to accumulate uniformly across the image.
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 image quality by evenly distributing noise across the image, reducing localized noise issues and improving the signal-to-noise ratio, resulting in higher quality digital images with reduced artifacts.
Implementation Method 1
the image is captured in its light conversion unit, which is a photosensitive diode. Charges generated from the light conversion unit are then transferred to a floating diffusion node.
Data Source
AI summary
The image qualify of an image frame from a CMOS image sensor array operated in global shutter mode may be enhanced by dispersing or randomizing the noise introduced by leakage currents from floating drains among the rows of the image frame. Further, the image quality may be improved by accounting for time dependent changes in the output of dark pixels in dark pixel rows or dark pixel columns. In addition, voltage and time dependent changes in the output of dark pixels may also be measured to provide an accurate estimate of the noise introduced to the charge held in the floating drains. Such methods may be employed individually or in combination to improve the quality of the image.