Active-Pixel Sensor Double Charge Transfer for High Dynamic Range
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Solution Overview
Problem
Current electronic image sensors face challenges in achieving a large dynamic range without saturation, especially in varying illumination conditions, and are prone to column-fixed reading noise, which affects image quality and requires complex processing.
Innovation Solution
A method that uses two capacitors for reading charges from pixels, where charges are integrated and transferred in two successive periods, with conditional sampling based on a threshold comparison to adjust the signal level, reducing noise and maintaining linearity across illumination levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If charges are integrated for a long duration to capture low illumination, then sensitivity to low light is improved, but saturation occurs in high illumination conditions
Solution Approach 1:
The patent implements dynamic integration time control by performing two successive charge transfers with different integration periods. The first transfer uses a longer integration time for low-light sensitivity, while the second uses a shorter integration time to prevent saturation in bright conditions. The system dynamically selects between these two readings based on signal level detection, achieving adaptability across varying illumination conditions.
Solution Approach 2:
The patent changes the integration time parameter by performing two successive charge transfers with different durations. The first transfer integrates charges for a longer period to enhance low-light sensitivity, while the second transfer uses a shorter period to avoid saturation. This parameter variation allows the system to optimize performance across different illumination levels.
2Reliability
If multiple successive images are captured with different integration times to achieve high dynamic range, then dynamic range is improved, but acquisition time and processing complexity increase
Solution Approach 1:
The patent performs two charge transfers and readings continuously within a single image acquisition cycle, rather than capturing multiple separate images. The first transfer and reading occur, then the second transfer and reading follow immediately. This continuous process achieves high dynamic range in one shot, eliminating the time loss associated with capturing and processing multiple successive images.
Solution Approach 2:
The patent merges two different integration time measurements into a single image acquisition cycle. Both the long-integration reading (for low light) and short-integration reading (for bright light) are obtained during one continuous operation, and the system selectively combines or chooses between them to produce a single high dynamic range image, rather than requiring multiple separate captures.
3Measurement precision
If complex processing is applied to reduce column-fixed reading noise, then noise reduction is improved, but processing complexity increases
Solution Approach 1:
The patent performs a preliminary reading of the column conductor potential before the charge transfer occurs. This first reading captures the baseline potential including any column-fixed noise. After the charge transfer, a second reading is taken. By having the preliminary reading in place before the transfer, the system can compare the two readings and eliminate the fixed noise component, achieving noise reduction without requiring complex post-processing algorithms.
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 allows for a significant reduction in column noise and preserves signal linearity, facilitating colorimetric corrections and automatic gain/exposure adjustments, while enabling the capture of images with both low and high illumination without saturation.
Implementation Method 1
a photodiode PD, a capacitive storage node ND... charges generated by the light in the photodiode
Data Source
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AI summary
The invention relates to active-pixel image sensors. To obtain a large dynamic range of operation, the pixels are read while performing a double charge integration, during periods of different values (Ti1, Ti2). The result of the first integration (period Ti1) is sampled (command SHS1) in a sampling capacitor, and the result of the second integration (period Ti2) is conditionally sampled (command SHS2) in the same capacitor. This second sampling depends on the observation of the potential of the column conductor after the charge integration corresponding to the longer period; this potential is compared with a threshold. If the comparison indicates a risk of saturation, the information gathered during the shorter period is gathered or preserved in the sampling capacitor with a view to multiplying it by a coefficient representing the ratio of the longer period to the shorter period. If the comparison indicates that there is no risk of saturation, the information gathered during the longer period is gathered or preserved in the sampling capacitor. The shorter period is in principle the first one.