Dark Current Calibration via Dual Pixel Array Segmentation
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Solution Overview
Problem
CMOS image sensors face challenges with dark current, which affects image accuracy and quality, especially in low light conditions, due to the accumulation of electrical charges at floating diffusions even in the absence of excitation light, leading to white pixels and incorrect dark current estimation.
Innovation Solution
The implementation of a method to acquire a dark current frame separately from signal frames, which can be used to subtract dark current from subsequent frames, utilizing a combination of high and low conversion gain readouts and an overflow transistor to isolate and drain dark current charges, allowing for accurate dark current estimation and correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel cells are miniaturized to achieve higher resolution and lower power consumption, then image sensor performance is improved, but susceptibility to dark current increases
Solution Approach 1:
The pixel array is divided into two separate arrays: a first pixel array for capturing image data and a second pixel array for measuring dark current. This segmentation allows independent optimization of each array's function, enabling accurate dark current measurement without interfering with image capture while maintaining miniaturized pixel dimensions for high resolution and low power consumption.
Solution Approach 2:
A dedicated second pixel array serves as an intermediary measurement system that captures dark current characteristics separately. This intermediary array provides reference data that can be used to correct and compensate for dark current effects in the primary image array, resolving the contradiction between miniaturization and dark current susceptibility.
2Speed
If dark current is not properly handled, then image capture speed is maintained, but image accuracy and quality deteriorate due to white pixels and saturation
Solution Approach 1:
The system performs preliminary dark current measurement by capturing a dark current frame from the second pixel array before processing image data. This preliminary action provides reference values that are subsequently used to correct the main image array data, ensuring accurate image quality without compromising capture speed through real-time processing.
Solution Approach 2:
A dark current frame is captured as a copy from the second pixel array, representing the dark current characteristics. This copy is then subtracted from the main image array data to remove dark current effects, preserving both image accuracy and capture speed by avoiding complex real-time corrections during image acquisition.
3Measurement precision
If dark current frame subtraction is implemented, then dark current estimation accuracy is improved, but device complexity increases due to additional readout circuits and processing
Solution Approach 1:
The patent combines the dark current measurement functionality with the existing pixel array structure by implementing a second array that mirrors the first. Both arrays share common readout circuitry and processing pipelines, merging resources to achieve accurate dark current estimation without proportionally increasing device complexity. The dual-array approach leverages existing infrastructure rather than adding entirely separate measurement systems.
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 effectively reduces the impact of dark current on image sensor accuracy by allowing for precise subtraction of dark current from image frames, improving image quality and reducing signal loss, especially in low light conditions.
Implementation Method 1
An image sensor includes an array of pixels having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light, in response generating corresponding electrical charges
Implementation Method 2
The overflow transistor is coupled between the photodiode and a voltage source, and is configured to isolate and drain charges accumulated in the photodiode
Data Source
AI summary
Image sensors capable of dark current calibration and associated circuits are disclosed herein. The method for calibrating dark current includes acquiring at least one dark current frame of a first plurality of pixels of a pixel array of the image sensor. The dark current frame contains readings of individual dark currents for the corresponding pixels obtained during an exposure period when a transistor is turned on disabling the photodiode. The method also includes acquiring at least one normal frame of a second plurality of pixels of the pixel array of the image sensor. The normal frame contains readings of individual signals for the corresponding pixels obtained during the exposure period when the transistor is turned OFF. The method includes subtracting the at least one dark current frame from the at least one normal frame.