Dark Pixel Sensor Layout for Pixel-Level Dark Current Calibration
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Solution Overview
Problem
Dark current in CMOS image sensors causes noise and defects in images due to heat-generated electrical currents, leading to inaccurate pixel readings, particularly when using average dark current measurements for calibration across all pixels.
Innovation Solution
Incorporating a plurality of dark pixel sensors within the pixel array to generate individual dark current measurements for visible light pixel sensors, allowing for precise calibration of each pixel or small subsets, thereby accounting for varying dark current levels across the array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If average dark current measurements are used for calibration across all pixels, then calibration process is simplified, but measurement precision deteriorates due to large variations in dark current levels across different pixels
Solution Approach 1:
The pixel array is divided into multiple segments, each containing visible light pixel sensors and at least one dark pixel sensor. This segmentation allows for localized dark current calibration that accounts for spatial variations in dark current levels across the array, improving measurement precision while maintaining manageable calibration complexity through modular organization.
Solution Approach 2:
Each segment is equipped with its own dark pixel sensor(s) to measure local dark current characteristics specific to that region. This local quality approach ensures that calibration parameters are tailored to the specific dark current levels of each segment, significantly improving dark current calibration accuracy compared to using a single average value for the entire array.
2Measurement precision
If dark pixel sensors are added to each segment of the pixel array, then dark current calibration accuracy is improved, but device complexity increases due to additional sensors and circuitry
Solution Approach 1:
The pixel array is organized into segments where each segment contains visible light pixel sensors and at least one dark pixel sensor. This segmentation strategy improves dark current calibration accuracy by providing localized measurements while controlling device complexity through a modular, scalable structure that can be implemented in standard CMOS manufacturing processes.
Solution Approach 2:
Dark pixel sensors are designed to perform multiple functions: measuring dark current for calibration, serving as reference pixels for noise characterization, and potentially functioning as additional sensing elements in certain operating modes. This multi-functionality justifies the added device complexity by maximizing the utility of each dark pixel sensor inclusion.
3Measurement precision
If individual dark current measurements are taken for each visible light pixel sensor, then calibration accuracy is maximized, but manufacturing precision requirements increase
Solution Approach 1:
Each segment is equipped with dark pixel sensor(s) that measure local dark current characteristics specific to that region's environmental conditions and manufacturing variations. This local measurement approach compensates for manufacturing precision variations by characterizing and correcting for local deviations, thereby achieving high calibration accuracy without requiring extremely tight manufacturing tolerances across the entire array.
Solution Approach 2:
The dark pixel sensors enable the system to self-calibrate by automatically measuring their own dark current levels and using these measurements to correct readings from adjacent visible light pixel sensors. This self-service calibration mechanism reduces the need for external calibration equipment and manual adjustment, thereby reducing overall manufacturing precision requirements while maintaining high calibration accuracy.
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 enables more accurate dark current calibration, reducing noise and defects in images by accounting for large differences in dark current levels, resulting in improved image quality and sensitivity.
Implementation Method 1
Complementary metal oxide semiconductor (CMOS) image sensors utilize light-sensitive CMOS circuitry to convert light energy into electrical energy. The light-sensitive CMOS circuitry may include a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode (referred to as a photocurrent).
Implementation Method 2
Some pixel sensors may include a near infrared (NIR) pass filter, which blocks visible light and passes NIR light through to the photodiode.
Data Source
AI summary
A pixel array includes a plurality of dark pixel sensors configured to generate dark current calibration information for a plurality of visible light pixel sensors included in the pixel array. The plurality of dark pixel sensors may generate respective dark current measurements for each of the plurality of visible light pixel sensors or for small subsets of the plurality of visible light pixel sensors. In this way, each of the plurality of visible light pixel sensors may be individually calibrated (or small subsets of the plurality of visible light pixel sensors may be individually calibrated) based on an estimated dark current experienced by each of the plurality of visible light pixel sensors. This may enable more accurate dark current calibration of the visible light pixel sensors included in the pixel array, and may be used to account for large differences in estimated dark currents for the visible light pixel sensors.