Image Sensor Dark Pixel Offset Correction for Column Fixed Pattern Noise
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Solution Overview
Problem
As the number of pixels in image sensors increases and their size decreases, they are prone to dark current noise, photon shot noise, and MOS device noise, leading to fixed pattern noise (FPN) and column fixed pattern noise (CFPN) due to deviations in transistor threshold voltage and pixel offsets, which affect image quality, especially in bright areas.
Innovation Solution
An image sensor design that includes a pixel array with active and dark pixel areas, an analog-to-digital conversion unit, an offset extractor, and a corrector to generate and correct pixel data codes, using dark pixel data to extract offsets and reduce noise by comparing active pixel data codes to a reference code, thereby minimizing column fixed pattern noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the number of pixels is increased and the size is decreased, then the resolution is improved, but dark current noise and fixed pattern noise increase
Solution Approach 1:
The pixel array is divided into active pixel areas for light sensing and dark pixel areas for offset measurement. This segmentation allows dark pixels to be dedicated to measuring and correcting offset errors without interfering with the imaging function of active pixels, thereby reducing fixed pattern noise while maintaining high resolution.
Solution Approach 2:
Offset values are measured and corrected in advance using dark pixels before actual image capture. By performing offset measurement and correction beforehand, the system eliminates fixed pattern noise from the final image without affecting the imaging process or requiring post-processing.
2Measurement precision
If the number of pixels is increased and the size is decreased, then the resolution is improved, but column fixed pattern noise increases
Solution Approach 1:
Dark pixels are distributed across different columns within the pixel array, allowing offset measurement for each column. This column-wise offset correction specifically addresses column fixed pattern noise by measuring and compensating for offsets in each column independently using the segmented dark pixel structure.
3Object-affected harmful factors
If offset correction is performed using dark pixels, then column fixed pattern noise is reduced, but device complexity increases
Solution Approach 1:
The offset measurement and correction functions are merged into the existing pixel array structure and signal processing pipeline. Dark pixels share the same readout circuits and processing pathways as active pixels, allowing offset correction to be performed using existing hardware without adding significant complexity to the device.
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 effectively reduces or prevents column fixed pattern noise in bright areas by accurately correcting pixel data codes, improving image quality by minimizing noise generated from pixel offsets and deviations.
Implementation Method 1
Each of the pixels may include a photodiode and a readout circuit connected to the photodiode
Data Source
AI summary
An image sensor may include a pixel array including an active pixel area including active pixels and a dark pixel area including dark pixels, an analog-to-digital conversion unit configured to generate a dark pixel data code by converting an analog output of each of the dark pixels and an active pixel data code by converting an analog output of each of the active pixels, an offset extractor configured to extract a final offset using the dark pixel data code, and a corrector configured to correct the active pixel data code using the final offset. The corrector outputs one of a correction pixel data code and a reference code as a corrected result from the active pixel data code and the reference code. The correction pixel data code is obtained by correcting the active pixel data code using the final offset.