Column Offset Correction for CMOS Image Sensor Fixed Pattern Noise
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Solution Overview
Problem
CMOS image sensors suffer from column fixed pattern noise due to mismatches in column output circuitry offsets and gains, which become more noticeable in low light conditions and are amplified by image gain, degrading image quality.
Innovation Solution
A method to determine and update column offset corrections using dark reference pixels, scaling these corrections based on changes in gain levels to compensate for column fixed pattern noise, thereby reducing noise artifacts in images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If gain is increased to make the image brighter in low light conditions, then image brightness is improved, but column fixed pattern noise is amplified and becomes more noticeable
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing offset correction values for multiple gain levels before image capture. When a gain change occurs during imaging, the system simply retrieves and applies the pre-compared correction values rather than performing real-time calculations, thus reducing noise without delaying the imaging process.
Solution Approach 2:
The patent utilizes parameter changes by maintaining multiple sets of offset correction values corresponding to different gain levels (e.g., 1x, 2x, 4x gain). The system dynamically selects and applies the appropriate correction values based on the current gain level, effectively compensating for gain-dependent column fixed pattern noise without requiring repeated dark signal readings.
2Measurement precision
If dark signal readings are repeatedly performed to update offset corrections for each gain level, then noise compensation accuracy is improved, but processing time and computational overhead increase
Solution Approach 1:
The patent performs dark signal readings and offset correction calculations in advance, storing the results in a lookup table with multiple gain levels. This preliminary action eliminates the need for repeated real-time calculations during imaging, significantly reducing processing time while maintaining compensation accuracy across different gain levels.
Solution Approach 2:
The patent implements a dynamic offset correction mechanism that automatically selects the appropriate pre-computed correction values based on the current gain level. This dynamic adaptation allows the system to maintain high noise compensation accuracy across varying gain levels without the time penalty of recalculating corrections each time the gain changes.
3Device complexity
If column offset corrections are not scaled when gain levels change, then device complexity is reduced, but image quality degrades due to inadequate noise compensation
Solution Approach 1:
The patent applies parameter changes by storing multiple sets of offset correction values, each optimized for a specific gain level. When the gain level changes, the system switches to the corresponding correction set, effectively scaling the compensation to match the current gain conditions. This approach maintains image quality without requiring complex real-time scaling calculations.
Data Source
AI summary
An image sensor includes multiple photoactive pixels and multiple dark reference pixels arranged in rows and columns to form a pixel array. A dark signal is read out of one or more dark reference pixels in each column and used to determine a column offset for one or more columns in the pixel array. Each time an image or frame of an image is read out, the column offset for the one or more columns is updated using dark signals read out from a given number of dark reference pixels. The column offset for the one or more columns is scaled when a gain level is changed for a captured image.