CMOS Image Sensor Black Level Compensation for Shading
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Solution Overview
Problem
CMOS image sensors face issues with spatial distortion and non-uniform dark current, leading to vertical and horizontal shading in images, particularly in rolling shutter mode and frame exposure, which affects image quality and requires frequent calibration.
Innovation Solution
A two-dimensional black level compensation pixel array design with opaque shields at the top and bottom rows and corner sub-arrays to capture and adjust for dark current variations, allowing for real-time correction of image signals to reduce shading effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If rolling shutter mode is used to read out image signals, then temporal dark noise is reduced through correlated double sampling, but spatial distortion occurs due to time offset between different rows
Solution Approach 1:
The patent applies preliminary action by performing correlated double sampling before the rolling shutter readout process. Dark reference signals are captured and stored in advance, then used to compensate for dark current variations during the actual image readout. This pre-prepared reference data enables noise reduction without compromising spatial accuracy, as the compensation is applied uniformly across all rows regardless of their temporal offset.
2Manufacturing precision
If global shutter mode is used to expose all pixels simultaneously, then spatial distortion is eliminated, but temporal dark noise increases due to simultaneous exposure of all rows
Solution Approach 1:
The patent applies segmentation by dividing the pixel array into multiple rows that are read out sequentially rather than simultaneously. Each row is processed independently through the correlated double sampling process, allowing dark current compensation to be applied row-by-row. This segmentation enables the system to maintain spatial accuracy like global shutter while reducing temporal dark noise through the sequential processing and reference signal subtraction.
3Measurement precision
If dark current estimation is performed to level correct for dark current variations, then black level compensation is achieved, but image quality degrades due to non-uniform dark current across pixels
Solution Approach 1:
The patent applies local quality by implementing per-pixel or per-row dark current estimation and compensation rather than a uniform correction across the entire array. Each pixel or row has its own dark reference signal stored in a dedicated register, allowing the compensation to be tailored to the specific dark current characteristics of each location. This localized approach maintains image uniformity while achieving accurate black level compensation.
4Manufacturing precision
If frequent calibration is performed to correct for non-uniform dark current, then image quality is maintained, but productivity decreases due to calibration overhead
Solution Approach 1:
The patent applies preliminary action by performing dark current calibration and storing reference signals in dedicated registers during manufacturing or initial setup. These pre-stored reference values are then reused during normal operation without requiring frequent recalibration. The system maintains image quality by applying these pre-computed compensation values through the correlated double sampling process, eliminating the need for repeated calibration cycles and thus improving productivity.
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 compensates for dark current variations across the image sensor array, improving image fidelity by reducing vertical and horizontal shading, enabling accurate black level calibration and maintaining image quality across different exposure modes.
Implementation Method 1
The imaging lens focuses light onto the image sensor to form an image, and the image sensor converts the light into electrical signals
Data Source
AI summary
An image sensor has an image sensor array and circuit design employing a method of black level compensation to address image shading related to global exposure image capture and rolling row by row readout schemes. An image sensor including the invented black level compensation pixel array and method may be incorporated within a digital camera.