CMOS Image Sensor Black Level Compensation for Vertical Shading
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Solution Overview
Problem
CMOS image sensors experience vertical shading due to non-uniform dark current across the pixel array, which can lead to image quality issues such as loss of information in dark areas and signal range sacrifice, especially in frame exposure mode where integration time differs vertically across the array.
Innovation Solution
The implementation of a pixel array design with covered rows at the top and bottom for black level compensation, along with a method to adjust image signal data using a black level compensation algorithm that extrapolates dark current values from these rows to correct for vertical shading, ensuring uniformity across the array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If frame exposure mode is used with simultaneous shutter and integration across the entire pixel array, then integration efficiency is improved, but vertical shading occurs due to integration time difference across rows
Solution Approach 1:
The pixel array is divided into multiple rows, each with independently controllable shutters. This segmentation allows each row to have its integration time individually adjusted, enabling compensation for vertical shading while maintaining overall frame exposure efficiency. The array is split into first, second, and third rows with different integration time characteristics that can be balanced through the compensation method.
Solution Approach 2:
The integration time parameter is varied across different rows of the pixel array. By adjusting the integration time of each row individually (first row has longer integration time, second row has intermediate, third row has shorter), the patent compensates for vertical shading effects. This parameter change allows each row to contribute uniformly to the final image despite different exposure durations.
2Measurement precision
If black level calibration is performed to set threshold for black representation, then black level accuracy is improved, but vertical shading from non-uniform dark current cannot be fully corrected
Solution Approach 1:
The patent performs preliminary measurement of dark current characteristics for each row before actual image capture. By measuring and storing the integration time-dependent dark current values for each row in advance, the system prepares compensation data that will be applied during image processing. This preliminary action enables accurate correction of vertical shading without affecting the main imaging operation.
Solution Approach 2:
The patent implements a feedback mechanism where measured dark current values from each row are used to calculate compensation amounts, which are then applied to correct the final image data. The system continuously monitors and adjusts row-specific compensation based on measured dark current characteristics, creating a closed-loop correction process that effectively eliminates vertical shading.
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 solution effectively reduces vertical shading by accurately compensating for dark current variations, enhancing image contrast and detail in dark regions, thereby improving overall image quality.
Implementation Method 1
The pixel unit uses photodiodes to perform photoelectric conversion to form photo-generated carriers and generate analog signals
Data Source
AI summary
An image sensor array and circuit design employs a method of black level compensation to address vertical image shading related to global exposure image capture and rolling row by row readout schemes. An image sensor having the invented black level compensation pixel array and method may be incorporated within a digital camera.