Pixel Read Method for CMOS Image Sensors
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Solution Overview
Problem
CMOS image sensors face challenges in discriminating brightness differences at high light intensities due to saturation discharge, limiting their sensitivity dynamics and requiring complex control methods with multiple integration periods to improve dynamic range.
Innovation Solution
Implementing a method with at least two integration periods, including one or more integration sub-periods, where photogenerated charges beyond a threshold are drained off, and output values are determined by accounting for charges at the end of each period and sub-period, using a circuit with a transistor connecting the photodiode cathode to a sense node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single integration period is used for pixel readout, then the circuit complexity is low, but the sensitivity dynamics and dynamic range are limited due to saturation discharge at high light intensities
Solution Approach 1:
The integration period is segmented into multiple sub-periods (first integration sub-period and second integration sub-period) with different durations. This segmentation allows the pixel to capture light at different integration times, enabling discrimination of both low and high light intensities within a single readout cycle, thereby improving sensitivity dynamics without requiring multiple separate readout operations
Solution Approach 2:
The patent implements dynamic control of the integration process by selectively activating different integration sub-periods based on the detected light intensity. The first integration sub-period has a longer duration for low light conditions, while the second integration sub-period has a shorter duration for high light conditions, creating a dynamic adaptation mechanism that optimizes measurement range
2Reliability
If multiple integration periods are implemented to improve dynamic range, then the sensitivity dynamics improve, but the memory requirements for processing pixel signals increase
Solution Approach 1:
The patent merges the results of multiple integration sub-periods into a single pixel output value through mathematical combination. By combining the charge measurements from the first and second integration sub-periods using a defined calculation method, the system achieves extended dynamic range while requiring minimal memory storage, as only the combined result needs to be stored rather than separate data from multiple full integration periods
3Measurement precision
If the photodiode integrates light for a long period to improve low light sensitivity, then the measurement precision for low light improves, but saturation occurs at high light intensities before the integration period ends
Solution Approach 1:
The integration period is divided into segments with different durations, allowing the system to use a longer integration sub-period for low light measurements and a shorter integration sub-period for high light measurements, preventing saturation while maintaining sensitivity
Solution Approach 2:
The patent changes the integration time parameter dynamically by selecting different integration sub-period durations based on the detected light intensity level, optimizing the measurement precision for each lighting condition while avoiding saturation effects
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 enhances the sensitivity dynamics of CMOS image sensors by improving luminosity level discrimination and reducing memory requirements for processing pixel signals, allowing for a wider dynamic range and efficient image reconstruction.
Implementation Method 1
Each pixel essentially comprises a photodiode used in reverse mode, having its junction capacitance discharged by a photocurrent according to a received light intensity
Data Source
AI summary
A method for reading a pixel, including at least two integration periods, at least one of said periods including at least one integration sub-period, wherein an output value of the pixel is determined by taking into account the amounts of photogenerated charges contained in the pixel at the end of each of said periods and the amount of photogenerated charges stored in a photodiode of the pixel beyond a threshold during said at least one sub-period.