Fractional-readout Image Sensor Subframe Readouts
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Solution Overview
Problem
Integrated-circuit image sensors face challenges in maintaining dynamic range and low-light sensitivity due to saturation issues and excessive noise in bright and low-light conditions, respectively, when using conventional full-readout methods.
Innovation Solution
The implementation of a fractional-readout image sensor that reads the pixel array multiple times per frame interval, utilizing subframe exposures and readouts to manage photocharge accumulation, allowing for both fractional and full readouts, thereby reducing noise and maintaining sensitivity across varying light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If full-readout method is used, then all photocharge is read out per frame interval, but saturation occurs in bright light conditions and excessive noise occurs in low-light conditions
Solution Approach 1:
The pixel array is divided into multiple subarrays that are read out at different times within a frame interval. Each subarray is exposed for a different duration, with earlier subarrays having shorter exposure times and later subarrays having longer exposure times. This segmentation allows the sensor to capture both bright and dark regions without saturation or excessive noise by distributing the photocharge accumulation across multiple readout operations.
2Measurement precision
If multiple readouts per frame interval are performed, then noise is reduced and dynamic range is enhanced, but device complexity increases
Solution Approach 1:
The pixel array is divided into multiple subarrays that can be independently read out. This segmentation allows the complex task of multiple readouts to be distributed across simpler, identical readout circuits for each subarray, making the overall system more manageable while achieving the noise reduction benefits of multiple exposures.
Solution Approach 2:
The sensor performs periodic readouts of different subarrays within each frame interval. This periodic action allows the system to achieve multiple exposures and noise reduction without requiring all pixels to be read out simultaneously, thereby reducing the instantaneous complexity of the readout circuitry while maintaining the benefits of multiple readouts.
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 dynamic range and low-light sensitivity of the image sensor by minimizing readout noise and avoiding saturation, while maintaining effective exposure intervals, resulting in improved signal-to-noise ratio (SNR) and image quality.
Implementation Method 1
each pixel element comprising a photodiode switchably coupled to a floating diffusion node... a first subframe exposure interval of a first polarity... a second subframe exposure interval of a second polarity
Data Source
AI summary
Signals representative of total photocharge integrated within respective image-sensor pixels are read out of the pixels after a first exposure interval that constitutes a first fraction of a frame interval. Signals in excess of a threshold level are read out of the pixels after an ensuing second exposure interval that constitutes a second fraction of the frame interval, leaving residual photocharge within the pixels. After a third exposure interval that constitutes a third fraction of the frame interval, signals representative of a combination of at least the residual photocharge and photocharge integrated within the pixels during the third exposure interval are read out of the pixels.