In-Pixel Analog Memory for High Dynamic Range CMOS Imaging
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Solution Overview
Problem
Conventional CMOS imagers face challenges in achieving high dynamic range image outputs due to pixel-to-pixel response variation and the need for variable exposure times, which often compromise spatial resolution or require additional transistors that affect pixel fill factor.
Innovation Solution
The solution involves modifying the CMOS imager pixels to allow selection of multiple exposure times on a per-pixel basis through non-destructive read operations, using additional transfer gates and controlling the storage gate to estimate pixel illumination and apply appropriate integration times, enabling high dynamic range operation without significantly impacting pixel fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional CMOS imager pixels use multiple transistors for high dynamic range operation, then dynamic range is improved, but spatial resolution and fill factor deteriorate due to increased pixel area requirements
Solution Approach 1:
The pixel operation is segmented into multiple exposure phases with different integration times. The method divides the dynamic range requirement into coarse (first integration time) and fine (second integration time) components, allowing the pixel to handle different light levels separately rather than requiring a single complex circuit to handle all ranges simultaneously.
Solution Approach 2:
The first integration phase is performed preliminarily to capture bright regions before the second integration phase captures dark regions. This preliminary action allows the system to establish a baseline exposure that prevents saturation in bright areas, enabling subsequent fine-tuning for dark areas without losing information in bright areas.
2Area of stationary object
If pixel full well capacity is reduced to increase pixel density, then spatial resolution is improved, but the ability to capture high dynamic range deteriorates
Solution Approach 1:
The pixel system dynamically switches between two integration times based on illumination conditions. Rather than being static in its full well capacity utilization, the system adapts its exposure duration dynamically - using a shorter first integration time for bright scenes and a longer second integration time for dark scenes, effectively expanding its dynamic range capability beyond what a single full well capacity would allow.
Solution Approach 2:
The method changes the integration time parameter between two distinct phases. By varying this temporal parameter rather than requiring changes in physical pixel structure or capacity, the system achieves high dynamic range performance with smaller pixels that have reduced full well capacity.
3Measurement precision
If non-destructive read operations are implemented for high dynamic range, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The pixel circuit performs self-measurement of its own illumination level through the non-destructive read operation during the first integration phase. The pixel uses its existing floating diffusion node to store and read back the intermediate charge level, enabling it to autonomously determine whether a second integration phase is needed without requiring external control for each pixel.
Solution Approach 2:
The floating diffusion node serves multiple functions: it acts as the charge storage region during integration, the readout node for non-destructive sensing, and the basis for determining whether second integration is required. This multi-functionality reduces the need for additional dedicated circuitry for each measurement function.
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 allows for high dynamic range imaging with reduced pixel-to-pixel response variation and maintains spatial resolution, enabling image capture beyond the typical linear response range of a pixel, while minimizing the impact on pixel size and fill factor.
Implementation Method 1
a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge
Data Source
AI summary
An imaging device having pixels that store charge from a photosensor under at least one storage gate during a sampling period. A driver used to operate the at least one storage gate, senses how much charge was transferred to the storage gate and sets a flag in an analog memory (e.g., stores a voltage in a floating diffusion region) based on the amount of sensed charge. The sensed charge is used to determine an integration time for the pixel.