CMOS Image Sensor Digital Frame Transfer Smear Reduction
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Solution Overview
Problem
CMOS image sensors suffer from smear and kT/C noise, and motion artifacts due to limitations in rolling shutter operation and the need for mechanical shutters in high-speed imaging applications.
Innovation Solution
A CMOS image sensor with integrated high-speed digital frame transfer and frame processing, utilizing a rolling shutter method and digital memory, which allows for fast image transfer and minimizes smear and kT/C noise by using a digital frame transfer imager with a frame memory connected via control and data buses, enabling parallel processing and correlated double sampling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a mechanical shutter is used to control exposure, then exposure control is achieved, but the device cannot operate at high frame rates and becomes unreliable
Solution Approach 1:
The patent replaces mechanical shutters with electronic shuttering mechanisms implemented through pixel-level control circuits. Each pixel or pixel group incorporates transfer gates and storage regions that can be electronically controlled to accumulate and transfer charge, enabling high-speed operation without moving parts. This substitution of mechanical systems with electronic control achieves both high reliability and high frame rates.
2Object-affected harmful factors
If a frame transfer CCD sensor is used to reduce smear, then smear is substantially reduced, but the sensor area is doubled and cost increases
Solution Approach 1:
The patent divides the pixel array into distinct regions with different functions: photoactive pixels for light detection, transfer regions for charge movement, and storage regions for temporary charge holding. This segmentation allows each region to be optimized for its specific function, reducing the need for redundant structures while effectively preventing smear through controlled charge transfer.
Solution Approach 2:
The patent applies different structural characteristics to different regions of the sensor. Photoactive regions have high light sensitivity, transfer regions have efficient charge transport properties, and storage regions have charge confinement characteristics. This local optimization reduces overall sensor area while maintaining smear reduction performance.
3Ease of operation
If vertical registers are used to accumulate photocharge, then image capture is enabled, but vertical streaks appear in the output image
Solution Approach 1:
The patent implements rapid charge transfer from photoactive pixels to storage regions immediately after the exposure period ends, before readout begins. This preliminary transfer action prevents charge accumulation in the vertical transfer registers during the readout process, thereby eliminating the vertical streaks that would otherwise appear in the output image.
4Object-affected harmful factors
If photodiodes are separated from vertical registers with masking to reduce smear, then smear is reduced, but complete masking is extremely difficult and some smear remains
Solution Approach 1:
The patent replaces optical masking with electronic control mechanisms. Instead of using physical metal masks to prevent light from reaching vertical registers, the patent uses electronically controlled transfer gates and timing sequences to prevent charge accumulation in transfer regions. This electronic approach achieves complete smear prevention without the complexity and limitations of physical masking.
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 reduces smear and kT/C noise, minimizes motion artifacts, and eliminates the need for mechanical shutters, enabling high-speed, high-resolution imaging with negligible distortion.
Implementation Method 1
Photocharge accumulates in photosensors (typically photodiodes) within vertical registers 12
Data Source
AI summary
A digital frame transfer imager having an image sensor and frame memory in the same chip. The image sensor has an integrated memory controller for controlling transfers of data between the sensor and the memory array. The imager utilizes a rolling shutter and multiple groups of analog-to-digital processing circuitry to readout data from the sensor and to output digital images substantially free from image smear, kT/C noise and other unwanted image artifacts.