Photodetector Persistence Mitigation via Light Saturation and Reference Subtraction
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Solution Overview
Problem
Photodetectors, such as CMOS and CCD image sensors, suffer from image persistence, which interferes with the quality of subsequent images by producing electrical signals from photons received during previous exposures, affecting precision in applications like astronomical surveys and medical imaging.
Innovation Solution
A method involving initialization and calibration steps where the image sensor is saturated with light to exceed its charge holding capacity, followed by resetting and capturing a black scene image to create a low noise reference image, which is then subtracted from subsequent image data to reduce persistence effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the image sensor is saturated with light to exceed its charge holding capacity, then persistence signals are reduced, but the device complexity and processing time increase due to required calibration steps
Solution Approach 1:
The patent applies preliminary action by performing saturation and calibration of the image sensor before actual imaging operations. The sensor is saturated with light to fill all charge traps, then a reference image is captured and stored for later subtraction. This preliminary preparation eliminates persistence artifacts in subsequent images without requiring complex real-time processing during actual imaging.
Solution Approach 2:
The patent uses copying by creating a reference image that replicates the persistence signal characteristics. This reference image is captured after saturation and serves as a template that can be subtracted from subsequent images. The copying approach allows the persistence signal to be modeled and removed without directly measuring it in each image, simplifying the overall process.
2Measurement precision
If multiple images are averaged to create a low noise reference image, then measurement precision improves, but the time required for calibration increases
Solution Approach 1:
The patent applies partial action by capturing a limited number of images (e.g., 5-10 images) rather than requiring extensive averaging. This provides sufficient reference accuracy for most applications while keeping calibration time reasonable. The number of images can be adjusted based on specific application requirements, allowing optimization between precision and time.
3Manufacturing precision
If the persistence signal is removed by subtraction, then image quality improves, but the device complexity increases due to additional processing steps
Solution Approach 1:
The patent applies the extraction principle by separating the persistence signal from the actual image data through subtraction. The reference image containing only persistence artifacts is subtracted from subsequent images, extracting and removing the unwanted persistence component. This leaves a clean image with only the actual scene information, improving image quality through a straightforward mathematical operation.
4Ease of operation
If the image sensor is reset after saturation, then the sensor is prepared for normal operation, but persistence artifacts remain in subsequent images
Solution Approach 1:
The patent converts the harmful persistence effect into a beneficial reference signal. By saturating the sensor and capturing the resulting persistence signal as a reference image, the harmful artifact becomes a useful template for removal. The persistence signal that would normally degrade image quality is instead captured and subtracted from subsequent images, transforming it from a problem into a solution.
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 significantly reduces persistence signals, resulting in images that are substantially free of persistence artifacts, enhancing the accuracy and reliability of photodetector measurements.
Implementation Method 1
A typical photo detector converts incident light into electrical charge. The amount of charge produced during an exposure period is generally proportional to the number of photons received by the photo detector during the exposure period.
Implementation Method 2
A possible process to explain the origins of persistence is the capture of charge by traps exposed to carriers as the depletion width shrinks when photo-generated charge accumulates on the electrically isolated photodiode. After the depletion width is reestablished by resetting the diode, trapped charge is slowly released, appearing to be a signal during a subsequent exposure.
Data Source
AI summary
Image sensor systems and methods that mitigate the effects of electronic image sensor persistence are provided. The mitigation of persistence is achieved by flooding the pixels of the image sensor with light, saturating all of the pixels, prior to the collection of desired image data. Moreover, after saturating the pixels, but prior to collecting the desired image, the image sensor is reset and operated to take an image of a black scene. Due to persistence from the prior saturation step, the image of the black scene appears as a gray image. The image sensor is then optionally reset and is again saturated. After another reset operation, a desired image is obtained. The gray image is then subtracted from the desired image, to substantially correct for the effects of persistence in that image.