SPAD Dark Current Compensation for Low-Light Color Imaging
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Solution Overview
Problem
Conventional CMOS and CCD image sensors suffer from high read noise and motion blur under low light conditions, affecting image quality and other operations in mixed-reality systems, while SPAD sensors face challenges with dark current noise and color image acquisition under low light.
Innovation Solution
Implement a SPAD array with dark current compensation techniques, including temporal filtering and subtraction of dark current images, and perform filtering operations on raw image data before demosaicing to reduce noise in SPAD imagery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS or CCD image sensors are used for low light imaging, then the device complexity is low and ease of manufacture is high, but the signal-to-noise ratio deteriorates due to high read noise dominating the detected signal
Solution Approach 1:
The patent changes the operating parameters of the image sensor by using SPAD pixels operated in Geiger mode with variable integration times. The system adjusts the integration time parameter to optimize the signal-to-noise ratio for different lighting conditions, allowing the sensor to detect single photons while managing dark current through controlled integration periods.
Solution Approach 2:
The patent implements dynamic adjustment of integration time based on ambient light conditions. The system dynamically selects between different integration time settings to adapt to varying scene brightness, enabling optimal performance across different lighting environments while maintaining low noise characteristics.
2Shape
If the frame rate is increased to reduce motion blur in low light imaging, then the image quality improves, but the read noise increases and dominates the detected signal
Solution Approach 1:
The patent changes the integration time parameter to optimize the balance between motion blur and read noise. By adjusting the integration time setting, the system can capture sufficient photons to maintain signal-to-noise ratio while limiting the temporal duration that contributes to motion blur, achieving optimal image quality under varying frame rates.
3Reliability
If SPAD sensors are used to reduce read noise and improve signal-to-noise ratio under low light conditions, then the signal-to-noise ratio improves, but dark current noise is introduced and affects image quality
Solution Approach 1:
The patent applies preliminary action by capturing a dark current reference image before the actual scene capture. This dark current image, obtained by blocking light from reaching the sensor, is then subtracted from the scene image to remove dark current noise. This preliminary measurement and subtraction process eliminates the harmful dark current effect while preserving the single photon detection capability.
Solution Approach 2:
The patent extracts the dark current component from the total signal by separating it into a distinct measurement. By capturing an image with blocked light paths, the system isolates the dark current contribution and removes it from the final scene image through subtraction, effectively eliminating this harmful factor.
4Measurement precision
If SPAD sensors operate in Geiger mode to detect single photons, then the sensitivity improves, but the dynamic range deteriorates due to binary output limiting intensity information
Solution Approach 1:
The patent uses preliminary action by capturing multiple images at different integration times or using a series of exponentially varying integration times. By combining these multiple measurements, the system reconstructs intensity information that would otherwise be lost in binary output, effectively expanding the dynamic range while maintaining single photon detection sensitivity.
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
Improves signal-to-noise ratio and reduces motion blur and dark current noise in SPAD imagery, enabling high-framerate image capture and enhanced color image quality under low light conditions.
Implementation Method 1
single photon avalanche diode (SPAD) pixels that detect photons and generate an electrical signal in response to a detected photon
Implementation Method 2
single photon avalanche diode (SPAD) - a SPAD is operated at a bias voltage that enables the SPAD to detect a single photon. Upon detecting a single photon, an electron-hole pair is formed, and the electron is accelerated across a high electric field, causing avalanche multiplication
Implementation Method 3
performing a temporal filtering operation on the image data to generate filtered image data
Implementation Method 4
obtaining dark current image data indicating that one or more SPAD pixels of the plurality of SPAD pixels have detected an avalanche event without detecting a corresponding photon
Data Source
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AI summary
A system for dark current compensation in SPAD imagery is configurable to capture an image frame with the SPAD array and generate a temporally filtered image by performing a temporal filtering operation using the image frame and at least one preceding image frame. The at least one preceding image frame is captured by the SPAD array at a timepoint that temporally precedes a timepoint associated with the image frame. The system is also configurable to obtain a dark current image frame. The dark current image frame includes data indicating one or more SPAD pixels of the plurality of SPAD pixels that detect an avalanche event without detecting a corresponding photon. The system is also configurable to generate a dark current compensated image by performing a subtraction operation on the temporally filtered image or the image frame based on the dark current image frame.