SPAD Sensor Color Imaging via Temporal Filtering
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Solution Overview
Problem
Conventional CMOS and CCD image sensors suffer from high read noise and motion blur, especially under low light conditions, which affects pass-through imaging and other operations in mixed-reality systems.
Innovation Solution
The use of single photon avalanche diode (SPAD) image sensors, which detect single photons and operate in a gated manner to reduce read noise, combined with techniques such as temporal filtering and dark current compensation, to improve image quality under low light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional CMOS or CCD image sensors are used for imaging under low light conditions, then the system can capture images, but the read noise dominates the detected signal and the signal-to-noise ratio decreases
Solution Approach 1:
The patent changes the operating parameters of the image sensor by using SPAD technology that operates in photon-counting mode with avalanche multiplication. This fundamental parameter change allows the sensor to detect single photons and amplify the signal through avalanche multiplication, thereby maintaining high signal-to-noise ratio even under low light conditions where conventional sensors fail.
Solution Approach 2:
The patent replaces the conventional analog charge accumulation and readout mechanism of CMOS/CCD sensors with a digital photon-counting mechanism using SPADs. Each SPAD pixel independently detects and counts individual photons, converting the analog imaging process into a digital counting process that is inherently more resistant to read noise.
2Productivity
If conventional image sensors operate at high frame rates, then productivity is improved, but motion blur increases and read noise dominates the signal
Solution Approach 1:
The patent implements continuous photon counting at high frame rates using SPAD pixels that can rapidly reset and count photons in successive time gates. The continuous operation of multiple SPAD pixels in parallel maintains high frame rates while the photon-counting mechanism ensures that read noise does not accumulate, preserving image quality even at elevated productivity levels.
Solution Approach 2:
The patent uses periodic time-gated photon counting where SPAD pixels are activated in successive time intervals to capture images at high frame rates. Each gate period is optimized to capture photons while minimizing dark current accumulation, and the periodic resetting of pixels prevents noise buildup, enabling high-frame-rate imaging without motion blur or read noise dominance.
3Measurement precision
If temporal filtering is applied to enhance image quality, then measurement precision is improved, but loss of time occurs due to processing multiple frames
Solution Approach 1:
The patent performs dark current compensation as a preliminary action by capturing a dark current image frame (with the sensor covered) and subtracting it from subsequent image frames. This preliminary compensation removes the dominant source of noise before temporal filtering is applied, thereby improving image quality while minimizing the time penalty since the dark current frame can be captured once and reused for multiple subsequent frames.
Solution Approach 2:
The patent implements temporal filtering that uses feedback from previously captured image frames to enhance the current frame. By comparing and combining information from multiple frames, the system reinforces true signal while suppressing random noise, improving measurement precision. The feedback mechanism is optimized to process only the necessary temporal information, minimizing time loss.
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
SPAD sensors enable high-frame-rate imaging without read noise dominance, and techniques like temporal filtering and dark current compensation further enhance image quality, reducing noise and improving signal-to-noise ratio.
Implementation Method 1
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
Implementation Method 2
the electrons may become stored in per-pixel capacitors, and the charge stored in the capacitors may be read out to provide image data
Data Source
AI summary
A system for obtaining color imagery using SPADs includes a SPAD array that has a plurality of SPAD pixels. Each of the plurality of SPAD pixels includes a respective color filter positioned thereover. The system is configurable to capture an image frame using the SPAD array and generate a 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, after performing the temporal filtering operation, generate a color image by demosaicing the filtered image.