Asynchronous SPAD Depth Imaging Mitigating Pile-Up Distortion
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Solution Overview
Problem
SPAD-based LiDAR systems face severe nonlinear distortions and depth errors due to ambient light, particularly in outdoor conditions, caused by the dead time of single-photon avalanche diodes, leading to pile-up effects that compromise imaging precision.
Innovation Solution
The system employs asynchronous single-photon depth imaging techniques by shifting the SPAD acquisition window relative to the laser pulses, allowing detection in later time bins and using computational methods to synchronize photon timing measurements, thereby distributing pile-up effects across all histogram bins and mitigating structured distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If synchronous SPAD-based LiDAR systems are used to achieve high timing resolution and depth precision, then measurement precision is improved, but severe nonlinear distortions and depth errors occur due to ambient light causing pile-up effects
Solution Approach 1:
The patent implements asynchronous detection by dynamically shifting the SPAD acquisition window relative to the laser pulse timing. Instead of fixed synchronous detection, the system varies the detection window position across multiple measurement cycles, allowing photons from different time bins to be detected in different cycles. This dynamic approach distributes the pile-up effect across all histogram bins rather than concentrating it in early bins, thereby maintaining depth precision while operating in ambient light conditions.
2Reliability
If the SPAD dead time is reduced to mitigate pile-up effects, then reliability in ambient light is improved, but the detector's inherent dead time cannot be eliminated and continues to cause detection losses
Solution Approach 1:
The patent converts the harmful pile-up effect into a beneficial distribution mechanism. By intentionally allowing the dead time to cause detection losses in synchronous operation, the system creates a known distortion pattern that can be computationally corrected. The asynchronous approach uses the same physical constraint (dead time) but distributes its effects uniformly across all time bins, transforming a source of severe nonlinear distortion into a manageable factor that preserves the linearity of the image formation model.
3Ease of operation
If conventional LiDAR systems use linear-mode detectors to avoid pile-up effects, then ease of operation is improved, but single-photon sensitivity and timing resolution are lost
Solution Approach 1:
The patent employs periodic modulation of the SPAD detection window relative to the laser pulse train. By cycling through different acquisition window positions across multiple laser periods, the system accumulates photon detections in an asynchronous manner. This periodic shifting allows the detector to sample photons from all time bins over multiple cycles, effectively converting the pulsed operation into a continuous measurement process that maintains linearity while preserving single-photon sensitivity and high timing resolution.
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 improves depth accuracy and precision in ambient light conditions, reducing root-mean-squared error and maintaining high signal-to-noise ratio, even in high ambient light environments.
Implementation Method 1
Detectors that are capable of detecting the arrival time of an individual photon, such as single-photon avalanche diodes (SPADs)
Implementation Method 2
The first photon detection times in each laser cycle can be collected and used to generate a histogram of the time-of-arrival of the photons
Data Source
AI summary
In accordance with some embodiments, systems, methods, and media for asynchronous single photon depth imaging with improved precision in ambient light conditions are provided. In some embodiments, the system comprises: a light source; a detector configured to detect arrival of individual photons, and enter a dead time after a detection; a processor programmed to: cause the light source to emit pulses toward a scene point at the beginning of light source cycles each corresponding to B time bins; cause the detector to enter an acquisition window at a first time bin position; cause the detector to enter another acquisition window at a shifted time bin position; record photon arrival times; associate each photon arrival time with a time bin; and estimate a depth of the scene point based on a number of photon detection events at each time bin, and a denominator corresponding to each time bin.