Photon-Flooded Single-Photon 3D Cameras

Abstract

Single-photon avalanche diodes (SPADs) are starting to play a pivotal role in the development of photon-efficient, long-range LiDAR systems.However, due to nonlinearities in their image formation model, a high photon flux (e.g., due to strong sunlight) leads to distortion of the incident temporal waveform, and potentially, large depth errors. Operating SPADs in low flux regimes can mitigate these distortions, but, often requires attenuating the signal and thus, results in low signal-to-noise ratio. In this paper, we address the following basic question: what is the optimal photon flux that a SPAD-based LiDAR should be operated in? We derive a closed form expression for the optimal flux, which is quasi-depth-invariant, and depends on the ambient light strength. The optimal flux is lower than what a SPAD typically measures in real world scenarios, but surprisingly, considerably higher than what is conventionally suggested for avoiding distortions. We propose a simple, adaptive approach for achieving the optimal flux by attenuating incident flux based on an estimate of ambient light strength. Using extensive simulations and a hardware prototype, we show that the optimal flux criterion holds for several depth estimators, under a wide range of illumination conditions. †

Asynchronous Single-Photon 3D Imaging

“…On the other hand, returning signal photons from the true laser peak add up "coherently" because their bin location remains fixed. Asynchronous acquisition with uniform shifting achieves lower error than synchronous acquisition with no and extreme attenuation [13], over a wide range of flux conditions. As a result, the accumulated histogram has enough photons in all bins ( Fig.…”

“…Photon pileup mitigation for SPAD cameras: Perhaps the most widely adopted approach for preventing pileup is attenuation, i.e., optically blocking the total photon flux incident on the SPAD so that only 1-5% of the laser pulses lead to a photon detection [2,3]. 1 Recent work [14,13] has shown that this rule-of-thumb extreme attenuation is too conservative and the optimal operating flux is considerably higher. Various computational [25,14] and hardware [1,3,35] techniques for mitigating pileup have also been proposed.…”

“…,N B ) of the first photon arrival times is constructed over many cycles, where N i denotes the number of times the first photon arrives in the i th bin. In low ambient light, the histogram, on average, is simply a scaled version of the incident waveform [13], from which, depth can be estimated by locating its peak. Effect of ambient light in synchronous acquisition: Under ambient light, the incident flux waveform can be modeled as an impulse with a constant d.c. offset, as shown in the top of Fig.…”

“…The first result (based on [13]) shows that if a shifting strategy can achieve a constant expected denominator sequence, it will have lower depth error than all other shifting strategies (including synchronous acquisition). The second result shows that there exists a shifting strategy that achieves a constant expected denominator: uniform shifting.…”

“…Recent work [13,14] has shown that there is an optimal incident flux level at which pileup in a synchronous SPAD-based 3D camera is minimized while maintaining high SNR. This optimal flux can be achieved by optically attenuating the incident photon flux.…”