# Asynchronous Single-Photon 3D Imaging

## TL;DR

This paper introduces asynchronous acquisition schemes for single-photon 3D imaging that significantly reduce depth errors caused by ambient light, by averaging out pileup distortions through deterministic or randomized measurement window offsets.

## Contribution

It proposes a novel asynchronous measurement approach and a generalized model to mitigate pileup effects in SPAD-based 3D imaging, improving depth accuracy substantially.

## Key findings

- Depth accuracy improved by up to an order of magnitude.
- Effective in high ambient light conditions.
- Validated through simulations and experiments.

## Abstract

Single-photon avalanche diodes (SPADs) are becoming popular in time-of-flight depth-ranging due to their unique ability to capture individual photons with picosecond timing resolution. However, ambient light (e.g., sunlight) incident on a SPAD-based 3D camera leads to severe non-linear distortions (pileup) in the measured waveform, resulting in large depth errors. We propose asynchronous single-photon 3D imaging, a family of acquisition schemes to mitigate pileup during data acquisition itself. Asynchronous acquisition temporally misaligns SPAD measurement windows and the laser cycles through deterministically predefined or randomized offsets. Our key insight is that pileup distortions can be "averaged out" by choosing a sequence of offsets that span the entire depth range. We develop a generalized image formation model and perform theoretical analysis to explore the space of asynchronous acquisition schemes and design high-performance schemes. Our simulations and experiments demonstrate an improvement in depth accuracy of up to an order of magnitude as compared to the state-of-the-art, across a wide range of imaging scenarios, including those with high ambient flux.