  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.
