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This paper presents recent progress in the development of a scanning time-of-flight imaging system employing
time-correlated single-photon counting (TCSPC) designed for the acquisition of depth information at kilometre
ranges. The device is capable of acquiring information on non-cooperative target surfaces at eye-safe average
optical power levels in the near-IR regime (<1 mW at 842 nm illumination wavelength). Target illumination
is periodic or non-periodic at typical repetition frequencies in the MHz domain, utilising a sub-ns pulse-width
laser diode. The system output is steered over the optical field of interest, and return photons from the target
are routed towards a single-photon detector. Measurements are performed with a silicon single-photon avalanche
diode (SPAD). Effective optical spatial and spectral filtering techniques permit operation in bright daylight
conditions.
Results in the form of depth images from a variety of targets, taken under various environmental conditions,
are presented. Achieved improvements of this first-generation system are discussed in terms of parametric
enhancement of quantities such as spatial and spectral filtering, internal optical attenuation and beam size.
We detail progress in the design process both based on theoretical assumptions and actual measurements at
distances between few 100's of metres and several km. The trade-offs between acquisition time, maximum range
and excitation laser power levels are discussed and projections made for this and future depth imaging systems.
State-of-the-art TCSPC hardware solutions facilitate the rapid transfer and storage of large quantities
of raw data. This renders possible real-time analysis with speed-optimised algorithms such as fast Fourier
transform-supported cross-correlation methods, as well as gathering additional information about the scene in
post-processing steps, based on approaches such as reversible-jump Markov-chain Monte Carlo (RJMCMC).
This algorithm dynamically adapts the number of degrees of freedom of a range measurement, resulting in
multi-surface resolution and the possible identification of targets obscured by objects such as foliage.
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