Time domain analysis of photon scattering and Huygens-Fresnel back projection.

Stand-off detection and characterization of scattering media such as fog and aerosols is an important task in environmental monitoring and related applications. We present, for the first time, a stand-off characterization of sprayed water fog in the time domain. Using a time correlated single photon counting, we measure transient signatures of photons reflected off a target within the fog volume. We can distinguish ballistic from scattered photon. By application of a forward propagation model, we reconstruct the scattered photon paths and determine the fog's mean scattering length µscat. in a range of 1.55 m to 1.86m. Moreover, in a second analysis, we project the recorded transients back to reconstruct the scene using virtual Huygens-Fresnel wavefronts. While in medium-density fog some contribution of ballistic remain in the signatures, we could demonstrate that in high-density fog, all recorded photons are at least scattered a single time. This work may path the way to novel characterization tools of and enhanced imaging in scattering media.

Three-dimensional single-photon imaging through obscurants.

We investigate the depth imaging of objects through various densities of different obscurants (water fog, glycol-based vapor, and incendiary smoke) using a time-correlated single-photon detection system which had an operating wavelength of 1550 nm and an average optical output power of approximately 1.5 mW. It consisted of a monostatic scanning transceiver unit used in conjunction with a picosecond laser source and an individual Peltier-cooled InGaAs/InP single-photon avalanche diode (SPAD) detector. We acquired depth and intensity data of targets imaged through distances of up to 24 meters for the different obscurants. We compare several statistical algorithms which reconstruct both the depth and intensity images for short data acquisition times, including very low signal returns in the photon-starved regime.

Influence of gating and of the gate shape on the penetration capacity of range-gated active imaging in scattering environments.

Range-gated active imaging is a well-known technique used for night vision or for vision enhancement in scattering environments. A lot of papers have been published, in which the performance enhancement of range gating has been demonstrated. However, there are no studies which systematically investigate and quantify the real gain brought by range gating, in comparison with a classical imaging system, in controlled smoke densities. In this paper, a systematic investigation of the performance enhancement of range-gated viewing is presented in comparison with a color camera representing the human vision. The influence of range gating and of the gate shape is studied. We have been able to demonstrate that a short-wave infrared (SWIR) range-gated active imaging system can enhance by a factor of 6.9 the penetration depth in dense smoke. On the other hand, we have shown that the combination of a short pulse with a short integration time gives better contrasted images in dense scattering media.

Quantification of the atmospheric scintillation for laser illumination in active imaging.

Most of the analytical scintillation models used by experts to simulate the illumination performances of active imaging systems are based on the use of monochromatic, punctual, and coherent sources. These analytical models seem pessimistic regarding lightpipe-based illumination techniques. Outdoor trials have been made with 1.57 µm laser illuminators with and without lightpipe to record illumination maps and associated refractive index structure parameter C(n)2 with a propagation distance of 1 km. Analysis shows a reduction of the scintillation by a factor of 2.5 comparing analytical models and laser illumination with lightpipe.

Long-range three-dimensional active imaging with superresolution depth mapping.

We present a technique to overcome the depth resolution limitation for 3D active imaging. Applying microsecond laser pulses and sensor gate width, a scene of several hundred meters is illuminated and recorded in a single image. The trapezoid-shaped range intensity profile is analyzed to obtain both the reflectivity and the depth of scene. We demonstrate a 3D scene reconstruction in a depth of 650 to 1550 m from only three images with an accuracy of <30 m. This depth accuracy is 10 times better than estimated from the classical resolution limit obtained for depth scanning active imaging with a similar number of images. Therefore, this technique enables superresolution depth mapping with a reduction of image data processing.