Influence of the obscurants and the illumination wavelengths on a range-gated active imaging system performance

Range-gated active imaging is a prominent technique for night vision, remote sensing or vision through obstacles (fog, smoke, camouflage netting.). Indeed, by means of the "range gating" or the "time gating" technique, it is possible to eliminate the backscattering effects during the propagation of the illuminating light through scattering environments such as rain, snow, fog, mist, haze or smoke. The elimination of the backscattering effects leads to a significant increase in the vision range in harsh environments. Surprisingly, even if a lot of authors estimate that range-gated imaging brings a gain when used in scattering environments, there are no studies which systematically investigate and quantify the real gain provided in comparison with classical imaging systems in different controlled obscurant densities. We put in evidence that the penetration depth improvement can drastically vary with the type of obscurant and with the illumination wavelength. For example, it can be improved by more than a factor of 10 for specific smokes to only a factor of 1.5 for water droplet based fog. In this paper, we thoroughly examined the performance enhancement of laser range gating in comparison with a color camera representing the human vision. On the one hand, we studied the influence of the different types of obscurants and showed that they lead to very different results. On the other hand, we examined the influence of the illumination wavelength. As the global attenuation of an obscurant is the sum of its absorption and its diffusion, we also report on some experimental results in which we tried to separate the influence of each of these two parameters. To demonstrate the influence of the absorption by maintaining the diffusion constant, we worked with the same type of smoke, but with different colors. To work with different levels of diffusion, we maintained the particle material constant and worked with different particle diameters.