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.

[1]  H Steingold,et al.  Backscatter effects in active night vision systems. , 1969, Applied optics.

[2]  J. L. Forand,et al.  Range-gated underwater laser imaging system , 1993 .

[3]  Paul F. McManamon,et al.  Review of ladar: a historic, yet emerging, sensor technology with rich phenomenology , 2012 .

[4]  Martin Laurenzis,et al.  Homogeneous and speckle-free laser illumination for range-gated imaging and active polarimetry , 2012 .

[5]  Frank Christnacher,et al.  A 3D Outdoor Scene Scanner Based on a Night-Vision Range-Gated Active Imaging System , 2006, Third International Symposium on 3D Data Processing, Visualization, and Transmission (3DPVT'06).

[6]  Martin Laurenzis,et al.  Beam shaping of laser diode stacks for compact and efficient illumination devices at the French-German Research Institute of Saint-Louis , 2014 .

[7]  Gary W. Kamerman,et al.  Laser radar: from early history to new trends , 2010, Security + Defence.

[8]  Pierre Mathieu,et al.  Using a Laser Underwater Camera Image Enhancer for Mine Warfare Applications: What is Gained? , 2002 .

[9]  Lester F. Gillespie,et al.  Apparent Illuminance as a Function of Range in Gated, Laser Night-Viewing Systems , 1966 .

[10]  Martin Laurenzis,et al.  Investigation of range-gated imaging in scattering environments , 2012 .

[11]  H. V. Hulst.  Light Scattering by Small Particles , 1957 .

[12]  Jens Busck,et al.  Underwater 3-D optical imaging with a gated viewing laser radar , 2005 .

[13]  P. Andersson.  Long-range three-dimensional imaging using range-gated laser radar images , 2006 .

[14]  A. Andersson,et al.  Experimental evaluation of underwater range-gated viewing in natural waters , 2006, SPIE Security + Defence.

[15]  Martin Laurenzis,et al.  Influence of gating and of the gate shape on the penetration capacity of range-gated active imaging in scattering environments. , 2015, Optics express.

[16]  Martin Laurenzis,et al.  Laser gated viewing at ISL for vision through smoke, active polarimetry, and 3D imaging in NIR and SWIR wavelength bands , 2013 .

[17]  C. Tropea,et al.  Light Scattering from Small Particles , 2003 .