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Natural and man-made obscurants like fog, cloud, smoke and dust are an impediment to the conduct of military operations, preventing effective pilotage, denying the ability to carry out surveillance and reconnaissance, and restricting situational awareness. Additionally, there is a growing interest in the ability to penetrate haze and fog for the safe navigation of autonomous vehicle applications.
There are several electro-optic technologies that offer improved ability to image through obscurants [1,2]. In this study the authors assessed 4 different active imaging technologies in the presence of an artificial smoke, and obtained 3D imagery of targets at ranges of 100m out to 1400m. The four systems tested were:
• a scanned time-correlated single photon counting (TCSPC) sensor using a InGaAs/InP single-photon avalanche diode (SPAD) detector operating at ~ 1.55 µm [2];
• a 32 32 InGaAs/InP SPAD array using TCSPC at ~ 1.55 µm;
• a coherent frequency modulated continuous wave (FMCW) scanned lidar system ~ 1.55 µm , [1];
• a CMOS SPAD array camera operating as a time gated imager operating at ~ 670nm.
The selection of sensors enables comparisons to be drawn between scanning and staring systems and direct detection and coherent detection, and between short-wave infrared and visible wavelengths.
Three-dimensional structured targets were placed at ranges of 100 – 150m and smoke was introduced between the targets and the sensors. The smoke transmission was measured with a separate laser device to correlate the imagery with the level of attenuation presented by the smoke and thereby relate the image quality to the degree of optical loss in the system. For the coherent lidar system, long range 3D images were obtained out to a distance of 1400m, and imaging through smoke of a target at 900m was achieved. Under the test conditions at least 2 of the systems have demonstrated the ability to obtain images through greater than 4 attenuation lengths of obscurant between transceiver and target, and work is progressing on image processing approaches to reconstruct images at greater levels of loss.
Imagery from the systems will be presented, the relative merits of the different techniques discussed, and the prospects for future practical systems will be explored.
[1] “Demonstration of frequency modulated continuous wave (FMCW) eye-safe, coherent LIDAR to See Through Clouds”, M.Silver, P.Feneyrou, L.Leviander, A.Martin and J Parsons, Optro, Jan 2018.
[2] “Depth imaging through obscurants using time-correlated single-photon counting”, R.Tobin, A.Halimi, A.McCarthy, M.Laurenzis, F.Christnacher and G.S.Buller, SPIE Vol 10659, April 2018
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