Performance assessment of various imaging sensors in fog

All systems operating in the visible and infrared bands of the spectrum are subject to a severe performance degradation when used in adverse weather conditions like fog, snow or rain. This is particularly true for active systems as rangefinders, laser designator, lidars and active imaging sensors where the laser beam will suffer attenuation, turbulence and scattering from the aerosols present in the atmospheric path. This paper presents the ALBEDOS active imaging performance in fog which was determined by observing reference targets through a 22-m controlled-environmental chamber, where fogs with various densities and droplet sizes were generated in a calibrated manner. ALBEDOS is an acronym for Airborne Laser-Based Enhanced Detection and Observation System and is based on a compact, powerful laser diode illuminator and a range-gated intensified CCD camera. It is capable of detecting and identifying people or objects in complete darkness and, to some extent, in adverse weather conditions. In this paper, we compare the efficiency of the range-gated active imager in fog with those of a far-infrared thermal imager and of a low-light level camera operating in a continuous mode.