Aerosol and vapor detection via infrared laser reflectance from common surfaces: all that shimmers is not gold

With their high spectral brightness, modern infrared lasers have enabled new methods for chemical trace detection, including from standoff distances. In this paper, we explore the possibility of using IR lasers or similar light sources as reflected from myriad “surfaces of opportunity” to achieve detection of aerosolized or gaseous effluents at standoff distances. Since mirrored reflectors cannot be deployed in many circumstances, we investigate how much (infrared) light is reflected from certain common surfaces so as to ultimately detect an intervening aerosol or vapor species. The amount of light, having either specular or diffuse components, reflected to the sensor determines the utility of the surface for spectroscopic applications. Such practical surfaces of opportunities (SOOs) were used as part of a research program developing optical methods for aerosol detection. The tested materials were both natural and anthropogenic in origin, including surfaces of both high- and low-reflectivity, e.g., aluminum plates, wallboard, siding, car body panels, marble, wood, painted surfaces, brick, etc., Both the total (diffuse + specular) and diffuse-only reflectance spectra from 7500−600cm −1 (1.33−16.7µm) were measured in the laboratory to investigate the possibility of the materials’ use as SOOs for standoff detection. The non-porous metallic surfaces, e.g., aluminum, traffic signs (back or non-coated side), etc., were, of course, very reflective across the infrared. For the non-metallic surfaces, reflectivities were mostly large in the shortwave, moderate in the midwave, and moderate to weak, e.g., 2% to 15%, in the longwave infrared. A quantum-cascade laser (QCL) spectrometer system for standoff detection, along with an aerosol chamber, were both developed as test beds. Using reflected IR laser light from different SOOs, we report for the first time the use of non-metallic surfaces for aerosol and vapor detection at standoff distances of up to 11 m.