Passive remote smoke plume opacity sensing: a technique

A new passive technique for the remote measurement of the visual opacity of plumes emerging from smokestacks is presented. It is based on the detection of the attenuation of the polarized component of Rayleigh-scattered background skylight as it traverses a smoke plume. A two-color difference measurement of the polarization of skylight through the plume is compared with a similar measurement of the unattenuated skylight adjacent to the plume. This method is independent of the intrinsic brightness of the plume resulting from the scattering of sunlight and cancels white-light polarization effects of plume-scattered radiation. A prototype instrument incorporating this method has been successfully field tested on an EPA smoke school generator and a power plant stack plume.