Substrate effects on two-dimensional laser beam backscattering

Experimental measurements of backscatterin g from media with optical depths ranging from thin to thick and composed of anisotropic scattering particles exposed to a laser beam are presented. The laser beam is incident normal to the first surface of a scattering medium whose substrate is a diffuse reflector. Measurements are made for two substrates (white - nearly 100% reflecting, and black - nearly 100% absorbing) to study the effect of substrate reflection on backscatter. Latex particles of uniform size (diameter = 0.18 /on) are used as scattering centers in a water solution to create the scattering medium. Results are presented for backscattered radiation in the normal direction as a function of optical radius from the laser beam, optical thickness of the scattering medium, and the character of the substrate for albedos near one. It is shown that the character of the substrate can be important for optical depths up to 20. The white reflecting substrate always produces greater backscattered radiation than the black absorbing substrate when effects of the substrate are detectable. A correlation is developed that predicts the onset of substrate effects on the backscattered radiation as a function of optical radius and optical depth.