Light scattering calculations exploring sensitivity of depolarization ratio to shape changes. I. Single spores in air

Calculations of the depolarization ratio, D(Theta, lambda) = 1 - (S22)/(S11), for light scattered from an ensemble or cloud of single aerosolized spores in air were studied using the discrete dipole approximation (DDA), sometimes also called the coupled-dipole approximation. Here S(ij) is the appropriate Mueller matrix element for scattering angle Theta and wavelength lambda. The effect of modest shape changes on D(Theta, lambda) was determined. The shapes compared were prolate ellipsoids versus right circular cylinders joined smoothly to end caps consisting of hemispheres of the same diameter as the cylinder. Using the same models, the graphs of (S34)/(S11) versus angle were compared with those for D(Theta, lambda). The latter shows sensitivity to length in some cases we examined, while (S34)/(S11) does not. Size parameters and optical constants suggested by measurements of Bacillus cereus endospores were used. An ensemble of spores was modeled with prolate spheroids. The results of this model were compared with results of a model using the same size and optical parameters, but for capped cylinders. The two models produced distinguishably different results for the same parameters. In calculations for all the graphs shown, averaging over random orientations was performed. Averaging over size distributions similar to those from experimental measurements was performed where indicated. The results show that measurements of D(Theta, lambda) could be quite useful in characterizing the shape of particles in an unknown aerosol and for distinguishing between two likely shapes, but not to reconstruct the shapes from the graphs alone without additional information.