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The complexity of biological agents can make it difficult to identify the important factors impacting
scattering characteristics among variables such as size, shape, internal structure and biochemical composition, particle
aggregation, and sample additives. This difficulty is exacerbated by the environmentally interactive nature of
biological organisms. In particular, bacterial spores equilibrate with environmental humidity by absorption/desorption
of water which can affect both the complex refractive index and the size/shape distributions of particles - two factors
upon which scattering characteristics depend critically. Therefore accurate analysis of experimental data for
determination of refractive index must take account of particle water content. First, spectral transmission
measurements to determine visible refractive index done on suspensions of bacterial spores must account for water (or
other solvent) uptake. Second, realistic calculations of aerosol scattering cross sections should consider effects of
atmospheric humidity on particle water content, size and shape. In this work we demonstrate a method for determining
refractive index of bacterial spores bacillus atropheus (BG), bacillus thuringiensis (BT) and bacillus anthracis Sterne
(BAs) which accounts for these effects. Visible index is found from transmission measurements on aqueous and
DMSO suspensions of particles, using an anomalous diffraction approximation. A simplified version of the anomalous
diffraction theory is used to eliminate the need for knowledge of particle size. Results using this approach indicate the
technique can be useful in determining the visible refractive index of particles when size and shape distributions are
not well known but fall within the region of validity of anomalous dispersion theory.
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