Journal of Quantitative Spectroscopy & Radiative Transfer 101 (2006) 462–470 Backscatter effects of surfaces composed of dry biological particles Andrey Ovcharenko a , Sergey Bondarenko a , Yuriy Shkuratov a,b , Cathy Scotto c , Charles Merritt c , Matthew Hart c , Jay Eversole c , Gorden Videen d,à a Institute of Astronomy, Kharkov National University, 35 Sumskaya Sreet, Kharkov 61022, Ukraine b Institute of Radio Astronomy of National Academy of Sciences of Ukraine, 4 Chervonopraporna Street, Kharkov, 61002, Ukraine c Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375-5320, USA d Army Research Laboratory, AMSRL-CI-EM, 2800 Powder Mill Road, Adelphi, MD 20783, USA Abstract We present the backscattering of particulate surfaces consisting of dry biological particles using two laboratory photopolarimeters that measure intensity and degree of linear polarization in a phase-angle range 0.2–601. We measure scattering properties from three samples composed of dry biological particles, Bacillus subtilis var. niger (BG) spores and samples of fungi Aspergillus terreus and Sporisorium cruentum spores. We find that the surfaces display a prominent brightness opposition effect and significant negative polarization near backscattering angles. The brightness and polarimetric phase curves are different for B. subtilis and the fungi. Published by Elsevier Ltd. Keywords: Backscattering; Opposition effect; Biological particle 1. Introduction The main goal of this research is to explore experimentally the scattering features of particulate surfaces consisting of dry biological particles. The use of elastic scattering may be used to detect potentially harmful biological particles in an early warning detector system. This manuscript continues our study of scattering properties of biological particles, which we began with DDA modeling [1,2]. Numerous powdered laboratory samples and the regoliths of celestial bodies reveal a prominent brightness surge and a negative polarization branch in the backscatter direction when they are illuminated with unpolarized light. We are especially interested in the degree of linear polarization P ¼ðI ?  I k Þ=ðI ? þ I k Þ, where I ? , and I k are the intensities corresponding to the perpendicular and parallel components of scattered light relative to the scattering plane. The negative polarization minimum may be a useful feature for characterizing samples. The principle mechanisms for this feature we consider to be the coherent backscatter enhancement of multiply scattered rays (e.g., [3–13]) and single-particle scatter [10–13]. ARTICLE IN PRESS www.elsevier.com/locate/jqsrt 0022-4073/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.jqsrt.2006.02.037 à Corresponding author. Tel.: +1 301 394 1871; fax:+1 301 394 4797. E-mail addresses: ovcharenko@astron.kharkov.ua (A. Ovcharenko), gvideen@arl.army.mil (G. Videen).