Measurements of the phase function of natural particles

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The phase functions of several types of natural particles have been measured between 5 and 172 degrees using a polar nephelometer with a Helium-Neon laser as the source of radiation. By cooling the laboratory to the outside temperature and using a vertical duct in the roof, natural snowflakes may be brought into the scattering volume. The phase functions of both falling and blowing snow exhibit the diffraction peak characteristic of particles whose size is considerably greater than the wavelength of the incident radiation as well as significant backscattering. The phase function associated with NaC1 cubes 335±85 micrometers on a side has been studied for various polarizations of both incident and scattered light. Comparison with the phase function calculated for an NaCl sphere with a diameter of 335 micrometers indicates that the cubes scatter more light between the angles of 30 and 150 degrees. This is attributed to external reflections by randomly oriented cubes. Because the phase function is normalized over a 47 solid angle by definition, the diffraction peak must be smaller for cubes than spheres. The effect of surface roughness on the phase function was studied by using particles of volcanic ash. Surface roughness increases the scattering above the Mie theory predictions for angles between 40 and 120 degrees, and also considerably broadens the back scattered peak. © 1981 SPIE.

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Proceedings of SPIE - The International Society for Optical Engineering