Abstract

A semiempirical approximation to the extinction efficiency Q_ext for randomly oriented spheroids, based on an extension of the anomalous diffraction formula, is given and compared to the extended boundary condition method or T-matrix method. Using this formula, Q_ext can be evaluated over 10⁴ times faster than by previous methods. This approximation has been verified for complex refractive indices m = n − ik, where 1.01 ≤ n ≤ 2.00 and 0 ≤ k ≤ 1 and aspect ratios from 0.5 to 4. We believe the approximation is uniformly valid over all size parameters and aspect ratios. It has the correct Rayleigh and large particle asymptotic behavior. The accuracy and limitations to this formula are extensively discussed.

© 1991 Optical Society of America

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