IMPROVED T-MATRIX COMPUTATIONS FOR LARGE, NONABSORBING AND WEAKLY ABSORBING NONSPHERICAL PARTICLES AND COMPARISON WITH GEOMETRICAL-OPTICS APPROXIMATION

We show that the use of a matrix inversion scheme based oil a special lower triangular-upper triangular factorization rather than on the sta ndard Gaussian elimination significantly improves the numerical stabil ity of T-matrix computations for nonabsorbing and weakly absorbing non spherical particles. As a result, the maximum convergent size paramete r for particles with small or zero absorption can increase by a factor of several and can exceed 100. We describe an improved scheme for eva luating Clebsch-Gordon coefficients with large quantum numbers, which allowed us to extend the analytical orientational averaging method dev eloped by Mishchenko [J. Opt. Sec. Am. A 8, 871 (1991)] to larger size parameters. Comparisons of T-matrix and geometrical optics computatio ns for large, randomly oriented spheroids and finite circular cylinder s show that the applicability range of the ray-tracing approximation d epends on the imaginary part of the refractive index and is different for different elements of the scattering matrix. (C) 1997 Optical Soci ety of America.