A NEW METHOD TO CALCULATE THE EXTINCTION PROPERTIES OF IRREGULARLY SHAPED PARTICLES

A new method to calculate the extinction of light by irregularly shape d particles, the generalized semi-analytical (GSA) method, is describe d. It allows a better approximation of the shape of a particle by inte rpolating explicitly the polarization field sampled at various points within the particle. The performance of the GSA method is compared to that of the discrete dipole approximation (DDA) method and of two othe r methods which solve for the surface polarization field in the zero-f requency limit: the Fuchs method, and the semi-analytical (SA) method. In that limit various test cases considered include a single sphere, a single spheroid, a pair of touching spheres, a single cube, and a si ngle short cylinder. The GSA method yields an improved accuracy compar ed to the other methods for moderate optical constants and shows less variability as a function of grid resolution than the DDA method. It i s also found that the DDA method tends to systematically overestimate the extinction, especially when the incident wave polarization vector is perpendicular to the major axis of elongated structures. In additio n, for finite size parameters, the GSA method and DDA method are compa red to Mie spheres and spheroids. Again, the DDA method is shown to ov erestimate the extinction systematically, whereas the GSA method yield s better accuracy for small size parameters. The likely sources of thi s discrepancy are discussed.