Surface-integral formulation for electromagnetic scattering in spheroidal coordinates

We derive surface-integral expressions for the Q matrices in spheroidal coordinates that allow us to compute the T matrix in spheroidal coordinates. This approach combines the advantages of the null-field method (also referred to as the extended boundary condition method) with those of the separation of variables method. For spheroidal particles we obtain explicit Q matrix expressions that display the expected symmetry properties and yield correct results in the spherical limit. Compared to surface-integral expressions for spheroids in spherical coordinates, our results are considerably simpler because the integrands do not contain radial functions.