Radiation forces in the discrete-dipole approximation

The theory of the discrete-dipole approximation (DDA) for light scattering is extended to allow for the calculation of radiation forces on each dipole in the DDA model. Starting with the theory of Draine and Weingartner [Astr ophys. J. 470, 551 (1996)] we derive an expression for the radiation force on each dipole. These ex pressions are reformulated into discrete convoluti ons, allowing for an efficient, O(N logN) evaluation of the forces. The tot al radiation pressure on the particle is obtained by summation of the indiv idual forces. The theory is tested on spherical particles. The resulting ac cumulated radiation, forces are compared with Mic theory. The accuracy is w ithin the order of a few percent, i.e., comparable with that obtained for e xtinction cross sections calculated with the DDA. (C) 2001 Optical Society of America.