The local field effect on the spontaneous emission rates of emitters embedded in a crystal dielectric is studied by microscopic numerical simulations. The ratio of the local radiation field to the macroscopic radiation field is calculated microscopically for the cases of doping a single luminescent ion and the embedding of a nanoparticle with a luminescent ion in the crystal. The results are compared with the two well-known macroscopic models, the real-cavity and virtual-cavity models. (c) 2007 Elsevier B.V. All rights reserved.