A toner particle is composed of submicrometer carbon black particles that are suspended in a polymeric resin. In flash fusing, the energy distribution has a broad wavelength distribution. Therefore the variation of the effective complex refractive indices with wavelength is required to determine the absorption coefficient. An approximation, performed by means of Maxwell Garnett theory, is used to estimate the indices. The spectral geometric optics approach is employed to determine the internal distribution of radiant absorption. The results indicate that the absorbed energy increases dramatically near 2 wt% carbon black content. In the region beyond the sharp increase, the absorbed energy plateaus. In the range of the sharp increase, considerable energy penetrates to the back of the particle. For the higher carbon weight ratios, most of the energy is absorbed just below the irradiated surface. For less than 2.5 wt%, the absorbed energy seems to be uniformly distributed. For fixing a toner particle, a large temperature increase at the interface between the toner particle and the paper is required. From this point of view, both uniformity and high percentage absorbed energy are desired. Both requirements can be satisfied by carbon content near 2.5 wt%, which is proposed as the optimum value.