The simplified intensity relationships listed below are based on the following assumptions. Firstly, aspect ratios are high enough to permit the infinite solid approximation. This is certainly not true when considering wavefront propagation, but can be used in power flow analysis. Secondly, heat loading is assumed to vary proportionally with absorbed pump power. As seen in the last section, this is a good approximation for neodymium although ytterbium systems can be more complex. Third, fluorescence emissions are approximated as flowing in the same direction as heat. This is a reasonable simplification since spontaneous emission uniformly illuminates the surface of the solid, which is dominated by the largest faces. For most laser configurations fluorescent emission is trapped within the medium and pump chamber where it adds to the waste heat. Highly saturated gain media produce much less spontaneous emission, but the high fluorescence intensities generated require careful consideration for each specific system. Finally, intensities are assumed to be uniformly distributed across the apertures. Uniform intensity is often essential to successful high power operation in order to maintain clean wavefronts. For cases where only a portion of the solid is illuminated, the definition of should be amended to the aspect ratio of the mode volume.