I present a model devoted to linking the properties of a quasi-three-level laser crystal to microscopic parameters. The role of the radiative diffusion is taken into account with a sample-size-dependent parameter, including the total internal reflections, allowing both radiative and nonradiative diffusions to be treated equally. The radiation self-trapping aspect is quantified with the number ${\eta }_{\mathrm{imp}}^{\prime }$ of imprisoned photons engendered by one absorbed pump photon. The losses toward the traps that are not always well known are described with the dipole–dipole interaction, giving a microscopic interpretation of the heat dispensed in the sample from the pump. One prediction is that the excited-state decay of the donor subsystem is exponential, in agreement with experiment. The main experimental data, which are the excited-state intrinsic and extrinsic lifetimes τ and ${\tau }^{\prime }$ and the extrinsic fractional thermal loading ${\xi }^{\prime }$ , can be calculated by adjusting a small number of parameters. This number can be reduced by use of spectroscopy. I have shown that this model can be applied with a reasonable success to ${\mathrm{Yb}}^{3+}$ -doped ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ .