Iterative procedure used in DIRTY to achieve a self-consistent radiative transfer solution. Primary emission refers to emission from stars and gas; secondary emission refers to emission from dust. The radiative transfer and dust emission routine are denoted as RT and DE, respectively. The iterated solution procedure is repeated until convergence is achieved (e.g., the direct plus scattered light changes are below a specified threshold), after which we combine escaped primary and secondary emissions to obtain the final SED.