Observational consequences of fine structure line optical depths on   infrared spectral diagnostics
  It has long been known that infrared fine structure lines of abundant ions, like the [O III] 88 micron line, can become optically thick in H II regions under certain high luminosity conditions. This could mitigate their potential as diagnostic tools, especially if the source is too dusty for optical spectroscopy to otherwise determine the system's parameters. We examined a series of photoionization calculations which were designed to push the nebulae into the limit where many IR lines should be quite optically thick. We find that radiative transfer effects do not significantly change the observed emission line spectrum. This is due to a combination of grain absorption of the hydrogen ionizing continuum and the fact that the correction for stimulated emission in these lines is large. Given these results, and the likelihood that real objects have non-thermal line broadening, it seems unlikely that line optical depth presents a problem in using these lines as diagnostics of the physical conditions or chemical composition. 
Nicholas Abel, Adam Bryant, Prabodh Dhakal, Ashley Gale, Alva Gibson,   William Goddard, Chad Howard, Ameya Kolarkar, Pey Li...