[en] The ability to diagnose the conditions in the fuel of an Inertial Confinement Fusion (ICF) capsule is a fundamental requirement for progress in ICF. Current ICF experiments are concentrating on high convergence capsules, and capsules will be larger on the more powerful lasers that are currently under development. Both of these developments make it harder to diagnose fuel conditions because of the larger column density in the target. This has led to increased interest in the use of line ratios as temperature and density diagnostics. For a line ratio to be a good temperature diagnostic, the ratio must be relatively insensitive to density and vary strongly with temperature. Given the atomic rates, it is relatively easy to find candidate line ratios in the coronal limit where all lines are optically thin. In this paper, the authors consider the effects of optical depth on line ratios. They show that, even if both of the lines used in the ratio are optically thin, the line ratio can depend on optical depth effects if one of the levels involved in the line ratio is pumped by an optically thick line. They present results from several different radiation transport models and attempt to draw conclusions about whether it is possible to calculate the effects of optical depth accurately enough that line ratios can still be used as diagnostics. They also consider the effects of uncertainties in atomic rates on predicted line ratios