On the basis of a modified version of the LOWTRAN 6 model, an absolute comparison is made of typical and background-limited IR sensors operating in the 3-5-microm and 8-12-microm wavebands in a tropical maritime environment. Allowance is made for slant paths and a variety of targets and backgrounds including hot targets and backgrounds spectrally different from the targets. It is found that with current detector technology the 8-12-microm waveband is superior for all except very hot targets at long ranges. The validity of various approximations is also investigated, and in particular it is found that the blackbody composite background approximation should not be used for slant paths.