The results of comparison between laboratory measurements and theoretical calculations of transmittance within the 15 mu m band of CO//2 are presented and analysed for a wide region of temperature (from 213 to 313 K), pressure and absorber amount. The calculations are performed by a direct method using contemporary data on parameters of fine structure of the band. Possible reasons of discrepancy between theory and experiment and the temperature dependence of the transmittance are analysed and methods of its approximative consideration by the solution of the inversion problem are discussed in detail.