Measures air temperature by comparing air radiance in the carbon dioxide 4.3 µm band with a reference blackbody at near-ambient temperature. It is generally unaffected by wetting caused by passes through wet clouds so the main use is in clouds and transitions between clouds and clear air when the Rosemount sensors are affected by wetting or evaporation.
The calibration is a 3 component linear least-squares fit to the aircraft reference temperature ATX. The fit coefficients consist of the detector signal, the blackbody temperature, and an internal optics bench temperature.