The back heat shield of the Exomars descent module is equipped with three COMARS+ packs located on a radial line . Each COMARS+ pack contains a heat flux sensor, a pressure gauge and two CNES infrared radiometers called ICOTOM. Each ICOTOM radiometer collects light from the back entry plasma in a 17° half-angle cone. Each pair is composed of a radiometer operating in the range 4.2-5 µm (B1) and a radiometer operating in the range 2,6-3.3 µm (B2). Some of the radiometers designed for the Exomars missions were not used for the mission on board but in laboratory wind tunnels in order to help to characterize the emission of CO2 plasmas and to better understand the Exomars mission measurements. Calibration experiments were carried out on non-flying ICOTOM in order to use them in plasma wind tunnels and to study their measuring behaviour with their own temperature. Embedded in a subsonic inductively-coupled plasma wind tunnel, the radiometers were exposed to CO2 plasma at various pressures and global specific enthalpies. Manufacturer's data were also post-processed in order to obtain direct flux density information from the ICOTOM electrical signal and to make ready the reception of the in-flight measurements. The experiments carried out with a blackbody-like electrical furnace as infrared source showed that the ICOTOM signals decrease with their own temperature and that effect is especially significant for the B1 band. The experiments carried out on the wind tunnel SOUPLIN allowed to expose the ICOTOM to CO2 plasmas with various pressure and global specific enthalpy conditions. Pressures were included in the range 1.2-12 kPa whereas global specific enthalpies were included in the range 5.5-13 MJ/kg. The radiometers were placed in a cooled holder, perpendicularly to the plasma jet, on the border of it. The results obtained show a significant increase of the ICOTOM signals with pressure especially in the lower part of the pressure range. The signals also increase with the global specific enthalpy but very slightly, following the opposite effect of a higher temperature and a higher CO2 dissociation. The ratio B1/B2 strongly decreases with pressure while it slightly decreases with global specific enthalpy. The signals and the ratio stabilize at higher pressures. Some calculations were carried out with CDSD4000 and HITEMP databases at equilibrium in order to compare the measured ratio with reference conditions.