To get a high optical quality beam from a High Power Laser (HPL), optical distorsions must be minimized. For a CW CO₂ HPL, the key parameter is absorptance. Generally, High Reflective (HR) coated mirrors exhibit absorptance less than one percent. A photothermal measurement method of absorptance (Mirage Effect) seems to be adequate to reach the required resolution. In this paper we present a method of absolute measurement of HR coatings absorptance at 10.6 microns. The measurement key point is calibration. The linearity of the deflection angle versus absorptance predicted by the theory led us to calibrate our experimental set up with a strongly absorbing sample which could be measured by reflection. (scattering neglected). The first step is to verify the signal linearity versus absorbed power and its evolution as a function of chopping frequency in order to neglect convective transfers. Measurements performed on different HR coated mirrors are presented.