Blackbodies are the appropriate tools for IR sensors calibration and test. A well-known property of these objects is their emissivity factor equals 1 while their transmission and reflection factors equal 0. Though some high emissive coatings with emissivity higher than 0.99 are now available on the market, a residual reflectivity factor always remains. The first part of this paper demonstrates the influence of the reflectivity factor on the radiated energy of a blackbody especially for blackbodies radiating at temperatures close to or below the ambient temperature. It happens that the difference between this radiated temperature, or apparent temperature, and the measured temperature may be of several tenths of degrees! Such a difference leads to great uncertainty in the calibration procedure of thermal sensors. The case of sensors tested and calibrated into climatic chambers for outdoor applications is particularly critical. The usual method to compensate this difference is to take emissivity and consequently reflectivity factor into the calculation of the theoretical irradiance received by the sensor. This calculation requires a live knowledge of the ambient temperature. While this may not always be the case, calculating the true irradiance i.e. the apparent temperature radiated by the reference source remains a complex calculation for major users of blackbodies. Indeed, they expect their blackbody source to be reliable and an actual reference source whatever the conditions of use. The second part of this paper presents a reminder about this calibration method of the absolute temperature of IR reference source and the correction method when ambient temperature doesn't change. The third part describes the integrated compensation method of the ambient temperature into the new controller of HGH's blackbody sources making these sources actual IR reference sources whatever the operating conditions.