This article describes the properties of the Carbon fibers that were used during the ground calibration of the High Frequency Instrument of the Planck satellite. It focuses on the properties of this new device used as radiation sources, and on the modelling of its thermal behaviour. Experimental data are presented and successfully compared with the proposed theory. Their small time constant, their stability and their emission spectrum pointing in the submm range make these fibers a very usefull tool for characterizing FIR instruments.