  We manufactured pulsed illuminators emitting in the far infrared for the
Planck-HFI bolometric instrument ground calibrations. Specific measurements
have been conducted on these light sources, based on Carbon fibers, to
understand and predict their properties. We present a modelisation of the
temperature dependence of the thermal conductivity and the calorific
capacitance of the fibers. A comparison between simulations and bolometer data
is given, that shows the coherence of our model. Their small time constants,
their stability and their emission spectrum pointing in the submm range make
these illuminators a very usefull tool for calibrating FIR instruments.
