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Far infrared(FIR) data on the optical properties of graphite are presentlylacking. An important step towards filling this gap was taken by Kuzmenko etal. (2008) who measured, on HOPG (Highly Oriented Pyrolitic Graphite) at normalincidence and from 10 to 300 K, the in-plane dielectric functions from 0.3 to200 mum, and the reflectance between 0.3 and about 300 mum. We show here how,using recent developments of the electron theory of graphene, extended tographite, it is possible to properly extrapolate the data farther even than1000 mum, in effect all the way to Direct Current. The plasma frequency as wellas the scattering rate of free electrons are shown to both decrease with T, butlevel off near 0 K, in agreement with theory. Along the way, we noticedsignificant discrepancies with the well-known and often used derivation ofPhilipp (1977) at room temperature, and also with previous data on temperaturedependence and absorbance of graphitic material samples in different physicalforms. Possible reasons for these discrepancies are discussed. Finally, theabsorption efficiency of small graphitic spheres is deduced for the spectralrange from 0.3 to 10000 mum. This may contribute to the discussion on modeldust candidates for recently observed astronomical far infrared emissions.

Some optical properties of graphite from IR to millimetric wavelengths