A prototype blackbody radiation source has been developed which incorporates a double cone cavity of extremely uniform surface temperature. This uniformity of surface temperature is the result of a unique method of supplying heat to the cavity. The principle of operation is based on heat pipe techniques which transfer heat by change of phase and mass transfer. The cavity is heated by serving as the condenser section of the heat pipe which is inherently a nearly isothermal device. Optical tests show that the isothermal cavity has a lambertian distribution over an inclusive angle of better than 60° while operating between 419°C and 760°C.
F. E. Bliss, S. Davis, and B. Stein, "Isothermal Cavity, Blackbody Radiation Source," Appl. Opt. 9, 2023-2027 (1970)