Measurement and calculation of the emissivity of a high-temperature black body

The emissivity of a high-temperature black body, used for the realization o f the spectral irradiance scale at the Physikalisch-Technische Bundesanstal t (PTB), has been examined experimentally by direct reflection measurements . Measurements at two laser wavelengths show a significant dependence on te mperature of the normal effective emissivity of the cavity. The emissivity is very close to the ideal value of unity at temperatures above 2000 K. Pre liminary experiments on the reflectance and emissivity of graphite, the mat erial from which the black-body cavity is made, were carried out at room te mperature. Using the results of these measurements as input parameters for Monte Carlo calculations of the effective emissivity of the cavity, good ag reement with the measurements is obtained.