A simple technique for measuring the infrared emissivity of black-body radiators

Black-body radiators, generally consisting of a nearly isothermal cavity wi th a surface of high intrinsic emissivity, are used in radiation thermometr y for the transfer of a temperature scale to a radiation thermometer from e ither a contact thermometer or another radiation thermometer. The emissivit y of these black bodies is typically greater than 0.99, and is usually esti mated by calculation. In the commonly used 7 mum to 14 mum band, several fa ctors conspire to make this calculation less satisfactory, and a technique is required to confirm the calculation by direct measurement of the cavity emissivity. A simple technique is presented for determining the average nor mal-hemispherical reflectance, and hence emissivity, of a cavity radiator o ver this wavelength region. An uncertainty of 0.0002 (k = 2) in emissivity is achieved, and experimentally measured and calculated emissivities are pr esented for the cavities used at the National Measurement Laboratory (NML, Australia) for radiation thermometer calibration. An assessment is also mad e of the reflectance of an "ice-cavity" radiator.