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

Black-body radiators, generally consisting of a nearly isothermal cavity with a surface of high intrinsic emissivity, are used in radiation thermometry for the transfer of a temperature scale to a radiation thermometer from either a contact thermometer or another radiation thermometer. The emissivity of these black bodies is typically greater than 0.99, and is usually estimated by calculation. In the commonly used 7 mum to 14 mum band, several factors 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 normal-hemispherical reflectance, and hence emissivity, of a cavity radiator over this wavelength region. An uncertainty of 0.0002 (k = 2) in emissivity is achieved, and experimentally measured and calculated emissivities are presented for the cavities used at the National Measurement Laboratory (NML, Australia) for radiation thermometer calibration. An assessment is also made of the reflectance of an "ice-cavity" radiator.