The double-heatpipe black body: a radiance and irradiance standard for accurate infrared calibrations in remote sensing

Infrared remote sensing has become an important tool for studies of Earth r esources, the environment and related issues. It is now well recognized tha t measurements in this field have to be accurately traceable to SI radiomet ric units in order to be comparable world-wide and reliable in the long ter m - in particular if the measurements are to detect small changes of key qu antities. To provide a highly stable standard source of infrared radiation, the Physikalisch-Technische Bundesanstalt (PTB), Germany, has investigated the uncertainties of its double-heatpipe large-area black body for radiome tric calibrations in the wavelength range from 1 mu m to 50 mu m. The spect ral emission of the black body can be traced either to the International Te mperature Scale ITS-90 or to the radiation thermometry cryogenic radiometer , with small uncertainties of about 50 mK at a black-body temperature of ab out 1000 degrees C. The total relative standard uncertainty of the black bo dy used as an irradiance standard in the infrared is less than 10(-3) for w avelengths between 1 mu m and 15 mu m and at most a few times 10(-3) for wa velengths up to 50 mu m. With a more sophisticated treatment of diffraction corrections and cryogenic shielding of background radiation, the uncertain ties can be reduced to a few parts in 10(4) for high-precision applications .