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 resources, the environment and related issues. It is now well recognized that measurements in this field have to be accurately traceable to SI radiometric units in order to be comparable world-wide and reliable in the long term - in particular if the measurements are to detect small changes of key quantities. 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 radiometric calibrations in the wavelength range from 1 mu m to 50 mu m. The spectral emission of the black body can be traced either to the International Temperature Scale ITS-90 or to the radiation thermometry cryogenic radiometer, with small uncertainties of about 50 mK at a black-body temperature of about 1000 degrees C. The total relative standard uncertainty of the black body used as an irradiance standard in the infrared is less than 10(-3) for wavelengths between 1 mu m and 15 mu m and at most a few times 10(-3) for wavelengths up to 50 mu m. With a more sophisticated treatment of diffraction corrections and cryogenic shielding of background radiation, the uncertainties can be reduced to a few parts in 10(4) for high-precision applications.