Using a new, high-temperature black-body radiator developed by the All-Russian Institute for Optophysical Measurements (VNIIOFI), Moscow, and applying newly developed methods and equipment, the black-body-based spectral irradiance scale of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, has been extended and its uncertainties have been reduced compared with previous results. The pyrolytic-graphite Planckian radiator BB3200pg, which can be operated at temperatures up to over 3300 K, is presented and characterized along with modifications to optimize uniformity and stability. The determination of the radiometric black-body temperatures is already well established by measuring the weighted irradiance using filter detectors without imaging systems. This method is compared and verified with results based on self-consistent spectral irradiance measurements using fast-scanning spectroradiometers. In addition, a laser-based method for the determination of the radiometric temperature is presented and compared with conventional procedures of radiationtemperature measurement. The implementation of this additional method into the radiometric chain of the PTB is illustrated. Improvements in the uncertainty budget of the spectral irradiance scale are discussed.

High-temperature black bodies used as primary standards of spectral irradiance have to meet several requirements. One important demand is that the spectral irradiance of the black body is uniquely determined by Planck's radiation law, where the (radiometric) temperature of the black body is the only parameter that determines its relative spectral distribution. By combining determinations of the radiometric temperature of a black body over a wide temperature range with the corresponding spectral measurements, black-body spectra for different temperatures can be compared. This allows the use of self-consistency checks to ascertain whether (i) the temperature determinations are coherent over a wide temperature range; and (ii) the different spectral distributions show any deviation from Planck's law. Using this method of self-consistent calibrations, new absorption bands in the spectrum of a black body were discovered at very high temperatures. The level of absorption increases with temperature and depends on the operating conditions and imperfections of the black-body system. Possible origins of this effect are discussed and modifications proposed to avoid such absorption bands.

Filter radiometers were used in a bilateral comparison of the radiometric temperatures of a hightemperature black body from 2100 K to 3100 K. The measurements took place over a ten-day period in October 1998 at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) using filter radiometers from the PTB and from the National Institute of Standards and Technology (NIST, Gaithersburg, USA). The comparison revealed that the radiometric temperatures determined using the NIST and the PTB filter radiometers differed progressively from 2.4 K at 2200 K to 5.1 K at 3200 K, with the NIST temperature values always lower than the PTB values. Differences in the measurements of absolute spectral responsivity were the main cause of the observed temperature differences.

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

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