At the PTB radiometric scales are compared which are based on electrodynamics (electron storage ring BESSY), on thermodynamics (black body of known temperature T90 of the International Temperature Scale) and on a thermal detector in combination with the electrical substitution principle (cryogenic radiometer). The uncertainty contributions of the different input parameters for establishing the independent radiometric scales are analysed in detail. According to this analysis spectral radiant powers in the visible are realized or expected to be realized (black body) with uncertainties (1 σ level) of 0,10% (BESSY), of 0,07% (black body, λ>650 nm) and of 0,007% (cryogenic radiometer). Filter radiometers calibrated at the three independent primary standards are used as transfer instruments for the comparison. The measured values of the responsivities of the filter radiometers are estimated to be uncertain by 0,11% (BESSY), 0,08% (black body, λ>650 nm), and 0,03% (cryogenic radiometer). Preliminary experimental results of the comparison between BESSY and the cryogenic radiometer are in agreement with the uncertainty analysis.

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The radiant intensity of VUV emission lines of a high-current hollow-cathode source has been determined for the 40-125-nm spectral range. The source is operated at a constant current of 1 A with an aluminum cathode. Different rare gases are alternatively used as the buffer gas at pressures of ~100 Pa. The radiant intensity has been determined by comparison with the calculable spectral radiant flux of the electron storage ring BESSY. Radiant intensities of the emission lines are in the 7-1400-µW/sr range. The long-term reproducibility of the radiant intensity of the source is within ±10% (2σ value). The systematic uncertainty of the radiometric calibration is better than 9% (√32σ value).

The prelaunch spectral-sensitivity calibration of the solar spectrometer SUMER (Solar Ultraviolet Measurements of Emitted Radiation) is described. SUMER is part of the payload of the Solar and Heliospheric Observatory (SOHO), which begins its scientific mission in 1996. The instrument consists of a telescope and a spectrometer capable of taking spatially and spectrally highly resolved images of the Sun in a spectral range from 50 to 161 nm. The pointing capabilities, the dynamic range, and the sensitivity of the instrument allow measurements both on the solar disk and above the limb as great as two solar radii. To determine plasma temperatures and densities in the solar atmosphere, the instrument needs an absolute spectral-sensitivity calibration. Here we describe the prelaunch calibration of the full instrument, which utilizes a radiometric transfer-standard source. The transfer standard was based on a high-current hollow-cathode discharge source. It had been calibrated in the laboratory for vacuum UV radiometry of the Physikalisch-Technische Bundesanstalt by use of the calculable spectral photon flux of the Berlin electron storage ring for synchrotron radiation (BESSY)-a primary radiometric source standard.