Present State of the Comparison between Radiometric Scales Based on Three Primary Standards

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.