The PTB high-accuracy spectral responsivity scale in the ultraviolet

The Physikalisch-Technische Bundesanstalt (PTB) has established spectral re sponsivity standards for photodetectors in the wavelength range between 200 nm and 400 nm based on cryogenic electrical-substitution radiometers. In o rder to obtain standards over a continuous wavelength range with low uncert ainty, combined use is made of both the dispersed synchrotron radiation of the storage ring BESSY I and quasi-monochromatic laser radiation. Taking ad vantage of the specific properties of the monochromatized synchrotron radia tion and the laser radiation, relative standard uncertainties between 10(-3 ) and 5 x 10(-3) are achieved in the calibration of trap detectors in the s pectral range 400 nm to 200 nm.