Improvements to the German national primary standard of radiant power above 200 nm

The set of cryogenic radiometers in the Physikalisch-Technische Bundesansta lt (PTB) at Braunschweig was recently completed by the addition of a modifi ed LaseRad II radiometer. The advantages of the improved facility for measu rement of optical radiant power are described. The paper outlines the radio metric calibration chain of the PTB, including a cryogenic laser and broadb and radiometer. The link of radiometric units to the International System o f Units (SI) volt and ohm in the PTB is presented. Results of current direc t international comparisons of cryogenic radiometers and an internal compar ison are summarized. The results demonstrate the degree of international eq uivalence at the lowest level of uncertainty available. The international a greement is within 2 x 10(-4).