PTB primary standard for optical radiant power: transfer-optimized facility in the clean-room centre

The new clean-room centre of the Physikalisch-Technische Bundesanstalt (PTB) provides the best environment for measurements of the highest accuracy. One part is specifically designed for radiometric measurements. It contains the PTB primary standard facility for radiant power. This facility is developed at the PTB by optimizing the combination of various components, one of which is a side-viewing small-sized cryogenic radiometer. The handy size and the movable stage of this radiometer allow the transfer of the scale from the primary standard to secondary standards with minimal errors. A combination of a He-Ne laser, an Ar + laser and a Ti:sapphire laser covers the spectral range 360 nm to 1090 nm. The new facility is described and a new method of transmittance measurement of the Brewster-angle window without disassembly of the window is presented.