A vacuum infrared standard radiation thermometer (VIRST) was jointly developed by the Physikalisch-Technische
Bundesanstalt (PTB) and the Raytek GmbH for temperature measurements from -150 °C to 170 °C under vacuum. The
radiation thermometer is a purpose built instrument to be operated with the PTB reduced-background infrared calibration
The instrument is a stand-alone system with an air-tight housing which allows operation inside a vacuum chamber,
attached to a vacuum chamber and on air. The radiation thermometer serves to calibrate thermal radiation sources, i.e.
blackbody radiators, for their radiation temperature by comparison with the vacuum variable-temperature reference
blackbodies inside the RBCF. Furthermore, since it can be operated under vacuum and on air, the instrument also allows
to compare the water- and ammonia-heat-pipe reference blackbodies of the PTB low-temperature calibration facility on
air with the vacuum variable-temperature blackbodies of the RBCF. Finally the instrument shall be used as a transfer
radiation thermometer to carry and compare the temperature scale of PTB by means of radiation thermometry to remote
sensing calibration facilities outside PTB.
The mechanical, optical and electrical design of the instrument are shortly reported. Results on the temperature
resolution, size-of-source effect, long-term stability, and the reference function are given. A comparison of the heat-pipe
blackbodies on air with the vacuum variable-low-temperature blackbody (VLTBB) and a comparison of the VLTBB
with the vacuum variable-medium- temperature blackbody (VMTBB) via VIRST are presented. The long term stability
of the infrared precision radiation thermometer after 17 months is shown.