Cryogenic radiometer facility of the CENAM and first international comparison

A new modern cryogenic radiometer facility has recently been installed at the Centro National de Metrologia (CENAM), Queretaro, Mexico. The radiometer head is a CryoRad II (CRI Inc., Boston, USA). In a specially designed laboratory with clean-room atmosphere, the facility is now ready for calibrations of detectors with power-stabilized laser radiation. This cryogenic radiometer facility will be the basis for most of the radiometric and photometric calibrations of the CENAM. A first radiometric comparison between the CENAM and the Physikalisch-Technische Bundesanstalt (PTB), Germany, was carried out with trap detectors at the He-Ne laser wavelength of 632.8 nm. Two reflection trap detectors, each with three photodiodes, were used: one with 10 mm x 10 mm and the other with 18 mm x 18 mm Si photodiodes. The results were very encouraging. At power levels of about 100 muW the calibrations showed a relative difference of 7 x 10(-5) or even less. The PTB calibrations are based on a modified LaseRad II (CRI) radiometer that has been compared with cryogenic radiometers of other types and from other national metrology institutes.