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SOLAR/SOLSPEC
(2009)
New PTB Set-up for the absolute calibration of the spectral responsivity of radiation thermometers
(2009)
To meet the increasing demand of calibrations in the field of radiation thermometry, the Physikalisch-Technische Bundesanstalt (PTB) operates two different setups for calibrations covering the temperature range from-60 C up to 3000 C. The first setup consists of several heat-pipe blackbodies in conjunction with platinum resistance thermometers, calibrated according to the International Temperature Scale of 1990 (ITS-90) and is applied for calibrations in the temperature range from-60 C to 961.78 C. In the temperature range from 962 C to 3000 C the calibration is based on high temperature blackbodies. The temperature of these blackbodies is determined in two independent ways: traceable to the ITS-90 gold fixed point or via absolutely calibrated filter radiometers (FR) traceable to the detector primary standard, the cryogenic radiometer. Taking advantage from the high accuracy of the FR calibration, these FRs have also been successfully used for the investigation of the thermodynamic accuracy of the ITS-90 in the range from 419 C to 962 C. For non-contact temperature measurements, the emissivity of the observed object is one of the most important parameters. Therefore, PTB has set up instrumentations for measuring the directional spectral and total emissivity of samples in air from 80 C up to 400 C over a spectral range from 4 µm to 40 µm and under vacuum from 0 C to 600 C and in the spectral range from 1 µm to 1000 µm.
The Physikalisch-Technische Bundesanstalt (PTB) has developed dedicated instrumentations and methods for the traceable calibration of space borne instruments in terms of the three fundamental radiometric units, i.e. spectral radiance (radiation temperature), spectral radiant intensity and spectral photon flux. The traceable calibration under conditions similar to the space environment is achieved by use of two major radiometric calibration facilities of PTB, the Spectral Radiance Comparator Facility (SRCF) and the Reduced Background Calibration Facility (RBCF) which are part of the Primary Temperature Radiator Facility of PTB and cover the wavelength range from the UV to the FIR (THz range). The improved detector instrumentations of the SRCF and RBCF, detailed calibration schemes and results of calibrations for space missions are presented.
The large area double-heatpipe blackbody (LABB) used for comparing thermodynamic temperatures with the International Temperature Scale of 1990 (ITS-90) at temperatures higher than 600 C is based on two sodium heat pipes. The lowest temperature at which the sodium heat pipes work properly is strongly correlated to the onset of sodium evaporation. This is expected for temperatures well above 500 C. Planning to use the LABB for extending the comparison of thermodynamic temperatures with the ITS-90 to temperatures as low as possible, we measured the isothermal behaviour of the blackbody cavity starting at heatpipe temperatures of 400 C. The temperature of the cylindrical part of the cavity was measured with a standard platinum resistance thermometer (SPRT) sensing the bottom of the cavity and up to three quarter of the cylindrical part of the cavity. The obtained temperature profiles are well explained assuming the occurrence of two superimposing effects: a real nonisothermal behaviour of the LABB for temperatures below 500 C and a cooling of the SPRT caused by radiation loss through the aperture of the cavity. Applying a Monte-Carlo simulation we calculated the emissivity of the LABB at different heatpipe temperatures below 600 C using the obtained non-isothermal temperature distribution at the cylindrical walls. It was found that even at temperatures as low as 415 C the accuracy of the LABB is sufficient for comparison of thermodynamic temperatures and the ITS-90.
