Abstract
The Physikalisch-Technische Bundesanstalt (PTB) designed a new calibration facility, the Reduced Background Calibration Facility 2 (RBCF2) and brought it recently into operation. It provides traceable calibrations of air born and space based infrared remote sensing experiments in terms of radiance temperature and spectral radiance. Traceable measurements from space require the use of calibrated stable detector systems and/or source-based calibration standards on board of the instrumentation. In any case they should be calibrated under space like conditions to ensure traceability with the smallest possible uncertainty. The RBCF2 enables therefore the calibration of radiators and detectors and cameras under cryogenic and/or vacuum conditions. The integration of the instrument under test into the RBCF2 can be done under ISO 5 clean room conditions.
The general concept of the RBCF2 is to connect different sources in the source chamber and detectors in the detector chamber via a liquid nitrogen cooled beam line. Source and detector chamber also incorporate cooling facilities. Translation units in both chambers enable the RBCF2 to automatically compare and calibrate different sources and detectors with stable comparison instruments at cryogenic ambient temperatures and under a common vacuum. Reference sources for comparisons are dedicated vacuum variable temperature blackbodies, for example the vacuum medium temperature blackbody (VMTBB, 150 °C to 430 °C), the vacuum low temperature blackbody (VLTBB, -173 °C to 177 °C), the large area heatpipe blackbody (LAHBB, -60 °C to 50 °C) featuring a radiating diameter of 250 mm, the liquid nitrogen blackbody (LNBB, -196 °C) and calibrated vacuum integrating sphere radiators for UV-VIS and SWIR applications. The radiation temperatures of the reference blackbodies and the radiance of the integrating sphere radiators are traceable to the ITS-90 via the primary standards of PTB. Using the calibrated vacuum infrared standard radiation thermometer (VIRST) direct calibrations of sources in terms of radiance temperature in the wavelength range from 8 µm to 14 µm can be performed.
For spectrally resolved measurements the radiation of the reference sources and the sources under test is imaged on a vacuum Fourier-Transform Spectrometer (FTS). The FTS covers the wavelength range from 0.4 µm to 1000 µm by employing detectors ranging from photomultipliers to liquid helium cooled bolometers. The different reference blackbodies enable measurements with respect to at least two reference temperatures, simultaneously. Hereby disturbances in the IR by background radiation resulting from inside the FTS can be effectively compensated. Sources can be also spatially mapped and characterized for the lateral distribution of their spectral radiance. The flexible design of the facility also allows large aperture camera characterizations and modifications for customer needs and the measurement of directional spectral emissivities over a wide temperature and wavelength range.
Recent calibrations of the large aperture on-board blackbodies of the airborne GLORIA limbsounder, the on-board calibrations assembly (OBCA) of the EnMAP satellite and of the prototype on-board blackbody for FORUM mission are shown to illustrate the capabilities.
The Reduced Background Calibration Facility 2 for Infrared Detectors, Cameras and Sources
The Physikalisch-Technische Bundesanstalt (PTB) designed a new calibration facility, the Reduced Background Calibration Facility 2 (RBCF2) and brought it recently into operation. It provides traceable calibrations of air born and space based infrared remote sensing experiments in terms of radiance temperature and spectral radiance. Traceable measurements from space require the use of calibrated stable detector systems and/or source-based calibration standards on board of the instrumentation. In any case they should be calibrated under space like conditions to ensure traceability with the smallest possible uncertainty. The RBCF2 enables therefore the calibration of radiators and detectors and cameras under cryogenic and/or vacuum conditions. The integration of the instrument under test into the RBCF2 can be done under ISO 5 clean room conditions.
The general concept of the RBCF2 is to connect different sources in the source chamber and detectors in the detector chamber via a liquid nitrogen cooled beam line. Source and detector chamber also incorporate cooling facilities. Translation units in both chambers enable the RBCF2 to automatically compare and calibrate different sources and detectors with stable comparison instruments at cryogenic ambient temperatures and under a common vacuum. Reference sources for comparisons are dedicated vacuum variable temperature blackbodies, for example the vacuum medium temperature blackbody (VMTBB, 150 °C to 430 °C), the vacuum low temperature blackbody (VLTBB, -173 °C to 177 °C), the large area heatpipe blackbody (LAHBB, -60 °C to 50 °C) featuring a radiating diameter of 250 mm, the liquid nitrogen blackbody (LNBB, -196 °C) and calibrated vacuum integrating sphere radiators for UV-VIS and SWIR applications. The radiation temperatures of the reference blackbodies and the radiance of the integrating sphere radiators are traceable to the ITS-90 via the primary standards of PTB. Using the calibrated vacuum infrared standard radiation thermometer (VIRST) direct calibrations of sources in terms of radiance temperature in the wavelength range from 8 µm to 14 µm can be performed.
For spectrally resolved measurements the radiation of the reference sources and the sources under test is imaged on a vacuum Fourier-Transform Spectrometer (FTS). The FTS covers the wavelength range from 0.4 µm to 1000 µm by employing detectors ranging from photomultipliers to liquid helium cooled bolometers. The different reference blackbodies enable measurements with respect to at least two reference temperatures, simultaneously. Hereby disturbances in the IR by background radiation resulting from inside the FTS can be effectively compensated. Sources can be also spatially mapped and characterized for the lateral distribution of their spectral radiance. The flexible design of the facility also allows large aperture camera characterizations and modifications for customer needs and the measurement of directional spectral emissivities over a wide temperature and wavelength range.
Recent calibrations of the large aperture on-board blackbodies of the airborne GLORIA limbsounder, the on-board calibrations assembly (OBCA) of the EnMAP satellite and of the prototype on-board blackbody for FORUM mission are shown to illustrate the capabilities.