Thermal Infrared Transfer Radiometer (TXR)

For verifications of radiance scales in the thermal-infrared spectral region, where near-room-temperature extended-area blackbody sources are used for calibrations, NIST developed the Thermal-infrared Transfer Radiometer (TXR). The TXR is a portable radiometer with two channels, one at 5 µm and the other at 10 µm. It uses a photovoltaic InSb detector for the 5-µm channel, and a photovoltaic mercury cadmium telluride (MCT) detector for the 10-µm channel. The detectors, filters, and reflective optics are built into a liquid-nitrogen cryostat, and the entire radiometer is vacuum/cryogenic compatible and designed to be deployed inside of the typical space-simulating vacuum chambers used for space-flight instrument calibrations. The TXR has a self-contained vacuum jacket and liquid-nitrogen reservoir, and so can be also used in a typical laboratory environment with the outer case of the cryostat at ambient pressure and room temperature. This allows it to be characterized and calibrated by a full array of infrared facilities in the Optical Technology Division at NIST, much of it in an ambient laboratory environment. It can be used for radiance scale verifications for blackbody sources either in vacuum chambers, or in ambient condition laboratories.

The need to test the TXR in a space-simulating chamber led to the development of the Medium Background Infrared (MBIR) facility at NIST. The MBIR facility includes the cryogenic blackbody having a 10.8 cm diameter black-coated cavity and a temperature range 180 K to 350 K for supplying extended area radiance to the TXR during testing.

The uncertainty for radiance measurements using the TXR is of the order of 0.2 % or better. The total uncertainty depends not only on the transfer uncertainty but also on the uncertainty of the primary radiance scale used to calibrate it at NIST. NIST is currently improving several methods for calibrating the TXR. One method uses either the ambient background water bath blackbody and/or the MBIR Large Area Blackbody for the absolute calibration. Another is a system-level approach using a laser-illuminated integrating sphere at the NIST IR-SIRCUS facility. The TXR has been successfully deployed about six different times during the past several years to several different aerospace calibration facilites, where it performed in-situ measurements of various sources in a space-simulating chambers. These measurements were used to verify the infrared radiance scales in currently used by several NASA, NOAA, DOE, and DOD satellite programs. The TXR is maintained at NIST when it is not on a deployment.

TXR Cryostat in MBIR chamber.

References

The NIST EOS thermal-infrared transfer radiometer.
J.P. Rice and B. Carol Johnson,
Metrologia 35, 505-509 (1998).