Low Background Infrared Research Articles

Calibration of low-background temperature IR test chambers used to calibrate space sensors

The low-background infrared (LBIR) facility at the US National Institute of Standards and Technology (NIST) has been calibrating infrared test chambers that are used to calibrate spacebased remote sensors. These test chambers typically operate at temperatures of 20 K to 80 K and have collimators in them to simulate faint objects at great distances in a low-background environment. Since 2001 the LBIR facility has used a transfer radiometer, the BXR, to calibrate nine IR test chambers. The BXR has shown that the output of these chambers can differ from the modelled output by ±10%. In this paper the methodology of the BXR calibration activity will be described along with specific examples to highlight important calibration performance parameters of the BXR and of other hardware in the calibration chain from the primary standard.

Low-background temperature calibration of infrared blackbodies

The Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) has performed ten radiance temperature calibrations of low-background blackbodies since 2001, when both the calibration facility and method of calibrating blackbodies were significantly improved. Data from nine of these blackbody calibrations are presented, showing a surprisingly large spread in blackbody performance. While some blackbodies performed relatively well, in no case did the measured radiance temperature agree with the temperature sensors in the blackbody core to within 0.3 K over the entire operating temperature range of the blackbody. Of the nine blackbodies reported, five showed temperature errors greater than 1 K at some point in their operating temperature range. The various sources of uncertainty, such as optical geometry and detector standard uncertainty, are presented with examples to support the stated calibration accuracy. Generic blackbody cavity design features, such as cavity thermal mass, cavity volume and defining aperture placement are discussed and correlated with blackbody performance. Data are also presented on the performance of the absolute cryogenic radiometers (ACRs) that are used as detector standards in the calibration of blackbodies. Recent intercomparisons of all the LBIR ACRs with a trap detector calibrated against the NIST primary optical power measurement standard show that ACRs used to calibrate blackbodies are suitable detector standards and contribute less than 0.02% uncertainty (k = 1) to radiance temperature measurements of the blackbody cavities.

Improved broadband blackbody calibrations at NIST for low-background infrared applications

The low-background infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) has continued to develop its facilities and knowledge base to meet the needs of the infrared community. Improvements in refrigeration capability at the LBIR facility have made it possible to perform calibrations of infrared sources and detectors in a stable 17 K background environment as compared to a relatively unstable 25 K environment available until about two years ago. This, combined with improved power measurement instrumentation, allows measurements of 1 nW with a standard uncertainty of 1% due to repeatability and reproducibility effects. A brief overview will be given of the changes to the LBIR facility that led to these improvements. The higher sensitivity in power measurement capability and some of the methods being used to generate low-power beams have highlighted new measurement issues that had previously been relatively unimportant. These issues include aperture quality, background scene temperature stability, beam shuttering, diffraction, and the noise floor of power measurement hardware. Demonstrations of common problems encountered will be shown and guidelines will be given for developing infrared sources that not only meet the needs of the user but can also be well calibrated.

Advanced absolute radiometers using superconducting transition thermometers

Recent work at the National Institute of Standards and Technology (NIST) with superconducting transition thermometers suggests that substantial improvements could be achieved in the detection capability of electrical-substitution radiometers, which are widely used as the most accurate absolute standards of radiant flux and irradiance. Cambridge Research & Instrumentation (CRI) has constructed liquid-nitrogen and liquid-helium-cooled prototype radiometers with responsivities of 550 mK/mW and 3.5 K/mW, and natural time constants of 30 s and 3 s, respectively. Our results with the liquid-nitrogen-cooled radiometer show that the high-Tc superconductor YBCO sensor has similar detection capability to a germanium resistance thermometer operating at 4 K. In this prototype radiometer, we were able to control receiver temperature to better than 1 µK and to attain a root-mean-square (rms) noise level of 1.6 nW. Our ongoing work focuses on the absolute accuracy achievable with such a liquid-nitrogen-cooled radiometer, using an optimized thermal design. The high detection sensitivity offered by the Nb sensor in the liquid-helium-cooled prototype radiometer achieved a temperature stability of 40 nK (rms), which implies a noise-equivalent power of 10 pW. Our measurements indicate that 1 pW should be feasible with optimized electronics, a lower-Tc sensor, and better control of heat-sink and scene drifts through implementation in the NIST Low Background Infrared (LBIR) chamber.

The new cryogenic vacuum chamber and black-body source for infrared calibrations at the NIST's FARCAL facility

The Facility for Advanced Radiometric Calibrations (FARCAL) at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD, comprises the Medium Background Infrared (MBIR) facility and the Training for Calibration Expertise in Radiometry (TraCER), the latter being a classroom setting. The MBIR is a 120 cm by 180 cm vacuum chamber with an internal cold shroud and an integral roll-out table. The chamber will contain a black-body source and an absolute cryogenic radiometer (ACR). Both components are mounted on the roll-out table. Measurements will be performed in the MBIR facility that cannot be accomplished using existing facilities, including the NIST Low Background Infrared (LBIR) facility and the room-temperature non-vacuum radiometric facilities. Initially the MBIR facility will be used for 80 K medium background measurements and 300 K ambient background measurements.

NIST Low-background infrared spectral calibration facility

The goal of the Low-background Infrared (LBIR) calibration facility at the National Institute of Standards and Technology is to provide the infrared user community with a calibration base for broadband and spectral radiant power measurements. The LBIR facility began broadband radiant-flux calibrations of infrared sources in 1990 using a cryogenic vacuum chamber to provide the low background at 20 K. A second cryogenic-vacuum chamber is being brought into service for spectral calibrations in the LBIR facility. This new spectral calibration chamber will house the spectral instrument and be the centre for the spectral calibrations of sources and detectors and for the measurement of optical properties of materials. The LBIR spectral instrument is composed of a KRS-5 prism predisperser for order-sorting, followed by a grating monochromator. This calibration capability is currently being developed in three areas: spectral calibration of infrared detectors for use in low-background applications; spectral resolution of radiation from black-body sources such as cavities and other geometries such as spheres and plates; and characterization of optical components of interest to the community such as filters and window materials. This work will be carried out over the spectral range 2 µm to 30 µm.

Intercomparison between the NIST LBIR Absolute Cryogenic Radiometer and an Optical Trap Detector

The goal of the Low Background Infrared (LBIR) calibration facility at the NIST is to provide the infrared user community with a measurement base for both broadband and spectral radiometric calibrations in a low background environment. The standard detector used in this facility is the Absolute Cryogenic Radiometer (ACR). The ACR is an electrical substitution radiometer which is capable of measuring between 20 nW and 100 μW of radiant power. By using an optical trap detector, an intercomparison with the ACR is made as part of a programme to monitor the long-term stability of the ACR. A comparison is also made between the optical trap detector and an in-house trap detector that was calibrated using the High Accuracy Cryogenic Radiometer (HACR).

Superconducting kinetic inductance radiometer

A bolometer that is based on a differential thermometer that senses temperature changes through changes in the kinetic inductance of a superconducting thin film is being developed. The temperature transducer is an inductance bridge patterned as an integrated circuit on a 1-cm/sup 2/ Si substrate. Two inductors from opposite arms of the bridge are patterned on a 2-mm/sup 2/ thermally isolated Si island which is supported by a 9- mu m thick Si:B membrane. The bridge is excited with audio-frequency current, and the bridge imbalance is detected with a commercial DC SQUID amplifier. The bridge is balanced by applying power to the thermally isolated island. This thermometer is the sensor for a prototype radiometer that will provide an absolute measure of IR power. The radiometer, which is designed for a NEP of about 10/sup -11/ W/ square root Hz, is intended to measure the spectrally dispersed power of a 300-K black body. This absolute radiometer is being developed for use at the Low Background Infrared (LBIR) Facility at NIST, Gaithersburg, MD. The noise floor of the temperature transducer for the radiometer has been measured to be 0.7 pW for a 100-s integration time. This is approximately 150 times lower noise than that of the commercial absolute radiometer currently used at the LBIR facility.