Steven R. Lorentz

Active cavity absolute radiometer based on high-Tc superconductors

To implement the detector-based radiometric scale in the new Medium Background Infrared (MBIR) facility at the National Institute of Standards and Technology (NIST), we have developed an electrical-substitution cavity radiometer that can be operated just above liquid-nitrogen temperature. This MBIR active cavity radiometer (ACR) utilizes a temperature-controlled receiver cone and an independently temperature-controlled heat sink. Being a thermal-type detector, low noise and drift of the radiometer signal depends mainly on low-noise temperature control of the receiver and heat sink. Using high critical-temperature (Tc) superconducting thin-film temperature sensors in the active control loops, we have achieved closed-loop temperature controllability of better than 10 µK at 89 K for a receiver having an open-loop thermal time constant of about 75 s. For a flux level of 1 µW to 10 µW, the rms noise floor over a measurement cycle time is below 20 nW. This is the lowest noise level yet reported for a liquid-nitrogen-cooled electrical-substitution radiometer, and it is the first demonstration of the use of high-Tc superconductors in such a radiometer. Potential uses for this ACR in the MBIR facility include absolute measurement of the broadband radiance of large-area 300 K cryogenic black-body sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved spectral responsivity measurements of infrared transfer standard radiometers.

Thermal Modeling of Absolute Cryogenic Radiometers

This work consists of a detailed thermal modeling of two different radiometers operated at cryogenic temperatures. Both employ a temperature sensor and an electrical-substitution technique to determine the absolute radiant power entering the aperture of a receiver. Their sensing elements are different: One is a germanium resistance thermometer, and the other is a superconducting kinetic-inductance thermometer. The finite element method is used to predict the transient and steady-state temperature distribution in the receiver. The nonequivalence between the radiant power and the electrical power due to the temperature gradient in the receiver is shown to be small and is minimzed by placing the thermometer near the thermal impedance

ACR II: Improved absolute cryogenic radiometer for low background infrared calibrations

A second-generation absolute cryogenic radiometer (ACR II) was developed for use at the Low Background Infrared calibration facility at the National Institute of Standards and Technology. The need for spectral calibrations of very sensitive [D* = 10(14) cm (Hz)1/2W(-1)] infrared detectors necessitated the use of a cryogenic infrared monochromator and a more sensitive radiometer. The improved low-power performance of the ACR II compared with the older absolute cryogenic radiometer (ACR) has also made it useful as the primary standard for the calibration of cryogenic blackbody sources that are used as low-power infrared sources. The responsivity of the new radiometers receiver is 210 K/mW with a type A (random component) standard uncertainty of at most 7 pW when making power measurements of less than 10 nW. The original ACR has a responsivity of 29 K/mW and has a type A standard uncertainty of approximately 100 pW when making a similar low-noise-power measurement. Other properties of the radiometers are also described and compared.

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.

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.

NIST-BMDO transfer radiometer (BXR)

An infrared transfer radiometer has been recently developed at the Low-Background Infrared Calibration (LBIR) facility at the National Institute of Standards and Technology (NIST) for the Ballistic Missile Defense Organization (BMDO) program. The BMDO Transfer Radiometer (BXR) is designed to measure the irradiance of a collimated source of infrared light having an angular divergence of less than 1 mrad. It is capable of measuring irradiance levels as low as 10-15 W/cm2 over the spectral range from 2 micrometer to 30 micrometer. The radiometer uses an arsenic-doped silicon blocked impurity band (BIB) detector operated at temperatures below 12 K. Spectral resolution is provided by narrow bandpass interference filters and long-wavelength blocking filters. All the components of the radiometer, which include a mechanical shutter, an internal calibration source and detector, a long baffle section, a spatial filter, two filter wheels and a two- axis detector stage are cooled with an active flow of liquid helium to maintain temperatures below 20 K. A cryogenic vacuum chamber has been built to house the radiometer and to provide mechanical tilt alignment to the source. The radiometer is easily transported to a user site along with its support equipment.

Radiometrically deducing aperture sizes

The desire for high-accuracy infrared sources suitable for low-background seeker/tracker calibrations pushes the limits of absolute cryogenic radiometry and blackbody design. It remains difficult to calibrate a blackbody at irradiance levels below 1 nW cm−2 using electrical substitution radiometry. Ideally, the blackbody temperature should be chosen so that most of the emitted power lies in the spectral range of interest. This constraint frequently necessitates the use of small apertures (less than 1 mm diameter) to achieve the required reduction in power. However, the dimensions of small apertures are difficult to determine accurately. Also, the effects of diffraction and systematic problems such as aperture heating and light leaks become amplified. To diagnose these problems and to calculate diffraction effects, aperture dimensions must be known accurately. We describe a technique of radiometrically deducing the diameter of small apertures in the blackbody in situ utilizing the cryogenic blackbody calibration.

Design, manufacture, and calibration of infrared radiometric blackbody sources

A radiometric calibration station (RCS) is being assembled at the Los Alamos National Laboratory (LANL) which will allow for calibration of sensors with detector arrays having spectral capability from about 0.4-15 micrometers. The configuration of the LANL RCS is shown. Two blackbody sources have been designed to cover the spectral range from about 3-15 micrometers, operating at temperatures ranging from about 180-350 K within a vacuum environment. The sources are designed to present a uniform spectral radiance over a large area to the sensor unit under test. THe thermal uniformity requirement of the blackbody cavities has been one of the key factors of the design, requiring less than 50 mK variation over the entire blackbody surface to attain effective emissivity values of about 0.999. Once the two units are built and verified to the level of about 100 mK at LANL, they will be sent to the National Institute of Standards and Technology (NIST), where at least a factor of two improvements will be calibrated into the blackbody control system. The physical size of these assemblies will require modifications of the existing NIST Low Background Infrared (LBIR) Facility. LANL has constructed a bolt-on addition to the LBIR facility that will allow calibration of our large aperture sources. Methodology for attaining the two blackbody sources at calibration levels of performance equivalent to present state of the art will be explained in the paper.

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).