Adriaan C. Carter

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.

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.

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.

Absolute cryogenic radiometer and solid-state trap detectors for IR power scales down to 1 pW with 0.1% uncertainty

Commercially available absolute cryogenic radiometers (ACRs) have combined uncertainties that grow rapidly above 1% (k = 1) at power levels below 10 nW. There are solid state detectors, however, used in sensors and radiometers that cannot be calibrated at levels as high as 10 nW because they begin to saturate. For this reason new detector-based standards are being developed to provide the low-background infrared calibration community with absolute traceability to powers down to 1 pW with ~0.1% (k = 1) combined uncertainty. The scale will be established using an ACR with a combined uncertainty of the order of 1 fW and a trap detector based on arsenic-doped silicon blocked impurity band devices with similar noise floor performance.

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.

Cryogenic Fourier transform infrared spectrometer from 4 to 20 micrometers

We describe the design and performance of a cryogenic Fourier transform spectrometer (Cryo-FTS) operating at a temperature of approximately 15 K. The instrument is based on a porch-swing scanning mirror design with active alignment stabilization using a fiber-optic coupled diode laser and voice-coil actuator mechanism. It has a KBr beamsplitter and has been integrated into an infrared radiometer containing a calibrated Si:As blocked impurity band (BIB) detector. Due to its low operating temperature, the spectrometer exhibits very small thermal background signal and low drift. Data from tests of basic spectrometer function, such as modulation efficiency, scan jitter, spectral range, and spectral resolution are presented. We also present results from measurements of faint point-like sources in a low background environment, including background, signal offset and gain, and spectral noise equivalent power, and discuss the possible use of the instrument for spectral characterization of ground-based infrared astronomy calibration sources. The Cryo-FTS is presently limited to wavelengths below 25 micrometers but can be in principle extended to longer wavelengths with changes in beamsplitter and detector.

Picowatt infrared power measurement using an absolute cryogenic radiometer

We report on initial measurements of the low-temperature thermal properties of a device that is similar to the experimental apparatus used for absolute cryogenic radiometry (ACR) within the Low Background Infrared Radiometry (LBIR) facility at NIST. The device consists of a receiver cavity mechanically and thermally connected to a temperature-controlled stage through a thin-walled polyimide tube which serves as a weak thermal link. In order to evaluate the functionality of the device for use in a cryogenic radiometer, we measured the thermal resistance and thermal time constant of the system within the temperature range of 1.8 - 4.4 K. The measured thermal resistance and thermal time constant at 1.883 K were 2400 ± 500 (K/mW) and 24 ± 6 (s). This value for the thermal resistance should result in about an order-of-magnitude increase in radiometer sensitivity compared with the present ACR within LBIR. Although the sensitivity should increase by about an order-of-magnitude, the measured time constant is nearly unchanged with respect to previous ACRs within LBIR, due to the reduced dimensions of the receiver cavity. Finally, the thermal conductivity inferred from the measured thermal resistance and geometrical parameters was computed, with an average value of 0.015 (W/m-K), and compared with other measurements of polyimide from the literature.

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.

Infrared transfer radiometer for broadband and spectral calibration of space chambers

The Low-Background Infrared (LBIR) facility at NIST has recently completed construction of an infrared transfer radiometer with an integrated cryogenic Fourier transform spectrometer (Cryo-FTS). This mobile system can be deployed to customer sites for broadband and spectral calibrations of space chambers and low-background HWIL testbeds. The Missile Defense Transfer Radiometer (MDXR) has many of the capabilities of a complete IR calibration facility and will replace our existing filter-based transfer radiometer (BXR) as the NIST standard detector deployed to MDA facilities. The MDXR features numerous improvements over the BXR, including: a cryogenic Fourier transform spectrometer, an on-board absolute cryogenic radiometer (ACR), an internal blackbody reference, and an integrated collimator. The Cryo-FTS can be used to measure high resolution spectra from 4 to 20 micrometers, using a Si:As blocked-impurity-band (BIB) detector. The on-board ACR can be used for self-calibration of the MDXR BIB as well as for absolute measurements of infrared sources. A set of filter wheels and a rotating polarizer within the MDXR allow for filter-based and polarization-sensitive measurements. The optical design of the MDXR makes both radiance and irradiance measurements possible and enables calibration of both divergent and collimated sources. Details of the various MDXR components will be presented as well as initial testing data on their performance.