The Low background infrared (LBIR) facility was originally designed to calibrate user supplied blackbody sources and to characterize low-background IR detectors and attenuators. Today, the facility continues to operate in this capacity providing calibrations of cryogenic blackbodies and transfer standard radiometers in low background environments for the National Missile Defense contractors of the Department of Defense. We are currently developing capabilities to characterize and calibrate large area blackbodies used for calibrating remote sensing instruments for climate research.
Cryogenic blackbody calibrations
The LBIR facility supports the calibration needs of the user community with two high vacuum, low temperature test chambers. Both the broadband calibration chamber (BCC) and the spectral calibration chamber (SCC) have an interior working volume that is approximately 150 cm in length and 50 cm in diameter, and can operate at ultra-high vacuum over a temperature range of 20 K to 100 K. Each chamber houses an Absolute cryogenic radiometer (ACR). A customer’s infrared blackbody source is directly calibrated for its radiance temperature versus its control temperature setting using the irradiance at the NIST ACR inside one of the calibration chambers. The LBIR facility also maintains an Antechamber that is used for calibrating blackbodies that are too large or too high power to be operated in the chambers.
A Cryogenic shutter assembly (CSA) is developed to couple the large area blackbody from customers serving the earth remote sensing community for calibrating the radiance temperature and characterizing the total emittance of the cavity. The radiance temperature is deduced by using the irradiance at the ACR aperture, and the emittance is deduced by using the technique of measuring the reflected light from a variable temperature hot plate in front of the cavity.
For more details about the calibrations and quality system, see Low background infrared calibration facility quality manual.
Research and development of cryogenic detector standards
The LBIR facility is equipped with a cryogenic prism-grating instrument that can spectrally resolve infrared radiation from 2 to 30 micrometers with a resolving power of 2% at any wavelength setting. An Absolute Cryogenic radiometer (ACR) is used as the detector standard. A low-temperature blackbody was developed as a source to power the spectral instrument for detector calibrations. Capable of functioning in a 20 K environment, the source has an operating range from 100 K to 450 K with a built-in aperture and filter wheels. The detector under test can be moved in and out of the monochromator output beam using a movable detector paddle in front of the ACR to carry out spectral calibrations.
The following detector standards are in development. A BIB-Trap detector is being developed using arsenic-doped silicon detectors in a trapping configuration to attain a flat spectral response and overall quantum efficiency greater than 98% over the 4 to 28 micrometer spectral range. A pico-watt ACR (pW-ACR) is being developed as an absolute cryogenic radiometer (ACR) able to calibrate 1 pW of optical power with 0.1% uncertainty. The new radiometer will be nearly 10,000 times more sensitive than the present ACRs at the LBIR facility yet have nearly the same response time.
Transfer standard radiometers
A BMDO transfer radiometer (BXR) and a missile defense transfer standard radiometer (MDXR) were developed for infrared test chambers that are too large to deliver to NIST. The BXR is a filter radiometer, and is calibrated against an ACR at the LBIR facility before being transported to a customer’s location. By transferring the calibration to the output of infrared collimators, NIST traceability is established for the customer’s test chambers. The MDXR has the same capabilities as the BXR, but is also equipped with a cryogenic Fourier transform spectrometer (FTS) to spectrally resolve the infrared beam from 2 micrometers to 30 micrometers. The LBIR facility is equipped with an IR Collimator that produces a 10 cm diameter IR collimated beam with a divergence below milliradian to calibrate these transfer standard radiometers. An ACR is used to calibrate the collimated beam.