We have constructed, calibrated, and tested a cryogenic low-background infrared (IR) radiometer for both spectral radiance and irradiance measurements over the 4- to 20-μm wavelength range. The primary purpose of the Missile Defense Transfer Radiometer (MDXR) is to measure absolute irradiance or radiance from cryogenic IR test chamber sources using a photoconductive Si:As blocked-impurity band (BIB) detector and a set of spectral filters. The MDXR also includes an absolute cryogenic radiometer (ACR) and a Fourier-transform spectrometer (FTS). For irradiance measurements, the ACR is used to provide the primary power scale for the BIB detector in conjunction with spectral filters, whereas the FTS/BIB configuration derives its scale from an internal blackbody source. The two measurement scales show agreement for the irradiance of highly collimated (<1 mrad) IR beams from 10 − 13 to 10 − 8 W / μm / cm2 within the combined relative uncertainties of 2.6% (coverage factor k = 1.) We have also calibrated the radiometer for radiance measurements using a large cavity fluid bath blackbody that overfills the spatial and angular extent of the radiometer entrance pupil. The radiometric calibration uncertainty analysis of the radiometer and its maintenance and stability are discussed.
The Missile Defense Transfer Radiometer (MDXR) is designed to calibrate infrared collimated and flood sources over the fW/cm2 to W/cm2 power range from 3 μm to 28μ m in wavelength. The MDXR operates in three different modes: as a filter radiometer, a Fourier-transform spectrometer (FTS)-based spectroradiometer, and as an absolute cryogenic radiometer (ACR). Since 2010, the MDXR has made measurements of the collimated infrared irradiance at the output port of seven different infrared test chambers at several facilities. We present a selection of results from these calibration efforts compared to signal predictions from the respective chamber models for the three different MDXR calibration modes. We also compare the results to previous measurements made of the same chambers with a legacy transfer radiometer, the NIST BXR. In general, the results are found to agree within their combined uncertainties, with the MDXR having 30 % lower uncertainty and greater spectral coverage.
We discuss the design, construction, and initial test results of a Si:As blocked-impurity-band (BIB) trap detector. The
trap consists of two rectangular BIB devices configured in a v-shaped geometry. This trapping geometry is designed to
ideally yield a minimum of 7 bounces before exit for incident light within an f/4 cone with 3 mm clear aperture. The
individual BIB devices consist of 70 μm thick active layers with As doping near 1.7×1018 cm-3, and have dark currents of
approximately 100 nA at an operating temperature of 9 K. A simple ray-tracing model of the trap, along with data on
the quantum yield of typical BIB detector elements, indicates that it is possible to achieve an external quantum efficiency
of > 0.99 over the 4 μm to 28 μm spectral range and significant suppression of the etalon fringes present in the spectral
responsivity of a single element. We have made initial responsivity measurements of the trap compared to a calibrated
5 mm diameter pyroelectric detector over the 3 μm to 17 μm spectral range using the fiber-coupled output of a Fourier-transform
spectrometer. We also discuss the results of comparison measurements between the trap detector and an
absolute cryogenic radiometer viewing the output of a calibrated blackbody source at discrete filter bands from 5 μm to
11 μ. In initial testing the performance of the trap is limited by the poor performance of the individual BIB detectors,
but the advantages of boosted quantum efficiency and suppressed etalon are realized by the trap.
The Low-Background Infrared (LBIR) facility at NIST has performed on-site calibration and initial off-site
deployments of a new infrared transfer radiometer with an integrated cryogenic Fourier transform spectrometer (Cryo-
FTS). This mobile radiometer can be deployed to customer sites for broadband and spectral calibrations of space
chambers and low-background hardware-in-the-loop testbeds. The Missile Defense Transfer Radiometer (MDXR) has
many of the capabilities of a complete IR calibration facility and replaces our existing filter-based transfer radiometer
(BXR) as the NIST standard detector deployed to customer facilities. The MDXR features numerous improvements over
the BXR, including: a cryogenic Fourier transform spectrometer, an on-board absolute cryogenic radiometer (ACR) and
an internal blackbody reference source with an integrated collimator. The Cryo-FTS can be used to measure high
resolution spectra from 3 to 28 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 external 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. Results of on-site calibration of the MDXR using its internal
blackbody source and an external reference source will be discussed, as well as the performance of the new radiometer in
its initial deployments to customer sites.
The Low Background Infrared (LBIR) facility has developed and tested the components of a new detector for calibration
of infrared greater than 1 pW, with 0.1 % uncertainty. Calibration of such low powers could be valuable for the
quantitative study of weak astronomical sources in the infrared. The pW-ACR is an absolute cryogenic radiometer
(ACR) employing a high resolution transition edge sensor (TES) thermometer, ultra-weak thermal link and miniaturized
receiver to achieve a noise level of around 1 fW at a temperature of 2 K. The novel thermometer employs the
superconducting transition of a tin (Sn) core and has demonstrated a temperature noise floor less than 3 nK/Hz1/2. Using
an applied magnetic field from an integrated solenoid to suppress the Sn transition temperature, the operating
temperature of the thermometer can be tuned to any temperature below 3.6 K. The conical receiver is coated on the
inside with infrared-absorbing paint and has a demonstrated absorptivity of 99.94 % at 10.6 μm. The thermal link is
made from a thin-walled polyimide tube and has exhibited very low thermal conductance near 2x10-7 W/K. In tests with
a heater mounted on the receiver, the receiver/thermal-link assembly demonstrated a thermal time constant of about 15 s.
Based on these experimental results, it is estimated that an ACR containing these components can achieve noise levels
below 1 fW, and the design of a radiometer merging the new thermometer, receiver and thermal link will be discussed.
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.
We present initial performance data from a cryogenic Fourier transform spectrometer (Cryo-FTS) designed for lowbackground
spectral infrared calibrations. The Cryo-FTS operates at a temperature of approximately 15 K and has been
integrated into an infrared transfer radiometer containing a calibrated Si:As blocked impurity band (BIB) detector.
Because of its low operating temperature, the spectrometer exhibits negligible thermal background signal and low drift.
Data from tests of basic spectrometer function, such as modulation efficiency, scan jitter, spectral range, spectral
resolution and sweep speed will be presented. We will also discuss calibration techniques and results pertinent to
operation of the Cryo-FTS as part of a calibration instrument, including background, signal offset and gain, and spectral
noise equivalent power. The spectrometer is presently limited to wavelengths below 25 micrometers but can be in
principle extended to longer wavelengths by replacing its KBr beamsplitter with another beamsplitter engineered for use
beyond 25 micrometers.
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.
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.
A cryogenic Fourier transform infrared spectrometer (Cryo-FTS) was developed for the Low Background Infrared
(LBIR) facility at the National Institute of Standards and Technology (NIST). This spectrometer was developed for the
Missile Defense Agency Transfer Radiometer (MDXR) that will be used to calibrate infrared sources that cannot be
transported to NIST for calibration. When used inside the MDXR, the Cryo-FTS provides relative spectral measurements
with a repeatability better than 1 % over the spectral range from 3 μm to 15 μm and at a spectral resolution of 0.6 cm-1.
This level of performance is enabled by the use of an advancec real-time resampling method.
The compact interferometer uses a compensated Michelson configuration and has an operating temperature range
between 10 K and 340 K with very low static beam redirection (< 215 μrad). The interferometer uses flat mirrors and a
KBr beamsplitter and compensator. This optics maintains low wavefront distortion for infrared beams of up to 2 cm
diameter and 5 mrad divergence. It integrates a digitally servo-controlled porchswing mechanism to provide an accurate
and repeatable optical path difference and is supported by a Wavefront Alignment (WA) system to correct for wavefront
residual tilt in real time using a fibre optic coupled metrology system. The interferometer provides modulation efficiency
of better than 44% with limited power dissipation (< 2.8 W) during operation.
A cryogenic Fourier transform infrared spectrometer (Cryo-FTS) was developed for the Low Background Infrared
(LBIR) facility at the National Institute of Standards and Technology (NIST). This spectrometer was developed for the
Missile Defense Agency Transfer Radiometer (MDXR) that will be used to calibrate infrared sources that cannot be
transported to NIST for calibration. When used inside the MDXR, the Cryo-FTS provides relative spectral measurements
with a repeatability better than 1 % over the spectral range from 3 μm to 15 μm and at a spectral resolution of 0.6 cm-1.
This level of performance is enabled by the use of an advancec real-time resampling method.
The compact interferometer uses a compensated Michelson configuration and has an operating temperature range
between 10 K and 340 K with very low static beam redirection (< 215 μrad). The interferometer uses flat mirrors and a
KBr beamsplitter and compensator. This optics maintains low wavefront distortion for infrared beams of up to 2 cm
diameter and 5 mrad divergence. It integrates a digitally servo-controlled porchswing mechanism to provide an accurate
and repeatable optical path difference and is supported by a Wavefront Alignment (WA) system to correct for wavefront
residual tilt in real time using a fibre optic coupled metrology system. The interferometer provides modulation efficiency
of better than 44% with limited power dissipation (< 2.8 W) during operation.
The Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) is
responsible for absolute IR radiometric calibrations (SI traceable) in low-background temperature (below 80 K)
environments. IR radiometric test hardware that needs to be operated in cryogenic environments is calibrated in
cryogenic vacuum chambers maintained by the facility to create environments that simulate the low-temperature
background of space. Transfer radiometers have also been developed to calibrate IR radiometric test hardware this is too
large to ship to NIST from their own IR test facilities. The first generation transfer radiometer, the BXR, is a filter-based
radiometer that uses an As-doped Si Blocked Impurity Band detector, and can calibrate IR test chambers to a total
uncertainty of less than 3 % (1 σ ) at powers as low as to 10-14 W/cm2. The BXR has evaluated 9 chambers and the
performance of a subset of these chambers will be discussed to a limited extent to demonstrate the need for calibrating
IR test chambers. The second generation transfer radiometer, the MDXR, and new primary standards allowing absolute
calibrations as low as 10-15 W/cm2 are in the final stages of development. The MDXR will have all the functionality of
the BXR and it will have a cryogenic Fourier transform spectrometer (FTS) for high resolution spectral capability.
Performance specifications and test results from development activity on the new primary standards will be discussed.
The authors have worked in the past year on integration, characterization, and calibration of The Johns Hopkins University Applied Physics Laboratory's (JHU/APL's) Infrared Seeker Space Calibration and Test facility, a cryogenic-vacuum chamber designed to test infrared seekers that detect targets against low-radiance backgrounds. The facility includes target-like infrared sources with well-known and controllable radiometric attributes and well-known and
controllable size, position, and motion. This paper summarizes the basic facility design, capabilities, concept of operations, current and projected uses, challenges, and lessons learned. It describes the chamber calibration and characterization activities conducted jointly by JHU/APL and the National Institute of Standards and Technology (NIST). In particular, this includes a description of the calibration and characterization methodology, modeling of the chamber optical path from the chamber target source module to the unit-under-test entrance aperture, ongoing calibration of the target source module at NIST with an absolute cryogenic radiometer, and planned end-to-end calibration of the chamber at JHU/APL using NIST's transfer radiometer and JHU/APL's field spectroradiometer.
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.
The capability of the Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology to spectrally calibrate infrared detectors was demonstrated with the spectral calibration of arsenic doped silicon blocked impurity band (BIB) detectors. The BIB detectors were calibrated over the 2 micrometer to 30 micrometer range, using light from a monochromator with a nominal 2% bandwidth. Photon fluxes used for the calibration ranged from 1013 photons/s/cm2 to 1014 photons/s/cm2. The large area detectors (10 mm2) calibrated in this paper were very linear up to 2.5 X 1014 photons/s/cm2, where they showed a 1% drop in signal from linearity. The calibrations contained less than 6 1% standard component of random noise uncertainty, and there was about a 6 5% standard component of uncertainty arising from systemic effects that will be discussed in detail. The calibrations were performed in ultra- high vacuum in a 20 K background environment by making direct intercomparisons between the power measured by an absolute cryogenic radiometer and the response measured by a detector irradiated by the same beam. A detailed description of measurement methodology and system apparatus is given. Detector linearity and uniformity are also discussed. The LBIR facility can now provide calibrated BIB detectors as transfer standards as well as evaluate and calibrate customer's large area detectors and detector arrays provided the detectors stay within certain physical limitations.
Collimated infrared sources covering the 2 micrometer to 30 micrometer range of wavelengths are necessary to simulate infrared radiation from distant objects. This is important because on-orbit servo and tracking systems make extensive use of infrared radiation for remote sensing. Collimators are used to calibrate infrared detectors in terms of absolute power within a given spectral range. The National Institute of Standards and Technology (NIST) operates and maintains the Low Background Infrared Calibration (LBIR) facility, which uses a 2 K electrical substitution radiometer, the Absolute Cryogenic Radiometer (ACR), that is the primary national standard for broadband and infrared spectral measurements. At this facility, users can calibrate blackbody sources with at most 1% uncertainty. However, users must then rely on optical systems at their own facility to collimate the radiation from the blackbody. The effect of the optics on the output of the beam must then be calculated from models. For this reason, NIST is developing a portable transfer radiometer (BXR) that can be taken onsite to directly measure the spectral output, thus eliminating intermediate steps in the calibration chain. NIST is also developing a source having 1 cm diameter collimated beam, for a preliminary calibration of the BXR at the LBIR facility from 2 micrometer to 8 micrometer. The source must fit into a volume of about 0.03 m3 (1 cubic foot), have an angular divergence of less than 700 (mu) rad, a power output greater than 10 nW, and demonstrate 1% repeatability or better. The development and characterization of this source is the main topic of this paper.
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