1. Infrared Calibration Transfer Standards for Flood and Collimated Sources
Simon G. Kaplan, Solomon I. Woods, Dana Defibaugh, Joseph P. Rice
Sensor Science Division
Infrared Technology Group
NIST, Gaithersburg MD 20899
Timothy M. Jung, Adriaan C. Carter
Jung Research and Development Corp.
Washington, DC 20009
Address audience
JACIE May 6, 2015

2. Calibration of infrared radiometers for remote sensing
onboard calibration source
readout electronics
fore optics
field stop
detector array
spectral selection
scene
Piecewise component model
System-level testing
Stability monitoring

3. System-level testing of remote sensing radiometers by measuring response to “ideal” sources
Extended-area blackbodies (BB) used for infrared calibrations:
Temperature range roughly within 190 K to 400 K (e.g. Earth)
Aperture diameter typically > 10 cm.
Designed to overfill the aperture and field of view of sensor for radiance calibration
Two typical effects which cause them to be non-ideal:
. Emissivity < 1 such that they emit less and reflect more (optical.)
. There is a temperature difference between the effective radiating
surface(s) and the thermometer (thermal.)

4. Flood source transfer radiometer: the NIST TXR
The Thermal-infrared Transfer Radiometer (TXR) was developed as part of a larger, multi-year calibration program between the NASA EOS Project Science Office and the NIST Optical Technology Division.
The TXR is a two-channel portable radiometer for providing thermal-infrared scale intercomparisons of large-area (> 4 cm dia. aperture) blackbody calibration sources.
The goal is to provide in-situ measurements of the radiance that the flight instrument actually sees during its chamber calibration.
TXR can also measure reflected background radiance from customer blackbody to estimate cavity emissivity.
The TXR has been deployed to the following:
Los Alamos National Laboratory, MTI calibration chamber
University of Miami for sea-surface temperature blackbody intercomparisons
ITT GOES Imager chamber for ECT blackbody comparison
Raytheon SBRS chamber for MODIS (and VIIRS) BCS blackbody comparison
University of Wisconsin for AIRI (scanning HIS) blackbody comparison

7. Calibration of the TXR in a vacuum chamber at NIST
Used the TXR in Medium Background IR (MBIR) facility
Light-tight shroud and optical table can be cooled to 80 K or left at room temperature
Viewed Large Area Cryogenic Blackbody (LABB)
Radiance scale is from LABB temperature sensors and emissivity
Measured TXR response at a large number of LABB temperature plateaus with shroud cold
Result is TXR response vs. LABB contact thermometer temperature ri(Tc)
Calibration was then immediately transferred to an on-board blackbody, the Check-Source (CS)
TXR Response to Blackbody CS
TXR
LABB

8. Calibration of TXR
Cold-shroud data were used to determine the three TXR calibration parameters
The calibration parameters are physically-based:
Two of them (for each channel) provide boxcar rsr limits and are fixed.
One of them (for each channel) is the absolute responsivity and varies from
day-to-day and cooldown-to-cooldown.
An on-board blackbody (called the Check-Source, CS) is used to correct for
the variation of absolute responsivity.
The NIST Water Bath Blackbody (WBBB) is used to correct the LABB
scale for higher accuracy and for better long-term scale maintenance.
Brightness temperature uncertainty is about 60 mK (k=1)
Brightness temperature resolution about 5 mK
LABB emissivity is based on measured radiance with warm-shroud background
Emissivity uncertainty values are well below 0.1% (k=1)
Total uncertainty for radiance is generally about 0.2% at 10 microns (k=1)
Field of view approximately 30 mrad
Uncertainty could potentially be improved using a better cryogenic blackbody

9. Water Bath Black Body used to maintain TXR scale
The NIST Water Bath Blackbody (WBBB) thermometry has 10 mK uncertainty
This is based on a removable/recalibrate-able standard PRT in the circulating fluid
Used over temperature range 15 °C to 80 °C, and in ambient environmental conditions
The WBBB has 10.8 cm diameter aperture, usually apertured down to 4 cm for TXR

10. Example Result from TXR at GOES measurements
Temperature of radiating surface ≠ thermometer reading.
Emissivity and temperature correction were measured.
Temperature correction qualitatively in agreement with GOES model.
Enables re-calibration of data with more direct tie to NIST.
TXR Measurement
GOES Model

11. New TXR capability: two-axis scans of large area blackbodies
Cryo-vacuum compatible two-axis tilt stage
Azimuth, Elevation Range of Motion ±10 deg, Resolution deg
Motors 1x10-11 torr vacuum rating
-270oC to +40oC temperature rating
All metal construction with MoS2 lubrication (except wiring and switches)
Used successfully in February 2015 for scanning of blackbody sources in CRIS test chamber at Exelis, Ft. Wayne, IN

12. Low-background point-source IR test chambers – calibration considerations
Mirror
Sensor
Collimated Infrared Beam
(used for sensor calibration and testing)
Mirror
Other
Possible
Optics
Blackbody
Infrared Source
Assembly
5. Reflected throughput
6. Effective focal length
7. Diffraction
8. Polarization
Aberrations, Adsorbed gases, Scattering, Stray light
Emissivity e(l,T)
Cavity temperature, T
Aperture area, A
Filter transmittance, t
Emission from aperture, chopper, filter
Cryogenic vacuum chamber 77 K or 20 K

13. Transfer radiometers for calibration of point-source collimated beam infrared test chambers
BMDO transfer Radiometer (BXR) ( )
Filter radiometer 2 mm to 14 mm
10 bands  1 mm width
Uncertainty 3.5 % to 4.5 %
Missile Defense transfer Radiometer (MDXR)
(2010-present)
Improved filter design
Spectral range 2 mm to 30 mm
Cryogenic Fourier transform spectrometer
ACR and blackbody source
Uncertainty 1 % to 2.5 %
BXR ( ) - talk about history of measurements and impact on users. My main contribution was characterization of filters. - filter based transfer radiometer ( 1 mm bandwidths) - irradiance calibration for nW/cm2 to fW/cm2 from 2 mm to 14 mm - rotating polarizer - uncertainty 3.5 % to 4.5 %
MDXR (2010-present) User community asked for better spectral coverage, lower uncertainty. I became project leader as MDXR was being assembled. Lessons learned from filter radiometer calibration on BXR about stacks of filters, sensitivity to precise mounting geometry (inter-reflections, interference, beam shifting), long-wavelength leakage addressed by separate long pass short pass wheels, better out-of-band blocking. Also onboard ACR makes transfer from 10CC to filter radiometer better – same defining aperture and primary mirror. Got lower uncertainty and better spectral coverage. - filter based radiometer (variable bandwidths 0.3 mm to 5 mm) - cryogenic Fourier Transform Spectrometer (CFTS) 3 mm to 28 mm - onboard blackbody source (200 K to 400 K) for internal reference
- onboard ACR for improved calibration
- rotating and fixed polarizers
- uncertainty 1.0 % to 3.0 % - 17 user chamber calibrations performed since June 2010
Current missile defense customers:
Raytheon Missile Systems
Arnold Engineering Development Center
Johns Hopkins Applied Physics Laboratory

14. NIST collimator chamber and MDXR filter radiometer calibration
Irradiance at output of 10CC is calibrated with primary standard ACR
Calibration is transferred to BXR or MDXR Radiometers
BXR and MDXR are taken to user facilities to calibrate infrared test chambers
Collimated spectrally defined beam used to calibrate MDXR

15. MDXR beam path – top view
Detector side
Beam entry side
Defining aperture (7 cm dia)
Primary paraboloid
Variable field stop wheel
Optics plate – actively cooled
Defining aperture. Vertically oriented optics plate. Folded optical design for portability (clever, Tim Jung). Still weighs about 400 kg and takes two days to cool down.

16. MDXR beam path – detector side
Translating periscope
Secondary paraboloid
3-axis stage
Filter wheels
(spectral and polarization)
Cryo-FTS
Tertiary paraboloid
BIB detector(s)
ACR
Variable field stop wheel
ACR moves in and out of beam to calibrate filter radiometer on band-to-band basis. Same filter set in MDXR and 10CC
Many moving parts must work at high vacuum (10^-8 mbar = 10^-11 bar ~ 10^-6 Pa) and 20 K and maintain optical alignment. Cryo-FTS must maintain interferometric alignment of 3 cm diameter scanning mirror over 1 cm path length at ambient in air or in cryogenic vacuum, survive shipping across the country. Complete cal of radiometer takes 3 weeks plus data processing. BXR was stable to 0.1 % year-to-year, MDXR much more complex, stable to about 0.5 % to 1.0 % despite lower absolute uncertainty. Everything must work at user site during chamber calibrations, typically 1 to 2 weeks per chamber.

17. Absolute Cryogenic Radiometer
thermometer
heat sink
receiver
Absolute Cryogenic Radiometer (ACR) ħω
thermal link
heater
Infrared power measurement traceable to electrical watt
Primary LBIR data product since Electrical substitution radiometry is basis of our scale for low-background infrared. ACR very low uncertainty 0.01 % in principle, but other effects limit uncertainty (stray light, diffraction, background subtraction, etc.) ACR slow (1 minute), sensitive to background temperature drift. Typical BB cal takes about 3 weeks including chamber cool down time. Chambers with calibrated BB sources still show differences 10 % or more in irradiance output so user community asked for NIST radiometer to check output of different chambers (about year 2000).
1 nW to 100 W power range

18. MDXR filter radiometer calibration factors
Explain meaning of Calibration factor – comment about corrections to model
The MDXR filter mode calibration is done on a band by band basis.
The filter set by which the MDXR is calibrated is the same that is used to calibrate the customer’s infrared test chamber.
The horizontal extent of the lines represent the approximate spectral width of the filter bands used for calibration measurements.
Scatter in cal factors largely due to responsivity of Si:As BIB detector whose spectral complexity (absorption bands, interference effects) not fully captured in model.
Collimator model
Calibration with ACR
MDXR Model
Calibrated MDXR

19. Radiometric calibration of cryogenic Fourier transform spectrometer
Defining aperture (7 cm dia)
Primary paraboloid
Blackbody source
1.0 mm aperture K
Collimator
Irradiance from internal MDXR blackbody viewed with an internal 7 cm collimator is used to calibrate the CFTS mode of operation.
Spectral response function versus separate cal factor for each filter band.
CFTS not required to hold scale by itself, but references to blackbody source, which is periodically checked for stability using ACR. Source-based versus FR which is detector-based.
Observing an external source
Observing internal blackbody source

20. Fourier transform spectrometer calibration uncertainties
Relative uncertainty source
Value at 10 mm
Internal BB stability (A)
0.0035
User source stability (A)
0.002
Detector nonlinearity (B)
0.0025
Alignment internal/external (B)
0.001
Polarization correction (B)
0.003
Defining aperture area (B)
Internal collimator geometry (B)
Internal collimator diffraction correction (B)
0.0018
Internal collimator mirror reflectance (B)
0.0057
BB temperature (B)
0.0046
BB emissivity (B)
Quadrature sum
0.0096
Lower uncertainty than FR at higher power levels, but less sensitivity due to noise issues compared to lock-in detection with FR. Systematic uncertainty dominated by knowledge of collimator mirror reflectance (0.5 %) and BB temperature (about 0.5 K)

21. BXR – MDXR calibration comparison (2011)
BXR-chamber measurement
MDXR-chamber measurement
“Simple” chamber with one blackbody source and mirrors; data for one of the larger apertures for source temperatures from 180 K to 400 K. Chamber output has been stable to within +/- 0.3 % over many years.
BXR-Chamber calibration uncertainties are to
MDXR-Chamber calibration uncertainties are to
BXR and MDXR agree to within their combined uncertainties.
The more careful design and build execution of the MDXR makes the observed trend vs. wavelength believable.
User chamber has been measured since 2005

22. MDXR Filter radiometer – Fourier transform spectrometer comparison
Filter radiometer traceable to watt
Spectrometer traceable to kelvin
Wavelength (um)
The absolute values of the calibration factors from the Fourier transform spectrometer and filter based operational modes agree with each other within their combined uncertainties.
The trends as a function of temperature and wavelength are also consistent.
Correction of wavelength or temperature bias of chamber spectral irradiance output is very important to our customers.
Newer user chamber (2014)

23. Conclusions
The NIST TXR measures radiance at 5 mm and 10 mm wavelengths from extended area blackbody sources for near-ambient temperatures with 60 mK uncertainty in brightness temperature.
Capability to perform two-axis scans with 30 mrad FOV, 5 mK resolution.
The NIST MDXR measures irradiance of collimated beam output from cryogenic infrared point source test chambers from 4 mm to 30 mm wavelength with uncertainties of 1 % to 2.5 %.
Radiance measurement capability of MDXR with up to 4 mrad FOV under development.
The absolute values of the calibration factors from the Fourier transform spectrometer and filter based operational modes agree with each other within their combined uncertainties.
The trends as a function of temperature and wavelength are also consistent.
Correction of wavelength or temperature bias of chamber spectral irradiance output is very important to our customers.