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 15.029 May 6, 2015

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

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.)

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

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

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

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

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

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 0.005 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

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

Transfer radiometers for calibration of point-source collimated beam infrared test chambers BMDO transfer Radiometer (BXR) (2001-2010) 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 (2001-2010) - 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

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

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.

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.

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 1990. 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

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

Radiometric calibration of cryogenic Fourier transform spectrometer Defining aperture (7 cm dia) Primary paraboloid Blackbody source 1.0 mm aperture 200-400 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

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) 0.00007 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)

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 0.035 to 0.045. MDXR-Chamber calibration uncertainties are 0.022 to 0.030. 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

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)

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.