Radiometric Metrology for Remote Sensing

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Radiometric Metrology for Remote Sensing

• Carol Johnson
• Optical Technology Division
• National Institute of Standards and Technology

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NIST Overview

• The U.S. metrology and standards laboratory
• Non-regulatory agency in Dept. of Commerce
• Promotes U.S. economic growth by working with
  industry to develop and apply technology,
  measurements, standards
• Four main components
• Measurement and Standards Laboratories
• Advanced Technology Program
• Manufacturing Extension Partnership
• National Quality Program

http//www.nist.gov/
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Interaction Dissemination
Examples Calibration Services Standard
Reference Materials Reference Information Database
s Special Publications Training
Conferences Special Tests
Irradiance Standard Lamps (FEL)
Documents what is traceability?
http//ts.nist.gov/ts/
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Metrology of Radiometry

• Scale realization
• electrical and dimensional metrology
  (detector-based)
• temperature metrology (source-based)
• Rules to remember
• compare like to like
• precision is not accuracy
• Thorough instrument characterization
• interdependent influencing parameters
• spectral, spatial, temporal, temperature,
  polarization, flux level
• Validate results
• through comparisons
• measurement of fundamental constants

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NIST Detector-based Irradiance

• Irradiance lamp standards are a primary means of
  radiometric scale dissemination from NIST to
  customers
• Reduced the NIST uncertainties, up to a factor of
  10 in the SWIR

Yoon, et al., Appl. Opt. 41 5879-5890 (2002).
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Comparison of Methods
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Like to Like RuleSpectrally

• Circled Region Upwelling in-water spectral
  radiance derived using the two spectrographs in
  the same system disagree in their region of
  overlap
• Spectral out-of-band an issue because the
  calibration and measured source differ in their
  relative spectral distributions
• Solution thorough instrument characterization
  using tunable lasers, Traveling SIRCUS (Spectral
  Irradiance and Radiance Responsivity Calibrations
  with Uniform Sources)

Red Spectrograph
Blue Spectrograph
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Like to Like Rule--Spatially
Issue Radiance calibration of 2D CCD framing
camera with Cassegrain-type foreoptics
Near field source of constant radiance that
overfilled the entrance pupil gave distance
dependent results (this is non-physical, radiance
is independent of distance!)
Use of a collimated source provided the required
characterization as well as a more accurate
radiance calibration
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SIRCUS Calibration Facility
A variety of tunable lasers
fiber-coupled into an integrating sphere
wavemeter
pump laser beam
cw dye laser

• Producing a
• spatially uniform,
• monochromatic,
• high flux,
• broadly tunable source
• of (known) radiance

Designed specifically for system level absolute
spectral responsivity calibrations of irradiance
or radiance systems Goal Calibrations at the
0.1 level
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SIRCUS Experimental Layout
Intensity Stabilizer



Chopper or Shutter
Radiometer under Test

Reference Radiometer



Spectrum Analyser


Wavemeter

Exit

Port


Computer
Speckle-removal System

Integrating Sphere
Lens


Monitor Detector (output to stabilizer)

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Wavelength (?m)
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ASD Bandpass using SIRCUS
The visible near-infrared spectrograph (VNIR)
utilizes a linear photodiode array. The bandpass
must be determined using laser excitation at many
finely-spaced wavelengths. Shown are the result
for five adjacent pixels. This was done on the
visible SIRCUS facility.
The short wave infrared spectral regions has two
scanning spectrometers, each with a cooled
detector. The bandpass can be determined using
laser excitation at a single wavelength, shown
here from IR SIRCUS at 1598 nm and 2348 nm.
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Radiometry Remote Sensing

• Radiometric measurements yield physical
  information of complex systems
• Required scientific accuracies are often state of
  the art
• Measurements require long time series anticipate
  small changes
• Characterization and calibration are pre-flight
• Radiometry is difficult and subject to systematic
  effects

http//spot/colorado.edu/koppg/TSI/
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Technical Response

• Portable radiometers (ultraviolet to thermal
  infrared)
• Portable sources (characterization, calibration,
  stability, solid state)
• Calibration and characterization of flight
  sensors ( one build)
• Special measurements of sensor hardware (filters,
  apertures, detectors)
• Participation in intercomparison activities
• Pilot in artifact comparisons (reflectance,
  aperture area)
• Training and community participation
• Peer Reviews
• Publication of results ( 50 papers since 1995)

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Portable Radiometers

• Spectral coverage instrument design per
  application
• Characterized calibrated
• linearity, stability, spectral response, spatial
  response, polarization sensitivity
• calibration requirements impacted development of
  new, tunable laser-based facility (SIRCUS) and
  in-house vacuum chambers
• Deployed since early 1990s in support of U.S.
  remote sensing
• Maintained by NIST, often as a shared resource
• Uncertainties for NIST instruments
• 0.5 in visible and near-infrared (VNIR)
• 1.7 in shortwave infrared (SWIR)
• 0.05 K in thermal infrared (TIR)

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Absolute Calibration for Thermal Infrared
System Capabilities Thresholds Objectives
Measurement range -2 to 40 ?C -2 to 40 ?C
Measurement precision 0.2 ?C 0.1 ?C
Measurement uncertainty 0.5 ?C 0.1 ?C
Long term stability 0.1 ?C 0.05 ?C
NPOESS IORDII
Measurement equation
S(T) signal R(?) depends on radiometer L(?,
T) depends on source
Vastly oversimplified (neglects background
sources) but illustrates the need for information
on the sensor and the calibration source(s).
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Rice Johnson, Metrologia 35, 505-509 (1998).
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Calibration of TXR at NIST

• Used TXR in Medium Background IR (MBIR) facility
  at NIST.
• Shroud can be cooled to 80 K or left at room
  temperature.
• Viewed 11 cm diameter cryogenic blackbody (BB).
• Radiance scale is currently from temperature
  sensors in blackbody.

TXR Response to Blackbody
TXR
BB
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Measuring Emissivity Using Reflected Radiance
Lmeasured Lemitted(Tblackbody)
Lreflected(Tbackground)
From intercept eband 0.99826 /-
0.00037
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Thermal Infrared Transfer Radiometer (TXR)
At U. Miami for NASA EOS, May 2001
At ITT for NOAA GOES, July 2001
At LANL for DOE, July 1999 August 2001
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Arrangement for TXR measuring emitted radiance
TXR Scene Plate Ambient Temperature
Scene Plate Cooled to below Ambient
Diffuse Black Paint (both sides)
TXR
Blackbody Under Test 190 K lt T lt 340 K
Diffuse Black Paint
MLI
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Arrangement for TXR measuring reflected radiance
TXR Scene Plate Ambient Temperature
Raytheon Scene Plate 100 K lt T lt 320 K
Diffuse Black Paint (both sides)
TXR
Blackbody Under Test T 190 K
Diffuse Black Paint
MLI
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Example Result from TXR at GOES

• 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.

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Temperature Correction is Independent of
Wavelength
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Summary

• Recent advances in radiometric calibration and
  characterization
• SIRCUS (UV to IR)
• stable and accurate transfer radiometers
• New era for sensor performance possible
• adoption of SIRCUS facilities by industry
• pre-flight validation and comparison of
  radiometric scales

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NIST Acknowledgements

• TXRJoe Rice
• Remote sensing calibration and validationSteve
  Brown, Joe Rice, Ted Early
• SIRCUSSteve Brown, Keith Lykke
• Detector radiometryJoe Rice, Jeanne Houston, Tom
  Larason, George Eppeldauer

• Source radiometryHoward Yoon, Charles Gibson
• Optical properties and BRDFTed Early, Leonard
  Hanssen
• Colorimetry PhotometryYoshi Ohno, Cameron
  Miller, Maria Nadal
• Aperture areaToni Litorja, Joel Fowler

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External Support

• Air Force/Navy/Army Calibration Coordination
  Group (CCG)
• NASA EOS Project Science (Jim Butler)
• NASA SeaWiFS Project Science (Chuck McClain)
• NOAA/NESDIS (Dennis Clark, Eric Bayler, Steve
  Kirkner)
• Others DOE, DOD, IPO, USDA, Scripps