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Field Spectroscopy, Hyperspectral Imaging,
Applications in Vegetation and Soils Analysis
Alexander F. H. Goetz
University of Colorado and
Analytical Spectral Devices Inc.
[email protected] and NanJing, China
June 28-29 and July 1-2, 2004
Lecture 1
Spectroscopy, Hyperspectral and Applications
• Day 1• Spectroscopy
fundamentals• Spectral Imaging• Hyperspectral Data
Analysis
• Day 2• Hyperspectral Data
Analysis cont.• Tradeoffs: Spatial,
Spectral Resolution, SNR
• Applications
Acknowledgements• Dr. Roger Clark, US Geological Survey
http://speclab.cr.usgs.gov• Dr. Greg Swayze, USGS [email protected]• Dr. Joe Boardman, AIG LLC www.aigllc.com• Dr. Fred Kruse, Horizon GeoImaging LLC
www.hgimaging.com• Dr. Brian Curtiss, Analytical Spectral Devices
Inc. www.asdi.com• Ms. Phoebe Hauff, Spectral International Inc,
www.specmin.com
Spectroscopy Fundamentals
Reflectance
• Instruments measure radiance L
2 1 1EL Wm sr m
L
E
Reflectance (2)
• In practice, the spectrometer is used to measure a white standard such as Spectralon®, which is sintered PFTE (polytetrafluoroethene)(Teflon®)
• It has a reflectance close to 100% over the 400-2500 nm region
• In the instrument, the radiance measured from the sample is ratioed with the Spectralon radiance to produce reflectance as a function of wavelength
ASD Spectrometers andSpectroradiometers
FieldSpec Pro
TerraSpec
High Intensity
Probe Attaches to
FieldSpec orTerraSpec
Argentina
7000 m
Peanut Field, Argentina
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
500 1000 1500 2000 2500
Leaves on SpectralonR
efle
ctan
ce
Wavelength, nm
PROCESSES THAT CAUSE ABSORPTION FEATURES
• Electronic• Interactions between electrons and
crystal fields• Vibrational
• Molecular vibrations• Fundamental• Overtone• Combination
ELECTRONIC PROCESSES
• Crystal field effects
• Charge transfer
• Semiconductor
• Color centers
CRYSTAL FIELD EFFECTS
• Energy levels of an ion• Split and displaced in crystal field• Determined by
• Valence state• Coordination number and symmetry
• Reflectance spectrum• Determined primarily by mineralogy not
cation• Depth of feature grain-size dependent
CRYSTAL FIELD EFFECTS
• Iron most important
• Most abundant
• Fe2+ , Fe3+ can substitute
• Mg2+
• Al3+
Ruby, Al2O3 + Cr+++
Emerald, Be3Al2Si6O18 + Cr+++
Electronic Transitions in Iron Minerals
Iron MineralsIron Minerals
Lepidocrocite
Ferrihydrite
Maghemite
Goethite
Hematite
CHARGE TRANSFER
• Electrons transfer from one atom to another
• Fe-O transfer responsible for reflectance falloff towards UV
SEMICONDUCTORS
• Absorption edge in reflectance spectrum• Created by width of forbidden energy
band gap• Incoming photons must have enough
energy to promote valence band electrons into conduction band
• Reflectance increases dramatically at wavelength corresponding to band gap energy
COLOR CENTERS
• Electron trapped in a structural defect such as a missing ion
• In fluorite (CaF2) a color center is formed when an F ion is missing and replaced by an electron
• Transition states created cause red-green absorption, hence purple color
VIBRATIONAL PROCESSES
• Fundamental vibrations
• For solids, generally occur beyond 2.5 m
• Si-O, Al-O occur in 10 m region, no effect in VNIR or SWIR
• OH, H2O, CO3 occur in 2.6-6 m region, overtones and combinations found in VNIR, SWIR
• 3N-6 possible degrees of freedom
• H2O has 3 fundamental vibrations at 2.66, 2.74, 6.08 m
OVERTONES AND COMBINATIONS
• Overtones
• Multiples of the fundamental frequency
• 21, 32, …..
• Combinations
• Sums and differences of fundamental or overtone frequencies
1 + 2 , 21 + 3, 1 + 2 + 3, ….
• Frequencies not wavelengths added
•
• Frequency units in cm-1
• 2.5 m = 4000 cm-1
,c
c
,c
c
WATER VAPOR
• Absorption fundamentals 1 = 3657.05 cm-1 = 2.734 m symmetric
stretch 2 = 1594.75 cm-1 = 6.271 m bend 3 = 3755.93 cm-1 = 2.662 m asymmetric
stretch• Important water vapor absorptions
2 + 3 = 1.865 m 1 + 3 = 1.379 m 1 + 2 + 3 = 1.135 m
• 21 + 3 = 0.942 m
LIQUID WATER
• Absorption fundamentals1 = 3219.57 = 3.106 m2 = 1644.74 = 6.08 m3 = 3444.71 = 2.903 m
HYDROXYL
• Absorption fundamental• 2.77 m stretch • Exact location depends on site on which it is
located• Overtone
• 2 ~ 1.4 m• Most common feature in terrestrial material
spectra• Combinations
• Al or Mg - OH bending modes• Features in 2.2 & 2.3 m region
SPECTRAL PROPERTIESSOME COMMON ABSORPTION FEATURES
FEATURE POSITION
Fe3+ 0.4 - 0.6 m, 0.66 m, 0.85 0.95m
Al - OH 2.15 - 2.22 m
Mg - OH 2.30 - 2.39 m
Fe - OH 2.24 - 2.27 m
Si - OH 2.25 m (broad)
H2O 1.9 m
CO3 2.30 - 2.35 m
NH4 2.0 - 2.13 m
LaboratorySpectra
Coatings (thin films)• Absorption features are square root 2 (0.707)
narrower width than thick particulate surfaces.
Coatings vary from transmissive thin films to full scattering thick layers; the natural width of spectral features varies by root 2.
The variety of absorption processes and their
wavelength dependence allows us to
derive information about the
chemistry of a mineral from its
reflected or emitted light.
SELECTED DIGITAL SPECTRAL DATA BASES
• JPL Laboratory reflectance spectra of 2000 natural and man-made materials, 0.4 to 14 micrometers
• Contact: Dr. Simon HookJPL, MS 183-5014800 Oak Grove DrivePasadena, CA 91109Phone: 818-354-0974Fax: 818-354-0966E-mail: [email protected] Web: http://speclib.jpl.nasa.gov/
SELECTED DIGITAL SPECTRAL DATA BASES
• CSIRO Spectral Library
Contact:
Dr. Jon HuntingtonCSIRO
Division of Exploration & Mining
P.O. Box 136
North Ryde, N.S.W., 1670
Australia
Phone: +61-2-94908839
E-mail: [email protected] Web: http://www.syd.dem.csiro.au/research/MMTG/
SELECTED DIGITAL SPECTRAL DATA BASES
• USGS (Denver) Spectral Library
Contact:
Dr. Roger ClarkU.S.G.S.P.O. Box 25046, MS 964
Denver, CO 80225-0046
Phone: 303-236-1332Fax: 303-236-1425E-mail: [email protected]: http://speclab.cr.usgs.gov/