Field Spectroscopy, Hyperspectral Imaging,

Applications in Vegetation and Soils Analysis

Alexander F. H. Goetz

University of Colorado and

Analytical Spectral Devices Inc.

goetz@cses.colorado.eduBeijing 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 gswayze@usgs.gov• 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: Simon.J.Hook@jpl.nasa.gov 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: Jon.Huntington@csiro.au 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: rclark@usgs.govWeb: http://speclab.cr.usgs.gov/