field spectroscopy, hyperspectral imaging, applications in vegetation and soils analysis alexander...

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/