Color and Spectral Signatures
Steve Dutch
University of Wisconsin-Green Bay
It’s a
Colorful
World
Color and Wavelength
Color Formalisms
• Physical Optics
– Scattering is most important in Remote Sensing
• Color Centers and Crystal-Field Effects
– Individual atoms interact with light
• Inter-Atomic Interactions
– Charge transfer
– Molecular orbitals
– Vibrations
• Band Theory
– Applicable to radar, not much to passive
remote sensing
Physical Optics
• Scattering
– Particulate Matter
• Dispersion
– Rainbows, etc.
• Interference
– Oil slicks, sea shells
• Diffraction
– Opal, CD’s
Scattering
Scattering
Anti-Crepuscular Rays
Major Types of Scattering
• Rayleigh scattering
– Small particle compared to wavelength of light
• Mie scattering
– Particle about the same size as wavelength of light
• Geometric scattering
– Particle much larger than wavelength of light
Rayleigh Scattering
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Molecules and nanoparticles
Proportional to 1/λ4
Blue vs. Red = (700nm/400nm)4 = 9.4
Why the sky is blue
Why isn’t the sky purple?
– Less sunlight
– Lower eye sensitivity
– Greater absorption
• UV Scattering useful in particle studies
Mie Scattering
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Responsible for most white colors
Cloud particles
Oil droplets in contaminated water
Suspended sediment
Algal blooms
Multiple and
Mie
Scattering
Backscattering
Atomic
Emission and
Absorption
Quantum Pinball
Fluorescence
(Sunlight and
laser induced
fluoresence in
chlorophyll)
Phosphorescence
Electrons and Light
Coloring Agents in Minerals
Iron is the Main Source of Color in
Rocks and Soils
Charge
Transfer
Charge Transfer
• Electron bumped between atoms
• Extremely effective absorption mechanism
• A major reason UV is not very useful for
surface remote sensing
• Very strong UV and blue absorption in iron
oxide
• Why we have “earth tones”
Molecular Orbitals
Molecular Orbitals
Chlorophyll
Water Vibration Modes
What Color is Water?
What Color is Ice?
Spectrum of Water