Spectral Signatures and Vegetation Analysis

Spectral Signatures

Electromagnetic radiation interacts with Earth's surface features through absorption, reflection, and transmission.

Surface Interactions

  • Absorption: Energy is absorbed by the surface, raising its temperature, and is re-emitted as heat.
  • Reflection: Energy is reflected off the surface (e.g., visible light reflecting off a leaf).
  • Transmission: Energy passes through the surface (e.g., light seen through a leaf).

Leaf Interactions Example

  • Visible light striking a leaf is partially reflected, creating the image we see.
  • The portion not reflected is absorbed or transmitted.
  • Absorbed energy raises the leaf's temperature and is re-emitted as heat.
  • A leaf's reflectance and absorption characteristics give it its color.

Spectral Signatures

  • A spectral signature is the pattern of spectral response of a material.
  • It is typically visualized with a graph.
  • The graph shows the percentage of radiation of different wavelengths reflected from an object.
  • By plotting spectral signatures of different materials together, the portions of the spectrum where their signatures differ can be readily identified.
  • Using multiple wavelengths in multi-dimensional space improves the ability to distinguish materials which is the basis for multispectral remote sensing.

Spectral Signature Graph

The graph typically plots:

  • Wavelength (nm or μm) on the x-axis.
  • Reflectance (%) on the y-axis.

Example Materials and Reflectance

  • Grasslands
  • Pinewoods
  • Red sand
  • Silty water

Example Wavelengths and Reflectance:

  • Grasslands: greatest reflectance at 0.82 \mu m, reflectance of 21\% at 0.6 \mu m
  • Pinewoods: greatest reflectance at 0.75 \mu m, reflectance of 18\% at 0.6 \mu m
  • Red sand: greatest reflectance at 0.59 \mu m, reflectance of 69\% at 0.6 \mu m
  • Silty water: greatest reflectance at 0.54 \mu m, reflectance of 10\% at 0.6 \mu m

Example Material Brightness:

  • Red sand is brightest at 0.6 \mu m.
  • Grasslands is brightest at 1.2 \mu m.

Spectral Analysis of Vegetation

Dominant Factors Controlling Leaf Reflectance

  • Water absorption bands: Occur at 0.97 \mu m, 1.19 \mu m, 1.45 \mu m, 1.94 \mu m, and 2.70 \mu m.

Leaf Structure

  • Upper epidermis and cuticle
  • Palisade parenchyma cells (containing chlorophyll)
  • Spongy parenchyma mesophyll cells
  • Lower epidermis
  • Intercellular air spaces

Impact of Leaf Components

  • Chlorophyll pigments in the palisade parenchyma mesophyll cells significantly impact the absorption and reflectance of visible light.
  • Spongy parenchyma mesophyll cells significantly impact the absorption and reflectance of NIR incident energy.

Reflectance Percentage Graph

The graph plots:

  • Wavelength \mu m on the x-axis.
  • Reflectance % on the y-axis.
  • Atmospheric Transmission % on a secondary y-axis.
Key Features
  • Chlorophyll absorption bands in the visible region.
  • NIR reflectance peak.
  • Water absorption bands in the middle-infrared region.

Colors of Autumn Leaves

  • Chlorophyll: Gives leaves their green color, breaks down in autumn.
  • Carotenoids (e.g., lutein, beta-carotene): Always present, responsible for yellows and oranges.
  • Anthocyanins: Produced in autumn, may protect leaves from excess light and cause the leaves to appear bright red.

Dominant Factors Controlling Visible Reflectance

  • Chlorophyll a peak absorption: 0.43 \mu m and 0.66 \mu m
  • Chlorophyll b peak absorption: 0.45 \mu m and 0.65 \mu m
  • Optimum chlorophyll absorption windows: 0.45 - 0.52 \mu m and 0.63 - 0.69 \mu m
  • Yellow carotenes & pale yellow xanthophyll pigments have strong absorptions in the blue \lambda’s.
  • \beta-carotene absorption spectra exhibits strong absorption at ~0.45\mu m.
  • Phycocyanin pigment absorbs primarily in the green and red regions at ~0.62\mu m.

Chlorophyll Dominance

  • When vegetation is healthy, chlorophyll pigments are dominant and can mask carotenes and other pigments.
  • During senescence or severe stress, chlorophyll dominance may be lost, causing other pigments to become dominant (e.g., at leaf fall).
  • Anthocyanin may also be produced in autumn, causing leaves to appear bright red.

Dominant Factors Controlling NIR Reflectance

  • In a typical healthy green leaf, NIR reflectance increases dramatically between 700 – 1200nm. Similarly, NIR absorption decreases.
  • In the NIR, healthy vegetation is normally characterized by:
    • High reflectance (40 – 60%)
    • High transmittance to underlying leaves (40 – 60%)
    • Relatively low absorption (5 – 10%)
  • High diffuse reflectance of NIR (700 – 1200nm) energy from plant leaves is due to internal scattering at the cell wall-air interface within the spongy mesophyll cells.
  • Leaves reflect so much NIR energy because:
    • The leaf already reflects 40 – 60% of the incident NIR energy from the spongy mesophyll.
    • The remaining 45 – 50% of the energy penetrates (i.e., transmitted) through the leaf and can be reflected once again by leaves below it.

Dominant Factors Controlling MIR / SWIR Reflectance

  • Water vapor in the atmosphere creates five major absorption bands across the NIR to middle-infrared (MIR) wavelengths: 0.97, 1.19, 1.45, 1.94, and 2.7 \mu m.
  • Water content in leaves creates water absorption bands at similar wavelengths.
  • There is also a strong relationship between the reflectance in the MIR region from 1.3 – 2.5 \mu m and the amount of water present in leaves.
  • Water in leaves absorb incident energy between the absorption bands with increasing strength at longer wavelengths.
  • Water is a good absorber of MIR energy, so the greater the water content of the leaves, the lower the MIR reflectance.
  • Conversely, as the amount of plant water in intercellular spaces decreases, this causes greater MIR leaf reflectance.

Summary of Dominant Factors

  • 0.4 – 0.75 \mu m: the various leaf pigments in the palisade parenchyma (e.g., chlorophyll a & b, and \beta-carotene).
  • 0.75 – 1.35 \mu m: the scattering (i.e., repeated reflectance and transmission) of near-infrared (NIR) energy in the spongy mesophyll.
  • 1.35 – 2.8 \mu m: the amount of water in the plant.

Basis of Vegetation Indices

  • Chlorophyll absorption in the red region.
  • Reduced Chlorophyll Absorption.
  • Carotenoid and Chlorophyll Absorption.
  • The Red Edge.

Vegetation Indices

Infrared/Red Ratio Vegetation Index

  • The near-infrared (NIR) to red simple ratio (SR) is the first true vegetation index.
  • It takes advantage of the inverse relationship between chlorophyll absorption of red radiant energy and increased reflectance of near-infrared energy for healthy plant canopies.

Normalized Difference Vegetation Index (NDVI)

  • The generic normalized difference vegetation index (NDVI) has provided a method of estimating net primary production over varying biome types, identifying ecoregions, monitoring phenological patterns of the earth’s vegetative surface, and assessing the length of the growing season and dry-down periods.
  • NDVI = (NIR - Visible) / (NIR + Visible)

Example NDVI Calculation:

  • Near-infrared: 50%
  • Visible: 8%
  • NDVI = (0.50 - 0.08) / (0.50 + 0.08) = 0.72

Example NDVI Calculation:

  • Near-infrared: 40%
  • Visible: 30%
  • NDVI = (0.40 - 0.30) / (0.40 + 0.30) = 0.14

Important Topics

  • Which surface interaction is used by Earth observing satellites to take images of the surface of the earth?
  • What are remote sensing scientists able to accomplish by comparing the spectral signatures of different materials?
  • What does NDVI stand for, and what parts of the EMS does it take advantage of when measuring vegetation?
  • What are the dominant factors controlling the spectral response of leaves in the visible, NIR, and MIR part of the spectrum?