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?