Remote Sensing II

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ON THE ROAD to PRECISION AGRICULTURE
Remote Sensing – How Plants Interact with Electromagnetic Energy

Everything around us (including us) emits electromagnetic energy (assuming they are at temperatures above absolute zero). However, much of this is at wavelengths outside the visible band. Sunlight is important in remote sensing. When sunlight interacts with the plant, three things can happen to that energy - it can be:

bulletreflected (like our image in a mirror)
bullettransmitted (like sunlight through a window)
bulletabsorbed (like a sun bather "soaking up the rays")

Much of the light energy that we sense using remote sensing technologies is a combination of sunlight energy reflected by the plant and energy emitted by the plant. The dominant plant pigments involved are the chlorophylls. These compounds absorb energy in bluish (400-500 nm) and reddish (600-700 nm) wavelengths, supplying energy for the photosynthesis process. Reflected energy is the greenish (500-600 nm) wavelengths. Very little of the energy in the visible wavelengths is transmitted through the plant tissue.

During stress or senescence, chlorophyll absorption declines, and the carotenes and xanthophylls (absorption band around 450 nm) predominate, giving the plant tissue a yellow reflectance. Anthocyanins absorb bluish and greenish wavelengths, giving rise to reddish color plant tissue when chlorophyll levels are low (such as during the fall color season in our hardwood forests).

Near-infrared (NIR) energy (wavelengths above about 750 nm) is predominantly transmitted through plant material, but enough is reflected to be useful in estimating the amount of biomass. As the number of leaf layers increase, NIR reflectance also increases.

Remotely sensed images of a plant canopy can be analyzed for these differences in absorption of visible light and reflection of NIR energy. Generally, controlled experiments producing various levels of plant stress are used to generate a spectral response curve. These curves define how the response of the plant changes under stress or senescence conditions. Suspect areas in a field can be identified and assigned geographic coordinates which are useful for scouting specific areas in the field.

One processing technique that seeks to take advantage of these reflectance relationships in the visible and NIR is called the Normalized Difference Vegetation Index (NDVI). The index is generally used to produce an estimate of biomass in the field.

    NDVI = (NIR - Red) / (NIR + Red)

However, we need to remember that there are many factors that can affect the apparent light reflectance from plants, including:

bulletthe angle of the sun above the horizon
bulletwater in the air (water absorbs NIR wavelengths)
bulletparticulates in the air (particularly scattering bluish wavelengths)
bulletshadowing from clouds

In short, with the exception of NIR reflectance we all have personal experience with differences in light reflection. Remote sensing technologies try to quantify what our eyes see, and give the differences observed a specific geographic reference.

Roger Brook

Abstracted from The Precision-Farming Guide for Agriculturists by John Deere Publ., 1997 and The Physical Basis of Remote Sensing by Dr. David Lusch, Center for Remote Sensing & GISci, Michigan State University.

 

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This page was last updated on 10/08/01.