Interaction of EMR with Earth's Surface

 

Electromagnetic radiation that passes through the earth's atmosphere without being absorbed or scattered reaches the earth's surface to interact in different ways with different materials constituting the surface.

There are three ways in which the total incident energy will interact with earth's surface materials.  These are

  • Absorption

  • Transmission, and

  • Reflection

Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission (T) occurs when radiation passes through a target. Reflection (R) occurs when radiation "bounces" off the target and is redirected.

How much of the energy is absorbed, transmitted or reflected by a material will depend upon:

    Wavelength of the energy

    Material constituting the surface, and

    Condition of the feature.

         In remote sensing, we are most interested in measuring the radiation reflected from targets.

Reflection from surfaces occurs in two ways:

  1. When the surface is smooth, we get a mirror-like or smooth reflection where all (or almost all) of the incident energy is reflected in one direction.  This is called Specular Reflection and gives rise to images.

  2. When the surface is rough, the energy is reflected uniformly in almost all directions.  This is called Diffuse Reflection and does not give rise to images.

 

Specular Reflection

Diffuse Reflection

Most surface features of the earth lie somewhere between perfectly specular or perfectly diffuse reflectors.  Whether a particular target reflects specularly or diffusely, or somewhere in between, depends on the surface roughness of the feature in comparison to the wavelength of the incoming radiation.

  • If the wavelengths are much smaller than the surface variations or the particle sizes that make up the surface, diffuse reflection will dominate.  For example, fine-grained sand would appear fairly smooth to long wavelength microwaves but would appear quite rough to the visible wavelengths. 

Let's take a look at a couple of examples of targets at the Earth's surface and how energy at the visible and infrared wavelengths interacts with them.

Vegetation:

A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths.

  • Leaves appear "greenest" to us in the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll in the leaves, so there is less absorption and proportionately more reflection of the red wavelengths, making the leaves appear red or yellow (yellow is a combination of red and green wavelengths).

  •  The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths. If our eyes were sensitive to near-infrared, trees would appear extremely bright to us at these wavelengths. In fact, measuring and monitoring the near-IR reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation may be.

Water:

Longer wavelength visible and near infrared radiation is absorbed more by water than shorter visible wavelengths. Thus water typically looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed at red or near infrared wavelengths.

  • If there is suspended sediment present in the upper layers of the water body, then this will allow better reflectivity and a brighter appearance of the water.

  • The apparent colour of the water will show a slight shift towards longer wavelengths.

  • Suspended sediment (S) can be easily confused with shallow (but clear) water, since these two phenomena appear very similar.

  • Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green, making the water appear more green in colour when algae is present.

  •  The topography of the water surface (rough, smooth, floating materials, etc.) can also lead to complications for water-related interpretation due to potential problems of specular reflection and other influences on colour and brightness.

  • We can see from these examples that, depending on the complex make-up of the target that is being looked at, and the wavelengths of radiation involved, we can observe very different responses to the mechanisms of absorption, transmission, and reflection.

Spectral Response of Materials:

By measuring the energy that is reflected (or emitted) by targets on the Earth's surface over a variety of different wavelengths, we can build up a spectral response for that object.  The spectral response of a material to different wavelengths of EMR can be represented graphically as a Spectral Reflectance Curve.

It may not be possible to distinguish between different materials if we were to compare their response at one wavelength.  But by comparing the response patterns of these materials over a range of wavelengths (in other words, comparing their spectral reflectance curves), we may be able to distinguish between them.  For example, water and vegetation may reflect somewhat similarly in the visible wavelengths but are almost always separable in the infrared.

         Spectral response can be quite variable, even for the same target type, and can also vary with time (e.g. "green-ness" of leaves) and location.

  • Knowing where to "look" spectrally and understanding the factors which influence the spectral response of the features of interest are critical to correctly interpreting the interaction of electromagnetic radiation with the surface.

   


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S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooq.amu@gmail.com