![Canada Centre for Remote Sensing Canada Centre for Remote Sensing](/web/20061103054102im_/http://www.ccrs.nrcan.gc.ca/esst_images/ccrs_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Canada Centre for Remote Sensing > Fundamentals of remote sensing
Introduction Radiation - Target Interactions
Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the Earth's surface. There are three (3) forms of interaction that can take place when energy strikes, or is incident (I) upon the surface. These are: absorption (A); transmission (T); and reflection (R). The total incident energy will interact with the surface in one or more of these three ways. The proportions of each will depend on the wavelength of the energy and the material and condition of the feature.
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. In remote sensing, we are most interested in measuring the radiation reflected from targets. We refer to two types of reflection, which represent the two extreme ends of the way in which energy is reflected from a target: specular reflection and diffuse reflection.
When a surface is smooth we get specular or
mirror-like reflection where all (or almost all) of the energy is directed away from the surface in a single direction. Diffuse reflection occurs when the surface is rough and the energy is reflected almost uniformly in all directions. Most earth surface features 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.
Leaves: 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 to 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.
![Spectral Response Pattern](/web/20061103054102im_/http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/images/spectrl_e.gif)
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. 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. By comparing the response patterns of different features we may be able to distinguish
between them, where we might not be able to, if we only compared them at
one wavelength. 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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"...now, here's something to 'reflect' on..."
![](/web/20061103054102im_/http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/images/dyk_1_5.gif)
... the colours we perceive are a combination of these radiation interactions (absorption, transmission, reflection), and represent the wavelengths being reflected. If all visible wavelengths are reflected from an object, it will appear white, while an object absorbing all visible wavelengths will appear colourless, or black.
![Graphic showing the moon being observed through a telescope](/web/20061103054102im_/http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/images/wq1_5.gif)
On a clear night with the crescent or half moon showing, it is possible to see the outline and perhaps very slight detail of the dark portion of the moon. Where is the light coming from, that illuminates the dark side of the moon?
The answer is ...
![Diagram showing how sunlight hits the earth, bounces up to the moon and then comes back to the earth and into your eye](/web/20061103054102im_/http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/images/wqa1_5.gif)
The light originates from the sun (of course), hits the earth, bounces up to the (dark side of the) moon and then comes back to the earth and into your eye. A long way around - isn't it?
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