Geospatial Data / Remote Sensing Tutorial / Electromagnetic Radiation
Electromagnetic Radiation
Objectives
- Explain the role of electromagnetic radiation in remote sensing
- Describe the wave nature of radiation
- Identify and describe each of the types of electromagnetic radiation
- Describe the ways energy interacts with the Earth and its atmosphere
Introduction
In the definition of remote sensing, we described a remote sensing system
as having several components, including an energy source, transmission
path, target, and sensor. These components work together to measure and
record information about a target without actually coming into physical
contact with it. In order for this to happen, something must act as a
medium for transmitting information from the target to the sensor. In
most instances of environmental remote sensing, that something is electromagnetic
energy.
Electromagnetic Energy
Electromagnetic radiation is energy that travels in waves. The nature
of energy traveling in waves can probably be best visualized by considering
the waves in the ocean as they pass under a boat floating on its surface.
Before any waves approach the boat, the boat is in a neutral, or "resting,"
state. As the wave approaches and passes under the boat, the boat is raised,
or displaced, in an upward direction. As the wave passes by, the boat
is lowered beyond where its original resting state was. After the wave
has passed the boat entirely it returns to its resting state. This is
analogous to waves of radiation, which pass through open space with very
similar qualities.
 Waves
have measurable properties that help us in describing radiation, including
wavelength, frequency, amplitude, and velocity. The point of maximum upward
displacement of a wave is called its crest, and the area of maximum downward
displacement called a trough. A wave's amplitude is defined as the magnitude
(or distance) of the vertical displacement caused by the wave. In more
general terms, amplitude can be thought of as the height of the wave.
The wavelength of a wave is defined as the distance between two successive
crests (or between two successive troughs). Frequency is a measure of
how many waves pass a fixed point in a given unit of time, and is therefore
dependent on the speed (velocity) at which the wave is traveling. Since
all electromagnetic energy travels at a constant speed (the speed of light),
the wavelength and frequency of different energy types are inversely related.
In other words, for electromagnetic energy, a general rule states that
the longer the wave, the lower the frequency, and the shorter the wave
the higher the frequency.
Electromagnetic energy is emitted, or given off, by all matter in our
universe at varying levels. In general, the higher the inherent energy
level in the energy source, the shorter the wavelength of the energy created
and the higher the frequency. This difference in energy wave characteristics
allows us to classify electromagnetic energy into groups that exhibit
similar wave property characteristics.
The Electromagnetic Spectrum
When all of the possible forms of radiation are classified and arranged
according to wavelength or frequency, the result is the Electromagnetic
Spectrum. The electromagnetic spectrum includes types of radiation that
range from extremely low energy, long wavelength, low frequency energy
like Radio energy to extremely high energy, short wavelength, high frequency
energy types such as x-ray and Gamma Ray radiation.
[NASA
Observatorium]
Radio Waves
Radio energy is the lowest energy level form of electromagnetic energy,
with wavelengths that range from thousands of kilometers to less than
a meter. Radio is actually divided into numerous sub-classifications that
are beyond the scope of this discussion, but suffice it to say that radio
energy has many common uses including communication and broadcasting.
One of the most common uses of radio technology related to remote sensing
is the study of the universe and the vast empty spaces between stars in
outer space (the interstellar medium). Hydrogen gasses in these spaces
at temperatures very close to Absolute Zero emit radiation in the form
of very low frequency radio energy. These radio waves are captured by
vast arrays of giant radio antennas arranged across acres of land and
which are aimed at a common point in the sky.
Radio energy is also used extensively in radar systems. A radar
system directs pulses of radio energy at a target and detects the reflection,
or echoes, of the energy as it bounces off its target. Weather radar systems
detect particles in the atmosphere, such as rain drops, ice crystals,
or birds. This makes radar especially useful in studying weather patterns,
locating storm centers, and observing the structure of storms. Radar systems
are also quite useful in studying the Earth from high altitudes, since
radio energy can readily penetrate the atmosphere. Radar sensors mounted
on high altitude aircraft and on space platforms such as the space shuttle
have been used to generate detailed 3D maps the surface of the Earth,
measure rainfall from space, determine the extent of ice floes in the
polar regions, and monitor environmental.
Microwave
Microwave radiation is the next type of energy on the electromagnetic
spectrum. It is commonly used as a means for communication and transmission
of information through open space, and it can be harnessed to cook food
in a specialized oven.
Its uses in remote sensing vary, and it is often used in active remote
sensing systems that direct microwave pulses at a target and measure the
reflection characteristics of the target. Other systems, such as the Tropical
Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI), measure
the minute amounts of microwave radiation emitted from the Earth's atmosphere
in order to quantify the water vapor, the cloud water, and the rainfall
intensity in the atmosphere.
Infrared Radiation
Infrared radiation, follows microwave radiation on the electromagnetic
spectrum. Infrared radiation can be emitted from an object or reflected
off a surface. Emitted infrared is detected as heat energy, and is therefore
known as thermal infrared. This would include the heat energy that is
felt radiating off a hot surface such as a stove burner. Reflected energy
is more similar to visible light energy and is known as near infrared
because of its location just outside (or near) the visible portion of
the electromagnetic spectrum.
Infrared sensors are especially important to remote sensing. The Earth's
surface emits thermal infrared radiation, which can be detected from aircraft
or satellite sensors after it passes through the atmosphere and travels
through space. This information can be used to construct temperature profiles
of the Earth's land and water surfaces and to observe the temperature
of cloud tops in the atmosphere, which provide us with clues about weather
patterns.
Vegetation on the Earth's surface reflects near infrared radiation in
varying amounts, depending on many factors including the pigment composition
of the plant species, the amount of water available to the plants, the
presence of pest or disease stress on the vegetation, and the time of
year (within the growing season). Thus, near infrared sensors can be used
to quantify vegetative cover on the surface and construct vegetation profiles
of the planet.
The image below illustrates both thermal and near infrared remote sensting.
This image of the entire Earth is designed to illustrate sea surface temperatures
and vegetation distribution across the planet. Sea surface temperature
measurements rely on the detection of thermal infrared radiation emitted
from the ocean surfaces. The warmest temperatures are a deep red, while
the coldest are a dark blue. The land surfaces are colored to depict global
vegetative distribution, with the most vegetation depicted in dark greens
and browns, while the lightest green color indicates the sparsest vegetation
distribution. These measurements rely on near infrared (and visible light)
energy reflected from leaf surfaces and detected by a satellite.
[Global sea surface temperatures and vegetation distribution]
Visible Light
Visible light is in the middle of the electromagnetic spectrum, and it
is the type of energy that the human eye is capable of detecting. It forms
a narrow band of wavelengths that are commonly separated into a collection
of component colors. Each color is really just a smaller range of wavelengths
within the visible light spectrum. These varying wavelengths of light
are detected by the human eye and translated into a color by the brain.
These colors are often described as including red, orange, yellow, green,
blue, and violet.
When white visible light passes through materials it can be bent, or
refracted, and the component colors can be separated. This is the basic
concept behind the creation of a rainbow or the formation of a colored
spectrum when light passes through a prism. Visible light that strikes
an opaque surface is selectively absorbed, meaning that some specific
wavelenghts (or colors) are absorbed while others are reflected. It is
the collective wavelengths of visible light reflected off a surface that
gives an object its color when it reaches the eye.
Similarly, environmental remote sensing satellites can detect visible
light from the sun as it reflects off the Earth. Visible light sensors
can be developed to detect varying wavelengths of light that are reflected
from the Earth's surface and then used to study weather patterns, vegetation
distribution, mineral content of soils, and phytoplankton production in
oceans. The following example is an image of central Maryland taken by
Landsat 7 that is a color composite which depicts the appearance of the
surface in red, green, and blue wavelengths.
(VIS composite Landsat 7 image)
Ultraviolet, Radiation, X-Rays, and Gamma Ray
Ultraviolet, X-Ray, and Gamma Ray radiation form the high end portion
of the electromagnetic spectrum. Each of these types of radiation are
naturally ocurring, and each tends to be detected in association with
very high energy events, such as star formation, nuclear reactions on
star surfaces, explosions of stars, and the destruction of materials as
they fall under the influence of a black hole.
Ultraviolet is located just beyond the range of visible light radiation
(hence the name: "ultra," meaning beyond, and "violet,"
the last color in the visible spectrum). Ultraviolet (UV) radiation from
the sun is a key culprit in causing skin damage and cancer. X-Ray radiation
follows UV on the spectrum. Perhaps the best known application of X-Ray
radiation is medical imaging of the body. Gamma Ray radiation is the form
of electromagnetic radiation that has the highest energy level. One widely
used application of gamma ray radiation is cancer treatement, which uses
exposure to gamma ray radiation in the attempt to destroy and eliminate
cancerous tumors from a patient's body.
The sun emits each of these forms of high energy radiation in levels
that would be deadly to any living organism on the Earth's surface. Fortunately,
each of these forms of energy are absorbed readily by materials in the
atmosphere and only reach the surface in minute quantites. Since the atmosphere
so readliy absorbs high energy radiation, it is not especially useful
for terrestrial remote sensing. One example of its use is the study of
the Earth's auroras, which are caused by high energy radiation from the
sun interacting with the particles in the extreme upper atmosphere of
the Earth. These interactions result in the creation of UV energy, which
can be detected by satellites such as NASA's Ultraviolet Imageer. A typical
data product from this satellite is shown below.
Another example of a remote sensing technique that relies on high energy
radiation is to compare views of the sun in various spectral bands. The
following sequence of images depics the sun in a visible image, ultraviolet
image, and X-ray image. Each image has been generated by a different sensor
at a different time.
Emission of Electromagnetic Radiation
All matter at temperatures greater than absolute zero (0 Kelvin, -273
C) continuously emits (gives off) electromagnetic radiation. Generally,
the hotter an object, the higher its energy level and the more it radiates.
The reverse is also true, the colder a material, the lower its energy
level. All materials in the universe emit energy of some form. A material
that emits no radiation at all is known as a black body, which is a theoretical
concept and does not exist in nature.
The energy level of the material determines the type of radiation that
is emitted from its surface. Surfaces actually emit radiation in a range
of wavelengths, with the highest amount of energy concentrated in one
portion of the spectrum, known as maximum emission. The surface temperature
of the emitting material determines the wavelength of maximum emission.
In general, the higher the surface temperature of a material, the shorter
the wavelength of the radiation at maximum emission.
Very high energy events such as stellar formation emit radiation at the
highest energy levels. This is why ultraviolet, x-ray, and gamma ray detection
is an important tool in studying the formation and evolution of stars
and galaxies. Conversely, the extremely cold clouds of gasses in the spaces
between galaxies (the interstellar medium) are best observed in very low
energy forms of radiation such as long wave radio.
In terrestrial remote sensing, the temperatures of the Sun and the Earth's
surfaces play an important role in determining what types of energy will
be collected. The Sun, with a surface temperature of approximately 6000
Kelvin, emits radiation that peaks in the visible portion of the spectrum.
The Earth, with a temperature of approximately 300 Kelvin, emits radiation
that peaks in the infrared portion of the spectrum. Since these two forms
of energy are the most readily available, the energy detected during terrestrial
remote sensing is either reflected visible energy from the Sun or thermal
infrared energy emitted from the Earth.
Energy Interactions
Electromagnetic waves that originate on the sun are radiated through
space and eventually enter the Earth's atmosphere. In the atmosphere,
the radiation interacts with atmospheric particles, which can absorb,
scatter, or reflect it back into space. Much of the sun's high-energy
radiation is absorbed by the atmosphere, preventing it from reaching the
Earth's surface. This absorption of energy in the upper atmosphere is
an important factor in allowing life to flourish on the Earth. Atmospheric
particles such as dust, sea salt, ash, and water droplets will reflect
energy back into space. Visible light can be scattered by particles in
the atmosphere, allowing only selected wavelengths to penetrate to the
surface.
A portion of the energy is able to penetrate the atmosphere, allowing
it to reach the Earth's surface. Radiation that is able to penetrate the
material and pass through it is said to be transmitted. Most wavelengths
of visible light energy from the sun are transmitted through the atmosphere,
allowing it to come into contact with the Earth's surface. Once this radiation
reaches the surface, it interacts with the surface materials where it
can be reflected back into space or absorbed and red-emitted as thermal
infrared energy.
Radiation is absorbed by surface materials through electron or molecular
reactions within the material. A portion of this energy is then emitted
by the material that absorbed it, usually at longer wavelengths, while
some of it remains and heats the target. For example, the ozone layer
in the upper atmosphere absorbs much of the high-energy ultraviolet radiation
emitted from the sun. This mechanism protects the earth's surface from
the damaging effects of this high energy radiation. Some of this energy
is then red-emitted as heat energy, resulting in a warmer layer in the
upper atmosphere than one might expect. Another example of absorption
occurs on the Earth's surface, which absorbs radiation from the sun at
varying levels. Much of the solar energy that is absorbed by the Earth's
surface is red-emitted as heat energy, and this is largely responsible
for keeping the temperatures near the Earth's surface at a level which
allows life to flourish.
Radiation is also reflected off various materials in the atmosphere and
on the planet's surface. Clouds, smoke, and other particles in the atmosphere
reflect varying levels of radiation back into space. This would explain
why cloudy days are often cooler than days when the skies are clear; the
clouds prevent the radiation from the sun from being transmitted to the
surface. This is also one of the key concepts behind the theory of the
dinosaur extinction that hypothesizes that dinosaurs disappeared when
a large asteroid struck the Earth, throwing tremendous amounts of dust
into the atmosphere which blocked light and heat from reaching the Earth's
surface. It is suggested that the resulting change in climate was responsible
for the elimination of the dinosaurs.
References
NASA
Earth Observatory: Remote Sensing
http://earthobservatory.nasa.gov/Library/RemoteSensing/
NASA
Observatorium Education-Reference Module: Electromagnetic Spectrum
http://observe.arc.ndasa.gov/nasa/education/reference/emspec/emspectrum.html
NASA
Observatorium Education-Reference Module: MultiSpectral Remote Sensing
http://observe.arc.ndasa.gov/nasa/education/reference/multi/spectrum.html
NASA
Observatorium Education-Reference Module: Thermal Infrared
http://observe.arc.ndasa.gov/nasa/education/reference//therm/therm_1.html
NASA
Observatorium Education-Reference Module: Reflected Infrared
http://observe.arc.ndasa.gov/nasa/education/reference/reflect/ir.html
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