ELECTROMAGNETIC
RADIATION
The electromagnetic radiation (EM) is the source of
all signals collected by most remote sensing instruments. The source of this
energy varies depending on the sensor characteristics. Most systems rely on the
sun to generate all the EM energy needed to image terrestrial surfaces. These
systems are called passive sensors. Other sensors generate
their own energy, called active sensors, transmitts that energy
in a certain direction and records the portion reflected back by features within
the signal path.
Electromagnetic energy can be generated by
changes in the energy levels of electrons, acceleration of electrical charges,
decay of radioactive substances, and the thermal motion of atoms and molecules.
Nuclear reactions within the sun produce a full spectrum of EM radiation which
is transmitted through space without major changes in its character until it
reaches the atmosphere.
EM radiation consists of an electrical field that
varies in magnitude in a direction perpendicular to the direction of
propagation.
WAVELENGTH - Distance from one wave crest to another.
Measured in meters or fractions of meters.
FREQUENCY - Number of wave crests passing a fixed
point in a given period of time. Measured
in hertz (1 cycle per second)
AMPLITUDE - Height of each peak. Measured as spectral
irradiance (watts per square meter per
micrometer) or (energy level per
wavelength interval)
The speed of EM energy is constant and frequency
and wavelength are related:
299,893 km/sec = WAVELENGTH *
FREQUENCY
The EM spectrum is arbitrarily segmented into
major divisions. There are no natural breaks in the em spectrum. These
separations are made by us for our convenience.
UV - LT .4 Micrometers
Causes fluorescence and is good in some
geological and vegetation applications.
Big sagebrush (artemisia spp.) fluorecess
under ultra-violet light. Some flowers also fluoress under UV light allowing
insects to locate nectar reservoirs.
Not much is done with UV for remote sensing
since these shorter wavelengths are easily scattered by the atmosphere making
spaceborne and some airborne sensors impractical.
VISIBLE - This is the small portion of the EM spectrum that
humans are sensitive to.
- BLUE (.4-.5 micrometers)
- GREEN (.5-.6 micrometeres)
- RED (.6-.73 micrometers)
INFRARED SPECTRUM - .72 - 15 micrometers
- There are three logical zones in the IR spectrum:
- NEAR INFRARED - reflected, can be recorded on film emulsions.
- MID INFRARED - reflected, can be detected using electro-optical sensors.
- THERMAL INFRARED - emitted, can only be detected using electro-optical sensors.
MICROWAVE - Radar sensors, wavelengths range from 1mm to 1m
For an excellent reading on the EM spectrum look at this site at the Jet Propulsion Laboratory.
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Author: R. Douglas Ramsey Doug@nr.usu.edu