GEOG 474
Energy Sources and Radiometric Principles
Introduction
4 basic components of a remote sensing system
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energy source
transmission path
target
sensor
Medium for transmitting information from target to sensor electromagnetic energy
Discuss
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basic definition of electromagnetic energy
sources of electromagnetic energy
models and characteristics of electromagnetic waves
Electromagnetic Radiation (emr)
Electromagnetic energy (radiation) is one of many forms of energy (such as chemical,
electrical, kinetic, magnetic, nuclear, or thermal).
EMR is the source of 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 earth's
atmosphere and land surfaces. These systems are called passive sensors. Other sensors
generate their own energy, called active sensors, transmits 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.
Emr consists of an electrical and magnetic field that varies in magnitude in a direction
perpendicular to the direction of propagation.
Electromagnetic Spectrum (ems)
EMS represents the continuum of electromagnetic energy from extremely short wavelengths
(cosmic and gamma rays) to extremely long wavelengths (microwaves). Spectrum is
arbitrarily segmented into major divisions. There are no natural breaks in the ems. These
separations are made by us for our convenience.
UV - 3 nanometers - .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 - small portion of the EMS that humans are sensitive to
BLUE (.4-.5 micrometers)
GREEN (.5-.6 micrometers)
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. (0.7 - 1.3
micrometers)
MID INFRARED - reflected, can be detected using electro-optical sensors. (1.3 - 3.0
micrometers)
THERMAL INFRARED - emitted, can only be detected using electro-optical
sensors. (3.0 - 5.0 and 8 - 14 micrometers)
MICROWAVE - Radar sensors, wavelengths range from 1mm to 1m
Units of Wavelength
The basic unit in which wavelengths are measured in the meter (m). In remote sensing, most
energy in the visible and infrared portions of the electromagnetic spectrum is measured in
micrometers (10-6 m). However, some wavelengths (such as radio and microwaves) are too
long for the micrometer to be a convenient unit of measure. For example, while the
wavelength of blue light is approximately 0.4-0.5 micrometers, a radio wave is in the
neighborhood of 100,000,000 micrometers long (100 m)! You should be aware that visible
wavelengths (including ultraviolet, visible, and near infrared) are frequently referred to in
units other than the micrometer. Astronomers use a unit called angstrom (10-10 m) to measure
these wavelengths. One micrometer equals 10,000 angstroms. Occasionally you may run
across this unit when reading satellite documentation from NASA, although most of the
information they have for remote sensing audiences uses micrometers. Also, some of the older
literature in remote sensing refers to micrometers as microns, and many of the biological
sciences still use "micron". One micron equals one micrometer.
Basic Principles of Electromagnetic Energy
Modern physics view EMR as having dual nature, enabling it to be independently described
as a wave or a particle.
Wave Model (basic wave theory - Maxwell's equations)
Shows EMR carried by a series of continuous waves that are equally and repetitively spaced
in time (harmonic waves)
Wave pattern is in the form of 2 fluctuating fields - one electric and the other magnetic. Each
has a sinusoidal shape because their plots resemble sine curves.
Paired fields are perpendicular to each other, and both are perpendicular to direction of wave
propagation (transverse waves)
Wave nature of EMR is characterized by:
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wavelength
frequency
Wavelength (lambda) - linear distance between 2 successive wave crests or
troughs
Frequency (v or f) - # of wave crests or troughs (cycles) that pass a fixed
point per second
Wavelength and frequency are related to the velocity of an electromagnetic wave (speed of
light) speed of light (c) = frequency (f) x wavelength (lambda) (1)
- frequency and wavelength are directly proportional to velocity which is essentially a
constant
- electromagnetic energy travels at the speed of light 2.99983x108 (3x108 ) ms-1 (186,000
miles s-1)
- wavelength and frequency have an inverse relationship
Particle Model
Emphasizes behavior of EMR as if EMR were composed of a collection of discrete, particlelike objects called quanta or photons, in which electromagnetic energy is transferred at the
speed of light.
Energy of a quantum is given as:
Q = h f = (h c) / lambda (2)
Q - energy of quantum [Joules - J]
h - Plank's constant [6.26x10-34 J s]
- direct relationship between frequency and energy (energy of a photon varies directly with
frequency)
- inverse relationship between wavelength and energy (energy of a photon varies inversely
with wavelength)
Relate wave model and quantum model of emr (Equation 1 and 2)
1. solving
2. substituting
for f
yielding
intoQ=hf yielding
This equation shows that the shorter the wavelength, the higher the energy.
For this reason, shorter wavelengths are easier to sense than very long ones such as passive
terrestrial microwave emissions
Summary
Remote sensing is concerned with the measurement of EMR returned by the Earth's natural
and man-made features that first receive energy from the sun or an artificial source such as a
radar transmitter.
Different objects return different types and amounts of EMR.
Objective of remote sensing is to detect these differences with the appropriate instruments.
Differences make it possible to identify and assess a broad range of surface features and their
conditions
Energy Sources
Electromagnetic waves are radiated through space from some source.
When the energy encounters an object, even a very tiny one like a molecule of air, one of
three reactions occurs.
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reflected off the object,
absorbed by the object, or
transmitted through the object.
Total amount of radiation that strikes an object is incident radiation reflected radiation + absorbed radiation + transmitted radiation (3)
In remote sensing, we are largely concerned with REFLECTED RADIATION. Reflected
radiation causes our eyes to see colors, causes infrared film to record vegetation, and allows
radar images of the earth to be created. Source of a vast majority of this reflected radiation is
the sun.
While the sun is the most obvious source of the electromagnetic energy measured in remote
sensing, it is not the only energy source one might encounter. This is because all matter at
temperatures greater than absolute zero (0 Kelvin) continuously emits electromagnetic
radiation. Generally, the hotter an object is, the more it radiates, but all objects with even the
slightest sub-molecular motion radiate some energy.
Remote Sensing uses electromagnetic energy from both natural and man-made sources.
Blackbody Model
Blackbody
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is a perfect absorber and emitter of radiation
all radiation incident on a blackbody object is re-emitted
emittance is a function only of temperature.
In nature, true blackbodies do not exist. However, many objects approximate
blackbodies.
Blackbody curves in figure
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show the amount of energy radiated at each wavelength for blackbodies of
various temperatures
red line is the blackbody curve for the earth, whose ambient temperature (the
"average" temperature given off by the soil, water, vegetation, and built
environment) is about 300 Kelvin
area under this curve is the total energy emitted across all wavelengths by the
earth. (Stefan-Boltzmann Law).
There is a direct relationship between the temperature of a blackbody and the
amount of electromagnetic energy it emits. The hotter the object, the more
energy it gives off.
Even though a perfect blackbody is only a theoretical construct, most objects in
nature behave like "imperfect" blackbodies. Consider the theoretical curve
created by a perfect blackbody and the actual curve created by the Sun.
Blackbody curves for hotter temperatures "peak" at lower wavelengths than do
the curves for cooler temperature objects (Wien's Displacement Law)
o
wavelength at which the largest portion of energy is emitted depends on
temperature.
Hotter objects emit more energy at lower wavelengths than do cooler
objects
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6000 K curve (Sun) peaks at 0.5 micrometers
300 K curve (Earth) peaks at about 9 micrometers
Radiation Laws
Plank's Radiation Law for Blackbodies gives the spectral radiance of an object as a function
of its temperature.
Wien's Displacement law
If we differentiate Plank's Radiation Law for blackbodies and set it equal to zero, we arrive at
a formula which gives the wavelength of maximum radiance for a blackbody of a given
temperature. This formula is referred to as Wien's Displacement Law.
Finally, if a blackbody is acting as a perfect emitter, the total emitted energy over the whole
spectrum is given by the
Stefan-Boltzmann law:
Source materials: Remote Sensing Core Curriculum and Utah State Geography Department
Remote Sensing Lecture Materials
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Last revised on September 22, 1999 by Tracy DeLiberty.