Principles: Electromagnetic Energy

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Energy Sources and Physical Principles
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• Energy Source or Illumination (A) - the first
  requirement for remote sensing is to have an
  energy source which illuminates or provides
  electromagnetic energy to the target of interest.
  
• Radiation and the Atmosphere (B) - as the energy
  travels from its source to the target, it will
  come in contact with and interact with the
  atmosphere it passes through. This interaction
  may take place a second time as the energy
  travels from the target to the sensor.

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• Interaction with the Target (C) - once the energy
  makes its way to the target through the
  atmosphere, it interacts with the target
  depending on the properties of both the target
  and the radiation.
• Recording of Energy by the Sensor (D) - after the
  energy has been scattered by, or emitted from the
  target, we require a sensor (remote - not in
  contact with the target) to collect and record
  the electromagnetic radiation.

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• Transmission, Reception, and Processing (E) - the
  energy recorded by the sensor has to be
  transmitted, often in electronic form, to a
  receiving and processing station where the data
  are processed into an image (hardcopy and/or
  digital).
• Interpretation and Analysis (F) - the processed
  image is interpreted, visually and/or digitally
  or electronically, to extract information about
  the target which was illuminated.

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• Application (G) - the final element of the remote
  sensing process is achieved when we apply the
  information we have been able to extract from the
  imagery about the target in order to better
  understand it, reveal some new information, or
  assist in solving a particular problem.

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ENERGY SOURCES
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Energy Sources

• Visible Light is only one form of electromagnetic
  energy.
• Radio waves, heat, ultra-violet rays and X-rays
  are other familiar forms.
• All of this energy is inherently similar, and
  radiates in accordance with basic wave theory.

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Wave Theory

• Electromagnetic radiation consists of an
  electrical field (E) which varies in magnitude in
  a direction perpendicular to the direction in
  which the radiation is travelling, and a magnetic
  field (M) oriented at right angles to the
  electrical field.
• Both these fields travel at the speed of light (c)

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Wavelength and Frequency

• Wavelength is measured in metres (m) or some
  factor of metres such as
• nanometers (nm, 10-9 metres),
• micrometers (mm, 10-6 metres) or
• centimetres (cm, 10-2 metres).
• Frequency refers to the number of cycles of a
  wave passing a fixed point per unit of time.
  Frequency is normally measured in hertz (Hz),
  equivalent to one cycle per second, and various
  multiples of hertz.

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Wave Theory

• From basic physics, waves obey the general
  equation
• c v l
• Since c is essentially a constant (3 x 108
  m/sec), frequency v and wavelength l for any
  given wave are related inversely, and either term
  can be used to characterise a wave into a
  particular form.

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Electromagnetic Spectrum
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Electromagnetic Spectrum

• The electromagnetic spectrum ranges from the
  shorter wavelengths (including gamma and x-rays)
  to the longer wavelengths (including microwaves
  and broadcast radio waves).
• There are several regions of the electromagnetic
  spectrum which are useful for remote sensing.

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Visible Spectrum

• The light which our eyes - our "remote sensors" -
  can detect is part of the visible spectrum.
• It is important to recognise how small the
  visible portion is relative to the rest of the
  spectrum.
• There is a lot of radiation around us which is
  "invisible" to our eyes, but can be detected by
  other remote sensing instruments and used to our
  advantage.

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Visible Spectrum

• The visible wavelengths cover a range from
  approximately 0.4 to 0.7 mm.
• The longest visible wavelength is red and the
  shortest is violet.
• It is important to note that this is the only
  portion of the EM spectrum we can associate with
  the concept of colours.

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VIOLET 0.400 - 0.446 mm BLUE 0.446 - 0.500
mm GREEN 0.500 - 0.578 mm YELLOW 0.578 -
0.592 mm ORANGE 0.592 - 0.620 mm RED 0.620
- 0.700 mm
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Visible Spectrum

• Blue, green, and red are the primary colours or
  wavelengths of the visible spectrum.
• They are defined as such because no single
  primary colour can be created from the other two,
  but all other colours can be formed by combining
  blue, green, and red in various proportions.
• Although we see sunlight as a uniform or
  homogeneous colour, it is actually composed of
  various wavelengths.
• The visible portion of this radiation can be
  shown when sunlight is passed through a prism

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Infrared(IR)Region

• The IR Region covers the wavelength range from
  approximately 0.7 mm to 100 mm - more than 100
  times as wide as the visible portion!
• The infrared region can be divided into two
  categories based on their radiation properties -
  the reflected IR, and the emitted or thermal IR.

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Reflected and Thermal IR

• Radiation in the reflected IR region is used for
  remote sensing purposes in ways very similar to
  radiation in the visible portion. The reflected
  IR covers wavelengths from approximately 0.7 mm
  to 3.0 mm.
• The thermal IR region is quite different than the
  visible and reflected IR portions, as this energy
  is essentially the radiation that is emitted from
  the Earth's surface in the form of heat. The
  thermal IR covers wavelengths from approximately
  3.0 mm to 100 mm.

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Microwave Region

• The portion of the spectrum of more recent
  interest to remote sensing is the microwave
  region from about 1 mm to 1 m.
• This covers the longest wavelengths used for
  remote sensing.
• The shorter wavelengths have properties similar
  to the thermal infrared region while the longer
  wavelengths approach the wavelengths used for
  radio broadcasts.

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Radiation Emission
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Emission of Radiation from Energy Sources

• Each energy/radiation source, or radiator, emits
  a characteristic array of radiation waves.
• A useful concept, widely used by physicists in
  the study of radiation, is that of a blackbody.
• A blackbody is defined as an object or substance
  that absorbs all of the energy incident upon it,
  and emits the maximum amount of radiation at all
  wavelengths.
• A series of laws relate to the comparison of
  natural surfaces/radiators to those of a
  black-body

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Stefan-Boltzmann Law

• All matter at temperatures above absolute zero
  (-273 oC) continually emit EM radiation. As well
  as the sun, terrestrial objects are also sources
  of radiation, though of a different magnitude and
  spectral composition than that of the sun.
• The amount of energy than an object radiates can
  be expressed as follows
• M s T4
• M total radiant exitance from the surface of a
  material (watts m-2)
• s Stefan-Boltzmann constant, (5.6697 x 10-8 W
  m-2 K-4)
• T absolute temperature (K) of the emitting
  material

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Stefan-Boltzmann Law

• It is important to note that the total energy
  emitted from an object varies as T4 and therefore
  increases rapidly with increases in temperature.
• Also, this law is expressed for an energy source
  that behaves like a blackbody, i.e. as a
  hypothetical radiator that totally absorbs and
  re-emits all energy that is incident upon
  it.actual objects only approach this ideal.

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Kirchoffs law

• Since no real body is a perfect emitter, its
  exitance is less than that of a black-body.
• Obviously it is important to know how the real
  exitance (M) compares with the black-body
  exitance (Mb)
• This may be established by looking at the ratio
  of M/Mb, which gives the emissivity (e) of the
  real body.
• M eMb

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Weins Displacement law

• Just as total energy varies with temperature, the
  spectral distribution of energy varies also.
• The dominant wavelength at which a blackbody
  radiation curve reached a maximum, is related to
  temperature by Weins Law
• l m A
• T
• lm wavelength of maximum spectral radiant
  exitance, mm
• A 2898 mm, K
• T Temperature, K

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Energy Interactions with the Atmpsphere
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Energy Interaction with the Atmosphere

• Irrespective of source, all radiation detected by
  remote sensors passes through some distance (path
  length) of atmosphere.
• The net effect of the atmosphere varies with
• Differences in path length
• Magnitude of the energy signal that is being
  sensed
• Atmospheric conditions present
• The wavelengths involved.

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Scattering

• Atmospheric scattering is the unpredictable
  diffusion of radiation by particles in the
  atmosphere.
• Three types of scattering can be distinguished,
  depending on the relationship between the
  diameter of the scattering particle (a) and the
  wavelength of the radiation (?).

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Scattering of EM energy by the atmosphere
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Rayleigh Scatter

• a lt ?
• Rayleigh scatter is common when radiation
  interacts with atmospheric molecules (gas
  molecules) and other tiny particles (aerosols)
  that are much smaller in diameter that the
  wavelength of the interacting radiation.
• The effect of Rayleigh scatter is inversely
  proportional to the fourth power of the
  wavelength. As a result, short wavelengths are
  more likely to be scattered than long
  wavelengths.
• Rayleigh scatter is one of the principal causes
  of haze in imagery. Visually haze diminishes the
  crispness or contrast of an image.

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Mie Scatter

• a ltgt ?
• Mie scatter exists when the atmospheric particle
  diameter is essentially equal to the energy
  wavelengths being sensed.
• Water vapour and dust particles are major causes
  of Mie scatter. This type of scatter tends to
  influence longer wavelengths than Rayleigh
  scatter.
• Although Rayleigh scatter tends to dominate under
  most atmospheric conditions, Mie scatter is
  significant in slightly overcast ones.

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Non-selective scatter

• a gt ?
• Non-selective scatter is more of a problem, and
  occurs when the diameter of the particles causing
  scatter are much larger than the wavelengths
  being sensed.
• Water droplets, that commonly have diameters of
  between 5 and 100mm, can cause such scatter, and
  can affect all visible and near - to - mid-IR
  wavelengths equally.
• Consequently, this scattering is non-selective
  with respect to wavelength. In the visible
  wavelengths, equal quantities of blue green and
  red light are scattered.

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Non-Selective scatter of EM radiation by a cloud
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Absorption

• In contrast to scatter, atmospheric absorption
  results in the effective loss of energy to
  atmospheric constituents.
• This normally involves absorption of energy at a
  given wavelength.
• The most efficient absorbers of solar radiation
  in this regard are
• Water Vapour
• Carbon Dioxide
• Ozone

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Absorption of EM energy by the atmosphere
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Atmospheric Windows

• Because these gases absorb electromagnetic energy
  in specific wavebands, they strongly influence
  where we look spectrally with any given remote
  sensing system.
• The wavelength ranges in which the atmosphere is
  particularly Transmissive are referred to as
  atmospheric windows

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Energy Interactions with Earth Surface Features

• When Electromagnetic energy is incident on any
  given earth surface feature, three fundamental
  energy interactions with the feature are
  possible
• Reflection
• Absorption
• Transmission

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The relationship between these three energy
interactions E i (l) E r (l) E a (l) E
t (l) E i Incident energy E r Reflected
energy E a Absorbed energy E t Transmitted
energy
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REFLECTANCE
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Reflectance

• The proportions of energy reflected, absorbed,
  and transmitted will vary for different earth
  features, depending on their different material
  type and condition.
• These features permit us to distinguish different
  features on an image.

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Reflectance from a surface

• The geometric manner in which an object reflects
  energy is also an important consideration. This
  is primarily a function of surface roughness.
  There are two broad categories of surface
  roughness
• Specular Reflectors
• Diffuse Reflectors

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Reflectors

• 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.

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Specular Reflectance
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Diffuse Reflectance
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Reflectance of Surfaces

• 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, diffuse
  reflection will dominate.

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Reflectance

• The reflectance characteristics of earth surface
  features can be quantified by measuring the
  portion of incident energy that is reflected.
• This is termed spectral reflectance, and is
  defined as
• Energy of wavelength (l) reflected from object
  x 100
• Energy of wavelength (l) incident upon the object

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Spectral Reflectance curve

• The graph of spectral reflectance of an object as
  a function of wavelength is termed a spectral
  reflectance curve
• Spectral reflectance curves are commonly
  collected in advance of a remote sensing survey
  in order to aid in both the identification of
  different surfaces and to decide on which remote
  sensor should be used to observe them.

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Spectra of Surfaces

• The spectral reflectance characteristics of
  three main types of environmental surface will be
  discussed further
• Vegetation
• Soil
• Water

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Vegetation Reflectance
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INTERACTION OF VISIBLE, NEAR INFRARED AND MIDDLE
INFRARED EM RADIATION WITH VEGETATION.

• Generally, a leaf is built up of layers of
  structural fibrous organic matter, within which
  are pigmented, water filled cells and air spaces.
  Each of these features
• Pigmentation
• Physiological Structure
• Water Content
• These all have an effect on the reflectance,
  absorbance and transmittance properties of a
  green leaf.

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Pigmentation

• Higher plants contain four primary pigments
• Pigment Wavelengths Absorbed
• Chlorophyll a 0.43 to 0.66 microns
• Chlorophyll b 0.45 to 0.65 microns
• b Carotene blue to green
• Xanthophyll blue to green

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Physiological Structure

• The discontinuities in the refractive indices
  within a leaf determine its near infrared
  reflectance.
• These discontinuities occur between membranes and
  cytoplasm within the upper half of the leaf, and
  more importantly between individual cells and air
  spaces of the spongy mesophyll within the lower
  half of the leaf.

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Result

• The combined effect of pigmentation and
  physiological structure is that all healthy green
  leaves have
• low reflectance in the red and blue
• medium reflectance in the green
• high reflectance in the near infrared
• This can and does vary slightly between species

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Water absorption in the near and middle infrared

• The are three major water absorption areas that
  affect the reflectance spectra of healthy leaves
• 1.4 microns
• 1.9 microns
• 2.7 microns
• two minor absorption features at
• 0.96 and 1.1 microns

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Soil Reflectance
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Interaction of visible, near infrared and middle
infrared EM radiation with soil.

• Most energy incident on a soil is either absorbed
  or reflected - there is usually little
  transmission.
• The reflectance of most soil types are similar,
  with an increase in reflectance with wavelength.
  The main factors that affect soil reflectance
  are
• Moisture content
• Organic content
• Soil texture
• Soil structure
• Iron oxide content

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Soil Texture, Structure and Moisture

• These factors are all interrelated
• Soils of different texture will have a different
  overall structure and roughness. Soil texture
  also affects soil moisture retention.
• The presence of soil moisture has the effect of
  reducing the reflectance of soils across the
  short-wave spectrum. This occurs until the soil
  is saturated, at which point it has no further
  effect.
• In the near and middle infrared, there is also a
  negative moisture effect on soil reflectance,
  particularly at 0.9, 1.4, 1.9, 2.2, and 2.7
  microns

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Moisture content
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Organic Matter

• Organic matter has a strong influence on soil
  reflectance.
• Soil organic matter is characteristically dark,
  and in its presence will decrease the reflectance
  across the short-wave spectrum.
• At a soil organic content of above 2 the
  decrease in reflectance may mask other soil
  absorption features
• For a soil with an organic content of above 5 ,
  the soil will be effectively look black.

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Iron Oxide

• Some iron is usually found in a soil.
• Iron oxide gives many soils a rusty red hue by
  coating or staining individual particles.
• Iron oxide selectively reflects red and absorbs
  green light. This ratio can be used to identify
  iron ore deposits.

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A OM high (gt2) fine texture B OM low (lt2)
low IO (lt1) C OM low (lt2) med IO (1-4) D
OM high (gt2) low IO (lt1) E IO high (gt4) fine
texture
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Spectral Propoerties of Water
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Interaction of visible, near infrared and middle
infrared EM radiation with water.

• Unlike vegetation or soil, the majority of
  radiant energy incident upon water is not
  reflected, but is either absorbed or transmitted.
  
• In visible wavelengths little is absorbed or
  reflected (lt 5), the majority being transmitted.
• Water absorbs near and middle infrared
  wavelengths strongly, leaving little radiation to
  be either reflected or transmitted.
• Most water/Land boundaries are therefore
  spectrally sharp

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Spatial Variability in Water Reflectance

• The factors that affect the spatial variability
  of reflectance of a water body are usually
  determined by the environment.
• The three most important factors are
• Depth of water
• Materials within the water
• Surface roughness of the water

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Spectral - Separation
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1
2
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grassland
pinewoods
red sand-pit
Percent reflectance at 850nm
silty water
Percent Reflectance at 550nm