The Electromagnetic Spectrum

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The Electromagnetic Spectrum

• EG5503
• (GIS Earth Observation)

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Lecture Topics

• What is the Electromagnetic Spectrum?
• The concept of wavelength
• Properties of EMR waves
• EMR and the Sun-Atmosphere system
• How does remote sensing exploit EMR

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What is the Electromagnetic Spectrum?

• The term radiation covers a wide variety of
  natural phenomena
• All radiation involves the exchange of energy
• The energy associated with electromagnetic
  radiation is called radiant energy
• Radiant energy may exist in the absence of matter

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What is the Electromagnetic Spectrum?

• All types of EMR are transmitted, or propagated,
  as waves
• In common with all waves, the two most
  fundamental properties of electromagnetic waves
  are length and frequency
• The longer the wavelength the lower the frequency
  and vice versa

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The concept of wavelength

• Wavelength is usually measured in metres (the SI
  unit of length), micrometres (1µm10-6m) and
  nanometres (1nm10-9m)
• The SI unit of frequency is hertz (cycles per
  second)
• The electromagnetic spectrum may be defined as
  the entire range of radiation wavelengths

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Electromagnetic spectrum with enhanced detail for
visible region of the spectrum Note the large
range of wavelengths encompassed in the spectrum
- it is over twenty orders of magnitude!
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Properties of EMR radiation

• Transfer energy from place to place
• Can be emitted and absorbed by matter
• Do not need a material medium to travel through
• Travel at 3 X 108 metres per second in a vacuum
• Can be polarised (made to vibrate in a plane)
• Can be reflected and refracted
• Can be diffracted (e.g. using a prism)
• Carry no electric charge

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EMR and the Sun-atmosphere system

• In order to understand how we can measure the
  physical environment with remote sensing, we must
  first understand solar radiation
• The amount of energy received by a surface
  perpendicular to the Suns rays at the Earths
  outer atmosphere is called the solar constant
  (about 1370 J m-2 s-1 average)

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EMR and the Sun-atmosphere system

• About 50 of incoming solar radiation is lost by
  the atmosphere scattered (30) and absorbed
  (20)
• Scattering involves the absorption and
  re-emission of energy by particles
• Absorption (unlike scattering) involves energy
  exchange

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EMR and the Sun-atmosphere system

• Wavelengths less than and greater than 0.8µm
  (800nm) are often referred to as shortwave and
  longwave radiation respectively
• The shortwave solar radiation consists of
  ultraviolet and visible
• The terrestrial longwave component is known as
  infrared

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EMR and the Sun-atmosphere system

• Just under 50 of the radiation reaching the
  Earths surface is in the visible range
• Components of visible light are referred to as
  colours
• Each colour behaves differently and white light
  can be separated out by use of a prism
• Colour separation occurs because of differential
  refraction

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EMR and the Sun-atmosphere system

• The human eye cannot see infrared radiation
• Infrared radiation is absorbed by water vapour
  and carbon dioxide in the troposphere
• The atmospheres relative transparency to
  incoming solar (SW) radiation, and ability to
  absorb/re-emit outgoing infrared (LW) radiation
  is the natural greenhouse effect

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Remote Sensing and EMR

• Remote sensing exploits the different
  characteristics of the electromagnetic spectrum
• Satellites use channels - a channel corresponds
  to a specific waveband, or portion of the
  electromagnetic spectrum
• The European geostationary weather satellite
  METEOSAT for example has 3 channels

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CHANNEL SPECTRAL RANGE USE Visible 0.45 to
1µm Daytime imaging Infrared 10.5 to 12.5
µm Temperature estimation and Imaging Water
Vapour 5.7 to 7.1 µm Tropospheric humidity
estimation
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AnyQuestions ?