Electromagnetic Radiation

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Electromagnetic Radiation
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What is EMR?

• Electromagnetic Radiation is a form of energy
  with the properties of a wave.

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What is EMR?

• The waves propagate through time and space in a
  manner rather like water waves, but oscillate in
  all directions perpendicular to their direction
  of travel.

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Electromagnetic Waves

• A wave is characterised by two principal
  measures wavelength and frequency The
  wavelength (lambda) is the distance in metres
  between successive crests of the waves. The
  frequency (nu) is the number of oscillations
  completed per second.

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Electromagnetic Waves
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Terms

• Crest The highest point of the wave.
• Trough The lowest point of the wave.
• Amplitude The height of the wave as measured
  between the trough and the crest.
• Wavelength The distance between two identical
  points on the wave.
• Period The time it takes for a wavelength to
  pass a stationary point.
• Frequency The number of wavelengths that pass a
  point in a set period of time.

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Wavelength

• The wavelength (lambda) is the distance in metres
  between successive crests of the waves.

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Electromagnetic Waves
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Frequency

• The frequency (nu) is the number of oscillations
  completed per second.

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Electromagnetic Waves
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Wavelength and Frequency

• Wavelength and frequency are related by the
  following formula

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Electromagnetic Spectrum
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Electromagnetic Spectrum Distribution of
Radiant Energies
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Electromagnetic Spectrum
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Gamma Rays
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Gamma Rays

• Gamma rays have wavelengths of less than about
  ten trillionths of a meter.
• They are more penetrating than X-rays.
• Gamma rays are generated by radioactive atoms and
  in nuclear explosions, and are used in many
  medical applications.
• Images of our universe taken in gamma rays have
  yielded important information on the life and
  death of stars, and other violent processes in
  the universe.

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X-Rays
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X-Rays

• X-rays are high energy waves which have great
  penetrating power and are used extensively in
  medical applications and in inspecting welds.
• X-ray images of our Sun can yield important
  clues to solar flares and other changes on our
  Sun that can affect space weather.
• The wavelength range is from about ten billionths
  of a meter to about 10 trillionths of a meter.

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UV Region
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Ultraviolet Radiation

• Ultraviolet radiation has a range of wavelengths
  from 400 billionths of a meter to about 10
  billionths of a meter.
• Sunlight contains ultraviolet waves which can
  burn your skin. Most of these are blocked by
  ozone in the Earth's upper atmosphere. A small
  dose of ultraviolet radiation is beneficial to
  humans, but larger doses cause skin cancer and
  cataracts.
• Ultraviolet wavelengths are used extensively in
  astronomical observatories.
• Some remote sensing observations of the Earth are
  also concerned with the measurement of ozone.

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Visible Spectrum
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Visible Spectrum

• The rainbow of colors we know as visible light is
  the portion of the electromagnetic spectrum with
  wavelengths between 400 and 700 billionths of a
  meter (400 to 700 nanometers).
• It is the part of the electromagnetic spectrum
  that we see, and coincides with the wavelength of
  greatest intensity of sunlight.
• Visible waves have great utility for the remote
  sensing of vegetation and for the identification
  of different objects by their visible colors.

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Infrared Region
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Infrared Spectrum

• Infrared is the region of the electromagnetic
  spectrum that extends from the visible region to
  about one millimeter (in wavelength).
• Infrared waves include thermal radiation. For
  example, burning charcoal may not give off light,
  but it does emit infrared radiation which is felt
  as heat.
• Infrared radiation can be measured using
  electronic detectors and has applications in
  medicine and in finding heat leaks from houses.
• Infrared images obtained by sensors in satellites
  and airplanes can yield important information on
  the health of crops and can help us see forest
  fires even when they are enveloped in an opaque
  curtain of smoke.

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Microwave Region
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Microwave Radiation

• Microwave wavelengths range from approximately
  one millimeter (the thickness of a pencil lead)
  to thirty centimeters (about twelve inches).
• In a microwave oven, the radio waves generated
  are tuned to frequencies that can be absorbed by
  the food. The food absorbs the energy and gets
  warmer. The dish holding the food doesn't absorb
  a significant amount of energy and stays much
  cooler.
• Microwaves are emitted from the Earth, from
  objects such as cars and planes, and from the
  atmosphere. These microwaves can be detected to
  give information, such as the temperature of the
  object that emitted the microwaves.

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Radio Waves

• Radio waves are used to transmit radio and
  television signals. Radio waves have wavelengths
  that range from less than a centimeter to tens or
  even hundreds of meters.
• Radio waves can also be used to create images.
  Radio waves with wavelengths of a few centimeters
  can be transmitted from a satellite or airplane
  antenna. The reflected waves can be used to form
  an image of the ground in complete darkness or
  through clouds

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Electromagnetic Interactions

• EMR that interacts with an object is called
  incident radiation

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• Electromagnetic energy is either, reflected,
  transmitted, or absorbed by the surface it
  strikes. Electromagnetic energy is
  either, reflected, transmitted, or absorbed by
  the surface it strikes.

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Electromagnetic Interactions

• (1) Transmission
• (2) Reflection
• (3) Absorption

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3 Interactions with an Object
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TRANSMISSION

• The movement of light through a surface .
• Transmission is wavelength dependent
• Transmittance is measured as the ratio of
  transmitted radiation to the incident radiation.
  Transmittance therefore is the proportional
  amount of incident radiation passing through a
  surface.

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Electromagnetic Interactions
Satellite electromagnetic sensors see reflected
and emitted radiation
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REFLECTION

• In our usage reflection is the bouncing of
  electromagnetic energy from a surface.
• The term reflectance is defined as the ratio of
  the amount of electromagnetic radiation, usually
  light, reflected from a surface to the amount
  originally striking the surface

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Reflection

• 2 Types
• (1) Specular
• (2) Diffuse

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Specular Reflection

• 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.

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Specular Reflection
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Diffuse Reflection

• Occurs when the surface is rough and the energy
  is reflected almost uniformly in all directions.

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Diffuse Reflection
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Reflection

• 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 that
  make up the surface, diffuse reflection will
  dominate.

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Absorption

• Some radiation is absorbed through electron or
  molecular reactions within the medium
• A portion of this energy is then re-emitted (as
  emittance), usually at longer wavelengths, and
  some of it remains and heats the target

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Electromagnetic Interactions
Satellite electromagnetic sensors see reflected
and emitted radiation
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Atmospheric Interactions
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Scattering

• - The redirection of electromagnetic energy by
  particles suspended in the atmosphere, or by
  large molecules of atmospheric gasses.- This
  redirection of light can be in any direction

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Scattering
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Three Types of Scattering

• (1) Rayleigh Scattering
• (2) Mie Scattering
• (3) Non-selective Scattering

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RAYLEIGH SCATTERING

• - Upper atmosphere scattering, sometimes called
  clear atmosphere scattering.
• Consists of scattering from atmospheric gasses
• Is wavelength dependent
• Scattering increases as the wavelength becomes
  shorter.
• Atmospheric particles have a diameter smaller
  than the incident wavelength.
• Dominant at elevations of 9 to 10km above the
  surface.
• Blue light is scattered about four times as much
  as red light and UV light about 16 times as red
  light.
• Causes the blue color of the sky and the
  brilliant red colors at sunset.

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Rayleigh Scattering
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MIE SCATTERING

• - Lower atmosphere scattering (0-5km)
• Caused by dust, pollen, smoke and water droplets.
• Particles have a diameter roughly equal to the
  incident wavelength.
• Effects are wavelength dependent and affect EM
  radiation mostly in the visible portion.

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NON-SELECTIVE SCATTERING

• - Lower atmosphere
• Particles much larger than incident radiation
• Scattering not wavelength dependent
• Primary cause of haze

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Non-Selective Scattering

• RGB light effected approximately the same

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GENERAL EFFECTS OF SCATTERING

• Causes skylight (allows us to see in shadow)
• Forces image to record the brightness of the
  atmosphere in addition to the target.
• Directs reflected light away from the sensor
  aperture and
• Directs light normally outside the sensor's field
  of view toward the sensors aperture decreasing
  the spatial detail (fuzzy images)
• Tends to make dark objects lighter and light
  objects darker (reduces contrast)

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ATMOSPHERIC ABSORPTION

• Mostly caused by three atmospheric gasses
• OZONE - absorbs UV
• CARBON DIOXIDE - Lower atmosphere absorbs energy
  in the 13 - 17.5 micrometer region.
• WATER VAPOR -Lower atmosphere. Mostly important
  in humid areas, very effective at absorbing in
  portions of the spectrum between 5.5 and 7
  micrometer and above 27 micrometer.

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Atmospheric Absorption
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ATMOSPHERIC WINDOWS

• Portions of the EM spectrum that can pass
  through the atmosphere with little or no
  attenuation. The figure below shows areas of the
  spectrum that can pass through the atmosphere
  without attenuation (peaks) and areas that are
  attenuated (valleys)

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Atmospheric Transmission