NEAR INFRA RED and Thermal Radiation

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• NEAR INFRA RED and Thermal Radiation
• Dr. M. M. Yagoub
• Dep. Of Geography, UAEU
• E-mail myagoub_at_uaeu.ac.ae
• E-mail myagoub_at_hotmail.com
• URL http//www.angelfire.com/mo/yagoub

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Overview

• NEAR INFRA RED (NIR)
• NEAR INFRA RED Applications
• THERMAL INFRA RED (TIR)
• Temperature
• Emissivity
• Thermal Energy Detectors (Radiometer)
• Thermal infrared (TIR) Applications
• Factors affect Thermal imagery

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NEAR INFRA RED (NIR)

• Infrared (0.7-1000 ?) - emitted thermal radiation
  (temperature of objects)
• The NEAR INFRA RED portion of the Electromagnetic
  Spectrum occurs in the 0.7 µm to 1.3µm region.
  NIR is within the reflective portion of the
  spectrum and can be recorded photographically,
  using special false colour photographic films.
• Conventional photographic emulsions and CCD
  arrays will typically not able to detect
  reflected or radiated electromagnetic energy
  beyond near infra red. A specially designed
  thermal sensor or radiometer is required to sense
  energy in the mid IR and thermal bands.
• By definition
• Mid InfraRed 1.3 um to 3.0 um - Reflective
  portion
• Thermal IR 3.0um to 14 um - Emissive portion
• While typically in the reflected portion of the
  spectrum MID IR imagery is influenced by high
  energy thermal emissions. Typically these are
  very hot features such as lava flows or intense
  bush fires with temperatures in the 400K - 1000K
  range (100C plus range. refer to blackbody
  diagram
• Typically in this portion vegetation effects are
  less than that of Infra red while geological
  features (rocks soils still maintain a high
  response).

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Geological features (rocks soils ) maintain a
high response in Near Infrared
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NEAR INFRA RED Applications

• Near Infra Red imagery offers a useful means for
  soil analysis, agricultural studies, vegetation
  (BIOMASS) studies and for land/water delineation.
  Visible bands are ideal for cartographic and
  general land use interpretation. While
  photography enables detailed studies, satellite
  imagery enables multi-spectral studies over
  larger areas
• To date mid infra red is limited to Landsat and
  SPOT 4 where 20 metre spatial resolution mid
  Infrared band 1.58 - 1.75 um
• Typical applications of the Landsat bands 5 7
  include.
•  Band 5 (1.55um - 1.75um) indicative of
  vegetation moisture content and soil moisture,
  differentiation of snow from clouds.
• Band 7 (2.08um - 2.35um)discrimination of rock
  types. Sensitive to vegetation moisture content
  and soil moisture content.

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Note about NIR

• Near Infra Red typically measures EM response of
  reflected energy. However for particularly hot
  features such as lava, intense bushfires etc.
  Near Infra red does become sensitive to emitted
  energy
• Near Infra red should not be confused with
  Thermal Infra red
• Vegetation appears RED in NIR composites only
  because the NIR response is much greater than
  that of the visible bands. Thus vegetation would
  still be measured as green in the visible
  portion. If we assign NIR a green colour then it
  will similarly appear green gt ie. NIR response
  for vegetation is HIGH

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THERMAL INFRA RED (TIR)

• Thermal infra red is the sensing of emissive
  energy or "temperature" energy
• It is governed by various laws and concepts
  including Blackbody concept, Stefan-Boltzmann
  Law, Wiens Law, and Kirchof's Law
• For satellite remote sensing purposes Thermal IR
  applications are typically limited to two
  distinct portions due to atmospheric
  interference, the two portions include
• 3 to 5 um range and 8 to 13 um range

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Temperature

• The sun is the most obvious source of EM
  radiation for remote sensing. However all matter
  at temperatures above absolute zero (00K or
  -2730C) continuously emits EM radiation.
• The amount of energy emitted by an object is
  directly proportional to its temperature. This
  property is expressed mathematically as the
  Stefan-Boltzmann Law
• The point to note is that radiation increases
  with temperature (a very high exponential
  increase) i.e. hotter objects emit more energy
• A blackbody is a hypothetical, ideal radiator
  that totally absorbs and re-emits all energy
  incident upon it

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Spectral distribution of energy radiated from
blackbodies of various temperatures
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Emissivity

• Real materials such as rocks, water, soil, road
  pavements do not behave as perfect black or grey
  bodies. They emit only a fraction of the energy
  emitted by a black body. This emitting ability
  of a material is called emissivity (e) and is a
  dimensionless scaling ratio
• Heat can be transferred in 3 ways
  convection, conduction, radiation
• In remote sensing we measure radiation using a
  radiometer.
• Other important thermal properties include
• thermal conductivity - rate at which heat will
  pass through a material
• thermal capacity - ability of material to store
  heat (absorbed energy)
• thermal diffusivity - rate at which temperature
  can change within a body (energy loss / gain
  characteristics )
• thermal inertia - thermal response, delay in
  reaching ambient temperature following exposure
  to energy
• During the day when surface-atmosphere exchange
  is high there is the likelihood that air
  temperature will affect the energy sensed
• A major consideration in thermal interpretation
  is the temporal variation (time - hourly, daily,
  seasonal) in response of features. Of
  significant interest is the diurnal response of
  features and thermal inertia.
• Thermal Inertia Comparison of Water and
  Vegetation During the day water bodies have a
  relatively cooler surface temperature than soils
  and rocks

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Thermal Energy Detectors

• There is a wide range of thermal detectors
  available The simplest systems are video based
  and have been used successfully in low cost
  remote sensing systems
• Typically low temperature systems require cooling
  (usually liquid nitrogen)
• Common Satellite Sensors in Thermal IR include
• Landsat Band 6 (10.4-12.5um)
• NOAA AVHRR Band 4 (3.55-3.93 um) (TIR)
• Large number of meteorological and ocean
  monitoring satellites with resolutions around
  2km-5km -10km including GOES, GMS, MOS 1,
  SeaWiFs, Nimbus CZCS (1 thermal), SeaSat, HCMM

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Characteristics of Photon Detectors in Common Use
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Thermal Radiometer
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Thermal infrared (TIR) Applications

• Thermal infrared (TIR) images of the Earth's
  surface can provide accurate distributions of
  surface spectral emittance and temperature (at
  local to global scales)
• Heat and moisture fluxes (exchanges)
• Climatological processes
• Evapo-transpiration
• Soil moisture variation
• Hydrology, oceanography and ocean currents
• Biomass distribution
• Vegetation monitoring
• Lithology geology
• Urban land use
• Natural disaster monitoring (volcano)
• Thermal pollution

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Factors affect Thermal imagery

• The high level of spectral sensitivity required
• The optimum times for thermal sensing are at
  night
• cloud shadowing - temperature in cloud shadow is
  often slightly lower than ambient, can result in
  false interpretation.
• slope aspect - orientation and facing direction
  of slope
• sun angle and diurnal temperature due to local
  weather conditions and seasonal variation

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References

• Campbell, J. B. , 1996. Introduction to Remote
  Sensing. 2nd ed.,Taylor and Francis, London.
• Lillesand,T. M. and R. W. Kiefer, 2000. Remote
  Sensing and Image Interpretation. 4thed., John
  Wiley and Sons, Inc. New York.