Radiometry and Blackbody Emission

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Radiometry and Blackbody Emission

• Greg Bishop

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Radiometry

• Radiometry describes radiated electromagnetic
  energy
• Radiometry is used to predict and/or calculate
  the transfer of radiation from one body to
  another

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Radiometry Terminology

• Emissivity (e), is a measure of how well a body
  gives off radiation
• Absorptivity (a), is a measure of how well a body
  absorbs radiation
• Absorptivity and emissivity are unit less with a
  range of 0 to 1
• For a perfect emitter/absorber the value is 1

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Radiometry Terminology

• Exitance (M) is defined as the power per unit
  area emitted from a surface
• Intensity (I) is defined as the power from the
  source per unit solid angle
• Incidance (E) or Irradiance power per unit
  detector area
• EF/Ac

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Blackbody

• Absorbs all the electromagnetic radiation that
  falls on it and it also radiates energy at every
  wavelength

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Planks Law of Blackbody Radiation in frequency
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Planks Law of Blackbody Radiation in wavelength
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Wiens Displacement Law

• Defines the relationship between Temperature and
  Wavelength
• Gives the wavelength where the maximum intensity
  occurs

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Stefan-Boltzmann Law

• The total energy radiated by a blackbody per unit
  area per unit time is proportional to T4

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Non-Ideal Blackbodies

• Emissivity of an ideal blackbody is 1
• The emissivity of a non-ideal blackbody can be
  close to unity
• Laboratory blackbodies are made by having a small
  hole with a large cavity inside
• The emission depends on the temperature inside
  the cavity

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Sample Problem

• Using a black body source with radius of the
  aperture r.5 cm and temperature of 800K find the
  Incidance on a detector R5 cm away with an area
  of .1cm2

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Questions?