Part 1 LN

• Thermal radiation principles


  The process of remote measurement of thermal state of objects
• Radiant temperature


  The external manifestation of an object's energy state that is remotely sensed using thermal scanning devices
• Kinetic temperature


  The internal manifestation of the average translational energy of the molecules constituting a body
• Blackbody radiation


  Any object having a temperature greater than absolute zero emits radiation whose intensity and spectral composition are a function of the material type and temperature
• Blackbody radiation


  • - The relationship between the wavelength of peak spectral exitance and temperature is given by Wien's Displacement Law
  • The total radiant exitance from the surface of a blackbody varies as the fourth power of absolute temperature (Stefan-Boltzmann law)
• Planck Radiation Law


  Describes the rate at which blackbody objects radiate thermal energy
• Emissivity


  The "emitting ability" of a real material, compared to that of a blackbody
• Emissivity


  • - Can have values between 0 and 1
  • Can vary with wavelength, viewing angle, and temperature
  • A greybody has a constant emissivity less than 1
  • A selective radiator has emissivity that varies with wavelength
• Atmospheric effects


  The atmosphere has a significant effect on the intensity and spectral composition of the energy recorded by a thermal system
• Atmospheric windows influence the selection of the wavelength intervals used in thermal remote sensing
• Across-track thermal scanning


  • - Spatial resolution and ground coverage
  • Tangential-scale distortion
• Thermal remote sensing is used to infer the temperature of a surface by measuring its radiant exitance
• Hyperspectral remote sensing is used to find out certain subtle mineral composition of rocks, vegetation
• Microwave remote sensing involves the concepts of radar altimeter and microwave radiometer
• Emissivity


  Spectral absorptance of a terrain element
• Reflectance


  Spectral reflectance of a terrain element
• Transmittance


  Spectral transmittance of a terrain element
• Kirchhoff's Radiation Law states that the spectral emissivity of an object equals its spectral absorptance
• Emissivity of an object


  Inversely related to its reflectance
• Radiant temperature (Trad)


  Measured by a thermal sensor
• Kinetic temperature (Tkin)


  Actual temperature of an object
• Radiant temperature (Trad)


  Always less than kinetic temperature (Tkin) due to emissivity effects
• Thermal sensors


  • Detect radiation from the surface (approximately the first 50m) of ground objects
  • May not be indicative of the internal bulk temperature of an object
• Detector materials


  • Mercury-cadmium-tellurium (HgCdTe) for 8-14 μm
  • Mercury-doped germanium [Ge(Hg)] for 2-14 μm
  • Indium-antimony (InSb) for 3-5 μm
• Detector cooling


  • Required to improve signal-to-noise ratio
  • Achieved using liquid nitrogen, helium, or radiant cooling systems
• Thermal scanner calibration


  1. Use calibration sources at different temperatures

  2. Determine temperature/radiance relations in advance in the laboratory
• Thermal scanner images display high radiant temperature areas as lighter tones, except for meteorological purposes where clouds (cooler than the earth's surface) appear light toned
• Thermal scanner


  A type of across-track multi-spectral scanner with detectors sensitive to thermal energy
• Aircraft scanners require periodic recalibration
• Image signal processing


  1. Goes to separate recording unit

  2. Goes through chopper for sampling from main beam
• Radiant temperatures are not normally converted to kinetic temperatures because we don't usually know the emissivities of the diverse surface materials sufficiently well to permit this
• Thermal scanner image


  Pictorial representation of the detector response on a line-by-line basis
• Convention for thermal scanner images

  High radiant temperature areas displayed as lighter toned image areas, for meteorological purposes this is typically reversed so that clouds (cooler than the earth's surface) appear light toned
• Thermal scanner


  A particular kind of across-track multi-spectral scanner, whose detector(s) only senses in the thermal portion of the spectrum
• Thermal scanners are restricted to operating in either (or both) the 3 to 5 μm or 8 to 14 μm range of wavelengths due to atmospheric effects
• Quantum or photon detectors


  Typically used for thermal scanning, capable of very rapid (less than 1sec) response, operate on the principle of direct interaction between photons radiation incident on them and the energy level of electrical charge carriers within the detector material
• For maximum sensitivity, the detector must be cooled to temperatures approaching absolute zero to minimise their own thermal emissions
• Dewar


  A double-walled insulated vessel that acts like a thermos bottle to prevent the liquid coolant from boiling away at a rapid rate
• Photon detectors in common use

  • Mercury-doped germanium (Ge:Hg)
  • Indium antimonide (InSb)
  • Mercury cadmium telluride (HgCdTe or MCT)
• Thermal scanners became commercially available during the late 1960s