GIS 104

• 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


  An ideal material that completely absorbs all incident radiation, converting it to internal energy
• Blackbody radiation


  The radiation emitted by a blackbody, whose intensity and spectral composition are a function of the material type and temperature
• Emissivity (ε)


  The "emitting ability" of a real material, compared to that of a blackbody, at the same temperature
• Greybody


  A material with an emissivity less than 1 but constant at all wavelengths
• Selective radiator


  A material whose emissivity varies with wavelength
• Atmospheric windows


  Wavelength intervals where the atmosphere is relatively transparent to radiation
• Across-track thermal scanning


  • - Spatial resolution and ground coverage
  • Tangential-scale distortion
• Planck Radiation (Blackbody) Law
• Many materials radiate like blackbodies over certain wavelength intervals
• The atmosphere has a significant effect on the intensity and spectral composition of the energy recorded by a thermal system
• Hyperspectral remote sensing is useful for finding subtle mineral composition of rocks and vegetation
• Microwave remote sensing involves the concepts of radar and microwave radiometry
• Emissivity


  Ratio of the energy radiated by a surface to the energy radiated by a blackbody at the same temperature
• As emissivity decreases


  Reflectance increases
• As emissivity increases


  Reflectance decreases
• Blackbody


  Ideal object that absorbs all incident radiation and emits the maximum possible radiation at a given temperature
• Greybody


  Object that absorbs and emits less radiation than a blackbody at the same temperature
• Selective radiator


  Object that absorbs and emits radiation selectively at certain wavelengths
• Kirchhoff's Radiation Law states that the spectral emissivity of an object equals its spectral absorptance
• For most remote sensing applications, objects are assumed to be opaque to thermal radiation
• Radiant temperature (Trad)


  Temperature measured by a thermal sensor
• Kinetic temperature (Tkin)


  Actual temperature of an object
• Thermal sensors detect radiation from the surface (approximately the first 50m) of ground objects
• Thermal detectors


  • HgCdTe (8-14μm)
  • Ge(Hg) (2-14μm)
  • InSb (3-5μm)
• Detector cooling


  Improves signal-to-noise ratio to a stable level
• Thermal scanner calibration


  1. Use calibration sources at different temperatures

  2. Determine temperature/radiance relations in advance

  3. Periodic recalibration required for aircraft scanners
• Thermal scanner image


  Pictorial representation of detector response on a line-by-line basis

  
• For meteorological purposes, thermal scanner images typically display clouds (cooler than earth's surface) as lighter toned
• 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 displaying 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