Micro Remote Sensing

• Radiant temperature


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


  An "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 from the surface of a blackbody, 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
• 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
• Greybody


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


  A material whose emissivity varies with wavelength
• Atmospheric effects


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


  1. Spatial resolution and ground coverage

  2. Tangential-scale distortion
• 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


  An ideal object that absorbs all incident radiation, regardless of wavelength or angle of incidence
• Greybody


  An object that absorbs and emits radiation, but not necessarily in the same proportion as a blackbody
• Selective radiator


  An 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
• As an object's reflectance decreases


  Its emissivity increases
• As an object's reflectance increases


  Its emissivity decreases
• Radiant temperature


  The temperature measured by a thermal sensor, which is less than the kinetic (actual) temperature of the object due to emissivity effects
• Kinetic temperature


  The actual temperature of an object
• Thermal sensors detect radiation from the surface (approximately the first 50m) of ground objects, which may not be indicative of the internal bulk temperature
• Thermal detectors


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


  • Require cooling to 30-77 K to improve signal-to-noise ratio
  • Calibrated using blackbody sources at different temperatures
• Thermal scanner image


  Pictorial representation of the detector response on a line-by-line basis, with high radiant temperature areas displayed as lighter tones
• A thermal scanner is a type of across-track multispectral scanner
• 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
• The detector is normally surrounded by a dewar containing liquid nitrogen at 77 K, which 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, 3 - 14 μm)
  • Indium antimonide (InSb, 3 - 5 μm)
  • Mercury cadmium telluride (HgCdTe or MCT, 8 - 14 μm)
• Thermal scanners became commercially available during the late 1960s