Thermal Images

1. Review of radiation laws:
   all objects at temperature above absolute 0^oK emit

   Stefan-Boltzmann law: W = sT⁴
           W-total emitted radiation, s-a constant, T-temperature in ^oK

        - the total emitted radiation from a blackbody is
            proportional to the fourth power of its absolute temperature

   Wien's displacement law: l = 2,897.8/T
          l-peak wavelength, T-temperature in ^oK

        - as temperature of objects increase, the wavelength of peak
            emittance becomes shorter

   Emissivity: e = M/Mb
          e-emissivity, M-emittance of a given object,
          Mb-emittance of blackbody
          e = 1 (blackbody)   e = 0 (whitebody, perfect reflector)

        - the ratio between the emittance of given object and that
            of blackbody at the same temperature

        Selective radiator:
        - an object with an emissivity varies with wavelength

2. Heat
        Kinetic temperature (^oF ^oC ^oK)
        - thermal energy of molecules within a substances

        Radiant temperature
        - the emitted energy

        Heat capacity C (cal.g^-1.^oC^-1)
        - the ratio of the change in heat energy per unit mass to the
            corresponding change in temperature at constant pressure

        Specific heat
        - the ratio of the heat capacity of a substance to that of a
            reference substance, i.e. pure water

        Thermal conductivity K (cal.cm^-1.sec^-1.^oC^-1)
        - the rate at that a substance transfers heat

3. Geometry of thermal images
        Tangential scale distortion
        - caused by varied viewing distance

        Aircraft instability
        - roll: side by side motion
        - pitch: head/tail motion
        - crab (yaw): by compensating drift

        Relief displacement
        - differs from that of aerial photography

        - vertical features displaced from the nadir for each scan
            vertical features on air photo displaced radially from the principal point

4. Thermal image interpretation
   limitations
        - thermal images contain noise and errors

        - differences in emitted energy is not directly related to
            differences in temp, must know emissivity of each material

        - sensors only record the radiance at the surface

  landscape factors
        - surface material

        - topography

        - vegetation cover

        - moisture

        Timing
        early afternoon:
        - high thermal contrast but with thermal shadow and slope
            orientation effects
        - water is cool, bare soil, meadow and forest are warmer

        before dawn:
        - lower thermal contrast but little slope orientation effects
            or thermal shadow
        - water is warm, open meadow and bare soil are cool,
            forests are warm
 
 

5. Reading: chpt 5
 

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