Remote Sensing - a tool for environmental observation

G the electrons and the emission of electromagnetic

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state (of e.g. the electrons) and the emission of electromagnetic radiation. The amount of energy radiated from an object is called radiant flux and is measured in watts per square centimetre. The concentration of the radiant flux of an object is the radiant temperature: T rad . Important: kinetic temperature and radiant temperature are not the same! The radiant temperature is always less than the kinetic temperature because objects do not behave like black bodies and do not comple- tely obey the Stefan Boltzman equation. This property of objects is called emissivity and is defined as the ratio between the radiant flux of the object and the radiant flux of a blackbody with the same (kinetic) temperature. Water is very close to behaving as a blackbody, Quartz act as a selective radiator its radiation varies with wavelength (figure 4.1 and 4.2). Emissivity values for some objects are: Pure water 0.993 Wet soil 0.950 Dry soil 0.920 Sand, quartz 0.914 Wood 0.900 Granite 0.815 Polished aluminium 0.060 4.3 Heat transfer Heat energy is transferred from one place to another by three mechanisms: conduction, convection and radiation: Conduction transfers heat through material by molecular contact. Heat transfer through a frying pan for cooking is an example. Convection transfers heat through the physical movement of heated matter. The circulation of heated water and air are examples of convection. Radiation transfers heat in the form of electromagnetic waves. Heat from the sun reaches the earth by radiation. In contrast to conduction and convection, which can only transfer heat through matter, radiation can transfer heat through a vacuum.
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55 Figure 4.1 Spectral emissivities and radiant exitances for a blackbody, a grey body and a selective radiator (Lillesand and Kiefer, 1994). Objects at the surface of the earth receive thermal energy mainly by radiation from the sun. There are daily (24 hours) and annually (1 year) cyclic variations in the duration and intensity of the solar energy and hence, in the energy received. Atmospheric transmission In the previous chapters it was already discussed that the atmosphere does not transmit radiation in the optical wavelengths equally throughout the spectrum. The same is true for the thermal infrared part of the spectrum. Carbon oxide (CO 2 ), ozone (O 3 ) and water vapour (H 2 O) absorb energy at certain wavelengths in the thermal bands. Two important atmospheric windows are distinguished in the thermal part of the spectrum: 3 to 5 μm and 8 to 14 μm (figure 4.3).
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56 Figure 4.2 Spectral radiance for water (a) versus a blackbody and (b) quartz versus a blackbody (Lillesand and Kiefer, 1994).
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57 Figure 4.3 Electromagnetic spectrum showing spectral bands used in the thermal region and gasses responsible for atmospheric absorption are indicated. Two thermal atmospheric windows are visible: 3 to 5 µ and 8 to 14 µ 4.4 Thermal properties of Materials Radiant energy striking the surface of a object at the earth surface is theoretically partly reflected, partly absorbed and partly transmitted through the material. Consequently, a
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