Remote Sensing - a tool for environmental observation

Figure 14 the electromagnetic spectrum lillesand

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Figure 1.4 The electromagnetic spectrum (Lillesand & Kiefer, 1994). Note the logarithmic scale! Wien’s displacement law The temperature of the object does not only determine the amount of energy radiated, but controls also the wavelength at which the maximum amount of energy is radiated, generally referred to as λ max . Figure 1.5 shows radiant curves for objects ranging in temperature from 6000 ° K (the sun) to 200 ° K (the coldest objects at the surface of the earth). Hence, with increasing temperature the total amount of radiant energy increases and the radiant peak ( λ max ) shifts to shorter wavelengths. This shift is described by Wien’s displacement law:

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9 λ max = A / T rad A : constant 2897 μm ° K; T rad : radiant temperature ° K. This formula is useful to calculate the wavelength of the radiant energy peak of objects. For example, the temperature of the earth is approximately 300 ° K (27 ° C) and its λ max is situated around 9.7 μm. Consequently, thermal infrared observations of the earth are carried out around 9.7 μm (thermal infrared region: 3-14 μm). Figure 1.5 shows the spectral distribution of energy radiated from a blackbody of various temperatures. Figure 1.5 Spectral distribution of energy radiated from a blackbody of various temperatures according to the law of Stefan-Boltzman and the displacement law of Wien (Lillesand & Kiefer, 1994).
10 1.4 Atmospheric transmission Electromagnetic energy emitted by the sun must pass the atmosphere before it reaches the surface of the earth. A sensor aboard a satellite or aircraft measures reflected radiation which also has to pass the atmosphere or at least a part of the atmosphere. The distance radiation has to pass through the atmosphere is called path length . Unfortunately, the atmosphere is not completely transparent for radiation. Radiation is absorbed and scattered in the atmosphere. Absorption and scattering are both a function of wavelength and path length. Furthermore, they depend on the conditions in the atmosphere at the time of data acquisition. As the conditions of the atmosphere vary largely in space and time, it is very difficult to assess the exact effect of atmospheric distortion upon the sensed images. As a result it also very difficult to correct images for these effects. Absorption The human eyes inform us that the atmosphere is essentially transparent to light. Therefore, it is often assumed that these conditions exist for all electromagnetic energy at any wavelength. However, the gases of the atmosphere absorb radiation at specific wavelengths: absorption bands (figure 1.6). Wavelengths shorter than 0.3 μm are completely absorbed, protecting us against lethal ultraviolet (UV) radiation. Water (H 2 O) and hydroxyl (OH) absorb radiation in specific bands called the water absorption bands. The most important water absorption bands are located at 1.4 and 1.9 μm and around 3.1 and 6.2 μm. Minor water absorption bands are located at 0.9 and 2.6-2.8 μm. Other absorbing gases in the atmosphere are CO 2 and O 3 .

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