Remote Sensing - a tool for environmental observation

6 figure 12 the 4 components of a remote sensing

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6 Figure 1.2 The 4 components of a Remote Sensing system (active and passive): the source of radiation, the atmosphere, the Earth surface or objects and the sensor. Three measurements are used to describe electromagnetic waves: - wavelength ( λ in micrometer: μm or nanometres: nm, 1μm=1000nm) which is the distance between the successive wave peaks; - frequency (v in Hertz: Hz) which is the number the number of wave peaks passing a fixed point per unit time; - velocity (c in m/s) which is a constant (speed of light: 3*10 8 m/s). The electromagnetic spectrum categorizes electromagnetic waves by their wavelengths (figure 1.3). The most important wavelengths for remote sensing are: - the visible wavelengths , an extremely small part of the spectrum divided in: blue: 0.4 - 0.5 μm green: 0.5 - 0.6 μm red: 0.6 - 0.7 μm - near infrared: 0.7 - 1.1 μm; - short-wave infrared: 1.1 - 2.5 μm; - thermal infrared: 3.0 –14.0 μm; - microwave region : 10 3 - 10 6 μm or 1 mm to 1 m.
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7 Figure 1.3 Electromagnetic radiation (or light) can be represented as waves. Most often names such as near infrared or microwave are assigned to parts of the spectrum for convenience. Use these names with care because they are not clearly defined with wavelengths. It is preferable to use nanometres (nm) or micrometers (μm) to indicate parts of the spectrum. Figure 1.4 shows the electromagnetic wavelengths categorized to wavelengths. Velocity, wavelength and frequency are related by: c = λ *v The energy of the wavelength (the photon model) is used to describe the energy of the radiation: Q = h*v or Q = (h*c)/v Q : Energy of a photon (Joules:J); h : Planck’ constant 6.626*10 -34 J sec; v : frequency; λ : wavelength (μm); v : velocity of light (3*10 8 m/s). From these formulae it can be seen that the energy of a photon is inversely proportional to its wavelength: the longer the wavelength involved, the lower its energy content. 1.3 Sources of electromagnetic energy Every object with a temperature above absolute zero (0 ° K or -273 ° C) radiates energy. Apart form the most commonly used source of radiant energy, the sun, all terrestrial objects are also sources of radiation (although their energy content is much lower). The amount of energy emitted is mainly a function of the temperature of the object and is described by the Stefan- Boltzmann law:
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8 W = σ *T 4 W : total radiated energy (Watts/m²); σ : Stefan-Boltzmann constant 5.67*10 -8 W m -2 K -4 ; T : absolute temperature ( ° K) of the object. Note the importance (power relation) of the temperature of the emitting object. The Stefan-Boltzmann relation is only valid for so-called black bodies. A blackbod y is an ideal material that absorbs all the radiant energy that strikes it. A blackbody is a physical abstraction because no object at the earth surface has an absorptivity of 1 and no object radiates the full amount of energy given in the Stefan-Boltzmann equation. Therefore, for real materials a property called emissivity ( ε ) has been defined as the ratio of the radiant flux of the real material and a blackbody. Remember that emissivity is wavelength dependent.
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