Interaction of EMR with atmosphere and earth surface
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Interaction of EMR with atmosphere and earth surface






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Interaction of EMR with atmosphere and earth surface Document Transcript

  • 1. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 1 INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACEElectro-magnetic radiation (EMR) or energy interactions with atmosphere and with the earth surface play a vital role inremote sensing. Energy interactions with the atmosphere dictate the spectral regions through which only we can do theremote sensing which are known as spectral windows (the spectral regions where atmosphere is more or lesstransparent). ENERGY INTERACTION IN ATMOSPHEREIrrespective of its source, all radiation detected by remote sensors passes through some distance, or path length, ofatmosphere. The path length involved can vary widely. For example, space photography results from sunlight that passesthrough the full thickness of the earths atmosphere twice on its journey from source to sensor. On the other hand , anairborne thermal sensor detects energy emitted directly from objects on the earth, so a single, relatively shortatmospheric path length is involved.Because of the varied nature of atmospheric effects, we treat this subject on a sensor-by-sensor. The atmosphere canhave a profound effect on, among other things, the intensity and spectral composition of radiation available to anysensing system. These effects are caused principally through the mechanisms of atmospheric scattering and absorption.(A). SCATTERING:Atmospheric scattering is unpredictable diffusion of radiation by particles in the atmosphere. Scattering is the processwhere an atom, molecule or particle redirects energy.TYPES OF SCATTERING 1. Rayleigh scattering – atmospheric atom 2. Mie scattering – aerosol 3. Non selective scattering – cloud MICHAEL HEMBROM
  • 2. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 2Rayleigh scattering:Atmospheric gas or small molecule scatters radiation by a process known as Rayleigh scattering. Rayleigh scatter iscommon when radiation interacts with atmospheric molecules and other tiny particles that are much smaller in diameterthan the wavelength of the interacting radiation. The effect of Rayleigh scatter is inversely proportional to the fourthpower of wavelength.Rayleigh scattering redirects the radiation about equal in all direction. Rayleigh scattering is responsible for blue skiesand red sunsets.Mie scattering:Another type of scatter is Mie scatter, which exists when atmospheric particle diameters essentially equal the energywavelengths being sensed. Water vapour and dust are major causes of Mie scatter. This type of scatter tends toinfluences longer wavelengths compared to Rayleigh scatter. Although Rayleigh scatter tends to dominate under mostatmospheric conditions, Mie scatter is significant in slightly overcast ones.Examples: water vapor, smoke particles, fine dust MICHAEL HEMBROM
  • 3. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 3Non selective scattering:A mo re b o thersome phenomenon is nonselective scatter, which comes about when the diameters of the particlescausing scatter are much larger than the energy wavelengths being sensed. Water droplets, for example, cause suchscater . They commonly have a diameter in the 5 to 100 µm (mocro meter, 10-6 m) range and scatter all visible andReflected IR wavelengths about equally. Consequently, this scattering is "nonselective" with respect to wavelength.In the visible wavelengths (Approx. 0.4 to 0.7 µm ), equal quantities of blue, green, and red light are scattered, Makingfog and clouds appear white.Examples: water droplets, ice crystals, volcanic ash,Smog(B). ABSORPTIONAtmospheric absorption results in the effective loss of energy to atmospheric constituents. This normally involvesabsorption of energy at a given wavelength. The most efficient absorbers of solar radiation in This regard is watervapour, carbon dioxide, and ozone. Because these gasses tend to absorb electromagnetic Energy in specific wavelengthbands, they strongly influence "where we look" spectrally with any given remote Sensing system. The wavelengthranges in which the atmosphere is particularly transmissive of energy are referredto as atmospheric windows. MICHAEL HEMBROM
  • 4. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 4(C). TRANSMISSION:A material transmits light when it allows the light to pass through it. Transparent materials allow all the light to passthrough them so that you can easily see what is on the other side. Examples of transparent materials are glass, water, andair. Translucent materials scatter the light that passes through them. You can tell that something is on the other side, butyou cannot see details clearly. Examples of translucent materials are wax paper, frosted glass, and some kinds of plastic.If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk ofthe material and reemitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted.If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of thematerial. Rather the electrons of atoms on the materials surface vibrate for short periods of time and then reemit theenergy as a reflected light wave. Such frequencies of light are said to be reflected. MICHAEL HEMBROM
  • 5. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 5 INTERACTIONS WITH EARTH SURFACE FEATURES Electromagnetic radiation that passes through the earths atmosphere without being absorbed or scattered reaches the earths surface to interact in different ways with different materials constituting the surface. When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible. There are three ways in which the total incident energy will interact with earths surface materials. These are • Absorption • Transmission, and • Reflection Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission (T) occurs when radiation passes through a target. Reflection (R) occurs when radiation "bounces" off the target and is redirected. How much of the energy is absorbed, transmitted or reflected by a material will depend upon: • Wavelength of the energy • Material constituting the surface, and • Condition of the feature.Reflection from surfaces occurs in two ways: 1. When the surface is smooth, we get a mirror-like or smooth reflection where all (or almost all) of the incident energy is reflected in one direction. This is called Specular Reflection and gives rise to images. 2. When the surface is rough, the energy is reflected uniformly in almost all directions. This is called Diffuse Reflection and does not give rise to images. Specular Reflection Diffuse Reflection Most surface features of the earth lie somewhere between perfectly specular or perfectly diffuse reflectors. Whether a particular target reflects specularly or diffusely, or somewhere in between, depends on the surface roughness of the feature in comparison to the wavelength of the incoming radiation. MICHAEL HEMBROM
  • 6. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 6 • If the wavelengths are much smaller than the surface variations or the particle sizes that make up the surface, diffuse reflection will dominate. For example, fine-grained sand would appear fairly smooth to long wavelength microwaves but would appear quite rough to the visible wavelengths.Vegetation: A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths. • Leaves appear "greenest" to us in the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll in the leaves, so there is less absorption and proportionately more reflection of the red wavelengths, making the leaves appear red or yellow (yellow is a combination of red and green wavelengths). • The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths. If our eyes were sensitive to near-infrared, trees would appear extremely bright to us at these wavelengths. In fact, measuring and monitoring the near-IR reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation may be.Water:Longer wavelength visible and near infrared radiation is absorbed more by water than shorter visible wavelengths. Thuswater typically looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed atred or near infrared wavelengths. MICHAEL HEMBROM
  • 7. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 7• If there is suspended sediment present in the upper layers of the water body, then this will allow better reflectivity and a brighter appearance of the water.• The apparent colour of the water will show a slight shift towards longer wavelengths.• Suspended sediment (S) can be easily confused with shallow (but clear) water, since these two phenomena appear very similar.• Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green, making the water appear more green in colour when algae is present.• The topography of the water surface (rough, smooth, floating materials, etc.) can also lead to complications for water-related interpretation due to potential problems of specular reflection and other influences on colour and brightness.• We can see from these examples that, depending on the complex make-up of the target that is being looked at, and the wavelengths of radiation involved, we can observe very different responses to the mechanisms of absorption, transmission, and reflection. MICHAEL HEMBROM
  • 8. INTERACTION OF EMR WITH ATMOSPHERE AND EARTH SURFACE 8Spectral Response of Materials:By measuring the energy that is reflected (or emitted) by targets on the Earths surface over a variety of differentwavelengths, we can build up a spectral response for that object. The spectral response of a material to different wavelengths of EMR can be represented graphically as a Spectral Reflectance Curve. It may not be possible to distinguish between different materials if we were to compare their response at one wavelength. But by comparing the response patterns of these materials over a range of wavelengths (in other words, comparing their spectral reflectance curves), we may be able to distinguish between them. For example, water and vegetation may reflect somewhat similarly in the visible wavelengths but are almost always separable in the infrared. • Spectral response can be quite variable, even for the same target type, and can also vary with time (e.g. "green-ness" of leaves) and location. MICHAEL HEMBROM