Spectral reflectance: When the solar radiation is incident upon the earth s surface, it is either

Transcription

1 Spectral reflectance: When the solar radiation is incident upon the earth s surface, it is either reflected by the surface, transmitted into the surface or absorbed and emitted by the surface. Remote sensing is based on the measurement of reflected or emitted radiations. Various earth surface materials reflect or absorb different amounts of radiation depending on their physical and chemical properties and wavelength of the incident radiation. The amount of radiation reflected can be studied using spectral reflectance. Spectral reflectance: It is the ratio of reflected energy to incident energy as a function of wavelength. Various materials of the earth s surface have different spectral reflectance characteristics. Spectral reflectance is responsible for the color or tone in a photographic image of an object. Trees appear green because they reflect more of the green wavelength. The spectral reflectance is dependent on wavelength, it has different values at different wavelengths for a given earth surface material. Spectral reflectance curve of vegetation: Spectral reflectance of green vegetation shows peaks and valleys (fig a). The peaks indicate reflection and the valleys indicate absorption. In general, healthy vegetations are very good absorbers of electromagnetic energy in the visible region. The absorption greatly reduces and reflection increases in the red/infrared boundary near 0.7 μm. The reflectance is nearly constant from μm and then decreases for the longer wavelengths. The absorption in the visible portion of the spectrum is due to the pigments in plant leaves. The pigment (chlorophyll) strongly absorbs energy in the wavelength bands at 0.45 and 0.67 μm within visible region (i.e. in blue and red bands). These are known as chlorophyll absorption bands. On the other hand, reflection peak occur for the green colour in the visible region, which makes our eyes perceive 1

2 healthy vegetation as green in colour. However, only 10-15% of the incident energy is reflected in the green band. In the NIR (Near Infra Red) region of the spectrum, at 0.7 μm, the reflectance of healthy vegetation increases dramatically. In the range from 0.7 to 1.3 μm, a plant leaf reflects about 50 percent of the energy incident upon it. The high reflection in NIR region is due to mesophyll cells of the leaves. Mesophyll cells reflect almost 60% of the NIR radiation reaching this layer. Most of the remaining energy is transmitted, since absorption in this spectral region is minimal. Healthy vegetation therefore shows brighter response in the NIR region compared to the green region. As the leaf structure is highly variable between plant species, reflectance measurements in this range often permit discrimination between species, even if they look same in visible wavelengths (fig b). If a plant is subjected to some form of stress that interrupts its normal growth and productivity, it may decrease or cease chlorophyll production. Stressed vegetation gives more reflectance in blue and red bands and less reflectance in NIR region. Hence, visible and NIR region can be used to distinguish between healthy and stressed vegetation (fig c). Beyond 1.3 μm, energy incident upon the plants is essentially absorbed or reflected, with little to no transmittance of energy. Dips in reflectance occur at 1.4, 1.9, and 2.7 μm as water in the leaf strongly absorbs the energy at these wavelengths. So, wavelengths in these spectral regions are referred to as water absorption bands. Reflectance peaks occur at 1.6 and 2.2 μm, between the absorption bands. 2

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4 Spectral Response Pattern of Soil: The spectral response pattern given by the soil is affected by flowing factors: 1. Soil water content: The spectral response of soil is affected by water content (moisture content). The reflectance of dry soil is more than wet soil. As the amount of water is soil increases the reflectance decreases and vice versa. The curve shows two water absorption bands at 1.4 and 1.9 μm. 2. Soil organic matter: Soil organic matter is dark and its presence will decrease the reflectance up to an organic matter content of around 4-5%. When the organic matter content of the soil is greater than 5%, the soil is black and any further increase in organic matter will have little effect on reflectance. 3. Soil texture: The look and feel of a soil is referred to as soil texture and is determined by the size and type of particles that make up the soil. Fine soils give higher reflectance than coarser soils. For example silt gives higher reflectance than sand. 4

5 4. Iron oxide content: Iron oxide gives rusty red color to soil. Hence soils that have more amount of iron oxide reflects red light ( μm) and absorbs green light ( μm). 5

6 Spectral Reflectance for Water: Water provides a semi-transparent medium for the electromagnetic radiation. Thus the electromagnetic radiations get reflected, transmitted or absorbed in water. The spectral responses vary with the wavelength of the radiation and the physical and chemical characteristics of the water. Spectral reflectance of water varies with its physical condition. In the solid phase (ice or snow) water give good reflection at all visible wavelengths. On the other hand, reflection in the visible region is poor in case of water in liquid stage. Water in the liquid form shows high reflectance in the visible region between 0.4μm and 0.6μm. Wavelengths beyond 0.7μm are completely absorbed. Thus clear water appears in darker tone in the NIR image. Locating and delineating water bodies with remote sensing data is done more easily in reflected infrared wavelengths because of this absorption property. Clear water absorbs relatively less energy having wavelengths shorter than 0.6 μm. However, as the turbidity of water changes (because of the presence of organic or inorganic materials), reflectance change dramatically. For example, water bodies containing large quantities of suspended sediments normally have much higher visible reflectance than clear water. 6

7 Likewise, the reflectance of water changes with the chlorophyll concentration involved. Increase in chlorophyll concentration tends to decrease reflectance in blue wavelengths and increase reflectance in NIR. These changes have been used in remote sensing to monitor the presence and to estimate the concentration of algae. Variation in the spectral reflectance in the visible region can be used to differentiate shallow and deep waters, clear and turbid waters, as well as rough and smooth water bodies. Reflectance in 7

8 the NIR range is generally used for delineating the water bodies and also to study the algal boom and phytoplankton concentration in water. Spectral reflectance curve of snow: Snow gives higher reflectance in visible region. The highest reflection is given in blue region which gradually decreases towards NIR region. The curve sows two absorption bands at 1 and 1.3 μm. Fresh snow has higher reflectance than firn (crystallized snow). In general, snow reflects more visible and NIR radiation than ice does. 8

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