UNIT 7b THERMAL

UNIT 7b THERMAL - 1 Geo 4131 Remote Sensing of the...

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Geo 4131 Remote Sensing of the Environment Dr. Charles Roberts THERMAL INFRARED REMOTE SENSING So far, we have talked about visible and near infrared wavelengths. Today, we are going to talk about thermal infrared. Thermal infrared is also called far infrared and emitted infrared because it is emitted, not reflected light. The interpretation of thermal infrared is fundamentally different than for reflected radiation because we are sensing emitted radiation, emitted heat. What is meant by heat? Heat energy = random motion of molecules: the hotter the substance, the more vigorous the motion. But this is not temperature as we understand it. We sense one kind of energy, called sensible heat. There is also latent heat which is heat stored in the molecular structure of matter. For example, a teacup full of hot tea might be 98 degrees but a bathtub full of 60 degree water has more latent heat, that is, heat stored that can be emitted by vaporizing the water in the bathtub. As far as the role of water in the biosphere, water is the only substance that is commonly found in three phases: mineral (ice), gas, and liquid. The major reason why the earth looks different then the moon is water. Water heats and cools faster than land which is critical for interpreting thermal imagery and the differentiation between water tone and land tone. The energy source that drives the water cycle is known as incoming solar radiation. All incoming radiation is reflected, transmitted, or absorbed and everything that is absorbed is emitted. A surface emits an amount of radiation in direct proportion to its surface temperature and its own characteristics. This is called spectral emissivity which measures not only the wavelength emitted, but the amount of energy. For example, the sun emits mostly shortwave radiation, in high amounts while the earth emits mostly longwave radiation, in low amounts. Infrared (IR) Wavelengths: Reflected Infrared Near Infrared Shortwave Infrared .7 to 3. μ Mid -Infrared Boundary between Long and Shortwave 3. to 7 μ Thermal Infrared Emitted Infrared Longwave Infrared 7. to 18 μ Blackbody Curve : The ideal thermal emitter... Blackbody curves transform heat energy into radiant energy at the maximum rate possible: absorbs, transmits, emits. This is useful because it gives us the maximum rate possible. There are blackbody curves for both the earth and sun. A surface emits an amount of radiation in direct proportion to its temperature; this is known as the maximum rate of spectral 1
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emissivity. The sun has a high temperature, emits mostly shortwave energy. The earth has a low temperature, emits mostly longwave energy. To define long and short wavelengths around the earth's and sun's emittance: Earth's surface: -40 to +40 centigrade. Maximum emission in 4+ micron range, or longwave.
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