Thermal Infrared Remote Sensing Radiant versus Kinetic temperature Blackbody radiation Atmospheric effect Principle of energy conservation Radiation from Real Materials Kirchhoff radiation law
Selected Applications of Thermal Infrared Remote Sensing
Thermal Infrared Remote Sensing • Thermal infrared EM radiationis emitted from all objects that have a temperature greater than absolute zero (K). • Our eyes cannot detect differences in thermal infrared energy • because they are primarily sensitive to short wavelength visible light • from 0.4 m to 0.7 m. • Thermal infrared sensorsare sensitive to thermal infrared radiation (3.0 - 14 µm).
Kinetic versus Radiant Temperature • The energy of particles of matter in random motion is called kinetic heat(also referred to as internal or true heat). All objects having a temperature above absolute zero (0 ˚K; or -273.16 ˚C) exhibit this random motion. • The amount of kinetic heat can be measured in kinetic temperature(Tkin) using a thermometer through direct contact with the object.
Kinetic versus Radiant Temperature • • The electromagnetic radiation exiting an object is called radiant exitance(M). • The concentration of the amount of radiant exitanceemitted from an object is its radiant temperature(Trad). • • There is a high positive correlation between the kinetic temperature of an object (Tkin) and radiant temperature (Trad). • Therefore, we can utilize radiometers placed some distance from the object to measure its radiant temperature which hopefully correlates well with the object’s true kinetic temperature. Thisis the basis of thermal infrared remote sensing.
Thermal Infrared Atmospheric Windows • The atmosphere allows a portion of the infrared energy to be transmitted from the terrain to the detectors. Regions that pass energy are called atmospheric windows. EM spectrum regions that absorb most of the infrared energy are called absorption bands. Water vapor (H2O), carbon dioxide (CO2), and ozone (O3) are responsible for most of the absorption. For example, atmospheric water vapor (H2O) absorbs most of the energy exiting the terrain in the region from 5 to 7 m making it almost useless for remote sensing.
Thermal Infrared Detectors •TIR detectors are made sensitive to thermal infrared radiant energyexiting the terrain in the two primary thermal infrared windows: 3 - 5 mand 8 - 14 m. • The Earth’s ozone (O3) layer absorbs much of the thermal energy exiting the terrain in an absorption band from approximately 9 - 10 m. Therefore, satellite thermal infrared remote sensing systems usually only record data in the region from 10.5 - 12.5 m to avoid the absorption band.
Thermal Radiation Laws • A blackbodyis a hypothetical, ideal radiator that totally absorbs and reemits all energy incident upon it. • No objects in nature are true blackbodies, however, we may think of the Sun as approximating a 6,000 ˚K blackbody and the Earth as a 300 ˚K blackbody.
Wein’s Displacement Law The relationship between the kinetic temperature of a blackbody (T) and its dominant wavelength (m) where peak exitance occurs is described by Wein’s displacement law:
Wein’s Displacement Law For example, the average temperature of the Earth is 300 ˚K (80 ˚F). We compute the Earth’s dominant wavelength as: max = 2898 m ˚K T max = 2898 m ˚K = 9.67 m 300 ˚K
Wein’s Displacement Law • The dominant wavelength provides valuable information about which part of the thermal spectrum we might want to sense in. For example, if we are looking for 800 ˚K forest fires that have a dominant wavelength of approximately 3.62 m then the most appropriate remote sensing system might be a 3-5 mthermal infrared detector. • If we are interested insoil, water, and rock with ambient temperatures on the earth’s surface of 300 ˚K and a dominant wavelength of 9.66 m, then a thermal infrared detector operating in the 8 - 14 m region might be most appropriate.
Developments from Planck’s Law:Stefan-Boltzmann Law The area under the Planck curve represents the total energy(M)emitted by an object at a given temperature (T) The Stefan-Boltzmann law calculate this energy for a blackbody at a given temperature (T).
Stephen Boltzmann Law Total radiant exitance (M) leaving the surface of a blackbody is proportional to the fourth power of its temperature (T). This is the Stefan-Boltzmann law. where σ is the Stefan-Boltzmann constant, 5.6697 x 10-8 W m-2 K-4. Thus the remote measurement of radiant exitance M from a surface can be used to infer the temperature T of the surface. It is this indirect approach to temperature measurement that is used in thermal sensing.
Radiation from real Materials & Emissivity • Real objects (such as rocks, soil, and water) emit only a fraction of the energy emitted from a blackbody at the same temperature. Emissivity, , is the ratio between the radiant exitance emitting from a real-world object and that from a blackbody at the same temperature:
Emissivity • • All real world materials have emissivities ranging from 0 to <1that fluctuate depending upon the wavelengths of energy being considered. • A graybody outputs a constant emissivity that is less than one at all wavelengths. • • Some materials like water have emissivities close to one (0.99) over the wavelength interval from 8 - 14 m. • Others such as polished aluminum (0.08) and stainless steel (0.16) have very low emissivities.
Spectral emissivity of a blackbody, a graybody, and a hypothetical selective radiator Spectral Emissivity, e Spectral radiant exitance distribution of the blackbody, graybody, and hypothetical selective radiator Spectral Radiant Exitance W m-2 um-1 2x reduction
Emissivity Two objects lying next to one another on the ground could have the same kinetic temperature but have different radiant temperatures when sensed by a thermal radiometer simply because their emissivities are different. The emissivity of an object may be influenced by a number factors, e.g. - color -- darker colored objects are usually better absorbers and emitters (i.e. they have a higher emissivity than lighter colored objects which tend to reflect more of the incident energy. - moisture content -- the more moisture an object contains, the greater its ability to absorb energy and become a good emitter. Wet soil particles have a high emissivity similar to water.
Principle of Energy Conservation • Incident (incoming) energy (i) is equal to the sum of the amount of energy reflected from the surface (r), the amount of energy absorbed by the surface (a), and the amount of energy transmitted through the surface (t). i = r+ +
• Dividing each of the variables by the original incident energy: i / i = (r/ i) +( / i) +( / i) allows us to rewrite the initial equation as: = r+ + where r is spectral reflectanceby the terrain, is spectral absorptance, and is spectral transmittance.
Kirchoff’s Radiation Law • • The Russian physicist Kirchhoff found that in the infrared portion of the spectrum the spectral emissivity of an object generally equals its spectral absorptance, i.e. ~. This is often phrased as: • “good absorbers are good emitters”. • In most remote sensing applications, objects are usually opaque to thermal radiation. Therefore, we may assume transmittance, = 0. Substituting emissivity for absorptance and removing transmittance from the equation yields: • = r+
Implications of Kirchoff’s Radiation Law • • If reflectivity increases then emissivity must decrease. If emissivity increases then reflectivity must decrease. • For example, water absorbs almost all incident energy and reflects very little. Therefore, water is a very good emitter and has a high emissivity close to 1. Conversely, a sheet metal roof reflects most of the incident energy, absorbs very little, yielding an emissivity much less than 1. • Therefore, metal objects such as cars, aircraft, and metal roofs almost always look very cold (dark) on thermal infrared imagery.
Relationship b/w Trad and Tkin • The radiant temperature of an object recorded by a remote sensor is related to its kinetic temperature and emissivity by the following relationship: Trad= 1/4Tkin
Thermal Infrared Multispectral Scanners • The diameter of the circular ground area viewed by the sensor, D, is a function of the instantaneous-field-of-view, , of the scanner measured in milliradians (mrad) and the altitude of the scanner above ground level, H, where: D = H x For example, if the IFOV of the scanner is 2.5 mrad, the ground size of the pixel in meters is a product of the IFOV (0.0025) and the altitude above ground level (AGL) in meters. IFOVs range from 0.5 to 5 milliradians
Daytime Optical and Nighttime Thermal Infrared Imagery of New York City Thermal Infrared Aerial Photograph
Daytime Optical and Nighttime Thermal Infrared Imagery April 26, 1981 4:56 am 1 x 1 m 2x reduction
Pre-dawn Thermal Infrared Image of Effluent Entering the Savannah River Swamp System Savannah River March 31, 1981 4:28 am; 3 x 3 m
Pre-dawn Thermal Infrared Image of a Residential Subdivision in Forth Worth, Texas 250 m AGL 1 mrad IFOV 6:45 am Jan 10, 1980 0.25 x 0.25 m