electromagnetic radiation - oregon state university
TRANSCRIPT
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Electromagnetic Radiation
Physical Principles of RemoteSensing
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Outline for 4/3/2003
• Properties of electromagnetic radiation
• The electromagnetic spectrum
• Spectral emissivity
• Radiant temperature vs. kinematictemperature
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Energy Transfer
• Energy is “the ability to do work”• Energy transfer:
– Conduction: transfer of kinetic energy bycontact between atoms or molecules
– Convection: transfer of kinetic energy byphysically moving the mass that containsthe energy
– Radiation: propagation via waves/particlesthrough a vacuum (or through a medium)
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Electromagnetic Radiation
• EMR is the source for most types of remote sensing– Sun and Earth are both passive sources of EM radiation
– Lasers and radar are active sources
• Generated by transformation of energy from otherforms– Kinetic: thermal motion of atoms and molecules (heat)– Electrical: radio frequency (dipole antenna)– Magnetic: electron tube (microwave)– Radioactive: decay of radioactive substances– Chemical/Laser: molecular excitation
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Electrical (E) and magnetic field (B) are orthogonal toeach other
Direction of each field is perpendicular to the directionof wave propagation.
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Electromagnetic Waves
Described by:
• Wavelength
• Amplitude
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Electromagnetic Radiation
• Its harmonic wave form can be describedaccording to the Maxwell equations:
†
†
Ex = E0 cos(wt - kz)Where,E is the electric fieldw= angular frequency (2pn), n = c/l, l = wavelengthc = speed of light in a vacuum (300,000 kms-1)k = wavenumber (2p/l)z = distancet = time
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Frequency vs. WavelengthThe product of wavelength and frequency is a
constant: n l=c
l = distance of separation between twosuccessive wave peaks
n = number of wave peaks passing in a giventime
c = speed of light in a vacuum (300,000 kms-1)
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Energy vs. Frequency
When considering the particle form of energy,we call it a photon
The energy of a photon is proportional tofrequency:
Q = h n n = c/lQ = hc/l
where, h = Planck’s constant = 6.626 10-34 Js
Thus, Q ~ 1/l
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The EM Spectrum
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Polarization
E and B fields are perpendicular to eachother but their orientation can change
• If both remain in their respective planes, theradiation is called “plane polarized”
• If they rotate around the axis of propagation,the radiation is called “circularly polarized” or“elliptically polarized”
• If their orientation changes randomly, it iscalled “randomly polarized” or unpolarized
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Polarization• Plane polarized light can be either
– vertically polarized (E0 is perpendicular to theplane of incidence)
– horizontally polarized (E0 is parallel to the plane ofincidence)
• Solar radiation is unpolarized (random) butcan become polarized by reflection, scattering,etc.
• Lasers and radars produce polarizedradiation
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Spectral Emittance
• All bodies whose temperature are aboveabsolute zero Kelvin (-273.2 oC) emit radiationat all wavelengths
• A “blackbody” is one that is a perfect absorberand perfect emitter (hypothetical, thoughEarth and Sun are close)
• Planck’s Law describes how heat energy istransformed into radiant energy
• This is the basic law for radiationmeasurements in all parts of the EM spectrum
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Planck’s Blackbody Equation
†
Ml =C1
l5 eC2 lT -1[ ]Ml = spectral radiant exitance (emittance), units are W m-2 mm-1
l = wavelengthT = the blackbody’s temperature in Kelvin (K)C1 = 3.74151 ¥ 108 W m-2 mm4
C2 = 1.43879 ¥ 104 mm K
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Blackbody Radiation
• According to Planck’s law, a blackbody willemit radiation in all wavelengths but notequally
• Stefan-Boltzmann Law:Emittance is proportional to physical temperatures = 5.670 10-8 W m-2 K-4
• Graybody:Object that reflects part of incident radiatione < 1.0
†
†
M = sT 4
†
M = esT 4
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Emissivity
• Describes the actual absorption andemission properties of real objects(“graybodies”)
• Is wavelength dependent• Emissivity = graybody emittance/blackbody emittance
• Emissivity establishes the radianttemperature Trad of an object
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Radiant Temperature vs.Kinematic Temperature
• Two objects can have the same kinematictemperature but different radiant temperatures
112.83000.02Mirror
288.93000.86Obsidian
293.83000.92Basalt, smooth
296.23000.95Basalt, rough
299.23000.99Water, distilled
3003001.0Blackbody
Radiant
Temperature
KinematicTemperature
EmissivityObject
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Wien’s Law
lmax = a /T
a = 2898 mm K• The wavelength of peak emittance is inversely proportional to the
kinematic temperature
• Sun’s temperature = 6000 K2898/6000 = 0.48 mm
• Earth’s temperature = 300 K2898/300 = 9.6 mm
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Sun’s Radiant Energy Distribution
Negligible> 1000Radio Waves
Negligible> 1000Microwave
0.415.6 - 1000Thermal Infrared
12.01.5 - 5.6Middle Infrared
36.80.7 - 1.5Near Infrared
43.50.4 - 0.7Visible
5.320.3 - 0.4Near Ultraviolet
1.950.2 - 0.3Middle Ultraviolet
0.020.01 - 0.2Far Ultraviolet
Negligible< 0.01Gamma and X-rays
Percent of TotalEnergy
WavelengthRange, mm
Name of SpectralRegion
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Solar Emittance Curve
Emission spectrum of a 6000K blackbody
Radiation leaving the surface of the sun
Solar radiation at sea level
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• For terrestrial remote sensing, the mostimportant source is the sunReflected solar energy is used0.3 - 2.5 mm
• The Earth is also an energy source>6 mm for self-emitted energy