Electromagnetic Radiation
Physical Principles of Remote
Sensing
Outline for 4/3/2003
•
•
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Properties of electromagnetic radiation
The electromagnetic spectrum
Spectral emissivity
Radiant temperature vs. kinematic
temperature
Energy Transfer
• Energy is “the ability to do work”
• Energy transfer:
– Conduction: transfer of kinetic energy by
contact between atoms or molecules
– Convection: transfer of kinetic energy by
physically moving the mass that contains
the energy
– Radiation: propagation via waves/particles
through a vacuum (or through a medium)
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 other
forms
–
–
–
–
–
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
Electrical (E) and magnetic field (B) are orthogonal to
each other
Direction of each field is perpendicular to the direction
of wave propagation.
Electromagnetic Waves
Described by:
• Wavelength
• Amplitude
Electromagnetic Radiation
• Its harmonic wave form can be described
according to the Maxwell equations:
E x = E 0 cos(wt - kz)
†
Where,
†
E is the electric field
w= angular frequency (2pn), n = c/l,
l = wavelength
c = speed of light in a vacuum (300,000 kms-1)
k = wavenumber (2p/l)
z = distance
t = time
Frequency vs. Wavelength
The product of wavelength and frequency is a
constant:
n l=c
l = distance of separation between two
successive wave peaks
n = number of wave peaks passing in a given
time
c = speed of light in a vacuum (300,000 kms-1)
Energy vs. Frequency
When considering the particle form of energy,
we call it a photon
The energy of a photon is proportional to
frequency:
Q=hn
n = c/l
Q = hc/l
where, h = Planck’s constant = 6.626 10-34 Js
Thus,
Q ~ 1/l
The EM Spectrum
Polarization
E and B fields are perpendicular to each
other but their orientation can change
• If both remain in their respective planes, the
radiation 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 is
called “randomly polarized” or unpolarized
Polarization
• Plane polarized light can be either
– vertically polarized (E0 is perpendicular to the
plane of incidence)
– horizontally polarized (E0 is parallel to the plane of
incidence)
• Solar radiation is unpolarized (random) but
can become polarized by reflection, scattering,
etc.
• Lasers and radars produce polarized
radiation
Spectral Emittance
• All bodies whose temperature are above
absolute zero Kelvin (-273.2 oC) emit radiation
at all wavelengths
• A “blackbody” is one that is a perfect absorber
and perfect emitter (hypothetical, though
Earth and Sun are close)
• Planck’s Law describes how heat energy is
transformed into radiant energy
• This is the basic law for radiation
measurements in all parts of the EM spectrum
Planck’s Blackbody Equation
Ml =
†
C1
l [e
5
C 2 lT
-1]
Ml = spectral radiant exitance (emittance),
units are W m-2 mm-1
l = wavelength
T = the blackbody’s temperature in Kelvin (K)
C1 = 3.74151 ¥ 108 W m-2 mm4
C2 = 1.43879 ¥ 104 mm K
Blackbody Radiation
• According to Planck’s law, a blackbody will
emit radiation in all wavelengths but not
equally
4
• Stefan-Boltzmann Law:
M = sT
Emittance is proportional
to physical temperature
†
s = 5.670 10-8 W m-2 K-4
• Graybody:
M = esT
4
Object that reflects part of incident radiation
e < 1.0
†
Emissivity
• Describes the actual absorption and
emission properties of real objects
(“graybodies”)
• Is wavelength dependent
• Emissivity = graybody emittance/blackbody emittance
• Emissivity establishes the radiant
temperature Trad of an object
Radiant Temperature vs.
Kinematic Temperature
• Two objects can have the same kinematic
temperature but different radiant temperatures
Object
Emissivity
Kinematic
Temperature
Radiant
Temperature
Blackbody
1.0
300
300
Water, distilled
0.99
300
299.2
Basalt, rough
0.95
300
296.2
Basalt, smooth
0.92
300
293.8
Obsidian
0.86
300
288.9
Mirror
0.02
300
112.8
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 K
2898/6000 = 0.48 mm
• Earth’s temperature = 300 K
2898/300 = 9.6 mm
Sun’s Radiant Energy Distribution
Name of Spectral
Region
Wavelength
Range, mm
Percent of Total
Energy
Gamma and X-rays
< 0.01
Negligible
Far Ultraviolet
0.01 - 0.2
0.02
Middle Ultraviolet
0.2 - 0.3
1.95
Near Ultraviolet
0.3 - 0.4
5.32
Visible
0.4 - 0.7
43.5
Near Infrared
0.7 - 1.5
36.8
Middle Infrared
1.5 - 5.6
12.0
Thermal Infrared
5.6 - 1000
0.41
Microwave
> 1000
Negligible
Radio Waves
> 1000
Negligible
Emission spectrum of a 6000K blackbody
leaving the surface
of the sun
SolarRadiation
Emittance
Curve
Solar radiation at sea level
• For terrestrial remote sensing, the most
important source is the sun
Reflected solar energy is used
0.3 - 2.5 mm
• The Earth is also an energy source
>6 mm for self-emitted energy