Download Electromagnetic radiation

ACET450 REMOTE SENSING
What is Electromagnetic Energy?
All objects with higher temperature than absolute zero (-273°C) emit electromagnetic
radiation. Electromagnetic radiation is emitted from the Sun to the Earth making life
possible.
The distribution of electromagnetic radiation over the electromagnetic spectrum is
not uniform and it depends on the source’s temperature.
Fig. 1: Electromagnetic spectrum
Visible light is so called because it is detected by eyes. Electromagnetic radiation of
other wavelengths is invisible to naked eye, but very useful in Remote Sensing.
Electromagnetic radiation is classified into types according to the frequency (the
number of occurrences of a repeating event per unit time) of the wave, these types
include (in order of increasing frequency): radio waves, microwaves, thermal infrared
radiation, infrared radiation, visible, ultraviolet radiation, X-Rays and Gamma rays. Of
these, radio waves have the longest wavelengths and Gamma rays have the
shortest.
In our everyday lifes we use items that make use of electromagnetic radiation: e.g. to
listen to the radio, watch TV, use the microwave, x-rays etc. All these are examples
of electromagnetic waves that have different wavelengths to each other.
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Longer
wavelength
Shorter
wavelength
Energy is the capacity to do work. It is expressed in Joules (J). Radiant energy is the
energy associated with EMR. The rate of transfer of energy from one place to
another (e.g. from the Sun to the Earth) is called the flux of energy (from Latin
meaning ‘flow’). It is measured in Watts (W). Radiant flux is the rate of transfet of
radiant (electromagnetic) energy.
Radiant energy that falls (is incident upon) a surface is called irradiance.
If the energy flow is away from the surface (e.g. thermal energy emitted by the
Earth or solar energy that is reflected by the earth), is called radiant exitance or
radiant emittance.
The distribution of electromagnetic radiation over the electromagnetic
spectrum is not uniform and depends on the source’s temperature.
What is a blackbody?
A blackbody is a hypothetical entity, which absorbs all incoming radiation, reflects
none and emits energy at full efficiency. The significance of these three laws Plank’s, Wien’s and Stefan-Boltzmann’s - lies in the fact that the Sun (and
stars) emit radiation in a similar way to the blackbodies.
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Planck’s law
The amount of energy emitted by a blackbody can be calculated for different
wavelengths using Plank’s law. Planck’s law describes the spectral exitance of a
blackbody.
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40
K
Peak
(Watts)
2000 K
2000
1600
1000
Spectral emittance (Wcm-2µm-1)
32
24
---
16
1.448
1.811
2.897
Flux
90.715
37.157
5.700
Join of peaks of the
spectral emittance curves
1600 K
8
1000 K
0
3
6
9
12
Wavelength (µm)
Figure 1: Spectral emittance curves for blackbodies at temperatures of 1000,
1600 and 2000 K (Mather, 1999)
•
Wien’s displacement law
Figure 1 shows the spectral exitance curves for blackbodies at different
temperatures. The higher the temperature of a blackbody the shorter the
wavelength at which the maximum spectral exitance is achieved, is. This is
known as Wien’s displacement law. The dotted line in Figure 1 joins the peaks of
the spectral exitance curves.
•
Stefan-Boltzmann law
The second feature of this Figure is that the energy emitted by the blackbody is
greater at every wavelength as the temperature increases. Therefore, the total
amount of radiant energy emitted increases with increase in temperature. This is
known as the Stefan-Boltzmann law.
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Absorption and scattering
The amount of solar irradiance that reaches the Earth’s surface is altered by
absorption and scattering. Absorption is a process during which the energy
present in electromagnetic radiation is converted into internal energy of the
absorbing molecule. Such absorbing molecules are water vapour, carbon dioxide
and ozone gases.
Scattering deflects the radiation and is caused by smoke, dust, salt (Mather,
1999). Figure 2 shows the solar irradiance outside the earth’s atmosphere and the
solar irradiance reaching the earth’s surface. The difference between the curves is
due to absorption and scattering.
2200
2000
Solar irradiation curve outside the atmosphere
1800
Intensity (W/m2 µm)
1600
Scattered by the atmosphere
1400
Solar irradiation curve at sea level
1200
1000
800
Absorbed by the atmosphere
600
400
200
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
Wavelength (µm)
Figure 2: Difference in the solar irradiance at the top of the atmosphere and
at sea level due to atmospheric effects (Mather, 1999)
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4.0
However, in the electromagnetic spectrum there are atmospheric windows where the
effects of absorption and scattering are minimised. Thus, remote sensing is
concentrated in bands within the atmospheric windows, which are free from these
two interactions. 3-5µm and 8-14µm band regions are the least affected by absorbing
molecules and are therefore useful in remote sensing. This is demonstrated in Figure
3.
100
Transmission %
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
CO2
O3
13
14
15
Wavelength (µm)
O2
123
CO2 H2O CO2 O3
H2O
H2O
H2O
Absorbing molecule
Figure 3: The effects of absorbing molecules to the electromagnetic spectrum
(Electro Optical Industries, Inc., 2000)
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CO2
CO2