CHAPTER 8:: RADIATION MEASUREMENTS
Introduction
Why is it important? It is important because radiation transfers energy in the atmosphere (the other two
ways are convection and conduction). Radiation is an important mechanism underlying the remote sensing
measurements of the atmosphere and surface. So for energy transfer it is useful to introduce the concept of
a flux.
Flux
One concept is closely related to energy transfer is flux. A flux is defined as a transfer of some quantity per
unit area per unit time. Some examples of fluxes include:
kg
m2s
J
W
J
Energy flux- 2  2  W 
s
m s m
Mass flux-
So consider a small volume of the atmosphere with fluxes at two interfaces.
Radiative characteristics
Radiation is an energy transfer mechanism which requires no physical medium, so the radiation
characterized as waves or particles (photons) that release energy when absorbed. Photons have no mass,
and travel at the speed of light, and they are characterized by their wavelength,  , and frequency, f ,
which is in hertz (cycles per second). They are related by the following relationship.

c
f
(6.1)
where c is the speed of light in a vacuum (299,792,458 meters per second). Equation 6.1 illustrates that
the longer the wavelength the lower the frequency.
The energy of a wave is illustrated by equation 6.2
E  hf
(6.2)
where E is energy, h is a constant, and f is the frequency. Then we can now look at the spectrum of
radiation:
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Cosmic, X, Gamma rays
 0.001m
Ultraviolet
0.001  0.4m
Visible
0.4  0.8m
Near Infrared
0.8  4m
Far Infrared
4  100m
Microwave
100m  1 *10 6 m
Radio
 10 * 10 6 m
The Solar spectrum, consists primarily on UV, Visible, and near infrared radiation.
0.001  4m . Solar
radiation peaks a 0.5m , and the largest portion of solar radiation emitted is in the visible spectrum.
Terrestrial radiation however is 4  100 m . So as the solar radiation peaks at 0.5m the terrestrial
radiation peaks at 10 m , and the largest percentage of terrestrial radiation emitted as infrared radiation.
Black Body radiation
All objects emit radiation, and an object that emits the maximum radiation possible is called a blackbody,
otherwise it is called a greybody. The distribution of radiant energy emitted is given by Plank’s law
(equation 6.3).
E* 
c1
 c  
5 exp  2   1
  T  
*
where E  is irradiance (or emission at a given wavelength) and it has the unites of
(6.3)
Wm 2 m ,
c1  3.74 *10 16 Wm 2 , and c2  1.44 *10 2 mK . The equation above can be used to show the
*
variation of E at different temperatures.
2
*
Above are the blackbody curves for T = 3000, 6000 and 12,000 K. There is a maximum in E  at some
wavelength that depends on temperature.
So how can we find the wavelength at which this maximum was to occur? Well using calculus we would
have to take the derivate and set it to zero.
dE 
0
d
Therefore
2897
T
 max 
(6.5)
Equation 6.4 is called Wien’s Displacement Law, where λ is in microns. Note that the difference between
the solar and terrestrial λmax values.
Sun’s
Temp = 5100K
Earth’s
Temp = 255K
*
If Plank’s law E  is integrated over all wavelengths, then

E *   E* d  T 4
0
(6.6)
Where σ = 5.7*10 Wm K , this is called the Stefan Boltzmann Law. Use Stefan Boltzmann and Wien’s
Displacement to characterize solar and terrestrial radiative characteristics. So consider that the sun is at
6000K and the earth at 260K, what is the difference in the irradiance? The maximum solar emission is at
0.48283333μm, and the maximum terrestrial emission is at about 11.14230769μm. Real objects are not
blackbodies so we would have to adjust the Stefan Boltzmann equation to fit that new information
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Where

-2
-4
E  T 4  E *
(6.7)
is emissivity and I is a ration of the actual emission to the blackbody emission.

E
; 0   1
E*
(6.8)
Radiative equilibrium temperature
Energy in  Energy out
Consider the earth illuminated by the sun.
The solar constant at the top of the atmosphere is
S o  1380 mW2
Then let the albedo of the earth be A~0.3, so that the energy in is
3
(6.9)
Ein  S o ( 1
 A )R 2 earth
absorptivity
Since the earth intercepts an area of
2
Rearth
The earth emits radiation in accordance with the Stefan Boltzmann Law.


2
Eout   T 4 4Rearth
Therefore
Where σ = 5.7*10-8 Wm-2K-4, and

4
(1  A) S o  4Tearth
is the emissivity of the earth’s atmosphere system.
A ray of radiation that is incident on a surface can be transmitted, absorbed, reflected
Fraction absorbed: absorbtivity
a 
E (absorbed )
E
r 
E (reflected )
E
Fraction reflected: reflectivity
Fraction transmitted: transmissivity
t 
E (transmited )
E
Since energy is conserved
a  r  t   1
For an opaque surface
a  r  1
At a given wavelength
   a
Further
a  r  t   1
If the reflectivity is averaged over a range of wavelengths (for example: the solar band), we can define it as
albedo A.
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(What evidence do you have that in general reflectivity is wavelength dependent?)

E
E

“greybody” emissivity
*
E
T 4
A
E reflected
Therefore
Eincident
*100%  Albedo
For the earth-atmosphere system what reflects solar radiation?
Surface
Soils
Desert
Grass
Remarks
Dark, wet
Light, dry
Long (1.0 meters)
Short (0.2 meters)
Agricultural crops,
tundra
Orchards
Forests
Deciduous (bare)
Deciduous (Leaved)
Coniferous
Water
Small Zenith angle
Large zenith angle
Snow
Old
Fresh
Ice
Sea
Glacier
So what gases absorb shortwave radiation?
Ozone
What gases absorb and emit longwave radiation?
Clouds
water vapor
CO2
Ozone
Methane
Albedo = A
0.05-0.40
Emissivity = 
0.90-0.98
0.20-0.45
0.160.26
0.18-0.25
0.84-0.91
0.900.95
0.90-0.99
0.15-0.20
0.150.20
0.05-0.15
0.03-0.10
0.10-1.00
0.400.95
0.30-0.45
0.20-0.40
0.15-0.20
0.970.98
0.97-0.99
0.92-0.97
0.92-0.97
0.820.99
0.92-0.97
Atmospheric gases absorb all energy at wavelengths emitted from surface except for 8-11 micron window
Oxygen, ozone, carbon dioxide, water vapor are great absorbers as shown below..
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Absorbtion of radiation in the atmosphere
.
The Atmospheric spectrum obtained with a scanning interferometer on board the Nimbus 4 satellite. The
interferometer viewed the earth vertically and the satellite was passing over the North Africa desert. (After
Hanel et al. 1972)
Making Radiation Measurements
There are three ways to make radiation measurements.
1. Thermal sensitive devise
2. Photoelectric cell (photodiode)
3. Photochemical sensor
What is the basic operating principle for the thermal devise?
Consider the energy budget for a plate in the atmosphere. How could we use a plate to measure broadband
radiation? For example, illuminate the surface with a bright light…
Ein  Eout
What besides radiation will affect temperature of the plate shown above? Convection and conduction, and
how could their effect on the temperature of the plate be removed? Using a glass dome (or an appropriate
material to cover the plate and limit other energy transfer measurements. What materials should be used?
These materials will differ depending on the application. A typical design
Since the thermopile is thermal sensitive, the received radiation is converted to heat, and thus, the change
of the temperature.
What is the basic operating principle for the photoelectric cell (photodiode)?
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A device that converts light into electricity. Two main types
of photoelectric cell are in use today: the phototube and the
solid-state photodetector.
A phototube is an electron tube in which electrons initiating
an electric current originate through photoelectric emission.
The simplest phototube is composed of a cathode coated
with a photosensitive material. Light falling upon the
cathode causes the liberation of electrons, which are then
attracted to the positively charged anode, resulting in a flow
of electrons (i.e., current) proportional to the intensity of the
light.
The simplest type of solid-state photodetector is the photoconductor whose resistance changes when it is
exposed to light. The solid-state photodetector has replaced the phototube for many applications because it
is small, inexpensive, and uses little power.
What is the basic operating principle for the photochemical sensor?
The photochemical sensor utilizes materials that tend to have chemical reaction due to the absorption of
light (including visible, ultraviolet, and infrared). The light excites atoms and molecules (shifts some of
their electrons to a higher energy level) and thus makes them more reactive. The bleaching of dyes or the
yellowing of paper by sunlight is a good example of photochemical reaction. It is harnessed by plants in
photosynthesis and by humans in photography.
Real instruments
Broadband Radiation Instruments:
Shortwave K
Longwave L
Total
Q=K+L
We can define an upward stream and a downward stream of radiation
Q  K   L 
Q  K   L 
K  = -solar incident
K  = reflected solar
L  = emission from sun’s face
L  = emission from atmosphere
*
We can also define the net radiation Q
Q *  Q  Q 
In addition to the broadband radiation, spectral radiometers can measure radiation at certain
frequency/wavelength, or smaller bands.
What are solar radiation measurements?
Light from the sky to the dome is either direct from, the sun, or it can be from everywhere else but from the
sun, or from the entire sky. We call it direct (beam), diffused (sky), and global (total). The Global is the
sum of the direct and the diffused.
The following results were obtained on a clear day, and it shows solar radiation irradiance measurements
for September 22, 1994.
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The below list are the names of broadband and global instruments:
Pyranometer — measures global-solar shortwave radiation
Pyrometer — measures global longwave radiation
Pyrheliometer — direct beam
Pyrradiometer — net radiation
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