Lecture 2: Basic principles of
electromagnetic magnetic
radiation (EMR)
Prepared by Rick Lathrop 9/99
Updated 9/04
Basic interactions between EMR and
the earth surface
•
Reflection:
specular reflection
q1 = q2
•
Absorption
or
scattering
q1 q2
emission
EMR re-emitted as thermal energy
•
Transmission
Shorter ls
refracted more
First law of thermodynamics
• Principle of conservation of energy
• Energy can neither be created or destroyed,
it can only be transformed
• Incident E = R + A + T
E
R
A
T
Adapted from Lillesand & Kiefer Remote Sensing and Image Interpretation
Units of EMR measurement
• Irradiance - radiant flux incident on a
receiving surface from all directions,
per unit surface area, W m-2
• Radiance - radiant flux emitted or scattered
by a unit area of surface as measured
through a solid angle, W m-2 sr-1
• Reflectance - fraction of the incident flux
that is reflected by a medium
Dual nature of EMR
• EMR as a wave
• EMR as a particle (photon)
Wave nature of EMR
• c=n*l
where
• c = 3 x 108 m/sec
 n = frequency,
measured in hertz (cycles/sec)
l = wavelength
• inverse relationship between wavelength
and frequency
EMR wavelength vs. frequency
as l gets shorter, v goes higher
l = 10 mm
n = 1013 Hz
l = 1.0 mm
n = 1014 Hz
l = 0.1 mm
n = 1015 Hz
Wave nature of EMR: translating
between wavelength and frequency
• c=n*l
where
• c = 3 x 108 m/sec
Example: n = 600 Mhz
l=?
n = c / l or
l = c /n
l = 3 x 108 m/sec / 600 x 106 hz =
l = 3 x 108 m/sec / 6 x 108 hz = 0.5 m
What EMR region is this wavelength? microwave
The electromagnetic spectrum
The electromagnetic spectrum
Comparative Sizes: from subatomic to human scales
Atom
Nucleus
Molecule
Human &
larger
Pinhead
Atom
Bacteria
Honeybee
adapted from NY Times graphic 4/8/2003
The visible spectrum
• The visible spectrum is
only a tiny window
• We are blind to 99.99%
of the energy in the
universe
• One of the strengths of
remote sensing is that we
have created devices that
allow us to see beyond
the range of human
vision
Herschel Discovers Infrared Light
•
Sir Frederick Herschel (1738-1822)
used a prism to split sunlight to
create a spectrum and then measured
the temperature of each color. He
also included a control just outside
the visible colors. He found to his
surprise that the control actually had
a higher temperature than the visible
colors. Based on this observation, he
concluded that there must be
additional light energy beyond the
visible, now known as near infrared.
Incidentally if the peak of sunlight energy is in the shorter visible wavelengths, why did
Herschel find the infrared to be hotter. Due to the nonlinear nature of refraction, his prism
concentrated the infrared light, while dispersing the shorter wavelength visible colors.
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_experiment.html
Gee Whiz:
Why do UV and not NIR rays
cause sunburn?
Particle nature of EMR
• E = h * n = (h * c)/l
where
• E = energy of a photon, measured in joules
• h = Planck’s constant 6.626 x 10-34 J sec
• inverse relationship between wavelength
and energy
Why do UV and not NIR rays cause
sunburn?
• E = (h * c)/l = (6.626 x 10-34 J sec)(3 x 108 m/sec)/l
= 19.878x10-26 J m / l
• UV l = 0.3 mm
– E = 19.878x10-26 J m / 0.3x10-6m = 66.26 x 10-20 J
• NIR l = 0.9 mm
– E = 19.878x10-26 J m / 0.9x10-6m = 22.09 x 10-20 J
UV has approximately 3x the amount of
energy per quanta
Gee Whiz:
Which emits more energy – the
Sun or Earth?
Relationship between
temperature and EMR
• M = s * T4
where
• M = total radiant exittance W m-2
 s = Stefan-Boltzman constant
5.6697 x 10-8 W m-2 K-4
• T = temperature in Kelvin (K)
– 0oC = 273.15K
Relationship between temperature and
EMR
• M = s * T4
where
• What is M for the Sun? T= 6000K
– (5.6697 x 10-8 W m-2 K-4)(6000K)4 =
– (5.6697 x 10-8 W m-2 K-4)(1.296 x 1015 K4) =
= 7.35 x 107 W m-2
• What is M for the Earth? T= 300K (27oC)
- (5.6697 x 10-8 W m-2 K-4)(3000K)4 = 4.59 x 102 W m-2
Relationship between temperature and EMR
Objects emit energy over a range of wavelengths. As the
temperature of the object increases, its radiant flux increases. The
wavelength of maximum flux depends on the temperature of the
object.
Blackbody at temperature T1
Radiant
Flux
T1 > T2
Blackbody at
temperature T2
Wavelength
Gee Whiz:
Why is the outside of a candle’s
flame red, while the inner flame
is blue?
Relationship between wavelength
and temperature






lm = A / T
where
lm = wavelength of max radiant exittance
A = 2898 mm K
T = temperature K
Inverse relationship between temperature and lm
Relationship between wavelength
and temperature
 lm = A / T
where A = 2898 mm K
• What is lm for the Sun? T= 6000K
 lm = 2898 mm K/6000K = 0.483um
 lm for the sun is in the visible
• What is lm for the Earth? T= 300K (27oC)
 lm = 2898 mm K/300K = 9.7mm
 lm for the earth is in the thermal IR
Gee Whiz:
Why do humans see in the
‘visible’ and not the NIR?
Human Color Vision
• Human eye contains 2 types of
photoreceptors: rods and cones
• Rods are more numerous and more sensitive
to the amount of visible light but are not
sensitive to color
• 3 types of cones: roughly sensitive to blue
(445nm), green (535nm) and red (575nm)
For more info on color vision go to:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colviscon.html#c1
Gee Whiz:
Why is the sky blue and
clouds white?
Atmospheric windows
•Specific wavelengths where a majority of the EMR is absorbed by the atmosphere
•Wavelength regions of little absorption known as atmospheric windows
Graphic from http://earthobservatory.nasa.gov/Library/RemoteSensingAtmosphere/
Atmospheric interference with EMR
• Shorter wavelengths strongly scattered,
adding to the received signal
• Longer wavelengths absorbed, subtracting
from the received signal
Signal
decreased by
absorption
Signal increased
by scattering
Ref
0.4
0.5
0.6
0.7
0.8
1.1 um
Adapted from Jensen, 1996, Introductory Digital Image Processing
Why is the sky blue and clouds white?
Incoming sunlight
Air molecules scatter
short l blue light,
longer ls transmitted
Rayleigh scattering
Clouds scatter all ls
of visible light,
appear white
Mie scattering
Breakdown of EMR components received at the sensor
Fundamental assumptions
• Objects that are related can be detected, identified, and described
by analyzing the energy that is reflected or emitted from them
•Measurements over
several bands make up a
“spectral response
pattern” or signature
•This signature is
different for different
objects
•This difference can be
analyzed
Gee Whiz:
Why are plants green?
Chlorophyll pigment is contained in minute structures called
plastids that are found in the leave’s parenchyma cells.
Chlorophyll differentially absorbs red and blue wavelengths
of light, there is less absorption in the green and nearly no
absorption in the near IR.
Graphic from: http://iusd.k12.ca.us/uhs/cs2/leaf_cross-section.htm
As light waves move from medium of one
density to another (e.g., from water to air), the
waves are refracted (i.e., changes direction).
Graphic from:
http://www.olympusmicro.com/pri
mer/lightandcolor/refraction.html
How plant leaves reflect light
As light moves from a hydrated cell to an
intercellular space it gets refracted, sometimes
multiple times. Eventually, some light may be
scattered back out through the upper leaf surface and
some transmitted down through the leaf.
NIR light (which is
not absorbed) is
scattered within
leaf: some reflected
back, some
transmitted through
Blue & red light
strongly absorbed by
chlorophyll. Green
light is not as
strongly absorbed
Cross-section of leaf
How plant leaves reflect light
Sunlight
B
G
R
Incoming
light
Reflected
light
Leaf
Transmitted light
NIR
An example-plant leaves
• Chlorophyll absorbs large % of red and
blue for photosynthesis- and strongly
reflects in green (.55mm)
• Peak reflectance in leaves in near
infrared (.7-1.2mm) up to 60% of
infrared energy per leaf is scattered up or
down due to cell wall size, shape, leaf
condition (age, stress, disease), etc.
• Reflectance in Mid IR (2-4mm)
influenced by water content-water
absorbs IR energy, so live leaves reduce
mid IR return
Spectral reflectance characteristics are both spatially and
temporally variable. For example, each leaf (even within
the same species) is different and can change. Thus you
should think of a spectral signature as more as a spectral
“envelope”.
Gee Whiz:
Why do plants turn yellow
as they senesce?
As a leaf senesces, the cellular structure
starts to break down and may change the
NIR as well as the visible reflectance.
A leaf’s chlorophyll (1) begins to break down as the leaf senesces (as
in the autumn). Accessory plant pigments (such as carotenoids and
anthocyanins) are also found in the leaf cells but are generally masked
by chlorophyll. Without chlorophyll, these pigments dominate.
Carotenoids absorb blue to blue green wavelengths and thus appear
yellow to orange (2). Anthocyanins absorb blue to green wavelengths
and thus appear magenta (purple) to red (3) .
Graphic from: http://www.fs.fed.us/conf/fall/leafchng_nf.htm
Extra Puzzler 1
• FM Radio waves have a frequency of
approx. 100MHz and this energy passes
through your body every second of every
day with no harm done! Why?
Extra Puzzler 1
• Radio wave energy passes through your body
every second of every day with no harm done!
Why?
• E = (h * n) = (6.626 x 10-34 J sec) (100MHz)
= 662.6 x 10-28 J = 6.626 x 10-26 J
• Remember NIR light (which is harmless)
has an quanta E of 2.209 x 10-19 J, or approx.
7 orders of magnitude higher.
Extra Puzzler 2
• If a lava flow has a temperature of
approximately 1000oC, what would be the
best wavelength to sense it?
Extra Puzzler 2
• If a lava flow has a temperature of approximately
1000oC, what would be the best wavelength to sense it?
 lm = A / T
lm = wavelength of max radiant exittance
A = 2898 mm K T = temperature K
 lm = A / T = 2898 mm K / 1273 K
 lm = 2.27 mm
 Which is in the short-middle infrared