PDF of Lecture 1

Definitions of Remote Sensing
• The science and art of obtaining useful
information about an object, area or
phenomenon through the analysis of data
acquired by a device that is not in contact
with the object, area, or phenomenon
under investigation.
– Lillesand, Thomas M. and Ralph W. Kiefer,
“Remote Sensing and Image Interpretation” ,
John Wiley and Sons, Inc, 1979.
• The measurement and analysis of
electromagnetic radiation reflected from,
transmitted through, or absorbed and
scattered by the atmosphere, the
hydrosphere and by material at or near the
land surface, for the purpose of
understanding and managing the Earth’s
resources and environment.
– Larry Morley – Teledetection International
Our definition
• Understanding the physical, chemical, and
biological properties of the earth and
planets from the way electromagnetic
radiation interacts with their surface
materials and atmospheres as viewed
from above.
Electromagnetic Radiation
• Velocity in vacuum: c = ~3.0 x 108 m per second.
• Does not require a specific medium to travel thru.
• Has both electrical and magnetic properties
Dual Nature of EMR (Light)
– Wavelength
– Frequency (i.e.,
number of waves per
– It ‘radiates’, reflects,
• Particle-like
– Interacts with matter
as discrete packets
of energy (photons)
Visible Light
• Wavelengths of 400-700 nm
• Bounded by UV and IR
The Electromagnetic Spectrum
• A means of ordering EMR according to
– Wavelength (λ) – m, cm, m, nm
– Frequency () – Hertz (Hz) = 1 cycle per second
– Energy (eV)
• Covers
Gamma rays
Visible Light
Reflected IR
Thermal IR (Heat)
Development of
Electromagnetic Theory
Isaac Newton
“And so the true Cause of the Length of that Image was detected to be no other, than
that Light is not similar or Homogenial, but consists of Difform Rays, some of which
are more Refrangible than others”. He coined the term ‘spectrum’.
From Voltaire's Eléments de la Philosophie de Newton, 1738
Isaac Newton
Issac Newton publishes four editions of Opticks. Query 29:
“Nothing is more requisite for producing all the variety of Colours, and degrees of Refrangibility than that the
Rays of Light be Bodies of different Sizes, the least of which may take violet the weakest and darkest of the
Colours, and be more easily diverted by refracting Surfaces from the right Course; and the rest as they are
bigger and bigger, may make the stronger and more lucid colours, blue, green, yellow, and red, and be more
and more difficultly diverted”.
"Are not the Rays of Light very small Bodies emitted from shining Substances?"
"Even the Rays of Light seem to be hard bodies; for otherwise they would not retain different properties in
their different Sides."
He develops the particle theory of light (also called the corpuscular theory of light). He is able to give
plausible explanations for properties of light such as color, reflection, and refraction.
He was not able to explain everything about light, diffraction bands outside the geometrical shadow
(discovered by Grimaldi in 1665) being one and Newton's rings being another.
An extremely important prediction implicit in Newton's particle theory is that, as light moves from air to water,
it SPEEDS up.
A wave theory of light existed in Newton's day. Its leading champion was Christiaan Huygens, but the theory
was incomplete. It only addressed a small fraction of the phenomena Newton discussed and was difficult to
understand. So, due to the wave theory's poor explanatory power and Newton's great authority within the
science world, the particle theory of light reigned supreme.
1800: William Herschel discovers the infrared portion of the spectrum. A year later,
Johann Ritter discovered ultraviolet light.
1814-1823: Joseph von Fraunhofer discovered the dark lines in the sun's spectrum
(Fraunhofer lines).
In 1821, he reported results of using a defraction grating to measure the wavelengths
of Na lines.
1815-1819: Augustin Fresnel independently rediscovered interference and begins to
study (and extend mathematically) the wave theory of light; completely refutes
particle theory of light.
1826-1849: John Herschel and W.H. Fox Talbot demonstrated, when a substance is
heated and its light passed through a spectroscope, that each element gave off its
own set of characteristic bright lines of color. Birth of the ‘Emission Spectrum’.
Spectrum of White Light
Spectrum of
Excited Hydrogen Gas
Line Spectra of Other Elements
1849-1850: Lóon Foucault worked to measure the speed of light and test Newton’s
particle theory. He was able to compare the two values needed to test Newton's
particle theory for light: light travels SLOWER in water than it does in air. In 1862,
Foucault determined the speed of light to be 298,000 ± 500 km/sec.
1862: Anders Jonas Ångström identified three lines in the visible portion of the
hydrogen emission spectrum, a red line, a blue-green line and a violet line.
James Clerk Maxwell
(1831 - 1879)1873: James Clerk Maxwell, a Scottish physicist, succeeded in unifying
electricity and magnetism.
I. Gauss' law for electricity
II. Gauss' law for magnetism
III. Faraday's law of induction
IV. Ampere's law
1888: Heinrich Hertz demonstrated the existence of the electromagnetic radiation that
Maxwell predicted by building an apparatus to produce radio waves.
1899: Max Planck discovered a new fundamental constant, which is named Planck's
constant (h), and is, for example, used to calculate the energy of a photon.
1900: Planck discovered the law of heat radiation, “Planck's law of black body
radiation”. This law became the basis of quantum theory, which emerged ten years
later in cooperation with Albert Einstein and Niels Bohr.
Development of Quantum Physics
1900 to 1930
– Development of ideas of quantum mechanics
• Also called wave mechanics
• Highly successful in explaining the behavior of atoms,
molecules, and nuclei
• Quantum Mechanics reduces to classical mechanics when applied
to macroscopic systems
• Involved a large number of physicists
– Planck introduced basic ideas
– Mathematical developments and interpretations involved such
people as Einstein, Bohr, Schrödinger, de Broglie, Heisenberg,
Born and Dirac
• Definition: A wave is a traveling disturbance
Any form of change or disturbance that propagates
(travels) from one region to another can be thought of
as a wave
Sound waves
Ocean waves
Defining a wave
Frequency = # wave crests
passing per second
Wavelength (λ)
Wavelength (λ)
speed = wavelength (λ) x frequency(f)
An electromagnetic wave is a disturbance propagated as
a variation in the local electric and magnetic fields
Electromagnetic (EM) radiation
• Visible light is just one ‘form’ of EM
• For all waves:
speed = wavelength (λ) x frequency(f)
• For all EM waves: speed = speed of light
The Electromagnetic Spectrum
The long and the short of it
• Electromagnetic (EM) waves
– longest wavelength - radio waves
– shortest wavelength - gamma rays
– and between these
X-rays, UV, visible light, IR, microwaves
Increasing wavelength
• Important point about waves
– they transmit information in the form of energy
Energy - a measure of the ability to do work
- units are Joules (J) = N m = m2 kg/s2
Energy has many forms
- energy of motion
- EM wave energy
- chemical energy
- heat energy
- nuclear energy
James Prestcott Joule (1818 -1889)
Radiant Energy
• The amount of energy E carried by an
electromagnetic wave is related to its
where h = Planck’s constant
= 6.626 x 10-34 Jsec
So, the greater the frequency the greater the
amount of energy carried by the wave
• Many applications are interested in energy
• For example:
– the temperature of a planet is governed by
the amount of EM energy it receives from
the Sun
– Hence:
• We would like to know exactly how much EM
energy the sun radiates into space
James Watt
(1736 - 1819)
Luminosity (L) = total amount of EM
energy radiated at all wavelengths into
space per second: units = Joules / sec. =
Flux (F) = energy received per square meter
per second: units = Watts/m2
• The problem is
– We want to determine the total energy
output (the luminosity L), but it can’t be
measured directly
The solution
Measure the flux (the energy received at
a detector per second per square meter)
and find a relationship between flux and
• Take a source of EM radiation e.g., the sun
– energy is radiated into space in all directions
– at a distance d from the star the radiated energy
will be spread out over the surface of a sphere of
radius d
Sphere of
radius d
Sun of
luminosity L
L = radiative energy at
source (luminosity)
Energy passing through
surface per m2 per sec = FLUX
• A really useful result: Energy flux a
distance d from source of luminosity L:
4π d
where 4π d2 is the surface area of a
sphere of radius d
A fundamental result
• Question: what is the Sun’s luminosity
• Idea for answer:
– measure Sun’s energy flux at the Earth’s orbit
Get F = 1370 W/m2
F = Sun’s energy flux at Earth’s orbit
we know the Earth is 1 AU from the Sun
theory: F = L / 4π (1 AU)2
Symbol means Sun related value
• The picture:
Sphere of radius 1 AU
about Sun
1 AU
Earth’s orbit
(where we measure the flux)
• Hence result:
L ≈ 4 x 1026 Watts
100 Watt
light bulb
Sun’s Luminosity
(energy output)
That’s a lot of
light bulbs!
OK, let us do a calculation
• Volunteer?
– Calculate Sun’s energy flux at the orbit of
Mars and compare it to that at Earth.
d(Mars) = 1.52 x 1 AU
L(Sun) = 3.85 x 1026 Watts
Hint: watch your units
The Sun - fundamental
• Mass - from Kepler’s 3rd law
– M = 1.99 x 1030 kg
• Radius - from angular diameter + distance
– 
• Luminosity - from flux at Earth’s orbit
– L = 3.85 x 1026 Watts
Some other useful quantities
Radiant energy is defined as the energy carried by electro- magnetic radiation
and expressed in the unit of joule (J).
Radiant flux is radiant energy transmitted as a radial direction per unit time and
expressed in a unit of watt (W).
Radiant intensity is radiant flux radiated from a point source per unit solid angle
in a radiant direction and expressed in the unit of Wsr-1.
Irradiance is radiant flux incident upon a surface per unit area and expressed in
the unit of Wm-2.
Exitance is radiant flux leaving a surface per unit area (outgoing power).
Radiant emittance is radiant flux radiated from a surface per unit area, and
expressed in a unit of Wm-2.
Radiance is radiant intensity per unit projected area in a radial direction and
expressed in the unit of Wm-2 sr-1.
Blackbody Radiation
An object at any temperature is known to emit electromagnetic
– Sometimes called thermal radiation
– Stefan’s Law describes the total power radiated
P= AeT
Stefan’s constant
– The spectrum of the radiation depends on the temperature
and properties of the object
• Blackbody is an idealized system that absorbs
incident radiation of all wavelengths
• If it is heated to a certain temperature, it starts
radiate electromagnetic waves of all
• Cavity is a good real-life approximation to a
Planck Curve and Blackbody
(Thermal) Radiation
A backbody radiates proportional to its
temperature. Spectral radiance, I( ,T), is
determined by Planck’s Radiation Law:
I , T =
2 π h c2
exp hc/ kT −1
h = Planck’s constant = 6.6260755 = 10-34 joule-second
k = Boltzmann’s constant = 1.3807 x 10-23 joule-kevins
Planck’s law defines the nature of blackbody radiation. Real objects are
not blackbodies so a correction for emissivity should be made.
Emissivity ranges between 0 and 1 depending on the dielectric constant of the object,
surface roughness, temperature, wavelength, look angle.
The temperature of the black body which radiates the same radiant energy as an
observed object is called the brightness temperature of the object.
Many natural surface materials are well approximated by blackbodies in the infrared
region. For instance, water has a thermal infrared emissivity of .98.
The spectral emissivity and spectral radiant flux for three objects that are a
black body, a gray body and a selective radiator.
Wien’s Displacement Law:
k = 2898 µm K, and T is the absolute temperature in degrees Kelvin
It is obtained by differentiating the spectral radiance.
It shows that the product of wavelength (corresponding to the maximum peak of
spectral radiance) and temperature, is approximately 3,000 (µmK is the best for
measurement of objects with a temperature of 300K.
This law is useful for determining the optimum (peak) wavelength for temperature
measurement of objects with a temperature of T. For example, about 10µm is the
best for measurement of objects with a temperature of 300K.
Stefan-Boltzmann Law:
Where σ is the Stefan-Boltzmann constant, 5.6697 x 10-8 W-2K-4
Gives the total amount of emitted radiation from a blackbody
Units: (Same as radiant emittance) W m-2
Proportional to T4
Obtained by integrating the area under the Planck Curve.