Chapter 2 RS.pptx

Figure. The satellite remote sensing process. A-Energy source or illumination
(electromagnetic energy); B- radiation and the atmosphere;
C- interaction with the target; D- recording of energy by the sensor;
E- transmission, reception and processing; F- interpretation and analysis;
G- application

Chapter 2. Electro-Magnetic Radiation (EMR) & RS
• Energy recorded by remote sensing systems
undergoes fundamental interactions that should be
understood to properly interpret the remotely sensed
data.
• For example, if the energy being remotely sensed
comes from the Sun, the energy:
– is radiated by atomic particles at the source (the Sun),
– propagates through the vacuum of space at the speed of
light,
– interacts with the Earth's atmosphere,

Electro-Magnetic Radiation (EMR) & RS
• interacts with the Earth's surface,
• interacts with the Earth's atmosphere once again,
and
• finally reaches the remote sensor where it interacts
with various optical systems, filters, emulsions, or
detectors.
• Hence, in RS Energy-matter interacts:
– at the atmosphere
– at the Earth’s surface
– at the remote sensor detector

Electro-Magnetic Radiation (EMR) & RS
Energy-matter
interactions in
the atmosphere,
at the study area,
and at the remote
sensor detector

How is Energy Transferred?
 Energy may be transferred in three ways: conduction, convection, &
radiation. a) Energy may be conducted directly from one object to
another as when a pan is in direct physical contact with a hot burner.
b) The Sun bathes the Earth’s surface with radiant energy causing the air
near the ground to increase in temperature. The less dense air rises,
creating convectional currents in the atmosphere.
c) Electromagnetic energy in the form of electromagnetic waves may be
transmitted through the vacuum of space from the Sun to the Earth.

Electro-Magnetic Radiation (EMR)
• EMR refers to all energy that moves with the
velocity of light in a harmonic wave pattern.
• The word harmonic implies that the component
waves are equally and repetitively spaced in time.
• This energy is detectable only in terms of its
interaction with matter.
• Hence, it reveals its presence by the observable
effects it produces when it strikes the matter (eg.
light, heat etc).
• Sun is the major energy source for Remote sensing.
• But all matter at temperatures above absolute zero
(0 0K or -273 0C) continuously emit EMR.

Electromagnetic Radiation Models
• To understand how EMR is created, how it
propagates through space, and how it interacts
with other matter, it is useful to describe the
processes using two different models:
• the wave model, and
• the particle model.
• Because, EMR can behave both as wave and
particles.
• Can be described as frequency, wavelength,
velocity and the amount of energy it carries.

1. EM Radiation Wave Model
 Electromagnetic radiation (ER)
travels as waves
 Waves are characterized by two
fields:
 Electric and Magnetic
 The two fields oscillate in time
 The two fields oscillate in
space perpendicularly to each
other and to the direction of
travel
 Waves travel with speed of
light:

• Electromagnetic radiation is generated when an electrical charge
is accelerated.
• The wavelength of electromagnetic radiation depends upon the
length of time that the charged particle is accelerated and its
frequency (v) depends on the number of accelerations per
second.
• Wavelength is formally defined as the mean distance between
maximums (or minimums) of a roughly periodic pattern and is
normally measured in micrometers (m) or nanometers (nm).
• Frequency is the number of wavelengths that pass a point per
unit time. A wave that sends one crest by every second
(completing one cycle) is said to have a frequency of one cycle
per second or one hertz, abbreviated 1 Hz.

Wave length and Cycle
Crest Trough

Wave length and Cycle
• The inverse relationship between
wavelength () and frequency (n).
The longer the wavelength the
lower the frequency; the shorter
the wavelength, the higher the
frequency.
• The amplitude of an
electromagnetic wave is the height
of the wave crest above the
undisturbed position.
• Successive wave crests are
numbered 1, 2, 3, and 4. An
observer at the position of the
clock records the number of crests
that pass by in a second.
• This frequency is measured in
cycles per second, or hertz
Note that frequency, n is inversely proportional to
wavelength,  The longer the wavelength, the
lower the frequency, and vice-versa.

• The electromagnetic energy from the Sun travels in eight
minutes across the intervening 93 million miles (150
million km) of space to the Earth.
• The Sun produces a continuous spectrum of
electromagnetic radiation ranging from very short
wavelength but extremely high frequency gamma to long
wavelength but very low frequency radio waves.
• The Earth approximates with a 300 K (27˚C) also emit
energy dominantly at approximately 9.7 micro meter
wavelength.

For long wavelengths, the amount of energy and frequency will be low,
and for short wavelengths, the amount of energy and the frequency is
high.
The energy of a photon, Q, is proportional to its frequency, n:
Q = h n
n = c/
Q = hc/
h = Planck’s constant = 6.63 10-34 Js
c = speed of light = 3.0 108 ms-1
Thus,
Q ~1/

EM-Spectrum
EM-Spectrum is the entire range of wavelengths of
electromagnetic radiation.
 It is the continuum of energy ranging from kilometers to
nanometers in wavelength.
The physical principles of interaction of the EMR with
targets are different over each spectral range.
 This continuum is commonly divided into the following
ranges, called spectral bands.
The Sun produces a continuous spectrum of EMR ranging
from very short, extremely high frequency gamma to
long, very low frequency radio waves.
Most sensors operate in the visible, infrared, and microwave
regions of the spectrum.

EM-Spectrum
Violet: 0.4 00 - 0.446 mm
Blue: 0.446 - 0.500 mm
Green: 0.500 - 0.578 mm
Yellow: 0.578 - 0.592 mm
Orange: 0.592 - 0.620 mm
Red: 0.620 - 0.700 mm

Band Wavelength Remarks
Gamma ray <0.03 nm Incoming radiation from the sun is completely absorbed
by the upper atmosphere, and is not available for Remote
Sensing.
Gamma radiation from radioactive minerals is detected
by low flying aircraft as a prospecting method.
X-ray 0.03 to 0.3
nm
Incoming radiation is completely absorbed by
atmosphere.
Not employed in Remote Sensing.
Ultraviolet,
UV
0.03 to 0.4
μm
Incoming UV radiation <0.3 mm is completely absorbed
by ozone in the upper atmosphere.
Photograph
ic
UV
0.3 to 0.4 μm Transmitted through the atmosphere.
Detectable with film and Photo detectors, but
atmospheric scattering is severe.
Visible 0.4 to 0.7
μm
Detected with film and photo detectors.
 Includes sun reflectance peak at about 0.5 mm.
Infrared, IR 0.7 to 300
μm
Interaction with matter varies with wavelength.
Atmospheric transmission windows are separated by
absorption bands.
Wavelength bands

Band Wavelength Remarks
Reflected IR 0.7 to 3 mm  This is primarily reflected solar radiation and contains
no information about thermal properties of materials.
 Commonly divided into the following regions:
• Near Infra Red (NIR) between 0.7 to 1.1 mm.
• Middle Infra Red (MIR) between 1.3 to 1.6 mm.
• Short Wave Infra Red (SWIR) between 2 to 2.5 mm.
 Radiation from 0.7 to 0.9 mm is detectable with film
and is called photographic IR radiation.
Thermal IR 3 to 5 mm
8 to 14 mm
These are the principal atmospheric windows in the
thermal region.
 Imagery at these wavelengths is acquired through the
use of optical-mechanical scanners, not by film.
Microwave 0.3 to 300 cm  These longer wavelengths can penetrate clouds and
fog.
 Imagery may be acquired in the active or passive
mode.
Radar 0.3 to 300 cm  Active mode of microwave Remote Sensing.
Wavelength bands

EM-spectrum useful for RS
• Different regions of the EM spectrum can provide
discrete information about an object.
• Remote sensor are engineered to detect specific
spectrum wavelength and frequency ranges.
• Most sensors operate in the visible, infrared and
microwave regions of the spectrum, sometimes
ultraviolet.
Ultraviolet:
• Shortest wavelength used in RS.
• Has short wavelengths (0.3 to 0.446 μm) and high
frequency and energy.
• Some may be affected by atmospheric absorption.
• Used in geologic (mineral identification) and
atmospheric science applications (tracking changes in
the ozone layer).

Visible Light:
• Radiation detected by human eyes.
• The only portion of the spectrum that can be
perceived as colors.
• Ranges from approximately 0.4 violet to 0.7μm red.
• Applicable in manmade and natural feature
identification and study.
Infrared:
 Ranges from approximately 0.7 to 100 μm.
 It is 100 times as wide as the visible portion.
 Based on their radiation properties, IR can be
the reflected IR (0.7 to 3.0 μm) and
the emitted or thermal IR (3.0 to
100 μm)

Reflected Infrared
• Shares radiation properties exhibited by the visible
portion.
• Valuable for delineating healthy verses unhealthy or
fallow vegetation, and
• For distinguishing among vegetation, soil, and rocks.
Thermal Infrared
• Radiation that is emitted from the Earth’s surface in
the form of thermal energy (heat).
• Useful for monitoring temperature variations in
land, water and ice.
• Monitor lost energy in urban area.

Microwave
• Ranging on the spectrum from 1μm to 1 m.
• The longest wavelength used for remote sensing.
• Used in the studies of meteorology, hydrology,
oceans, geology, agriculture, forest and ice and for
topographic mapping.
• Microwave emission is influenced by moisture
content, hence, it is useful for mapping soil
moisture, sea ice, currents, and surface winds.

Interaction Mechanism
• A number of interactions are possible when EME
encounters matter (solid, liquid or gas).
• The interaction could be surface and/or volume
phenomena.
• The surface and volume interactions with matter can
produce a number of changes in the incident EMR:
• magnitude,
• direction,
• wavelength,
• polarization and phase.
• RS science detects and records these changes.
• The resulting images and data are interpreted to
identify remotely the characteristics of the matter that
produced the changes in the recorded EMR.

As EMR interacts with a target the following may
happen:
• Radiation may be transmitted. The velocity of EMR
changes as it is transmitted from air, or a vacuum, into
other substances.
• Radiation may be absorbed by a substance and give
up its energy largely to heating the substance.
• Radiation may be emitted by a substance as a function
of its structure and temperature.
• Radiation may be scattered, that is, deflected in all
directions and lost ultimately to absorption or further
scattering in the atmosphere.
• Radiation may be reflected.

• RS sensors are intended to measure the reflected
and/or emitted energy from the target for identifying
their nature, condition and type.
• B/c the resultant interaction is specific for that form
of matter depending on the properties and
atomic/molecular structure.

EMR- Interaction
A) EM Interaction with Atmosphere
• Before the Sun’s radiation reaches the Earth’s surface, three RS
relevant interactions occurs in the atmosphere:
• Absorption, transmission, and scattering.
• The transmitted radiation is then either absorbed by the surface
material or reflected.
• The reflected radiation is also subject to scattering and absorption in
the atmosphere before reaching a remote sensor
• Gases mainly absorb EM energy
– known concentrations and location (cycles) enable to predict
influence (per wavelength).
• Aerosols mainly scatter EM energy
– variable and difficult to model (human and natural changing
influence)
• Either way we sense less than what reached the Earth’s atmosphere

EM Spectrum (Scattering)
• Atmospheric scattering occurs when particles or gaseous
molecules present in the atmosphere cause EM radiation to be
redirected from its original path.
• This can affect the speed of radiation, its wavelength, intensity,
spectral distribution, and/or direction.
• The amount of scattering depends on several factors
– The wavelength of the radiation
– The size and amount of particles and
– The distance the radiation travels through the atmosphere.
• Three types of scattering occurs in the atmosphere depending
on the size of particles in the atmosphere causing it. They are of
different relevance to RS.
– Rayleigh Scattering
– Mie scattering
– Non-selective scattering
EMR- Interaction

Rayleigh scattering
– Particles are very smaller than wavelength of the radiation.
– e.g. small specks of dust or nitrogen and oxygen molecules.
– The effect is inversely proportional to the fourth power of the
wavelength, shorter wavelengths scattered more than the
longer. For example, blue light (0.4 m) is scattered 16 times
more than near-infrared light (0.8 m).
– Common to the upper atmosphere.
– Why the sky appear blue during day time and red/yellowish
during sunrise and sunset?

Rayleigh scattering
 Particles are very smaller than
wavelength of the radiation
 Rayleigh scattering is responsible for the blue
sky.
 The short violet and blue wavelengths are
more efficiently scattered than the longer
orange and red wavelengths.
 When we look up on cloudless day and
admire the blue sky, we witness the
preferential scattering of the short
wavelength sunlight.
 Rayleigh scattering is responsible for red
sunsets and sunrise.

Mie scattering
 Particles are just about the same size as the wavelength
of the radiation. Particles & radiation has same size.
 E.g. Dust, pollen, smoke and water vapor.
 Longer wavelengths scattered more.
• Common to the lower atmosphere where larger particles
are more abundant, and dominates when cloud conditions
are overcast.

Nonselective scattering
 Particles are much larger than the wavelength of the
radiation (e.g. water droplet and large dust).
 All wavelengths scattered equally
 Causes fog and clouds to appear white to our eyes because
blue, green, and red light are all scattered in approximately
equal quantities (blue+ green +red light = white light).
Note: Scattering can severely
reduce the information content
of remotely sensed data to the
point that the imagery looses
contrast and it is difficult to
differentiate one object from
another.

Absorption
• Absorption is the process by which radiant energy is
absorbed and converted into other forms of energy.
• An absorption band is a range of wavelengths (or
frequencies) in the electromagnetic spectrum within
which radiant energy is absorbed by atmospheric
substances such as water (H2O), carbon dioxide (CO2),
oxygen (O2), ozone (O3), and nitrous oxide (N2O).
• The most common atmospheric constituent that
absorb radiation is Ozone (O3), water (H2O) and carbon
dioxide (CO2).

Absorption
• The cumulative effect of the absorption by the
various constituents can cause the atmosphere to
close down in certain regions of the spectrum.
• This is bad for remote sensing because no energy
is available to be sensed.
• Hence, atmosphere selectively absorb EMR in
specific wavelength bands.
• Influence where (in the spectrum) we can "look"
for remote sensing purposes.
• Better to define more effective wavelengths for RS
which are not severely affected by absorption.

Absorption
• From below fig. it can be seen that many of the
wavelengths are not useful for remote sensing of the
Earth’s surface, simply because the corresponding
radiation cannot penetrate the atmosphere.
• Only the wavelengths outside the main Atmospheric
transmission absorption ranges of the atmospheric
gases can be used for remote sensing.

Atmospheric Window
• The spectral bands for which the atmosphere is relatively
transparent.
• The atmosphere selectively transmits energy of certain
wavelengths.
• Atmospheric windows are present in the visible part (0.4
µm - .76 µm) and the infrared regions of the EM
spectrum.
• The atmosphere is transparent again beyond about λ =
1mm, the region used for microwave remote sensing.

Conti…
Selection of sensors for remote-sensing task must
consider
• The available spectral sensitivity of the sensors,
• The presence or absence of atmospheric windows in
the spectral range(s) in which one wishes to sense,
and
• The source, magnitude, and spectral composition of
the energy available in these ranges.
• However, the choice of spectral range of the sensor
must be based on the manner in which the energy
interacts with the features under investigation.

Interaction of EMR with the Earth’s Surface
• Radiation from the sun, when incident upon the
earth’s surface is either
• reflected by the surface,
• transmitted into the surface,
• absorbed and/or
• emitted by the surface.
Interaction of Energy
with the earth’s surface.

Interaction of EMR with the Earth’s Surface
• Reflection, absorption and transmission will vary for
different earth features and is wavelength dependent,
this help us to distinguish different features.
• The wavelength dependency is critically important b/c
two indistinguishable features in one wavelength
region may be very different in another wavelength
band.
• The reflected and/or the emitted energy has practical
use in remote sensing technology.

Interaction with Earth’s surface Conti..
• The EMR, on interaction, experiences a number
of changes in magnitude, direction, wavelength,
polarization and phase.
• These changes are detected by the remote
sensor and enable the interpreter to obtain
useful information about the object of interest.
• The remotely sensed data contain both spatial
information (size, shape and orientation) and
spectral information (tone, color and spectral
signature).

Interaction of EMR with Target conti…
• Why the proportion of reflected-absorbed-transmitted
radiation will vary with wavelength and material type?
• The reflection intensity depends on the surface refractive
index, absorption coefficient and the angles of incidence
and reflection.
• Of all the interactions in the reflective region, surface
reflections are the most useful and revealing in remote
sensing applications.
• These character is different for different features.
• Hence, different earth’s features are easily identified.

Reflectance: is the process whereby radiation “bounces
off” an object like a cloud or the terrain.
• Actually, the process is more complicated, involving re-
radiation of photons in unison by atoms or molecules
in a layer one-half wavelength deep.
• Reflection differs from Scatter in that the direction
associated with scattering is unpredictable, whereas the
direction of reflection is predictable.
• Reflection reveals fundamental characteristics that are
important in remote sensing.
• First, the incident radiation, the reflected radiation, and a
vertical to the surface from which the angles of incidence
and reflection are measured all lie in the same plane.
Second, the angle of incidence and the angle of reflection
are equal.

There are two major types of reflecting surfaces:
Specular Reflection
• Reflects from smooth surface (i.e. the average surface profile
is several times smaller than the wavelength of radiation
striking the surface). eg. Water body, asphalt etc.
• Reflection direction is in one direction
• Incident angle and reflection angle is equal.
Diffused Reflection
• Reflected from rough surface
• The reflected rays go in many directions
• Produces diffused radiation
Note: In the real world, usually a combination of both types is
found.

• The amount of each interaction will be a function of
the incoming wavelength, the composition of the
material, and the smoothness of the surface.
• Of all the interactions in the reflective region, surface
reflections are the most useful and revealing in
remote sensing applications b/c it tell us the physical,
biological and chemical properties of the matter.
• Reflection occurs when a ray of light is redirected as it
strikes a non-transparent surface.

Reflectance Conti…
• The reflectance characteristics of the earth’s surface
features are expressed by spectral reflectance, which
is given by: ρ (λ ) = [ ER(λ ) / EI (λ ) ] x 100
• Where, ρ (λ ) = Spectral reflectance (reflectivity) at a particular
wavelength.
ER(λ ) = Energy of wavelength reflected from object
EI (λ ) = Energy of wavelength incident upon the object
 Everything in nature has its own unique distribution of
reflected, emitted and absorbed radiation.
 Can be used to distinguish one thing from another or
to obtain information about shape, size and other
physical and chemical properties.
 Via detecting and recording of radiant energy
reflected or emitted by objects or surface material.

• Generally, two points about the interaction of
radiation with earth’s surface should be noted.
1. The proportions of energy reflected, absorbed,
and transmitted will vary for different earth
features, depending upon their material type and
conditions.
• These differences permit us to distinguish
different features on an image.
2. The wavelength dependency means that, even
within a given feature type, the proportion of
reflected, absorbed, and transmitted energy will
vary at different wavelengths.

• RS measures how much of what “color” of light is
coming from what place on the ground
• Surface feature color can be characterized by the
percentage of incoming EME it reflects at each
wavelength across the electromagnetic spectrum.
• This is said to be spectral reflectance curve or
“spectral signature, it is the plot between ρ (λ) and λ.
• This varies with the variation in the chemical
composition and physical conditions of the feature,
which results in a range of values.
• Different features has different reflectance curve, this
help us to identify features and their condition.

Chapter 2 RS.pptx

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