Electromagnetic Remote Sensing Process
• The generalised processes involved in
electromagnetic remote sensing system or passive
remote sensing system, namely, data acquisition and
Fig . 1: Electromagnetic Remote Sensing Process with overview on
The data acquisition process comprises distinct elements, namely,
a) energy sources,
b) propagation of energy through the atmosphere,
c) Energy interactions with earth's surface features
d) airborne/space borne sensors to record the reflected energy
e) generation of sensor data in the form of pictures or digital
Energy Sources and Radiation Principles
• Visible light is only one of many forms of electromagnetic
• Radio waves, ultraviolet rays, radiant heat, and X-rays are
other familiar forms.
• All this energy is inherently similar and propagates in
accordance with basic wave theory.
• This theory describes electromagnetic energy as traveling in a
harmonic, sinusoidal fashion at the “velocity of light” c.
• Radiation is characterized with respect to frequency and
• Waves obey the general equation,
C = νλ
where, C is velocity of light 3.0 x 108 m/s, ν is frequency and λ is
Fig. Electromagnetic wave
• Electromagnetic spectrum continuation of the energy that
ranges from meters to nanometers in wavelength, travels, at
the speed of light and propagates thorough a vacuum such as
Fig.3: Electromagnetic spectrum
Region Wavelength Remarks
Gamma ray <0.03 nm Incoming radiation is completely absorbed by upper
atmosphere and is not available for remote sensing
X- ray 0.03 to 3.0 nm Completely absorbed by atmosphere. Not employed in
Ultraviolet 0.03 to 0.4 µm Incoming wavelength less than 0.3µm are completely
absorbed by ozone in the upper atmosphere
0.3 to 0.4 µm Transmitted through atmosphere. Detectable with film
and photo-detector, but atmospheric scattering is
Visible 0.4 to 0.7 µm
0.4- 0.5 µm (Blue)
0.5- 0.6 µm (Green)
0.6- 0.7 µm (Red)
Imaged with film and photo-detector. Includes reflected
energy peak of earth at 0.5 µm.
0.7 to 3.0 µm
0.7 – 1.3 µm (NIR)
1.3- 3.0 µm (MIR)
The band from 0.7 to 0.9 µm is detectable with
film and is called photographic IR band.
3 to 5 µm
8 to 14 µm
Principal atmospheric window in thermal region.
Images at these wavelengths are acquired by
optical mechanical scanners and special vidicon
systems but not by film.
1 mm to 1 m Longer wavelengths can penetrate clouds, fog, and rain.
Images may be acquired in the active and passive mode.
Electromagnetic spectral regions
Book no. 4, Table 1.3
• This theory suggests that electromagnetic radiation is
composed of many discrete units called photons or quanta. The
energy of quantum is given by,
Q = hv
Where Q is energy of a quantum, Joules, h is Plank’s
constant, 6.626 x 10-34 J sec, v is frequency.
• The wave and quantum models of EM radiation can be
Q = hc/ λ
• All matter at temperatures above absolute zero (-273o
continually emit EM radiation.
• Like the sun, terrestrial objects are also sources of
radiation, though of a different magnitude and spectral
composition than that of the sun.
The amount of energy that any object radiates can be expressed
M = σT 4
M = total radiant exitance from the surface of a material (W m-2)
σ= Stefan-Boltzmann constant, (5.6697 x 10-8 W m
T = absolute temperature (K) of the emitting material
Weins Displacement law
• The spectral distribution of energy varies also with temperature.
• The dominant wavelength or wavelength at which a blackbody
radiation curve reached a maximum, is related to temperature by
Weins displacement Law:
λm = A /T
λm = wavelength of maximum spectral radiant exitance,μm
A = 2898 mm, K
T = Temperature, K
Black Body Radiation as a function of
Temperature and Wavelength
Fig.4: Spectral distribution of energy radiated from blackbodies of various
• The sun emits radiation in the same manner as a blackbody
radiator whose temperature is about 6000 K (Fig.4).
• Many incandescent lamps emit radiation typified by a 3000
K blackbody radiation curve.
• The earth’s emits radiation is about 300 K (Fig.4).
Energy Interactions in the Atmosphere
• All radiation detected by remote sensors passes through some
distance, or path length, of atmosphere. The path length
involved can vary widely.
• For example, space photography results from sunlight that
passes through the full thickness of the earth’s atmosphere
twice on its journey from source to sensor.
• An airborne thermal sensor detects energy emitted directly
from objects on the earth, so a single, relatively short
atmospheric path length is involved.
• Atmospheric scattering is the unpredictable diffusion of
radiation by particles in the atmosphere.
• Three types of scattering can be distinguished,
depending on the relationship between the diameter of
the scattering particle (a) and the wavelength of the
• Rayleigh scatter is common when radiation interacts with
atmospheric molecules (gas molecules) and other tiny
particles (aerosols) that are much smaller in diameter that
the wavelength of the interacting radiation.
• The effect of Rayleigh scatter is inversely proportional to the
fourth power of the wavelength. As a result, short
wavelengths are more likely to be scattered than long
• Rayleigh scattering is the dominant scattering mechanism in
the upper atmosphere.
• Rayleigh scatter is one of the principal causes of haze in
Rayleigh Scattering a < λ
• Mie scattering occurs when the particles are just about
the same size as the wavelength of the radiation.
• Dust, pollen, smoke and water vapour are common causes
of Mie scattering which tends to affect longer wavelengths
than those affected by Rayleigh scattering.
• Mie scattering occurs mostly in the lower portions of the
atmosphere where larger particles are more abundant,
and dominates when cloud conditions are overcast.
a < = > λ
• Non-selective scattering occurs when the particles (e.g.
water droplets and large dust particles ) are much larger
than the wavelength of the radiation.
• Non-selective scattering gets its name from the fact that
all wavelengths are scattered about equally.
• This type of scattering causes fog and clouds to appear
white to our eyes because blue, green, and red light are
all scattered in approximately equal quantities.
Non-selective scattering a > λ
• In contrast to scatter, atmospheric absorption results in
effective loss of energy to atmospheric constituents.
Generally involves absorption of energy at a given
• The wavelength ranges in which the atmosphere is
particularly transmissive of energy are referred as
• When electromagnetic energy is incident on any given earth
surface feature, three fundamental energy interactions with the
feature are possible (an element of the volume of a water body)
• Various fractions of the energy incident on the element are
reflected, absorbed, and/or transmitted.
Energy interactions with earth surface features
Fig. 5: Basic interactions between electromagnetic energy and an earth surface
• Fractions of the energy are reflected, absorbed, and/or
• Applying the principal of conservation of energy, we can state
the interrelationship between these three- energy
EI denotes the incident energy,
ER denotes the reflected energy,
EA denotes the absorbed energy and
ET denotes the transmitted energy,
with all energy components being a function of wavelength.
• The reflectance characteristics of earth surface features may be
quantified by measuring the portion of incident energy that is
• This is measured as a function of wavelength and is called
• A graph of the spectral reflectance of an object as a function of
wavelength is termed a spectral reflectance curve.
• Fig. assume that task of selecting an airborne sensor system to
assist in preparing a map of a forested area differentiating
deciduous versus coniferous trees.
Fig. Generalized spectral reflectance envelopes for deciduous (broadleaved)
and coniferous (needle-bearing) trees
• It refers all energy that moves with the velocity of light in a
harmonic wave pattern.
• Each frequency is associated with a different standing wave
pattern. These frequencies and their associated wave patterns
are referred to as harmonics.
• In interaction, electromagnetic energy behaves as
through it consists of many individual bodies called
as photons that have such particle like properties as
energy and momentum.
• When light refracts as it moves through media of
different optical densities, it is behaving like waves.
1. Transmitted, that is passed through substance. The ratio of
two velocities called index of refraction (n),
n = Ca/Cs
Ca = velocity in a vacuum, Cs = velocity in substance
2. Absorbed, giving up its energy largely to matter.
3. Emitted by substance, usually at longer paths a function of
its structure and temperature.
4. Scattered, that is, deflected in all directions with
dimensions of relief, or roughness, comparable to the wave
length of incident energy produce scattering.
5. Reflected, that is, returned form surface of material with
angle of reflection equal and opposite to angle of incidence.
1. Explain the electromagnetic Remote Sensing
2. Explain wave theory and particle theory.
3. Give Stefan-Boltzmann Law and Weins
4. Define scattering and explain its types.