Chapter 2

CH.
2: INTERACTION OF ELECTROMAGNETIC ENERGY
Department of Geography and

Environmental Studies
College of Social Sciences and
Humanities
JIMMA UNIVERSITY
RADIATION LAWS AND INTERACTION
OF EMR WITH EARTH’S SURFACE
AKA: Backscatter
or path radiance
From the above discussion we must conclude that
reflectance behavior of objects does not only alter due to a
change of object properties but also due to different
illumination conditions, different position with regard to the
sensor and different atmospheric conditions.
Remote sensing image analysts should keep this in mind

while interpreting the images.
Energy Interaction with the Atmosphere
(scattering, absorption, refraction)
Before radiation used for remote sensing reaches the Earth's

surface it has to travel through some distance of the Earth's
atmosphere.
Particles and gases in the atmosphere can affect the

incoming light and radiation.
 These effects are caused by the mechanisms of scattering

and absorption.
 Hence all radiation that is detected by remote sensors

passes through some distance in the atmosphere.
The atmosphere can have a profound effect on the intensity
and spectral composition of radiation available to any sensing
system.
These effects are caused principally through the mechanisms

of atmospheric scattering and absorption.
Atmospheric scattering is an unpredictable diffusion of

radiation by particles in the atmosphere.
Types of scattering:
A. Selective scattering: affects selectively some portions of the

electromagnetic spectrum and can be divided into two:
(i)Rayleigh scattering: occurs when particles are very small

compared to the wavelength of the radiation. These could be
particles such as small specks of dust or nitrogen and oxygen
molecules.
Rayleigh scattering causes shorter wavelengths of energy to
be scattered much more than longer wavelengths.
Rayleigh scattering is the dominant scattering mechanism in

the upper atmosphere.
 The fact that the sky appears "blue" during the day is because
of this phenomenon.
 As sunlight passes through the atmosphere, the shorter

wavelengths (i.e. blue) of the visible spectrum are scattered
more than the other (longer) visible wavelengths.
At sunrise and sunset the light has to travel farther through
the atmosphere than at midday and the scattering of the shorter
wavelengths is more complete; this leaves a greater proportion
of the longer wavelengths to penetrate the atmosphere.
(ii)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.
(B). Non-selective scattering: The final scattering mechanism of
importance is called nonselective scattering.
This occurs when the particles are much larger than the wavelength
of the radiation.
Water droplets and large dust particles can cause this type of
scattering.
 Nonselective 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 (blue + green + red light = white
light).
Atmospheric absorption, on the other hand, results in the
effective loss of energy to atmospheric constituents.
This normally involves absorption of energy at a given

wavelength.
Generally, some constituents absorb the energy incident upon

them totally, while others absorb only part of it depending on
wavelength and the property of the material.
 The latter is called selective absorption.
This means that not all energy is available for remote sensing
purposes.
The major causes of atmospheric absorption are H2O, CO2, and

Ozone. For example, water absorbs EMR at 1.4, 1.9 and 2.7 μm
(these regions are called the ‘water absorption band’).
The wavelength ranges which are relatively less affected by
atmospheric absorption are referred to as atmospheric windows
11
Atmospheric windows and absorption wavelengths by H2O,

O3 and CO2
It is these atmospheric windows that are used for remote sensing
purposes.
The most important atmospheric windows for remote sensing data

acquisition are: The visible portion of the spectrum, to which our eyes
are most sensitive, corresponds to both an atmospheric window and
the peak energy level of the sun.
Note also that heat energy emitted by the Earth corresponds to a

window around 10 µm in the thermal IR portion of the spectrum, while
the large window at wavelengths beyond 1 mm is associated with the
microwave region.
Types of Remote Sensing
 Passive Remote Sensing : Doesn’t employ any external source of
energy.
 Active Remote sensing: Has its own source of energy.

Controlled illumination signal
Day/night operation

Chapter 2

Uploaded by

Copyright:

Available Formats

Chapter 2

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Chapter 2

Uploaded by

Copyright:

Available Formats

CH.

2: INTERACTION OF ELECTROMAGNETIC ENERGY

Department of Geography and

Remote sensing image analysts should keep this in mind

Before radiation used for remote sensing reaches the Earth's

Particles and gases in the atmosphere can affect the

 These effects are caused by the mechanisms of scattering

 Hence all radiation that is detected by remote sensors

These effects are caused principally through the mechanisms

Atmospheric scattering is an unpredictable diffusion of

A. Selective scattering: affects selectively some portions of the

(i)Rayleigh scattering: occurs when particles are very small

Rayleigh scattering is the dominant scattering mechanism in

 As sunlight passes through the atmosphere, the shorter

Dust, pollen, smoke and water vapour are common causes of

Mie scattering occurs mostly in the lower portions of the

This normally involves absorption of energy at a given

Generally, some constituents absorb the energy incident upon

 The latter is called selective absorption.

The major causes of atmospheric absorption are H2O, CO2, and

Atmospheric windows and absorption wavelengths by H2O,

The most important atmospheric windows for remote sensing data

Note also that heat energy emitted by the Earth corresponds to a

 Active Remote sensing: Has its own source of energy.

You might also like