Principles of Remote

UNIT 2 INTERACTION OF EMR WITH Sensing

EARTH AND ATMOSPHERE

Structure
2.1 Introduction
Objectives
2.2 Energy-Atmosphere Interaction
Refraction
Scattering
Absorption
2.3 Energy-Earth Interaction
Reflection
Transmission
2.4 Important Terminologies
Radiant Energy
Radiant Flux
Radiant Intensity
Irradiance and Exitance
Radiance
Albedo
2.5 Activity
2.6 Summary
2.7 Unit End Questions
2.8 References
2.9 Further/Suggested Reading
2.10 Answers

2.1 INTRODUCTION
In the previous unit, you have studied about history and processes of remote
sensing and electromagnetic energy and its properties. You have also studied
about the models of electromagnetic radiation (EMR). Now you know that
EMR is the basis of remote sensing and most of the remote sensing sensors
use Sun’s energy (radiation) for collecting information about objects on the
Earth’s surface. Sensors record the radiation coming from the Sun after
interacting with the atmosphere and the Earth’s surface. Particles and gases in
the atmosphere can affect the incoming light or radiation. The radiation
reaching the remote sensor is modified significantly because of the processes
taking place in the atmosphere and the Earth’s surface. This unit discusses in
detail about how these radiations interact in the atmosphere and at the Earth’s
surface. An account of the important terminologies used in remote sensing is
also given.

Objectives
After studying this unit you should be able to:
• explain the interaction of EMR with atmosphere; 23
Introduction to Remote • describe how absorption and scattering together attenuate the electromagnetic
Sensing
radiation through different mechanisms;
• discuss atmospheric windows and their importance; and
• describe how the radiation interacts with the Earth’s surface.

2.2 ENERGY-ATMOSPHERE INTERACTION

On hot sunny days, the As you know the word ‘atmosphere’ refers to the gas layers surrounding the Earth.
atmosphere near the Constituents of atmosphere are nitrogen, oxygen, carbon dioxide, ozone, water
Earth’s surface is very hot vapour and other gases. EMR coming from the Sun has to pass through the Earth’s
because of the turbulent
hot air with different
atmosphere twice before being detected by the satellite sensor – once on its
densities and hence journey from the Sun to the Earth and second time after being reflected / emitted by
different refractive the Earth’s surface to the sensor. Particles and gases present in the atmosphere
indices. The difference in interact with the incoming light and reflected / emitted radiation. Our interest in this
refractive indices makes interaction is related to the fact that atmospheric components diffuse, refract,
the air bubbles act as
lenses, slightly deviating
reflect, absorb and emit EMR changing the original radiance of the objects
the transmitted light. This observed by a remote sensor. The interaction of EMR with the atmosphere is
deviation causes the important to remote sensing for two main reasons:
mirage effect as seen in
the following figure. • information carried by EMR reflected/emitted by the Earth’s surface is
modified while traversing through the atmosphere, and
• interaction of EMR with the atmosphere can be used to obtain useful
information about the atmosphere itself.

Fig. 2.2: Mirage seen in the

The change in incident radiation on its way towards satellite sensor is
road in a hot sunny day governed by several atmospheric effects as shown in Fig. 2.1. These effects are
(source: www.weather caused by the mechanisms of refraction, reflection, scattering, absorption and
scapes.com)
transmission.

Fig. 2.1: Interaction of EMR in the Atmosphere

We will now discuss about these mechanisms.

2.2.1 Refraction
Atmospheric refraction is the deviation of electromagnetic wave from a
straight line as it passes through the atmosphere due to variation in air density,
which varies with altitude. Atmospheric refraction near the ground produces
mirages (Fig. 2.2) and can change the look of distant objects. Atmospheric
refraction causes astronomical objects to appear higher in the sky than they are
in reality. It affects complete spectrum of EMR in varying degrees. For
example, in visible light, blue is more affected than red. The amount of atmospheric
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example, in visible light, blue is more affected than red. The amount of Interaction of EMR with
Earth and Atmosphere
atmospheric refraction is a function of temperature, pressure and humidity.
The presence of turbulence in the air makes atmospheric refraction
inhomogeneous. This is the cause of twinkling of the stars and deformation of If the sun (or moon) is low
the shape of the Sun at sunset and sunrise (Fig. 2.3). Atmospheric refraction is above the horizon, the
minimum in the zenith and maximum at the horizon. In day-to-day life, we optical path of light
through the atmosphere is
often experience refraction; e.g., when we insert a spoon in a water-filled
very long, and the
bowl, it appears slightly elevated (Fig. 2.4). atmosphere usually has a
layered structure of
different temperature
gradients and pressure.
Refraction of the light by
these layers can cause the
sun’s disk to be deformed,
flattened or distorted
(Fig. 2.3).

(a) (b) (c)

Fig. 2.4: (a) When white light passes through a prism, its components, i.e., VIBGYOR
are visible on the other side of the prism illustrating the refraction Fig. 2.3: Deformities seen in
phenomenon; (b) Another example from everyday life is when you insert a Sun’s shape when it is in
spoon or straw in a glass of water, it appears slightly bent at the water surface; horizon (source:
and (c) Representation of refraction of light at the interface of two media www.weatherscapes.com)

2.2.2 Scattering
Anisotropy is the property
Most of the light that reaches our eyes comes not directly from its sources but of being directionally
indirectly by the process of scattering. You see diffusely scattered solar dependent, as opposed to
radiation when you look at clouds or at the sky. The land and water surfaces isotropy, which implies
and the objects surrounding us are visible through the light they scatter. An identical properties in all
electric lamp does not send us light directly from the luminous filament but directions.
usually glows with the light that has been scattered by the glass bulb. Unless
you look at a source, such as the Sun, a flame, or an incandescent filament Isotropic:Scattering is
with a clear bulb, you see light that has been scattered. In the atmosphere, you different from reflection in
see many colourful examples of scattering generated by molecules, aerosols, the sense that the direction
associated with scattering
and clouds containing droplets and ice crystals. Blue sky (Fig. 2.5), white is unpredictable whereas
clouds, and magnificent rainbows and halos, to name a few, are all optical the direction of reflection
phenomena due to scattering. Scattering is a physical process associated with is predictable.
the light and its interaction with matter. It occurs at all wavelengths covering
the entire electromagnetic spectrum. Scattering redirects
incident EMR and deflects
reflected EMR from its
path.

Aerosols are airborne

particulate matter.

Fig. 2.5: Scattering of blue colour from sunlight 25

Introduction to Remote continuously abstract energy from the incident wave and re-radiates that energy in
Sensing
all directions. Therefore, a particle may be thought of as a point source of the
scattered energy. In the atmosphere the particles responsible for scattering cover
the sizes from gas molecules (~10-8cm) to large raindrops and hail particles (~1
cm). The relative intensity of the scattering pattern depends strongly on the ratio of
Scattering is a very particle size to wavelength of the incident wave. If scattering is isotropic, the
important consideration in
remote sensing scattering pattern is symmetric about the direction of incident wave. A small
investigations because it anisotropic particle tends to scatter light equally into the forward and rear
can severely reduce the directions. When the particle becomes larger, the scattered energy is increasingly
information content of concentrated in the forward directions with greater complexities.
remotely sensed data to
the point that the imagery Radiation scattered from a particle is a function of several things such as shape, size
loses contrast and then it and index of refraction of particle, wavelength of radiation, and surface geometry.
becomes difficult to For a spherical scatterer, the scattered radiation is a function of only viewing angle,
differentiate one object
index of refraction, and the size parameter defined as
from another.
÷ = 2 ð r /ë .................................... (1)
where,
÷ is the size of particle,
Scattering is a process in
which EMR interacts with r is the radius of the sphere, and
particles or large gas ë is the wavelength of the radiation.
molecules present in the
atmosphere and cause it to Depending on the size parameter, the following two types of scattering take place.
be redirected in all
• Selective scattering
direction from its original
path by reflection and • Non-selective scattering
refraction.
a) Selective Scattering
Scattering depends on When the scattering is wavelength-dependent, it is known as selective
several factors such as the
scattering. Selective scattering are of two types – Rayleigh scattering and
wavelength of the
radiation, the abundance Mie scattering.
of particles or gases, and • Rayleigh Scattering
the distance the radiation
travels through the Rayleigh scattering is named after the English physicist, Lord Rayleigh,
atmosphere. who offered its explanation. Rayleigh scattering is caused by very small
particles and gas molecule with radii for less than the wavelength of EMR
of interest (Fig. 2.6). Primarily, it occurs due to oxygen and nitrogen
molecules in the sky; thus it is also known as molecular scattering.
Rayleigh scattering can be considered to be elastic scattering since the
photon energies of the scattered photons is not changed. Scattering in
which the scattered photons have either a higher or a lower photon energy
is called raman scattering. Usually, this kind of scattering involves
exciting some vibrational modes of the molecules, giving lower scattered
photon energy, or scattering off an excited vibrational state of a molecule
which adds its vibrational energy to the incident photon.
The intensity of light is inversely proportional to the fourth power of the
wavelength of the light.
I α 1 / ë4 ................................................ (2)
where,
I and ë are scattering intensity and wavelength of incident radiation,
respectively.

26
 Interaction of EMR with
Earth and Atmosphere

The sky appears blue

because as sunlight passes
through the atmosphere,
the shorter wavelengths
(i.e. blue) of the visible
spectrum are scattered
more than the other
(longer) visible
Fig. 2.6: Angular patterns of the scattered intensity for Rayleigh scattering wavelengths. However,
(source: www.islandnet.com/~see/weather/almanac/arc2008/alm08oct.htm) during sunrise and sunset,
the sky appears red
Since the extent of scattering is inversely proportional to the 4th power of because during the sunrise
wavelength, shorter wavelengths such as blue light in the visible and sunset the light has to
travel farther through the
spectrum are affected the most. Rayleigh scattering is also responsible for atmosphere than at midday
red sunsets. During sunsets, sunlight passes through a longer path of air and the scattering of the
than at noon. Since the violet and blue wavelengths are scattered more shorter wavelengths is
during their longer path through the air than at noon, hence, what we see more complete; this leaves
at sunset is the residue, i.e., the wavelengths of sunlight that are hardly a greater proportion of the
longer wavelengths light
scattered away specially the oranges and reds. (red) to penetrate the
atmosphere.
• Mie Scattering
Mie scattering occurs when the particles in the atmosphere are of the
same size as the wavelengths being scattered. It is caused by particles
with radii between 0.1 and 10μm such as dust, smoke and salt (aerosols).
Dust, pollen, smoke and water vapour are common causes of Mie
scattering, which tends to affect longer wavelengths. Mie scattering
occurs mostly in the lower portions of the atmosphere where larger
particles are more abundant, and dominates when cloud conditions are
overcast. The amount of scattering is greater than Rayleigh scattering.
The violet and blue light are scattered away more with an increasing
amount of smoke and dust particles in the atmosphere and only the longer
orange and red wavelength light reaches our eyes.

b) Non-Selective Scattering
Non-selective scattering are wavelength-independent. It is caused by
particles (water droplets and ice fragments in cloud) whose radii exceed
10μm. The scattering is independent of the wavelength; all the
wavelengths are scattered equally and not just blue green and red. Non-
selective scattering takes place in the lower portion of the atmosphere
where there are particles more than 10 times the wavelength of the
incident EMR. The most common example of non-selective scattering is
the appearance of clouds as white. As clouds consist of water droplet
particles and the wavelengths are scattered in equal amount, the clouds
appear as white.

Water droplets and large dust particles can cause this type of scattering and
thus cause fog and clouds to appear white to our eyes because blue, green, and
red light are all scattered in approximately equal quantities.
Scattering creates an effect of haziness in remote sensing images, which reduces
contrast in images. It also creates ‘adjacency effect’ in which signal recorded in a
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Introduction to Remote pixel partly incorporates the scattered signal from the neighbouring pixels. It is
Sensing
required to introduce an important term here, i.e., ‘path radiance’. The radiation
that has been scattered in the Earth’s atmosphere and has reached the sensor
Distortions in remote without contacting the Earth’s surface is known as path radiance. It is essential to
sensing image due to remove the path radiance from remote sensing images before any image analysis
atmospheric phenomena
particularly for multi-date images.
and the relevant
correction processes are 2.2.3 Absorption
discussed in Unit 11 of
MGY-002. By the time EMR is recorded by a sensor, it has already passed through the Earth’s
atmosphere twice (once while travelling from the Sun to the Earth and second time
while travelling from the Earth to the sensor). When light travels through
Strength of absorption atmosphere, a gradual reduction in its intensity occurs. The reduction in intensity
is represented by
with distance in a medium (i.e., atmosphere) is called attenuation of light as
absorption cross section
shown in Fig. 2.7. This attenuation occurs mainly because of the scattering and
ó a in units of cm2. It is
basically division of absorption of light in atmosphere. Absorption is the process by which radiation
absorption coefficient (radiant energy) is absorbed and converted into other forms of energy such as heat
and number density or chemical energy. Absorption is wavelength-dependent. Absorption of light
(number of molecules occurs because part of the incident light is transformed into the energy of motions of
per unit volume). It the atoms in the medium. It can take place in the atmosphere or on the terrain.
represents a molecule’s
effective area for
absorption of radiation.

Fig. 2.7: Attenuation of a light wave in an absorbing medium

To understand it better, let us take an example. Grass appears green because it

scatters green light more effectively than red and blue light. Apparently, red
and blue light incident on the grass is absorbed. The absorbed energy is
converted into some other form, and it is no longer present as red or blue light.
In the visible spectrum, absorption of energy is nearly absent in molecular
atmospheres. Clouds also absorb very little visible light. Both scattering and
absorption removes energy from the beam of light. Thus, beam of light is
attenuated, and we call this attenuation extinction.
There are three main atmospheric constituents, which absorb solar radiation.
The three constituents are ozone (O3), carbon dioxide (CO2), and water vapour
(H2O). Ozone gas, which plays an important role in the Earth’s energy
balance, has maximum concentration in the stratosphere (at the altitude of
about 20 to 30 km). Ozone absorbs high energy; it prevents short wavelength
portion of the ultraviolet spectrum to transmit through the lower atmosphere.
Carbon dioxide, which occurs mainly in lower atmosphere, absorbs radiation
in the mid and far infrared regions of the electromagnetic spectrum. Maximum
absorption occurs in the region from about 13 to 17.5 μm. Abundance of water
vapour significantly varies with time and location. However, it is commonly
present in the lower atmosphere. Water vapour contributes significantly to the
absorption of radiation particularly in several bands in the region between 5.5 and 7
28 μm.
It is important to note here that all media show some absorption. Media which Interaction of EMR with
Earth and Atmosphere
absorb all wavelengths more or less equally are said to show general
absorption whereas media which absorb some wavelengths more strongly than
others are said to show selective absorption.
The ability of a medium to absorb energy is measured as the absorptance and
is expressed as
Absorbed radiation
Absorptance (α) = —————————
Incident radiation
From remote sensing point of view, absorption in the visible, near Infrared and
thermal Infrared regions of EMS is important. Significant amount of
absorption in visible and near infrared (NIR) band is basically due to
molecular oxygen and ozone, water vapour, carbon dioxide and some other
minor gases. Water vapour, carbon dioxide, ozone, methane and
chlorofluorocarbons absorb radiation in thermal infrared band.
As you know, windows are used as a motion for air ventilation and lights.
Similarly, there are certain portions of electromagnetic spectrum where light
can travel through atmosphere with much absorption. The absorption by
various constituents in the atmosphere results in limiting portions of the EMR
from reaching the Earth. Hence, the Earth’s atmosphere is not completely
transparent to EMR. For remote sensing, this limits us to portions of the EMS
where radiation is not strongly absorbed. This portion of the atmosphere is
called Atmospheric Windows.
Position, extents and effectiveness of atmospheric window are determined by
the absorption spectra of atmospheric gases. Energy outside the atmospheric
windows is severely attenuated by the atmosphere and hence cannot be
effective for remote sensing. The most important atmospheric windows are the
visible window (0.4 – 0.7 μm), the 3.7 μm window, the microwave windows
(2 – 4 mm and >6 mm), and the 8.5 –12.5 μm window as shown in Fig. 2.8.
The visible window is mainly affected by ozone absorption and by molecular
scattering. The 8.5 – 12.5 μm infrared window is punctuated by the 9.6 μm
ozone absorption band, and is affected by water vapour absorption.

Fig. 2.8: Atmospheric windows in the EM spectrum. Chemical notations (CO2, O2, etc.)
indicate the gases responsible for blocking the radiation at a particular
wavelength (source: http://earthobservatory.nasa.gov/Features/
RemoteSensing/ remote_04.php)

The dark (black) areas in Fig. 2.8 denote regions of the EMS where
atmosphere absorbs most of the radiation and light (grey) areas are the
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Introduction to Remote The dark (black) areas in Fig. 2.8 denote regions of the EMS where
Sensing
atmosphere absorbs most of the radiation and light (grey) areas are the
atmospheric windows.

The infrared channels are most often between 1 and 30 μm. The most
common infrared band for meteorological satellites is in the 10 – 12.5 μm
window, in which the atmosphere is relatively transparent to radiation
upwelling from the Earth’s surface. Even in the atmospheric window regions,
scattering by the atmospheric constituents produces spatial redistribution of
energy.

In a broad sense, remote sensing of the Earth’s surface is generally confined to

certain wavelength regions as given in Table 2.1.

Table 2.1:Remote sensing of the Earth’s surface with respect to wavelength

region
S. No. Atmospheric Window (μm) Region
1 0.3 – 0.4 μm UV
2 0.4 – 0.7 μm Visible
3 0.7 – 3.0 μm Reflected infrared
4 3.0 – 5.0 μm Thermal infrared
5 8.0 – 11.0 μm Thermal infrared
6 1.0 mm – 1.0 m Microwave

Spend Check Your Progress I

5 mins
1) Point out the mechanisms that are responsible for interaction of EMR and
atmosphere.
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2) What do you mean by scattering?

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30
by the Earth’s surface. The absorbed short wave (visible) radiation by Earth’s Interaction of EMR with
Earth and Atmosphere
surface is emitted as a long wave radiation (infrared band). The complete process is
shown in Fig. 2.9. This physical process changes the magnitude, direction,
wavelength, polarisation and phase of the EMR. 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, colour and spectral
signature).

There are three major regions in EMR-Earth interaction that are important in
remote sensing. The visible and NIR spectral band from 0.3 ìm to 3 ìm is
known as the reflective region. In this band, the Sun’s radiation sensed by the
sensor is reflected by the Earth’s surface. The band corresponding to the
atmospheric window between 8 ìm and 14 ìm is known as the thermal
infrared band. The energy available in this band for remote sensing is due to
thermal emission from the Earth’s surface. Both reflection and self-emission
are important in the intermediate band from 3 ìm to 5.5 ìm.

Fig. 2.9: Interaction of EMR with Earth’s surface

In the microwave region (1-30 cm) of the spectrum, the sensor is normally a
radar, which is an active sensor, as it provides its own source of EMR. The
EMR produced by the radar is transmitted to the Earth’s surface and the EMR
reflected (back-scattered/radar return) from the surface is recorded and
analysed. The microwave region can also be monitored with passive sensors,
called microwave radiometers, which record the radiation emitted by the
Earth’s surface and its atmosphere in the microwave region. We will now
discuss about two phenomenon i.e., reflection and transmission.

2.3.1 Reflection
When light travelling in a medium (i.e., atmosphere) encounters a surface
leading to a second medium (Earth’s surface), part of the incident light is
returned to the first medium from which it came. This phenomenon is called
reflection. In other words, reflection is the phenomenon in which the incident
radiation is returned back to the same medium due to the discontinuity of
electromagnetic characteristics at the interface of two media. Reflection occurs
when a ray of light is re-directed as it strikes a surface as shown in Fig. 2.10.
Understanding of reflection is important, since about a third of the energy from the
sun is reflected.
31
Introduction to Remote
Sensing

(a) (b) (c)

Fig. 2.10: An illustration of reflection of light. You can see inverted images of the (a) letters
and (b) the glass object on the floor. You are able to see the inverted images of the
objects on the shiny floor because of the reflection of light (c) (èi is angle of
incident ray and èr is angle of reflected ray)

Let us recall the laws of reflection here. As you know, the first law of reflection
states that if the reflecting surface is very smooth, the reflection of light that occurs is
called specular or regular reflection. The laws of reflection are as given below:
• the incident ray (èi), the reflected ray and the normal to the reflection surface
at the point of the incidence lie in the same plane as shown in Fig. 2.10(c). This
plane is called the plane of incidence.
• the angle of reflection(èr) (the angle which the reflected ray makes to the same
normal) is equal to the angle of incidence (the angle which the incident ray
makes with the normal) as shown in Fig. 2.10(c).

The ability of a medium to reflect energy is measured as the reflectance and it is

defined as a ratio between reflected radiation and incident radiation [ñ(ë)]:

Reflectance radiation
Reflectance ñ(ë) = —————————
Incident radiation

Reflectance [ñ(ë)] is the ratio of reflected energy to incident energy and hence
is a measure of how much radiation is reflected off a surface. Its value ranges
from 0 to 1. Value of 0 means that 0% of incident radiation is reflected off the
surface and the value of 1indicates that 100% of the incident radiation is
reflected.
Spectral reflectance [ñ(ë)] is the ratio of reflected energy to incident energy as
a function of wavelength. Various materials of the Earth’s surface have
different spectral reflectance characteristics. Spectral reflectance is responsible
for the colour or tone in a photographic image of an object. Trees appear green
because they reflect more of the green wavelength. The values of the spectral
reflectance of objects averaged over different, well-defined wavelength
intervals comprise the spectral signature of the objects or features by which
they can be distinguished. To obtain the necessary ground truth for the
interpretation of multispectral imagery, the spectral characteristics of various
natural objects have been extensively measured and recorded.

The spectral reflectance is dependent on wavelength. It has different values at

different wavelengths for a given terrain feature. The reflectance
characteristics of the Earth’s surface features are expressed by spectral
32 reflectance, which is given by:
The spectral reflectance is dependent on wavelength. It has different values at Interaction of EMR with
Earth and Atmosphere
different wavelengths for a given terrain feature. The reflectance
characteristics of the Earth’s surface features are expressed by spectral
reflectance, which is given by:
ñ(ë) = [ ER(ë) / EI(ë) ] x 100 ....................................... (3)
where,
ñ(ë) is spectral reflectance (reflectivity) at a particular wavelength,
ER(ë) is energy of wavelength, reflected from object, and
EI(ë) is energy of wavelength, incident upon the object.
The plot between ñ(ë) and ë is called a spectral reflectance curve. This varies
with the variation in the chemical composition, physical conditions and EM
properties of the object, which results in a range of values. The spectral
response patterns are averaged to get a generalised form, which is called
spectral response pattern for the object concerned. Spectral signature is a term
used for unique spectral response pattern, which is characteristic of a terrain
feature. Fig. 2.11 shows a set of typical reflectance curves for three basic types
of Earth surface features, viz., the healthy vegetation, the dry bare soil (grey-
brown and loamy) and clear lake water.

Fig. 2.11: Typical spectral reflectance curves for vegetation, soil and water (source:
Liliesand and Kiefer, 1993)

The spectral characteristics of these three main Earth surface features are
discussed below.
Nature of reflection depends on sizes of surface irregularities (i.e., roughness
or smoothness) with respect to the wavelength of the radiation considered. If
the surface is smooth in comparison to wavelength, then specular reflection
occurs (Fig. 2.12a). In specular reflection, almost all the incident radiation is
redirected in a single direction. For such reflection, angle of incidence is equal
to the angle of reflection. Specular reflection can occur with surfaces such as
smooth metal and calm waterbody. If the surface is rough relative to the
wavelength, then energy is scattered more or less equally in all directions as
shown in Fig. 2.12 d. This property of light is known as diffuse reflection. So,
33
Introduction to Remote whichever angle we observe from, a perfectly diffuse reflector would have
Sensing
equal brightness in all the directions. It is largely by diffuse reflection that we
see non-luminous objects around us. Uniform grass surface is a good example
of diffuse reflectors. Perfectly diffuse reflectors are also called as Lambertian
surface since the concept of perfectly diffuse reflecting surface is derived
from the work of J.H. Lambert. He observed that the perceived brightness of a
perfectly diffuse surface does not change with the angle of view. This
behaviour of light is known as Lambert’s cosine law.
It is important to note here that the two laws of reflection are obeyed in
specular reflection. They do not hold in case of irregular or diffuse reflection.
Much of the reflection of solar radiation takes place from the top of clouds and
Johann Heinrich Lambert other materials in the atmosphere and hence a significant amount of this
(1728-1777) was a Swiss
mathematician, physicist
energy is reradiated back to space.
and astronomer. He
conducted many
experiments designed to
describe the behaviour of
light. Lambert was the first
to introduce hyperbolic
functions into
trigonometry. He was also (a) (b) (c) (d)
the first mathematician to
address the general Fig. 2.12: Different types of scattering surfaces (a) perfect specular reflector; (b) near
properties of map perfect specular reflector; (c) near perfect diffuse reflector; and (d) perfect
projections. He also diffuse reflector (Lambertian surface)
developed a theory of the
generation of the universe To have true reflection, a real discontinuity in the index of reflection is
that was similar to the required. The spatial scale of discontinuity, compared to the wavelength of the
nebular hypothesis. radiation, must also be significant for a perfect reflection to take place. The
energy reflects off at an interface, at the same angle at which it initially strikes
the surface, as seen in Fig. 2.12. Of all the interactions in the reflective region,
surface reflections are the most useful and revealing in remote sensing
In optics the refractive applications. The reflection intensity depends on the surface refractive index,
index of a substance is a
measure of the speed of
absorption coefficient and the angles of incidence.
light in that medium and is As you have now understood, reflection exhibits certain fundamental
expressed as a ratio of the
characteristics (as stated in laws of reflection) that are important in remote
speed of light in vacuum
relative to the medium i.e. sensing.
n = speed of light in a
vacuum/speed of light in 2.3.2 Transmission
medium. When electromagnetic radiation is incident on Earth’s surface, part of the
energy gets scattered from the surface (which is known as surface scattering)
and a part of the energy gets transmitted into the medium. In homogeneous
materials, the radiation is simply transmitted but in inhomogeneous materials,
the transmitted radiation gets further scattered (which is known as volume
scattering). The signal received by sensors is a combination of both the
processes, i.e., surface and volume scattering.
Transmission of radiation occurs when radiation passes through a substance
without significant attenuation (Fig. 2.13). The ability of a medium to transmit
energy is measured as the transmittance and it is defined as the ratio between
transmitted radiation and incident radiation (ô):
Trammitted radiation
Transmittance (ô) = —————————
34 Incident radiation
Introduction to Remote and moisture. Changing humidity and the dust content in the air at a given point
Sensing
throughout the year determine the annual atmospheric transmittance at that point.
The atmosphere is most transmissive in winter and least transmissive in summer. A
significant decrease in atmospheric transmittance is observed as a result of
increasing air pollution, especially when the dust content increases.
Check Your Progress II
1) What is the range of thermal infrared band?
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5 mins ......................................................................................................................
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2) Spectral reflectance is the ratio of ..............................................................

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2.4 IMPORTANT TERMINOLOGIES

It is essential for us to get introduced to some important radiometric terminologies
which are often used in remote sensing.

2.4.1 Radiant Energy

It is the quantity of energy carried by the EMR. It is a measure of the capacity of
radiation to perform work (heat, movement, etc.). Radiant energy refers to the
quantity of energy propagating into or through a surface of a given area in a given
period of time. It is represented as Q and its unit is Joules (J). When radiant energy
is considered at a particular wavelength, it is called spectral radiant energy and is
represented as Që.

2.4.2 Radiant Flux

It is the rate of flow of radiant energy onto or through a surface. To understand it
better, it may be compared to the rate of flow of water past a position along a pipe.
It is represented by Ö and is measured in Joules per second (J s-1) or watts (W).

When the radiant flux is considered at a wavelength, it is called spectral radiant

flux and is represented as Öë. It is measured in Joules per second per micron (J s–1
m–1) or watts per micron (W m-1).

2.4.3 Radiant Intensity

We can further refine our measurement of radiant flux by including a direction.
Radiant flux leaving a source per unit solid angle in a given direction is called radiant
intensity. It is represented by I and its unit is watts per steradian (W sr-1).

2.4.4 Irradiance and Exitance

36 Now we can refine the measurement of radian flux by including the size of the area.
It is the amount of radiant flux incident (arriving) upon per unit area of a surface. It Interaction of EMR with
Earth and Atmosphere
is represented as E and its unit is watts per meter square (W m-2). When it is
considered at a wavelength, it is represented as Eë and is expressed as:
Eë = Ö ë / A …………………………. (4)

The amount of radiant flux emitted (leaving) from a unit area of a surface is called
exitance. It is represented as M. When it is considered at a wavelength, it is
represented as Më and is expressed as

Më = Ö ë / A ………………………. (5)

2.4.5 Radiance
It is the most precise radiometric measurement in remote sensing. It is the radiant
flux per unit solid angle leaving a per unit projected source area in a given direction.
In other words, it is the radiant intensity per unit of projected source area. To better
understand the concept of radiance, you can compare it with what you would see if
you were in an airplane and looking at the ground through a telescope. You would
only see the energy that exited the ground and came through the telescope in a
specific solid angle (Ù).

Radiance is represented as L and its unit is watts per meter square per steradian
(W m-2 sr-1). When the radiance is considered in a particular wavelength, it is
represented as Lë.

L l = ( l ) ………………....... (6)
f /W
A cos q

2.4.6 Albedo
It is defined as the ratio of the electromagnetic energy reflected or diffused by a
surface to the total incident energy. Albedo of objects varies from object to object.
Fresh snow has higher albedo (in the range of about 75 to 95%) and dark soil has
lower albedo (in the range of about 5 to 10%). Albedo also varies with the Sun’s
angle and the variations are large with tilted angles of the solar rays from 0º to 30º.

Some of the above discussed and frequently used terminologies are summarised in
Table 2.2.

Table 2.2: Important radiometric terminologies often used in remote sensing

Term Concept Symbol Unit
Radiant Energy The quantity of energy propagating into, off of Q J
or through a surface of given area in a given
period of time
Radiant Flux Rate at which photons (quanta) strike a surface, Ö J s–1 or W
or in other words, rate of flow of radiant energy
onto or through a surface
Radiant Intensity Radiant flux leaving a source per unit solid I W sr–1
angle in a given direction

37
Introduction to Remote Table 2.2: Important radiometric terminologies often used in remote sensing
Sensing
Term Concept Symbol Unit
Radiant Energy The quantity of energy propagating into, off of Q J
or through a surface of given area in a given
period of time
Radiant Flux Rate at which photons (quanta) strike a surface, Ö J s–1 or W
or in other words, rate of flow of radiant energy
onto or through a surface
Radiant Intensity Radiant flux leaving a source per unit solid I W sr–1
angle in a given direction
Irradiance Radiant flux incident (arriving) upon per unit E W m–2
area of a surface
Radiant Exitance Radiant flux emitted (leaving) from a unit area M W m–2
of a surface
Radiance Radiant flux per unit solid angle leaving a per L W m–2 sr–1
unit projected source area in a given direction
Reflectance Ratio of reflected energy to incident energy ñ(ë) %
Albedo The ratio of the electromagnetic energy A %
reflected or diffused by a surface to the total
incident energy

2.5 ACTIVITY
You have read about interaction of electromagnetic radiation with the
atmosphere and the Earth’s surface. You are now aware of different types of
phenomenon taking place in journey of electromagnetic radiation from Sun to
Earth and back to satellite sensor. You might be enthusiastic to do some
activities to better understand these phenomena.

1) As we know now, the colour of an object is not actually within the object
itself. Rather, the colour is in the light that shines upon it and is ultimately
reflected or transmitted to our eyes. So you can try to see a yellow
coloured object (yellow is a mixture of red and green colour) in a red
light. You can note down the colour changes observed.

2) You can build your own filter wheel (using different coloured films)
through which you can examine a colour image.

3) You can observe the changes in the colour of sun rays from early morning
to afternoon and again till evening. The process of scattering will be
clearly understood to you.

2.6 SUMMARY
Let us summarise what we have studied in this unit:
• Scattering, reflection and absorption are important phenomena which take
place when EMR interacts with the atmosphere and the Earth’s surface.

• In the atmosphere, scattering and absorption are the prominent

mechanisms for attenuation of the radiation.
38
 Interaction of EMR with
2.7 UNIT END QUESTIONS Earth and Atmosphere

1) How does scattering differ from absorption, although both attenuate the
radiation?
2) Define atmospheric windows and state the infrared window which is used
for remote sensing.
3) Why does the sky appear blue and Sun in the horizon red?
4) What is spectral reflectance?
Spend
30 mins
2.8 REFERENCES
• http://earthobservatory.nasa.gov/Features/RemoteSensing/ remote_04.php
• Lilliesand, T.M and Kiefer, R.W. (1993), Remote Sensing and Image
Interpretation, 3rd Ed., John Willey and Sons, 820p.
• www.everythingweather.com/atmospheric-radiation/transmission.shtml
• www.islandnet.com/~see/weather/almanac/arc2008/alm08oct.htm
• www.weatherscapes.com
• All the websites were retrieved on May 17, 2011.

2.9 FURTHER/SUGGESTED READING

• Joseph, G. (2005), Fundamental of Remote Sensing, University Press,
486p.
• Jensen, J.R. (2009), Remote Sensing of the Environment, 2nd Ed., Indian
edition by Dorling Kindersley India Pvt. Ltd., 592p.
• www.jars1974.net/pdf/02_Chapter01.pdf.

2.10 ANSWERS
Check Your Progress I

1) Refraction, reflection, scattering, absorption and transmission.

2) Scattering is a physical process by which a particle in the path of an EM

wave continuously abstract energy from the incident wave and re-radiates
that energy in all directions.

Check Your Progress II

1) The band corresponding to the atmospheric window between 8 ìm and 14

ìm is known as the thermal infrared band.

2) Reflected energy to incident energy.

Unit End Questions

1) Scattering is basically redistribution of incident energy through the process of

39