Page 14 - RemoteSnsing&GIS by PK Gogoi

10
can easily reach on the earth’s surface.These are
the portions of EMR having different
wavelengths,and not blocked by any
atmospheric elements in the atmosphere.These
unblocked portions of EMR or spectrum is
called ‘atmospheric windows’. 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.
nergy emitted by the Earth corresponds
Eto a window around 10 um in the thermal
IR portion of the spectrum. Keeping in view of
the such atmospheric windows of spectrum ,
remote sensing sensors are designed for
acquiring data.
.3. Interaction with the Target (C) - Once the
2energy reaches the target or earth’s surface
through the atmosphere, it interacts with the
target(earth’s surface) depending upon the
properties of both the target and the radiation.
.3. Interaction mechanism of EM radiation
2with ground and spectral response curve :
pproximately half of the solar energy
Acoming from the sun is reflected or
absorbed by the earth’s atmosphere. Remaining
half reaching the earth surface is either reflected,
absorbed, or transmitted. The three forms of
interaction,which is called ‘Incident Ray’ (I) of
of EMR ,are absorption (A); transmission (T);
and reflection (R). The total incident energy will
interact with the surface of the earth in one or
more of these three ways. .How much of the
energy is absorbed, transmitted or reflected by a
material will depend upon: Wavelength of the
energy (EMR), material constituting the surface
9
he specular or mirror-like reflection occurs maximum chlorophyll and thus, it reflects more
Tover the surface of smooth topography, like green. But, on the contrary, during autumn, tree
our National Highways, where all (or almost all) leaves produce less chlorophyll ,and reflecting
of the energy is directed away from the surface the red wavelengths. Thus it appears red or
in a single direction. On the other hand, Diffuse yellow (yellow is a combination of red and green
reflection occurs when the surface is rough and wavelengths).It is to be further added that all
the energy is reflected almost uniformly in all healthy trees having green leaves can reflect
near-infrared wavelengths. By observing the IR
of the earth , and condition of the feature. The reflectance from trees ,scientists can determine
equation of incident rays is expressed as EI (l) health(lavishness) of vegetation cover.
= EA (l) + ET (l) + ER(l) where, EI(I) = n remote sensing, due to interaction of
Indecent Ebergy, EA= Absorbed Energy , ET IIncidence ray(I) with that of the target (T),-
Transmitted Energy and ER Radiated Energy that is the earth’s lithosphere - transfers EMR
directions. The nature of reflection, therefore,
depends on the nature of topography of the
earth’s features.Whether a particular target
reflects ‘specularly’ or ‘diffusely’, or
somewhere in between, depends on the surface
bsorption (A) occurs when radiation roughness of the feature in comparison to the
A(energy) is absorbed into the target while wavelength of the incoming radiation. If the
transmission wavelengths are much smaller than the particle
(T) occurs sizes of the earth’s features, then from the
terrestrial surface, diffuse reflection will occur.In
when radiation
viewing remote sensing imageries it may either
passes through
be appeared smooth to long wavelength
a target.
microwaves, or be appeared quite rough to the
Reflection (R)
visible wavelengths.
occurs when
r a d i a t i o n hlorophyll, presents in the leaves of energy through the features of the earth and
"bounces" off the target and is redirected. In Cplant, absorb radiation of the red and subsequently, ire-radiates back to the
remote sensing, we are most interested in blue wavelengths almost completely but reflects atmosphere in the form of long -wave terrestrial
measuring the radiation reflected from targets. its green wavelengths. That is why leaves radiation. In the following paragraph, a pictorial
eflection from a target is occurred in two appear ‘green’. There is a variation of reflected representation of energy reflectance of different
Rways – one in specular reflection and other colour from the leaves from season to objects will be discussed.
is diffuse reflection. season.During summer, plant leaves produces