Hello everyone This is the fourth lecture of introduction to remote sensing course and
in this particular lecture we will be covering topics of electromagnetic spectrum which is
very important to understand the entire gamete of remote sensing and solar reflection which
will include of course this passive part that is solar reflection and then emission that
is thermal emission as well So we will be looking in detail different portions of EM
spectrum
Now as you know the first requirement for remote sensing is to have energy source and
that might be external energy source or internal energy source to illuminate the target unless
the suns energy is being emitted by the target So two ways one is either the solar energy
you have or natural heat being emitted by the earth’s object but it has to have energy
source In case if radar remote sensing or active remote sensing the energy is coming
through the satellite
So there also you have external energy One needs to have in case of remote sensing some
form of energy source to illuminate the target and to get the signatures of any object that
are present on the earth through these sensors so that is very must And all EM radiation
has fundamental properties and behaves in predictable ways according to the basics of
wave theory
We will not go into wave theory part because altogether it is a different topic but we
will be covering different portions of EM spectrum specially those which are having
more applications or direct applications in current remote sensing and electromagnetic
radiations So therefore there are two characteristics of EM radiation first is the wavelength and
frequency particularly important for understanding remote sensing Sometimes people us one term
another or simultaneously but it is not possible to use interchanging
Because it is electromagnetic radiation so you are having the way you have waves which
are in magnet we put them in magnetic waves we call them and then electric waves are there
which are having relationship which are perpendicular to each other Suppose this is the direction
of movement of the wave then in this particular example the magnetic waves are in this pink
color and the electric waves which are perpendicular to magnetic waves are travelling in yellow
color This distance between two peaks makes the wavelength which we will see in a bit
This is how in a simulation I can show that how electric and magnetic fields are having
relationship and this is how they travel This is the magnetic field here this is electric
field two types of electromagnetic waves are travelling and this is line of propagation
direction of propagation so electromagnetic radiation consists of two electrical fields
that is E which varies in magnitude in direction of perpendicular to the direction in which
the radiation is travelling and a magnetic field M which is oriented at right angle to
the electrical field and this is how they keep travelling Both these fields travel at
speed of light so that’s one thing one has to remember that they travel at speed same
and at speed of light equivalent to speed of light
Now two characteristics of electromagnetic radiation are particularly wavelength and
frequency which you can see that different wavelengths are shown and their frequencies
Wavelength is the length of one wave cycle as you can see and denoted by lamda and which
is measured at distance between successive wave crest These two parts in this case lamda
is much longer and here it is shorter so wavelength is usually represented by Greek letter lamda
Wavelength is measured in meters or some factors of meters Sometimes we call as micrometer
some people may prefer to call nanometer or meters depending but we always put meters
or some factors of meters whenever we talk about wavelength Whereas the frequency refers
to number of cycles of wave passing through fixed point for unit of a time This is the
major difference and between these two electromagnetic here the radio based are having much more
wavelength and wavelength is much higher whereas gamma rays the frequency is higher wavelength
is less so there is an inverse relation which we will see later
So frequency is measured in hertz whereas wavelength is measured in meters or factor
of meters equivalent to one cycle per second and in various multiples of hertz wavelength
and frequency are related by the following formula that is C equals to lamda V where
our lamda is the wavelength which we measure in meters or factors of meters V is the frequency
cycle per second in hertz and C is speed of light and this relationship tells us that
the shorter the wavelength the higher the frequency and you have also seen that here
the wavelength in case of gamma rays is very short but having high frequency
In case of radio waves in this particular example having longer wavelength but less
frequency so the longer the wavelength the lower the frequency is there Now these characteristics
or understanding these about electromagnetic radiation in terms of their wavelength and
frequency is crucial to understanding the information which we try to extract from remote
sensing data One has to remember in which part of visible spectrum we are working or
near infrared infrared thermal infrared and other things are there
So this is what whole spectrum is shown here Here is the longer wavelength shorter wavelength
are shown in meters and water molecules of very small size but they do affect and there
are different names given like longer wavelength we call them as radio waves and for reference
the set of wavelength or the size of objects are also shown then infrared then visible
then ultraviolet maybe x-rays and other things so here remote sensing doesn’t work till
ultraviolet because atmosphere energy is absorbed so basically it starts from visible to thermal
infrared or even radar
Now sources of energy can vary starting from radio frequencies then you have solar energy
light bulbs and so on so forth and this energy and frequency Here the wavelength when it
is shorted the frequency is going to be higher because there’s inverse relation When the
wavelength is longer the frequency is going to be lower
Now different parts of electromagnetic spectrum like here radio waves are shown in this part
then you have microwave infrared visible and here In these figures what we are trying to
show very distinctly these parts These are very transitional if we look at visible part
of EM spectrum what we will find is that the colors change is very smoothly but for our
own convenience we divide and we put SAR boundaries between one color to another or one band to
another
So one has to remember and that is why here these boundaries are not deliberately shown
very sharper They gradient changing of this transition is there So radar remote sensing
is basically covering this part of EM spectrum starting from 1 meter to few centimeters that
is microwave radar SAR synthetic aperture radar remote sensing then visible of course
many satellites including SPOT or Landsat AVHRR NOAA and so on and then of course ultraviolet
x-ray remote sensing is not possible
Now ultraviolet part which is here in EM spectrum and which covers wavelength but the problem
is the ultraviolet portion of the spectrum has the shortest wavelength and the radiation
is just beyond the violet portion of the visible length and that is what we call ultraviolet
but except some earth materials like certain rocks or minerals they emit the energy and
the fluorescence and which we can detect in ultraviolet light otherwise once the energy
has to pass through the atmosphere the ultraviolet in this part of EM spectrum the energy is
absorbed and we canott have a sensor satellite based sensor but on left conditions people
will use ultra violet lights to detect certain minerals like fluoride or others because of
having fluorescence capabilities
Visible is the most popular one as I was mentioning here though we sometimes call red band blue
band green band but if you see the colors are changing very smoothly very smooth transition
Gradually things change but we can put the SAR boundaries slice down so the light which
our eyes are also sensitive and we call it as visible spectrum can detect which we can
see different colors It is important to recognize how small the visible portion is So if you
look at the entire EM spectrum a very tiny portion of EM spectrum where our eyes are
sensitive where visible channels are present but most of the remote sensing is concentrated
mainly in this part so this is very important thing
There is lot of radiation around is which is invisible to our eyes especially in infrared
thermal infrared but can be detected by other remote sensing instruments and used to our
advantage Our eyes are not capable but their sensors onwards satellites are capable of
looking beyond visible channels or visible part of EM spectrum So visible wavelength
covers a very small portion of EM spectrum only 0.4 micrometer to 0.7 micrometer That’s
the only part which is available for us and for remote sensing But lot of remote sensing
has been done in this part
This is the longest wavelength or visible wavelength is the red which is here and the
shortest is the violet and common wavelength what we perceive as particular colors from
visible portion of the spectrum are here It is important to know that this is the only
portion of the spectrum we can associate the concept of colors basically otherwise the
white color is made from this vibgyor so there we use the word color or colors otherwise
in any other parts of EM spectrum we cannott use the word color
So violet which is having the shorter wavelength is 0.4 to 2.446 no remote sensing sensors
are there It starts with blue 0.44 and 2.5 if you remember when I was showing about the
Landsat MSS or IRS LISS 1 or LISS 3 sensors their first channel started from 0.45 2.5
something like that So then blue after blue you come to the green part of visible spectrum
0.5 to 0.78 Then yellow part of visible spectrum 0.5 to 9 which is not covered then orange
then red part is covered specially infrared part also people go there
So blue green and red are primary colors and wavelength of visible length These are the
colors which most of the devices used edited color scheme uses Our eyes are also sensitive
many sensors are there in this part but it’s a very tiny part of EM spectrum which is available
without much problems These colors are defined as no single primary color can be created
from other two but all other colors can be formed combining blue green and red in various
portions That is why they are called primary colors
Although we can see sunlight as a uniform or homogenous color it is composed of various
wavelengths of variation in primary with violet visible infrared portion of spectrum and they
as you know the visible portion of radiation can be shown in the components of colors and
sunlight is passed through a prism and then all components of white light will come as
a vibgyor
So the next portion which is again important is the infrared and there are sensors which
work in infrared part of EM spectrum so IR region which covers the wavelength range from
0.7 to 1000 micrometer or 1 meter and more than 1000 times as wide as visible portion
Visible portion was very narrow but many channels are there many sensors are there Now this
infrared region can be divided into two main categories One is reflected infrared; another
one is thermal infrared which is the emitted energy whereas the reflected is the solar
energy Then radiation is reflected this infrared region is used for remote sensing purposes
is very similar to ratio of visible portion
Whereas the emitted one is used differently and the reflected IR covers wavelength approximately
0.7 micrometer to 3 micrometer and this thermal IR region is quiet different than the visible
reflected IR portions and this energy is essentially the radiation which is emitted from the earth
surface in the form of heat Thermal sensor IR covers the wavelength from 3 micrometer
to 100 micrometer and there are channels in various satellites like Landsat TM it has
thermal sensors ETM is having thermal sensors ETM plus NOAA AVHRR sensors they are all equipped
with thermal channels having wavelength between 3 micrometer to 100 micrometer
Now we come to this microwave and here the longer wavelength we are discussing the portion
of spectrum of more recent remote sensing is microwave region from above 1 mm to 1 meter
That region is especially because of SAR interferometry it has become very useful and popular and
this covers the longest wavelength used for remote sensing Shorter wavelengths have properties
similar to thermal infrared region while the longer wavelength region approach the wavelength
used for radio broadcast so this brings to the end of this different part of EM spectrum
We started with visible of course ultraviolet their sensors cannot work because of presence
of atmosphere then infrared near infrared thermal infrared and then finally the micro
ignitions So thank you very much.