Electromagnetic Spectrum, solar reflection and thermal emission


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.

Leave a Reply

Your email address will not be published. Required fields are marked *