 Hello everyone, welcome to today's lecture in the course remote sensing principles and applications. In this lecture we will be talking about the basic class in remote sensing which helps us to define the various energy quantities. Just as a quick recap, in the last class we have discussed about what electromagnetic radiation is, how to characterize like the wavelength frequency of these electromagnetic radiation, then what electromagnetic spectrum is, then what are the classification of electromagnetic spectrum. All these topics we covered in the last class. This class we will be continuing ahead with how electromagnetic radiation is being generated in nature and how we are going to use it for remote sensing purposes. So, how electromagnetic radiation is produced in nature? In nature EMR is produced by various processes and mechanisms. A safe example, when we switch on a bulb inside our living room, we are converting electrical energy into light energy. So, light energy is nothing but electromagnetic radiation. We go to take X-rays. So, there the X-ray technician will switch on the X-ray mission which will generate X-ray radiation and it will be irradiated on our body for getting an X-ray image. So, X-ray is nothing but electromagnetic radiation. So, these are simple processes that we observe in our everyday life which produces electromagnetic radiation. In addition to these all the natural objects surrounding us including our own body emits electromagnetic radiation continuously because of the internal temperature we have or the internal energy we possess. So, electromagnetic radiation is naturally being produced by all objects and this is what we are going to use for our remote sensing needs. Safe example, sun by virtue of its energy content and temperature emits EMR which reaches the earth surface and drives various processes on the earth surface. Similarly, earth also has its own internal energy and temperature which helps the earth surface to emit electromagnetic radiation. So, are there any means to quantify the amount of radiation emitted by an object? Yes, there are few basic class which helps us to quantify the radiation emitted by any object at a given temperature T. Those three basic laws are the Planck's law, the Stephen Boltzmann law and the Wien's displacement law or Wien's law in a simpler sense. What are these law? First we will start by looking at the Planck's law. So, the Planck's law tells us if an object is at a given temperature T then the particular object will emit certain amount of radiation in a particular wavelength that is Planck's law will tell us the spectral variation of the radiation emitted by the object. That is in the last class I told you that most of the natural objects when it emits electromagnetic radiation it would not emit in a single particular wavelength, it will emit energy with a mix of wavelengths. So, the amount of energy emitted by an object will vary with respect to wavelength and Planck's law will help us to estimate what is the amount of energy emitted by an object at a given wavelength. Then if we want to know the total amount of radiation emitted by the object then Stephen Boltzmann law will help us. Stephen Boltzmann law is nothing but the integration of Planck's law over the entire range of electromagnetic spectrum that is Planck's law will give us the spectral variation of emitted energy. If we integrate that particular equation over the entire range of wavelengths known to us that is between 0 to infinity we will get the total radiant flux density coming out from the object and that is Stephen Boltzmann law. So, that is explained here. So, if we integrate Planck's law between the limits of 0 to infinity we get Stephen Boltzmann law and Stephen Boltzmann law suggests that the total radiant flux density of an object is proportional to the fourth power of temperature that is as the object's temperature increases the amount of energy emitted by an object will also increase correspondingly. So, this first we will see with few example plots. So, in this particular slide we have two graphs each telling us what is the amount of electromagnetic radiation emitted by different objects at various temperatures. If we look at this particular figure here we have objects temperature ranging from 1000 Kelvin to 2000 Kelvin. So, as you can see as the temperature increases the what to say the amount of energy emitted by an object increases correspondingly as given by Stephen Boltzmann law and also these curves do not intersect that is as the object temperature increases at any given wavelength the amount of energy emitted by an object will be increasing. So, these curves normally won't intersect they will be like lying one above the other as the object temperature increases at all given wavelengths the energy emitted will increase that is one thing. And one more thing you can notice each and every point on this particular figure or this particular plot is estimated using Planck's law that is Planck's law will tell us if an object is at say 2000 Kelvin at given wavelength say 1 micrometer what will be the energy emitted at 2 micrometer what will be the energy emitted. So, each point on this curve can be estimated using Planck's law and then Stephen Boltzmann law we can calculate what is the total amount of energy emitted that is the total area contained within this curve. So, the total area will be given by Stephen Boltzmann law then again if you observe these figures we can know that for each curve there is one particular wavelength at which the peak emission occurs. So, here what I am marking in red circles is the particular wavelength at which peak emission occurs for each and every object at various temperatures. So, this characteristic wavelength at which peak emission occurs is given by Wien's displacement law or Wien's law. So, Wien's displacement law will help us to find out the wavelength at which maximum emission takes place and one more important observation we have to do is as the objects temperature increases this particular wavelength at which the peak emission occurs moves continuously towards the shorter side or shorter wavelengths we can see. So, as the object temperature increases the wavelength at which the peak emission occurs will be moving continuously towards shorter wavelengths. So, the few important observations and just summarizing it again for object at 2 different temperatures if we plot these Planck's equation as like graphs then the those 2 curves will not intersect with each other that is for any given wavelength the total the amount of energy emitted by an object at a higher temperature will always be higher than the energy emitted by object at lower temperature these curves will not intersect then the total area contained within this curve will increase when the objects temperature increase then there is a particular characteristic wavelength at which peak emission occurs as the object temperature increases this characteristic wavelength will shift towards shorter side or shorter side of wavelength. So, this is like the major observations we have to do with respect to these laws and here in this slide I have given the basic equations for the 3 laws. So, the first one is the equation for Planck's law second one is the equation for Stephen Boltzmann law and third one is the Wien's displacement law. So, lambda m is the maximum moment of sorry the wavelength corresponding to the peak radiation coming out from an object is given by a by t that is all. From the 3 laws we are able to find out what is the energy emitted by an object that is spectral variation of energy what is the total amount of energy and what is the maximum amount what is the wavelength at which the maximum radiation is emitted. Is there a way to find out the energy emitted corresponding to that particular peak wavelength please pass the video for a few seconds and think over it that is how to find the maximum amount of radiation estimated sorry emitted by an object corresponding to the peak wavelength of emission please pass the video for few seconds and think over it. The solution to this particular question is given in this slide that is you have to substitute Wien's displacement law in Planck's law that is Wien's displacement law will give us the wavelength at which peak emission occurs. So, if we can substitute this line into Planck's law and solve the equation we will get the energy emitted by an object at the wavelength of peak emission that is lambda max at lambda max what is the amount of energy emitted by an object. So, this equation also will take form very similar to Stephen Boltzmann law here that is the m corresponding to lambda max for an object at a given temperature t is is equal to b t power 5 where b is another constant the units are given here and t to be substituted in Kelvin. Before we proceed further I just wanted to ensure that few things are kept in mind regarding these laws. First thing is the units with which we get the output from this law and in which the units we should provide inputs that is the major thing regarding units and then the value of constants we should be really careful about the units and constants when we work with these 3 laws in order to get accurate results. First thing to notice the Planck's law will give the spectral gradient existence with a unit of watt per meter square per meter of wavelength. Like when we just look at this unit it may look little odd to us that is it reads watt per meter square per meter inverse. So, cannot we cancel them together no it is not it is the amount of energy or the power emitted by an object that is watt is the unit for power per meter square area of object per meter wavelength. So, this meter corresponds to wavelength this meter corresponds to the area of the object per unit area of the object and W is the unit for power we know is watt. So, this is the energy emitted sorry the power emitted by unit area of the object per unit wavelength that is given by Planck's law. Stephen Boltzmann law we are integrating everything with respect to wavelength from 0 to infinity. So, this third meter square will go off. So, we will get the units in watt per meter square the total power emitted by an object per unit meter square area. So, these are the units of outputs from Planck's law and Stephen Boltzmann law. And the one more thing we have to note carefully is all the variables that we substitute in these two equations must be in their SI units that is wavelength should be substituted in units of meters, temperature should be substituted in the unit of Kelvin and so on. So, everything in this particular equation has to be done in their SI units only. So, normally in the last lecture when I introduce you to electromagnetic radiation I said for remote sensing purposes we would not use a unit of meter to denote wavelength we will be using nanometers, micrometers or even centimeters for microwave and so on. But when we substitute those values in these equations we must substitute in units of meters for a stick at accurate results. Then if we want to calculate the spectral gradient existence in linear units other than meters we have to divide the answer by corresponding conversion factor that is by normal convention we will be interested to calculate what is the power emitted by an object per unit micrometer wavelength or per unit centimeter of wavelength rather than a meter of wavelength. If we want to calculate that we have to divide the output from Planck's law by the corresponding units that is if we want to convert Planck's law into watt per meter square per micrometer of wavelength we have to divide the output from Planck's law by 10 power 6 because 1 micrometer is equal to 10 power minus 6 meters so this conversion factor we have to use. So, these are some important things we have to remember about using T's laws. Then some of the important constant values that we often use in this particular course are given here and I have like again repeatedly said everything with units and T absolute temperature given everything mentioned. Just remember one thing only in Wien's displacement law this particular constant has been calculated with units of micrometers. So, the output from Wien's displacement law will directly be in micrometers just by mere definition of this constant with unit of micrometer the output from Wien's displacement law also will be in micrometer but anyway we can convert them between various units based on our needs. We know the conversion factor how to convert between micrometer, nanometer, centimeter, meter all these conversion factors well known to us so we can convert. So, now what we are going to see is from this basic class we now came to know what is the amount of radiation emitted by an object. The next topic to be understood is what are the source of EMR available for us to do remote sensing. So, remote sensing can basically be carried out either in active manner or passive manner. What is active manner? Active manner means like I have a sensor which observes radiation but the sensor itself has the capacity to produce EMR and it will send the TMR towards the object of interest and will get the signal back. So, the sensor itself will produce EMR send it towards the object receive it back and store it. So, such sort of sensors which can produce EMR as well as receive EMR we call them as active sensors. A very simple example we use in our everyday life is a camera with a flash attached to it. So, if we switch on the flash and take a picture what happens the camera itself produce white light which travels towards the object falls on the object and gets reflected back and gets stored in the camera itself. So, camera is a very good example for active sensor. Similarly, in remote sensing also we can produce the intended electromagnetic radiation with a given wavelength or a frequency and use it for remote sensing purposes. On the other hand a sensor can merely observe the radiation coming in from the earth's surface either the radiation can be emitted by coming in from the sun or directly emitted by earth such sensors which merely observes the radiation coming in from an object or known as passive sensors. So, passive sensors a good example from our everyday life is a camera with a flash switched off. So, without flash means the camera has to rely on the energy from the surroundings either the lights we use or sun's radiation. So, remote sensing can be carried out both in active way and passive way. In active way the sensor itself will have the capacity to produce EMR send it towards the target and receive it back whereas in passive mode the sensor has to rely on external sources of energy. So, what are these external sources of energy for passive mode of remote sensing? So, in passive mode of remote sensing the external source of energy is primarily the radiation coming in from the sun. So, when we do remote sensing in the visible NIR, SWER and to some extent in MWIR domain that is visible near infrared, short wave infrared and mid wave infrared. In these portions the energy that we receive or we sense using passive sensor or essentially coming in from the sun, sun provides us the energy required to do remote sensing. On the other hand if we perform remote sensing of earth's surface in the long wave infrared portion or if we do passive remote sensing in the microwave portion then such energy levels are naturally emitted by earth's surface features itself are not coming in from the sun. So, sun and earth are essentially the primary source of electromagnetic radiation for passive mode of remote sensing and in the shorter wavelengths that is visible NIR and SWR wavelengths the primary energy source is sun. In longer wavelengths such as microwaves or thermal infrared that is long way infrared the primary source of energy is earth itself. In active mode we do active remote sensing invisible as well as microwave domains that is invisible we produce laser lights and use undo laser remote sensing. Similarly, we do microwave remote sensing using active microwave sensors. So, these are also possible fine. So, what do we study in passive remote sensing basically? First we will talk about passive remote sensing then we move to active. In passive especially in shorter wavelengths of visible NIR and SWR wavelengths what we observe is the amount of solar radiation reflected by the object of our interest that is see this is like the object of our interest some land surface here there is sun we have our sensor here. So, solar radiation will be coming in this will interact with the object of interest and the portion of it will be reflected back. So, the sensor will essentially see the reflected solar radiation. So, this is true primarily in the visible NIR and SWR bands. On the other hand in TIR or to be more precise in the LWR bands and microwave bands also sun's incoming radiation is almost 0. So, whatever is emitted whatever is observed by remote sensing sensor is primarily emitted by earth surface. So, in shorter wavelengths we are observing the reflection nature of an object how much the object is reflecting in longer wavelengths in LWR or microwave domains we are naturally observing what is the emitted radiation from earth surface features itself. So, this is the source we use for our natural remote sensing purposes. So, this is just to drive home the point the amount of this is like blackbody radiation curve for sun and earth the blue line is for sun and the brown line is for earth we can see like sun is a very good source of energy especially in shorter wavelengths that is why like in SWR bands in wavelength less than 3 micrometers basically whatever we sense in passive remote sensing is primarily driven by sun's energy whereas in longer wavelengths earth emission also takes place and whatever we observe is primarily from earth. So, this is for like an ideal blackbody whatever curve is given here in later classes we will see what blackbody is and how much solar radiation will deviate from this curves all these things we will see later but just to give you information that in shorter wavelengths sun's incoming radiation is much higher. So, we are primarily sensing reflected solar radiation whereas in longer wavelengths we are primarily sensing earth emitted radiation. So, this is again gives you the emitted energy by sun and earth at longer wavelengths that is here the x axis is millimeters it goes all the way up to 1000 millimeters. So, this is like just the Planck's law plotted for different variation for temperature corresponding to sun and earth like roughly we assume sun to be at a temperature of 5800 Kelvin and earth we assume to be at a temperature of 300 Kelvin under a plot of these curves just to give you like some information about what is the amount of radiation coming in from this objects. So, just to summarize what we have learnt in today's lecture we have learnt the basic laws which helps us to calculate the amount of radiation emitted by an object and how to use them for calculating the amount of radiation emitted at a given wavelength the total amount of radiation and also to calculate the wavelength at which peak emission occurs. Then we also saw the primary source of energy we use for our remote sensing purposes. So, with this we stop this lecture. Thank you.