 Hello everyone, welcome to the next lecture in the course remote sensing principles and applications. Till now we have discussed in detail about image remote sensing image acquisition systems, the basic principles of data collection, what data is being collected and how to convert the data into surface reference all those concepts. Essentially with respect to data our focus is on the solar reflective portion of the electromagnetic spectrum that is wavelength less than 3 micrometers. Also the image acquisition systems that we have seen till now like the visc room scanner, the push room sensor all those concepts are essentially the ways in which images in this particular solar reflective portion and also the thermal infrared portion how data will be collected in these two portions like wavelength up to say 14 micrometers. So now we are going to continue with the topic of analyzing the data in the solar reflective portion of the electromagnetic spectrum rather than analyzing I will say that how this particular reflectance is being recorded that is we now know that satellite sensors collect radiance and we have also seen the steps to convert this radiance to surface reflectance and this is our primary interest like acquiring the surface reflectance of various features is of the primary interest to remote sensing users especially when we are dealing with wavelengths less than 3 micrometers. Starting from this lecture and maybe next few lectures we are going to see what causes certain objects to look like in a certain way that is we now know what the reflectance will be what we are going to see now is why the reflectance is like that and how commonly occurring earth surface features will behave in different wavelengths starting from visible to SWIR range of electromagnetic spectrum. So, the topic of the lecture we are going to see starting from today is analyzing or understanding the spectral reflectance curves of few commonly occurring earth surface features the visible NIR and SWIR domain. While discussing the interaction of EMR with earth surface features I introduced you the concept of spectral reflectance curve. So, a spectral reflectance curve is nothing but recording the wavelength and the spectral reflectance observed in the corresponding wavelength. So, that is example is given in this particular slide here you can see that in x axis we have wavelength in y axis we have reflectance. So, if we plot this for like various wavelengths especially within the range of say 0.42 up to say 2.5 or even 3 micrometers we call this particular plot as spectral reflectance curve. So, we have also seen that this spectral reflectance curve is mostly unique for various earth surface features and that is why sometimes we call this spectral reflectance curve also as spectral signature. So, here you can see for different features like this is for grass which is a form of vegetation and this is for water body this tiny dotted line along the bottom is for water body and then for sand it looks something like this for concrete it looks something like this. So, essentially what we can observe is different features behave differently when EMR is irradiated over it or when EMR interacts with different features it will behave differently and hence we get like a unique spectral reflectance pattern or unique spectral reflectance curve for different features. In the series of lectures we are going to analyze why such spectral reflectance curves are observed we are going to go a little bit in detail and reason out why the spectral reflectance curve has a particular pattern for a particular feature. So, we are going to discuss about the spectral reflectance curves of vegetation, soil, water and snow. So, we are going to describe in detail about the spectral reflectance curve of these four features which is or which are mostly abundant on the earth surface and also we are going to discuss about what factors influence the spectral reflectance curve of these four class of features. In addition to this at the end of this particular lecture or this particular topic we are also going to get introduced to the concept of spectral indices. So, this is what we are going to cover in the series of lectures starting from today maybe for next 3 to 4 lectures. So, first we are going to start with the spectral reflectance curves of vegetation. So, one of the earliest and primary application of remote sensing was vegetation monitoring and vegetation has been studied in detail about how it will look in the solar refractive portion of EMR what factors will affect it. And vegetation as a whole if you look at like a global scale vegetation as a whole can influence earth surface or earth system to a larger extent. Vegetation plays a major role in controlling the earth system itself like it can change the water cycle, it can change the carbon cycle only from vegetation we derive our food that is essentially the crops. So, vegetation covers like the entire spectrum or the entire gamut of starting from trees, plants, crops, grasses and so on. Even though vegetation has like many different complex forms starting from like tiny algae to like huge trees it can take variety of shapes and everything. The spectral reference curve of vegetation has like a one particular unique pattern. So, this is like general of healthy green vegetation or in particular we are going to discuss about the spectral reference curve of healthy green leaf. We will start with understanding the spectral reference of one leaf and then slowly we will build our understanding of how it will look when we move up in the sky that is when we move away from single leaf to looking at one plant or one full canopy and so on. So, at first we will start by looking at the spectral reference curve of one single healthy leaf that is given in this particular slide. So, here in the x-axis we have wavelengths ranging from 0.4 to roughly 2.5 micrometers and reflectance is plotted along the y-axis. So, this is from like observed data in laboratory measurements I have taken it from a spectral library called equestress library. So, you can like search it in internet and you can like easily access the spectral library. So, this spectral library contains like for various features on earth surface for hundreds of features on earth surface how the reflectance will vary with different wavelengths. You will get it in the form of a table you can just plot them and analyzing detail have different features look. So, this spectral library I will use to like a good extent in the series of lectures you can easily refer it in the internet. So, coming back to the spectral reference curve of vegetation. The spectral reference curve of vegetation is typically called to have a peak and valley configuration. If you look at this it has lot of small small peaks like here like here all these portions here and here and so on and also lot of valleys like here, here, here and so on. So, that is why this particular reflectance pattern got a name of peak and valley configuration. We can divide this particular curve into 3 portions according to or 3 divisions according to the wavelength range that is we will first analyze what happens in visible band then we will analyze what happens in the NAR band and finally we will analyze what happens in the SWIR band roughly 0.4 to 0.7 micrometers 0.7 to 1.4 micrometers and from 1.4 to say up to say 2.5 or 3 micrometers 3 range we will divide this curve into 3 different portions and then we will analyze in detail. Before doing that it will be beneficial for us to understand some information about the structure of a leaf. So, if you look at like a structure of a leaf a single leaf this is like the electron microscope image of one single leaf. It shows that it has different different portions that is a leaf has like one top portion what we call as epidermis. Similar to our skin has like a top layer even the plant leaf has a top layer we call it as epidermis and some varieties of leaves have a vaccine coating on top of it called cuticle. So, this is like a vaccine coating present on top of the leaf which prevents the leaf from over draining of water. So, this will be like a thick coating over like desert plants and shrubs it may be like a thin coating over like plants and trees present in heavy rainfall region. If you look here, here you can observe like two different types of cell one like elongated lengthy cells on top half of the leaf. So, this is like the top portion of the leaf and this is below this particular red line this bottom portion of the leaf. Here you can see the cells present are actually looking different. So, here it is like having like long elongated shape here it is having like irregular shape. The cells that are present on the top portion we call it as the palisade parenchyma cells. The cells present on the bottom portion we call it as the spongy parenchyma mesofil cell this like this particular portion we call it as mesofil. So, we call it as spongy parenchyma on the top we call it as palisade parenchyma. In the top portion we have one of the most important pigment in the vegetation that is the chlorophyll and we all know that chlorophyll is one of the most essential pigment or chemical substance to be present in the leaf that enables the leaves to do photosynthesis which produces food on its own that gives leaves its characteristic green color and so on. So, the chlorophyll the most important pigment in the leaf is present in the palisade parenchyma cells that is the elongated cells mostly on the top portion of the leaf. If you look at like the bottom portion the spongy parenchyma portion we will have we can observe lot of air gaps or air space is present in between the cells. So, this particular air space when water is present it will be filled with water that is when plants take water from the root system the water will come and occupy this particular air space and when plants dry out this air space will be removed of water and it will again became like gaps filled with air. So, this is like a very basic structure or basic introduction to the structure of leaf. Now we go back to the spectral reflectance curve and possibly analyze or understand what controls the spectral reflectance curve for different, different portions. So, here this one more spectral reflectance curve of healthy green vegetation. So, within the visible range that is in the wavelength of 0.4 to 0.7 micrometers the primary factor that influence the spectral reflectance curve is the leaf pigments or absorption due to leaf pigments. So, pigments are nothing but like chemical substances which is present in the leaf chlorophyll is one of the most commonly occurring pigment that is present in the leaf. Similarly, there can be like other type of pigments like kerotene and so on. So, within the visible range it is the presence or absence of leaf pigments which influences the spectral reflectance curve to a large extent. Once we enter into the NIR portion that is starting from 0.7 to 1.4 micrometers the leaf internal structure or the scattering happening within the leaf controls the spectral reflectance portion. So, in the visible it is the pigments in the NIR range it is the leaves internal structure and in the SWIR range that is after 1.4 micrometers it is the leaf water content that controls the spectral reflectance curve that is wavelength greater than 1.4 micrometers this is roughly 0.7 to 1.4 micrometers and this is 0.4 to 0.7 micrometers. So, these three factors one is the pigments then the leaves internal structure and then the leaves water content influences the spectral reflectance curve of healthy green vegetation. Also in this particular slide and the atmospheric absorption is superimposed on the spectral reflectance curve like atmospheric it is not just like atmospheric transmissivity on this particular part of the y-axis on the secondary axis. We can see that there are for most of the portions like up to like NIR the atmosphere is kind of fairly transparent with almost like 50 percent transmissivity and almost like in most of the bands if the transmissivity is up to 80 percent but there are like really strong absorption bands of atmosphere around this 1.4, around this 1.9 and around this 2.7 micrometers. So, when we study the spectral reflectance curve of vegetation in ground in laboratory measurement we will get the entire curve but when we do it in the atmosphere like when we do it from satellites because of influence of atmosphere we may not be able to do remote sensing of the entire solar reflected portion we may have to leave certain bands so that the atmospheric influence is reduced or essentially we have to do remote sensing of vegetation in the atmospheric windows avoiding the bands in which atmosphere is highly absorbed being. Okay, we will move ahead. Now we have got a very broad general interaction about the leaves internal structure and also what factors controls the reflectance or spectral reflectance curve of vegetation in different portion of EMR especially in the solar reflected domain. So, now we will go little bit in detail and study each portion individually. First we will start with the visible portion of the spectrum. As I have already told you the most important factor that controls the vegetation reflectance is the pigment absorption in the visible portion of EMR and pigments essentially mean chemical substances present within the cell or present within the leaf and one of the most important pigment or the most abundant pigment is chlorophyll. So, this particular curve on the left hand side gives the absorption curve of chlorophyll. So, this is not the reflectance curve this is the absorption curve how chlorophyll absorbs in different wavelengths starting from ultraviolet and all the way up to red band. If we closely observe this the absorption is pretty high in blue wavelength roughly around like 0.4 to 0.45 micrometers and similarly the absorption is fairly high in the red wavelength that is from 0.6 to 0.7 micrometers and absorption is very low in the green band of the spectrum 0.5 to 0.6. So, essentially a leaf containing lot of chlorophyll pigments will be absorbing blue wavelength and red wavelength to a significant extent and will be reflecting green wavelength again to a large extent. So, that is chlorophyll pigments absorb blue and green wavelength and reflect sorry this should be absorbed blue and red wavelength and reflect green wavelength. So, this is especially for chlorophyll. So, two kinds of chlorophyll pigments are there chlorophyll A and chlorophyll B both of them has like a typical characteristics. But if you look at this the chlorophyll B pigment has like a really strong absorption in the blue band in comparison to chlorophyll A. On the other hand chlorophyll A has a really strong absorption in the red band in comparison with chlorophyll B pigment. So, based on the presence of different types of chlorophyll the reflectance can vary but essentially the overall feature that is absorption in blue and red wavelength and reflection in green wavelength will be remaining. Apart from chlorophyll there can be other pigments that can be present in the leaf say beta keratin, pychorethrin, pycosanin and so on. So, as the pigment changes or as the leaf changes through different stages of its growth cycle and the pigments contained within the leaf will change and hence the leaves characteristic colour also will change. So, like essentially we have seen for a healthy green leaf chlorophyll will be the most abundant pigment present within the leaf if the leaf is extremely healthy. So, the absorption features or the reflection feature of chlorophyll will be dominating like it will be absorbing lot of red and blue wavelength and it will be reflecting green wavelength that is what gives healthy leaves its characteristic green colour chlorophyll is abundant in healthy green leaf. But if the leaf progresses through different stages of its growth cycle we call it as senescence or phenological cycle if the leaf matures what will happen is different pigments will come in chlorophyll will drop other pigments may start to increase in the quantity when that happens the leaves colour may change depending on the pigments that is becoming abundant now. So, here if you look at like let us say like beta keratin is being present in the leaf. So, you can see that beta keratin absorbs a lot in the blue wavelength again, but its absorption becomes extremely low in green and red portion combined together. So, when beta keratin becomes like the dominant pigment within the leaf it will absorb blue and it will reflect green and red portions and it may have like a leaf may have like a yellowish appearance. Similarly, if pychoerythrin is like the primary pigment present in the leaf it absorbs primarily in the green band with very low absorption in blue and red bands. So, again it may have it may produce a completely different colour to the leaf. So, the absorption by different different pigments is what drives the spectral reflectance curve of vegetation in the visible part of the spectrum. And also this particular absorption of certain characteristic wavelengths gives the leaves its characteristic colour like absorption of blue and red leaving green out will give the leaf the green colour this is done by chlorophyll. Absorption of blue only allowing green and red to pass may give the leaf an yellowish colour that is because of like the beta keratin and so on. So, the presence or absence of certain pigments will drive or will dominate the spectral reflectance property of vegetation. So, as the vegetation goes through different cycles of its growth phase or if the vegetation undergoes certain amount of stress the pigments present within the leaf will change it may some of the pigments instead of chlorophyll may come in and hence the colour of the leaf will start to vary. We will observe it visibly with our eyes as our eyes are tuned to observing visible portion of EMR. So, this particular figure is gives another example of how leaf with different different colours look or how the spectral trans curve of leaves in different different colours look. See for a healthy green leaf you can see reflectance is like pretty high in the green portion that is around like 0.55 micrometers and it is like low in the blue and also it is low in the red portion you can observe it from here yes up to here up to 0.7. But if you look at like other leaves that is they say take this reddish purple leaf you can see that the reflectance is pretty low in almost all portion of EMR giving like a combined mix of like a dark colour that is why the leaf may appear like reddish purple or for a red leaf the reflectance in red portion is very high with very low reflectance in green portion. So, this is because of presence of different different pigments present within the leaf essentially that will dominate or that will control the reflectance portion of reflectance pattern of leaf especially in the visible range. Next we will move on to discuss about the spectral reflectance property of leaf in the NIR portion of electromagnetic spectrum. So, if you look at the basic spectral reflectance curve maybe I will go back to that particular slide. So, if you look at like this particular curve on the slide we can observe that the reflectance is quite low in the visible part that is around like less than say 10 to 12 percent maximum itself is like say 12 percent in the visible portion. If you transfer to NIR portion after this 0.7 micrometers you can see the reflectance increases drastically it reaches 50 percent and again it slowly begins to decrease as we increase in the wavelength. But if you look at this particular curve in general the reflectance starting from NIR all the way up to like SWIR portion is relatively higher when compared to the reflectance in the visible band and it is the highest in the NIR portion the reflectance is highest in the NIR portion. The reason for this is plants typically need light for its function we all know that it has to do photosynthesis it needs its internal energy which it derives from sunlight and sunlight is a mixture of or mixture of various wavelengths we also know that with the primary wavelength or the wavelength in which maximum energy coming is green portion typically. So, what happens is leaves derives most of its energy requirement from the visible portion of EMR that is between 0.4 to 0.7 micrometers leaves do lot of absorption and uses that particular energy for its functioning and photosynthesis that is why leaves reflect very little amount in the visible portion the reflectance is pretty low like less than like 15 percent in almost the entire visible range and this is still lower in the blue and red band in comparison to green. But leaves will not absorb EMR with the same efficiency in the NIR portion of the spectrum because a significant amount of solar energy will come in the NIR portion of the spectrum also and if leaves absorbs all these things with the same efficiency with which it absorbs visible wavelength then it will overheat the leaves and leaves may die causing an irreparable damage. So, that is why in order to prevent the leaf from dying or from preventing the leaf from irreparable damage leaves generally has a tendency of high reflectance and high transmittance in the NIR portion of electromagnetic spectrum. So, essentially in the NIR portion it is the leaves internal structure that controls the reflectance that is leaves do not want to store NIR or leaves do not want to absorb because it is not going to use it most of the users for leaves or most of the energy requirement for leaves comes in the visible portion of the spectrum itself that is why it absorbs a lot of visible portion of EMR reflecting very little but it does not want to store and use NIR with a full efficiency. So, what the leaves structure will do is like when I showed you the leaves internal structure I told you that the top portion contains this chlorophyll cells like the palisade parenchyma cells which contains chlorophyll the lower half of the portion contains air gaps plus spongy parenchyma cells. So, when NIR enters through a leaf NIR energy enters through a leaf first thing is leaf has a high transmissivity like leaves allow NIR to pass through it it is not going to absorb it when it passes through it will cross the top half of the leaf or the top portion of the leaf then it will enter the portion the lower portion where it also encounters air gaps present air gaps or water it will be present. So, there will be like a change in the medium like we will just go back to the initial part of the classes where I discussed about how EMR will interact with different features like the initial lectures when we discuss about properties of EMR. I told you that when EMR travels from one medium to another medium at the surface that divides medium one and medium two it will undergo reflection that we have already discussed. So, some part may be transmitted inside some part may be absorbed and some part may be reflected and this is what we gave we studied as like reflectance plus transmittance plus absorbance is equal to 1 this is we have already studied. But what we have to remember is when there is like a change in medium a portion of EMR is reflected back and some portion is transmitted inside the medium. If you take leaf absorption is quite low it has lot of reflection and transmittance. So, when let us say this is like a single leaf EMR is going to instant on it. So, what will happen maybe we will go back to the slide where we discussed the internal structure that will be much easier to explain. So, let us say if EMR is coming into it is especially in the NIR portion what will happen is the leaf will allow or leaf will transmit a large portion of EMR and whenever this NIR energy encounters a discontinuity or a change in medium say from a plant cell to like air gap or if it is filled with water whatever it may be it will undergo multiple reflection say from here it may be reflected here it can again be reflected here and it can undergo like this a lot of multiple reflections. So, the leaves internal structure how the cells are oriented within the leaf how the air gaps are oriented within the leaf will cause the energy in the NIR portion to undergo multiple reflection within the leaf and also combining this with high transmittance the leaf after undergoing multiple reflectance will be either reflected totally in the upward direction like some part of it may be reflected again in the top part or some may be reflected or transmitted down to the bottom part. So, essentially what I want to convey is leaf has high transmittance and high reflectance especially in the NIR portion. So, when energy enters into the leaf what will happen is it will transmit through and when it encounters a change in medium that is from leaf cell to air gap or whatever when it encounters a different medium it will undergo multiple it will undergo reflection and because of the random orientation of leaf pigments and leaf air spaces NIR will undergo multiple reflections and finally some fraction of NIR will come out of the top part of the leaf itself giving like a total reflectance some portion of energy will be allowed to transmit completely through the leaf and will be coming out of the bottom part of the leaf. So, essentially what you have to remember is a large portion of NIR is either reflected totally like if you look at a leaf as a whole leaving the internal part away a large portion of EMR will be reflected or transmitted from the leaf this is what you have to remember. So, in this particular lecture we have started discussing about the spectral reflectance property of vegetation especially of a leaf we have covered the reflectance property in the visible portion and we have started discussing about the NIR portion and we will continue this and further topics in the next lecture. Thank you very much.