 Welcome back. In the last class, we just reviewed the concept of filters, illumination filters and interference filters and we also looked at the definition of optical path length and so on. And before we started the optical microscope topic and we also extensively looked at the different kinds of lens defects. So, I just started introducing you the equipment, the light optical microscope. I would like to continue to do that. So, let us look at what I just showed in the last class. The first one I introduce is the metallurgical microscope which is an inverted type and I just I just want you to have a feel of how this equipment look like. Then we will slowly get into the details of the parts details as well as the operation details as we go along. So, I think we have just seen that let me speed up this. I also told that this is an attached with an CCD camera also with some polarizers and analyzers and so on. These attachments we will take it up as and when we deal with the variation of the microscope. So, what you are now seeing is an illumination stage where the sample is kept. And this is a simple vertical type metallurgical microscope which I introduced in this the last class. So, this is just to have a look at it again. This is a specimen stage these are all different objectives and this is your ocular and the another one is which is attached with the image analysis system that also I just introduced just to recap. You can see that objective lens again specimen stage. So, these objectives will have 3 or 4 depending upon the microscope ranging from 5x to 50x to 100x. And you can see that all the objectives are marked with their specification of their magnification refractive index details. And depending upon our interest we can choose any one of this objectives to view the microstructure. This is again a leveling press to keep the specimen flat on the microscope and this is what we have seen already. A metallic polished specimen is being pressed with the plasticizer to make it evenly placed so that your reflection can be from the flat surface. Now this is how the specimen is placed under the objective and then you choose the lowest one to examine the microstructure details through the eyepiece. You can directly view through the eyepiece as well as you can look at the monitor computer monitor because it is being interfaced with the CCD camera. So, either you can look through the ocular directly or you can look at the computer monitor because it is attached with the CCD camera. Now this microscope is the this is where I just left in the last class. So this is optical transmission microscope which has got 2 illuminating system one is halogen lamp this one and this is mercury vapor lamp. So it is being shown closely for your clarity halogen lamp mercury vapor lamp these 2 are just used for a specific application I will just mention whenever it is applicable. So have a look at it how this the structure the architecture is very different from the simple vertical microscope what you are now seeing is a polarizer which is being engaged and then in a normal bright field illumination you disengage this so that this is kept for the light to pass through and what you are now seeing is below the a set of a condenser and filters and apertures belong to different modes of operation you can see that it is being numbered 1, 2 and 3 and 4 and 5 and so on. So depending upon the operation mode you will turn this condenser and aperture to a particular slot which is given as a white dot here so that you will be able to perform that particular operation. So in this condenser set of condenser apertures the 1 and 2 is for a phase contrast mode and 3 is for DIC mode that is differential interference contrast mode I will we will see in much more detail when we deal with that particular variant of the microscopic technique and 4 and 5 apertures are meant for bright field illumination. So you have this condenser set of apertures here and then appropriate apertures are being chosen depending upon the mode of operation. So this is for your clarity much more closer view of this condenser apertures being rotated 1, 2 for phase contrast mode 3 DIC mode I will let you know the details of what kind of apertures and condenser details as and when I just talk about the theory of this mode of operation I just do not introduce the kind of hardware you should know you should not think that every microscope variant will have a different microscope as a whole it is just that aperture and then condenser setup and filters which makes the variance of the microscopy. So now again this is a polarizer being engaged and disengaged and this is the specimen stage remember this is a transmission optical microscope. So you have the transparent window where you can keep your very thin transparent sample on this glass slide and then you can start viewing it. So this is the specimen stage which can be adjusted in X and Y moment for with this knob you can clearly see that X and Y moment. So you should have an idea this is done just to give you a feel of as if you are actually in the laboratory to operate this microscope. I will also do give you the actual experiment so that you will have a complete understanding of operation of this equipment. So again this is the whole specimen stage is being adjusted you can see that this is an another axis movement now you see that your all your objectives are kept in the vertical position compared to the normal vertical type metallurgical microscope it is put in inverted positions but you can see that you have about 6 slots one these 2 slots are kept empty but other 4 slots are filled with different kinds of objective aperture you can also see that the details of the objectives are written in these letters where the magnification details that is 4x and then this is some kind of refractive index details and then typically this has got 4x and then 10x and 20x and this is 40x or 50x. So depending upon the kind of microscopes you can have whether it is 4 objectives or 5 and it is up to 6 so this has got about possibility of keeping 6 objectives but you have right now we have it is only 4 objectives. So if you have up to 50 and 100x eventually you will have the magnification up to 1000x because your IPs will have 10x magnification what is that now you are seeing is this is a part called analyzer the operator is just inserting the analyzer this analyzer is being used when we operate that microscope in a differential interference contrast mode DIC mode and you can do this a constructive and destructive interference preference using this knob turning this knob you can make the preference of constructive and destructive interference we will see in appropriate time how it is useful to use this parts. So now we will see another set of condenser aperture just below this shutter this shutter is meant for operating this the bottom illumination which is mercury lamp in this microscope it is being used only for fluorescence mode when you want to operate perform a fluorescence microscopy then this is kept on this is an on position mercury will have will be on and when you put it back then it is in a halogen lamp mode. So here again you have another set of condenser apertures similar to what we have seen in the top of or above these objective apertures 1 and 2 for phase contrast and 3 for DIC and 4 and 5 for bright field illumination so you should keep both the condenser and aperture filters above and below the objective on a same position to perform a particular mode of operation for example if it is if you are going to operate a bright field illumination either you choose a condenser window here 4 or 5 similarly you have to rotate this on the top of or above the objective condenser lens also 4 or 5 then only your mode of operation will be correct it is just for your clarity this rotation is being shown once again. So the next detail we would like to see is the focusing knob this is the bigger one is course focus and the smaller one is the fine focus it will basically move the stage towards the objective or either way you will start focusing the specimen and this is a coarser this is fine the smaller one is fine and the next detail is the the eyepiece illumination I mean magnification either you have 1x if you pull this knob out then it is 1x and if you push it inside sorry if you pull it it is 1.66 if you push it inside it is 1x for eyepiece so this facility is possible and this is your brightness control this is more intensity to the lower intensity so this is an ocular piece which has got about which has got some variable I mean 10x to 15x. So now you have some idea about a parts of optical transmission microscope and as well as the simple vertical or inverted microscopes what are the kind of parts you have a illumination system you have a specimen stage you have objective you have an eyepiece and so on and if it is integrated with the computer or the CCD camera then you will have those also will be part of the optical microscope. So now I would like to show a bright field illumination in this optical microscope in actual experiments before I do that I would like to do some board work in in a bright field illumination the primary thing is the reflectivity reflectivity is the key factor in a bright field illumination. So for the when you say reflectivity what happens when the light falls on the specimen then the light which is being reflected which are getting entering into the objective then that place will appear bright so the more the reflectivity from the object the more the image quality and so on. So for that the objective also should be completely illuminated so for that we will see some of the some of the basics of a condensers and illuminators especially for an opaque specimens like a metallurgical system or material system bright field illumination reflectivity is a key factor. So this is a key point and before we will just exploit this concept we will look at the condensers and illuminators we will talk about illuminators. So you see up to 25 times of magnifications you do not require a condenser to illuminate the objective but beyond that otherwise the objective which is having higher magnification you need a condenser. So let us write no condenser is necessary up to the magnification of 25x this is another point so the overall resolving work is depending on numerical apertures of both objective and illuminating systems. So we know that already just to give a connect to the concept of reflection illumination of objective is also equally important concept. So for that we are looking at the some of the basics and so we can write if the numerical aperture of the objective exceeds that of illuminating system the resolving power we can write is given by r equals 0.5 times lambda divided by objective numerical aperture plus condenser numerical aperture. So this clearly tells that thus if your microscope is to give its optimum performance the numerical aperture of the condenser must equal that of objective. So let me read it again we are talking about illuminating the objective at higher magnification that is more than 25x the overall resolving power is depending on the numerical apertures of both objective and illuminating system. If the numerical aperture of objective exceeds that of illuminating system then the resolving power r is given by 0.5 times lambda divided by objective of numerical aperture and then condenser numerical aperture. If a microscope is to give its optimum performance and put it in the code the numerical aperture of the condenser must equal to that of objective. So that takes care of the illumination of the objective then what we talk about this reflectivity also will be optimum. So in normal microscope if you talk about the illuminating system and if you there are two things we will we will confine our discussion only to the metallurgical microscope primarily we will see what are the other variations in the other variants. First let us take illumination of opaque specimens. See if you look at the working distance of any metallurgical microscope which we are going to see I will show you it is very very small. So illuminating the specimen between the objective and the specimen surface is rarely possible. So oblique illumination can be employed. So what happens if you employ an oblique illumination suppose if you have the surface like this suppose we are now illuminating like this. This will go and this will go and suppose you have the objective here. So if it is an oblique illumination on a surface like this if it is not an even surface the reflected rays not primary reflected rays will not enter the objective but only the reflection which is from the step or an uneven surface only will get into the objective only those features will appear bright in the feature. So see the best is the normal illumination through the normal direction we will see just what are the illuminations which are primarily employed in the metallurgical microscope and then we will proceed to the bright field illumination. Let me draw the schematic suppose this is the this is a light ray which is coming from the source. So normally it is passes through a glass slip and then they are all reflected then you have the objective and then you have the specimen surface this is object let us be a object this is objective lens and then it goes and then it reflected back and it goes to eyepiece this is an inclined slip illuminator inclined slip illuminator for a in a normal conventional metallurgical microscope the other type of illumination is this here the glass slip is kept at 45 degree and you have this glass slip this comes down and you have the objective and you have the object and then this goes to eyepiece and this is called Smith illuminator and the third and final one is simple one the light source comes and then it enters a prism and then directly goes to the objective. So you have these three kind of illumination possible in the conventional metallurgical microscope this is called I mean the inclined glass slip illumination and you see that with this only 20% of the light is being used for the image formation and this setup is just because if you use a polarized light and in this case if you use a polarized light the plane of polarization get rotated by this glass slip which is overcome by this design that is for a Smith illuminator the more intense illumination is possible by introducing a prism in the optical tube but then it also obstructs the the ray path in the tube but these are the primary illumination paths are being considered for the conventional metallurgical microscope. So now we will look at some of the example of right field illumination I will take you to the microscopy lab again and then we will see so what you are now seeing is I am going to just show you the right field illumination and the identifying the microstructure there are three samples one is steel specimen another is a cast iron another is aluminum specimen we will see how this right field illumination gives what kind of information we will use again the vertical type microscope and I will take up this sample preparation techniques in a separate class and this is just I am introducing a specific mode of operation. So we are talking about the right field illumination now so you should have some idea about how this right field looking like so we will keep this so now the sample is being kept the polish sample being kept on an inverted microscope and what you now see is this is another I mean this is a cast sample and you can even use this clip to be stationed the specimen and then you use the appropriate objective lens to start with it will be the lowest probably the 5x objective you use and you can rotate the torrent and what you now see is selecting the appropriate objective and then viewing it on this specimen and this is for aluminum sample. So all these three microstructures we are going to look at it that is why it is just shown how the samples are being kept and then just it is just showing the operational mode how you look at it since it is a reflection microscope and I say as I said it is an OPEC samples. So we will see how the right field microstructure appear so you see the microstructure is grabbed by the CCD camera and now you are seeing it in the monitor this is the cast iron microstructure where you see the people who have some materials background will understand what I am saying otherwise the people who do not have the material when you do not have to worry what it is all that you have to appreciate is you are able to see this microstructure because of the reflection that the region which is appearing very bright or getting the lights are getting into the objective the one which is appearing dark they are escaping the objective that is something you can see. So here the for the people who understand the material the information is this is white cast iron and then you have so now you see that you are increasing the magnification you are able to see the third phase details so you have basically a two phase in this case it is a ferrite and cementite the white is cementite in the black is pearlite sorry pearlite and cementite sorry and then you increase the magnification to 500 X so you start seeing much more detail of the specimen see you see that there is a magnification increases you are you can see that the depth of focus is also having some issue because the specimen flatness is questioned here and you can see that at the end of the corner you are not able to focus as good as in the center region of the specimen. So now you go to the 500 X or even more 1000 X you will see that much more detail I think it is getting blurred beyond this we will look at the next okay this is better 1000 X and this is for the medium carbon steel specimen is the lowest magnification or 100 X you see that ferrite and white is ferrite and black is pearlite as I mentioned the people do not have the background and materials you do not have to worry it is just that the phase which is appearing bright that means the reflectivity of that phase is very high you can say that that means the rays which is coming coming from this white surface are entering the objective the one which is appearing dark escape the objective that is all you have to remember and then appreciate for any specimen which is having this kind of a surface undulation you will have this kind of a contrast again this is a 200 X magnification and you see the details of the microstructure is getting better and better. So now you go to 500 X you see the much more detail inside the the white region all close boundaries are being revealed so clearly and this is 1000 X magnification you can even see that you are able to resolve this the black portion which is pearlite and you are able to see much more details of the sample. So you just see okay this is a better 1000 X you can see much more clearly the details of the black phase and so we will now move on to the next sample this is the aluminum sample is a in as cast structure you are able to see that it is a cast structure the people who do not have this background for this materials I would say them it is it is a solidified form of aluminum microstructure you see that we call it these things are called in metallurgical terms are dendrites and then you can see that the details much more details of this specimen at as you go from a lower magnification to higher magnification much more clear details are visible you can see that inside this the other black phase you will be able to see much more details of the microstructural information. So with that I would like to move on to the next technique what we have now seen is a bright field elimination in a conventional metallurgical microscope the kind of information you get what I have just demonstrated and in the next class I will start with the first another variant of optical microscope and then I will also similarly take you to this microscopy and then show the actual demonstration of the different contrast which you obtain from the microscope thank you.