 Good day everyone, I am Dr. Indanil and today I will be discussing about cathode ray tube monitor. Now the reason for choosing this topic is many of the present generation residents are relatively unfamiliar with this topic so it's a little less explored. The reason being most of the department's CRT tube monitors are now almost outdated and extinct. They are all replaced by flat screen LCD and LED display monitors. But the reason that we have to learn about this is in the year 2005 these were there in the form of computer monitors, our television screens and fluoroscopy display monitors. These all were cathode ray tube, picture tube monitors. And these are important for university exams. They may be asked in university exams or in practicals or in objective structured examinations. So we need to learn and I will simplify the concept as better as possible and in a ready to read and write format. What is the structure of the cathode ray tube? These you can classify this into five constituents, five components. The major component is the vacuum glass envelope. It is almost similar to the X-ray tube design. So we have a glass envelope with vacuum inside and there is an electron gun which constitutes the cathode part. And there is an anode assembly. Anode assembly is in the form of two sets. One set of anodes are placed nearer to the cathode. And the other set of anode is in the form of a graphite inner graphite lining and the fluorescent screen combo. I have labeled all the parts accordingly according to the colors. So you can pause the video, take a minute and try to in by what each stands for and where they are placed. So the cathode is an electron gun and there are two sets of anode assemblies. Then we have a control grid. The control grid is attached to the electron gun and as the name implies it is used to control or modulate the intensity of the electron beam that is emitted from the electron gun. When the filament is heated, the electrons are liberated and then these anodes provide the driving force for the electron beam. They propel the electron beam towards the fluorescent screen. And when the electron beam hits the fluorescent screen, it consists of a phosphor and this emits a burst of light. A short burst of light is emitted. In this way, so many multiple bursts of lights, they form a visible image pattern on the screen. And we have external coils. These are very useful because these are important. They help to focus and deflect the beam. They will navigate and steer the beam accordingly. The beam has to be moved horizontally and vertically all over the screen in order to make an image pattern. So these magnetic coils, they will help to deflect and focus the beam on the screen. And there is an aluminum reflector which serves to transmit the electron beam. They allow the electron beam to pass through but reflect away the light. So that is the function of the aluminum reflector that is placed on the back side of the screen. So we have a cathode, an anode assembly, the coils, the control grid and the fluorescent screen. So we have discussed the anatomy. Now we will look at how it functions, the physiology of the screen. Now the electron dot that the beam pattern that strikes this fluorescent screen is not random. It follows a pattern called raster scan pattern. What is this raster scan pattern? If you are familiar with an old typewriter, you might have observed how they type. It reaches the end of the page and again there is a written carriage lever. They will pull it and again it goes back to the next row. So it follows a similar pattern. The electron dot first begins at the top left corner. Then it continues to make a burst of lights pattern. Then it reaches the end of the right corner. Then it goes down and again it starts to make another line. In this way a standard TV image consists of 525 horizontal lines. So it makes 525 horizontal lines in this pattern. There is a peculiar pattern that is the odd field that means the line 1, line 3, 5, 7 and so on up to line number 525. The odd field is first written in this way and then it is followed by the even field. That means line 2, 4, 6, 8, 10 up to line 524 are written. So first the odd field is written and then that is followed by the even field. Each field takes about 1 by 60th of a second. So if you do the simple math, both odd and even fields combined will take 1 by 30th of a second. This is called one frame. The combination of one odd and even field which fills the entire page is called the frame. Like that in within 1 second 30 frames can be formed in this way. So 30 frames per second will give you rise to a flicker free display. The human eye can detect flicker if it is below 20 frames per second. So 25 to 30 frames per second it should be flicker free to the human eye. Now I will show you live animation how the raster scan pattern is done. As you can see each dot starts from the top left, proceeds to the right. Line 1 is completed, now line 3. Now again it proceeds to line 5. So the odd field is completed. After completing it again goes up, line 2 and line 4 is completed. Now if you can see the shades of the dots or intensities is modulated by the control grid. So it is made either brighter or darker or in intermediate. The shades vary according to the intensity of the electron beam that is modulated by the control grid. Here you can observe that a simple alphabet the letter n is in the form of a raster display. So this is how the patterns can be formed by the electron beam. You can also observe that after completing the first line it goes down to the next horizontal line. This is called horizontal retracing. This is called horizontal retracing. And after completion of all the lines it goes up vertically to complete the second line. That is called vertical retracing. So you have a horizontal retracing that is going down to the next line. And you have vertical retracing that is at the end of the field it again goes up to form the next line or for the next frame. After each frame is completed it is refreshed and a second frame is ready for the electron beam pattern. As I told you these are nothing but short bursts of light. They will fade away after some time. By the time the frame is completed they will fade away. And it is good to go for the next frame. By now you have observed that there are rows horizontal and vertical columns. So vertical resolution depends upon the number of scan lines. The more the number of scan lines the better the vertical resolution. And horizontal resolution depends upon entity called band pass. What is band pass? It is the number of times that the electron beam can be modulated in one second. That means in one second how many times I can change the intensity of the electron beam. So the more the better isn't it? If I can change it the modulation is more. I can show you a better clearer picture and a true representation of the brightness of the object. Our fluoroscopy monitor standard typical monitors have 4.5 megahertz band pass. Band pass is measured in terms of hertz because one second how many times it can be modulated. So what I mean by 4.5 megahertz is in one second I can modulate 4.5 million times. The electron beam pattern is modulated 4.5 million times in one second. Coming to the spatial resolution, the monitor spatial resolution is 4. It is actually the weakest chain in the link. This is the weakest part. It has only one line pair per millimeter. Whereas the image intensity of the tube has 5 line pairs per millimeter. The image intensifier tube that is connected to the video camera. That video signal is sent to the television monitor. So at the starting stage we have 5 LP per annum. But at the end when it reaches it degrades. It has its capacity is only one line pair per millimeter. So as you can see the image quality is degraded as it proceeds from the IA tube to the last link in the chain, the monitor. So this is an example of a typewriter. You can see how the raster scan pattern is seeing this. First it is typing the line from left to right. And after reaching the end it again goes back to the second line. This is similar to the raster scan pattern. Thanks for watching.