Monochrome LCD Displays - what is behind Monochrome LCD Displays, how they work
the most standard and common display would be transmissive display so we have a transmission of the light from the backlight through the LCD as we said the LCD will be polarizers LCD glass and other polarizer electrodes and then we have the surface of the screen this black part and we have the the viewer the user that is seeing the screen Welcome at Riverdi University! My name is Kamil and i'm the founder and CEO of Riverd. Today we'll be talking about the most basic LCD displays that are of course monochrome LCD displays. So we will talk about basic technology what is behind there, how they work, how the pixels work, and this will be the foundation for our next session and lesson regarding the TFT, so we will add colors later and active matrix but right now we'll be talking about the simple and the most simple LCD displays and technologies around it. Yes, so here here is the short agenda for today. So we'll talk about TN, STN, FSTN and DFSTN and other technologies. Then we'll talk about positive and negative displays, how they different from each other, we'll talk about the light, let's say in a display, so we'll talk about reflective, transflective and transmissive displays and of course the backlights, viewing angles, this is very important part ,and the colors, because even in monochrome LCD displays we can have different colors. Of course it's still a monochrome LCD display so there will be one color, one background, one character color, but they can be different colors, no need to be only black and white. So let's start with the basics.
The technology of the LVD displays are related to liquid crystals and the liquid crystal is a very special material, that can change the polarization of the light. It's actually liquid, it's really liquid, it's put in between two glasses and that we have in every LCD display. we put LCD crystals inside and because of special properties we can change the polarization of the light using electrical voltage. Yes so we we generate electrical field in between
these two glasses and then we move the crystals inside and in the same time we move, and we change the polarization of the light. So the most basic LCD display is a TN that we call twisted nematic. Yes, so it's the the first and like oldest technology that we still use nowadays. Twisted nematic displays are used in even in a TFT displays, so it's the oldest but used until today and its working principles are the the simplest. So, first of all we need to have two glasses where we put inside the liquid crystal, as
i said already we need to have electrodes, so of course they are transparent, made usually by ITO on the glass, so a special transparent conductive surface that allow us to generate electrical field in between these two glasses, two electrodes and then of course we cannot just change the polarization of the light, it won't be enough to set on and off pixel. Yes we need to have first polarized light yes, so what we do additionally to really have a display, we need to add two polarizers, one we add on the bottom, and one we add on the top of the screen, and once we have the polarizer we have the polarized light. So let's say in this case we see that then the light is being twisted with this crystals when they are in an off state. So this display, we will call it normally white, because we see there is no
voltage right now applied, but the pixel is light. It's white so the the light is being transferred through. So what we know from this picture is that the polarizers are right now in the right angles and they are 90 degrees to each other. Yes, so first the light goes through the first polarizer, then it turns on the liquid crystals and go out through the other polarizer and we see the pixel. But the the magic happens when we actually apply the voltage yes. So we have the second image here and when we apply the voltage, the liquid crystals become organized, and the the light is not, the polarization of the light is not changed anymore, it's not switched to the like 90 degrees position. So because we have two polarizers in a 90 degrees to each other the light is going through the first polarizer, but it's not being turned and it's blocked by the other polarizer this time yes. So by applying the voltage we can actually switch on and off
pixel. One once we can do it, we we have a display yes. So we have a one pixel then from one pixel we can we can build like matrix of the pixels and actually presents any image. What is more important and useful, this control is not only OFF and ON states. We can actually control the states in between, so we will have something like a grayscale, so we can go from the white pixel to the black pixel with some steps. They could be like you know 16 steps or 256 usually. We do it, like you know, through the bits, so it will be 16, 256 or more. For TFT for example it will be like millions, because we will have
24 bits for example, which will give us 16 million combinations of the different pixel, or three pixels, RGB pixels brightness yes. We can we can say, so this technology actually allows us to have a display like you know working and present some images with the grayscale, but in a normal LCD, the monochrome LCD displays it's usually not used, because the technology is not perfect enough, it's a passive technology, there are no transistors connected to the to the cell itself and controlling the brightness with the grayscale is really hard, so usually it's not done here. It's used in the other technologies, like TFT that we'll be talking later, but here we only usually have only ON and OFF states to have the clear image, so let's go further. Once we know how the display works, how the the cell is working, and we know that the most basic is a twisted nematic display. We have some other variants, there are many of them actually, but the next I would say that is super popular, would be STN that is Super Twisted Nematic, is the technology that allow us to reduce the voltage to control the the display, which is very useful when we have battery powered devices, or it's also useful because we can connect the display directly to most microcontrollers, that work for example on three volts or even lower voltages, so we don't need any special drivers and the other like very important advantage is, it has better contrast, so we can see the image more clear. We talked about the contrast in the previous video, so you can learn more there, but the contrast, the voltage and actually it can be faster yes. So, STM can be faster, that means we can refresh it faster,
and the image, if we have it, can even move or be more clear when we change something on the screen. Okay here we have another variation of monochrome displays, it's called FSTN or DFSTN, that is like Film compensated Super Twisted Nematic, so it's additional layer actually that we add on the glass, that is making the display even better yes, and what I mean better here, is like having the higher contrast than the regular TN or STN. Once we add the film we can go to the to the more black, let's say dark background. As you can see here is an FSTN display from Winstar company, that is our partner, and the next display is a DFSTN, so it's additional film, so we have double FSTN, so two FSTN layers, and the contrast is even better, as you see it's more black. These technologies of course are add cost to the display,
so the most the cheapest one will be TN, then STN will be a little more expensive, and the FSTN and DFSTN are more expensive even, and these technologies even if they all look very good here, they are not perfect, because when we go, and we change the angles of a display, then we we see that the contrast is being lost very fast. So this kind of technologies are useful, but in some applications where we actually look on the screen straight, most useful let's say yes. If we want to see the display from the angles there are other technologies now. Of course TFT is one of them,
or all the technologies that are way better than the monochrome LCD displays, but monochrome LCD displays are very popular still because this technology is well known, established and cheap yes. So, these displays are usually the the lowest cost displays that we can buy on the market. Next screen. Okay so another variation is HTN, so High Twisted nematic, something that is a little bit different, but similar to STN. It's it was mainly made for automotive industry, and the voltage here is even lower, but actually HTN have the main advantage, that is very wide temperature, so it's just just an example that we have many of the different options when we choose TN based displays, either STN and FSTN, HTN and other technologies as TN. It always depends on what kind of application we have, what kind of environment, I mean temperatures, what kind of light and the colors that we need okay. So let's jump right now to the
explanation what is the Positive and Negative LCD display. So basically, as you can see, it means like the active state of the pixel yes, what we see as a pixel and the background. So, in positive display it's like the most simple, it's like a calculator let's say. So what we see
on the background is just the LCD in a normal state, usually some light, kind of light color, or the background color from the backlight yes. In this example will be like yellow green background, the most common backlight for the STN displays, and the pixels are black, so this is the positive LCD display. In negative LCDdisplay we have completely opposite, so the light is being blocked in. The background here we have the blue, because it's like a property of STN display that when we put the white light under it and we make a negative colors, the natural color of the LCD will become blue. But when the pixels are not blocking the light, we see the white pixels yes. So actually the backlight here is a white backlight yes and they it's not
blocked where we have the pixels. So we have positive and negative LCD displays, they are again like. Where we can use them? Positive we usually use when we have the like regular environment with the light and we we don't need the backlight yes. So, in
positive display it's possible to switch off the backlight and we still see the image, it's like as i said like a calculator for example, we don't need a backlight there because it's a reflective display or transflective, we'll be talking about it later. But in a negative display we need always the backlight yes. So when you choose the display and you think which one would you like to use in your application, think about the power consumption. If you have a battery powered device, then probably the positive display will be better, because it will allow you to switch off the backlight, which is the main current drain, the place where we need to put the most power, and for negative you need to have the backlight always on yes, so you cannot switch it off, if we switch off the backlight in a negative LCD display, we will not see anything, so it's rather not for battery powered devices okay. Now we will talk about three different technologies used in monochrome LCD displays, and how the light is being transmitted, and what we, and how we see the the image yes. So the most standard and common LCD display would be
transmissive display, so we have a transmission of the light from the backlight through the LCD. As we said, in the LCD there will be a polarizer, LCD glass and other polarizer electrode, and then we have the surface of the screen, this black part, and we have the the viewer, the user that is seeing the screen, So in a transmissive LCD we have transmission of the light. As i said it's the most simple one, but in a transmissive LCD display we need to have the backlight always on, so this kind of display will be for example negative, but not necessary, could be a positive display as well, but the backlight in transmissive display need to be always on okay, The next display that is pretty simple as well, would be a reflective display. So this kind of display in the most basic example have no backlight at all. So, the example here will be again a calculator,
we don't have the backlight there usually at all, we just use the environmental light which is being reflected. And how it's being reflected? It goes through the LCD and then it's being reflected on the mirror, that is on the back of the screen yes. So the back of the screen is a mirror and the light goes back yes. So we have a reflection here and some variation of the reflective display could be a reflective display with the frontlight yes. So we have the same light here from the back, so we have the mirror, then we have LCD, but we have the LCD that is actually the front lighted, not the backlight here, so it's a frontlight and it's giving us the light that is being reflected later inside this mirror. And the last one, as I said we have three of them, so the last one will be transflective.
It's like a combination between the transmissive display and the reflective display, so it uses both phenomena let's say. One is to use reflection, and one is to use transmission of the of the light. So, to use it both we have, we need to have something that is called semi-mirror, the mirror that is actually reflecting half of the light more or less, and it's also transparent to another half of the light yes. So in this kind of display we can use external light to be reflected and then we can switch off the backlight, but for example, during the night, when we have no external light, we can switch on the backlight and then we can see the the screen again.
Thie kinds of displays in monochrome LCD displays, i would say, are the most common. They need to be positive, the same as reflective displays, they need to be positive to achieve this reflection, but they are common, as I said, because we don't need the backlight during the day, we can use backlight only during the night let's say, or when it's dark, so we can save quite a lot of power yes. So this display is the most common okay. The next one we will talk about the backlight types. So basically for monochrome LCD displays we have two types of backlights. One is the most popular right now, which will be the edge light, edge backlight, so we have the LEDs only on the edge of it, not on the screen, because LCD is here, but under the LCD we have a diffuser, and the LEDs are on the edge of this diffuser, and they are pumping the light inside the diffuser, then the light is being reflected by the mirrors and go out through the LCD. And this is the most common right now, the cheapest way also to build the backlight,
and very efficient, because nowadays LEDs are very efficient in power consumption, but also like the light generating. But the other, more let's say mature technology of all the technologies from the past, was to use the the backlight LEDs usually on the whole surface of the of the LCD, so we literally have the light of the LCD from back yes, from the whole surface, and it's rather not used anymore, because this is more expensive, we need a big PCB or FPC to put LEDs behind LCD, we need many LEDs to have the light that's uniform enough. So, this technology is not used for monochrome LCD displays anymore, it's only used right now for TFTs, for something that is called Local Dimming, in very special TFTs, maybe we'll talk about this in some further video. So, as I said right now the edge backlight is used okay. The next important point is like viewing angles. So uh for LCD technology it is quite important
to understand, that the image that we see, when we look straight on a display can be completely different than we see from some angles, and this is usually very well described in a in a data sheet, we talk about the viewing angles and they need to be defined there yes. So it should be, like you know, 50 degrees, 60 degrees, 70 degrees or the best TFT display will have like 89 degrees, but it will be defined. For monochrome LCD displays usually the numbers are lower, and basically the viewing angle is like the maximum angle, that the user see the clear image yes on the screen, So very simple definition, of course when we measure it, we have the more defined definition because it's usually being measured by a camera, so we measure the real contrast when we move the camera with the angles over the screen, but later of course for the user it's the same. So when we are too far with the angle, then we will not see the image clear or we will not see the image at all. So it's very important to remember okay. I mentioned in the beginning, that we have also colors for monochrome displays. Of course, as I said, there is only one color for the backlight, and one color for the characters, but as you can see here on the backlight type there could be more, could be many of different colors, so we can have a white...