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Page 1 of 4  You should be versed with today’s TFT monitor market to be certain that you really know what you want and what you need. Of course that there always be someone who will be satisfied with anything and those who always expect too much. In worst case, we should know where all the money gone. In best scenario, you’ll get almost ideal price /performance ratio in relation to your needs and budget. First we want to remind you on few basic work principles for TFT monitors. That will help you to understand causes of the potential problems and explain to you how you can easier to recognize them.
Let There Be Light
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Vocabulary
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LCD – Liquid Crystal Display
TFT – Thin Film Transistor; transistors applied in thin layers, films
Transistor – semiconductor device commonly used to amplify or switch electronic signals. It has three wires: gate, which actually acts like a switch, current source and third is drain – wire for distributing current from the source when we switch the gate.
Truecolor – palette in which are all three basic colours, also known as channels, represented as 8-bit and because of that this palette is also called 24-bit. 28 = 256, 2563 = 16.7millions. We can calculate maximum number of nuances that can be presented in this way. 32-bit palette is Truecolor plus one more 8-bit channel that determines transparency.
Dither – implies on attempting of creating illusion of bigger palette (palette is actually limited) leaning only on imperfections of human eyesight. One way is, let’s say, to alternately fill surface with red and blue dots. Looking from distance this surface will looks purple, colour that we didn’t use at all. Instead of spatial imperfectness of human eyesight, we could also direct on temporally eyesight dilatoriness. Similar effect will get if we would change fast enough surfaces of red and blue colours. In this way works FRC (Frame Rate Control) algorithm that can produce also unwanted blinking or noise in the picture.
Clear Type – anti-aliasing technique, like all others, is directed on solving problems of displaying curved, oblique and other “inconvenient” lines with the help of little squares (pixels). It takes in consideration subpixels around the text and by controlling them creates illusion of smooth shapes. It is invented especially for LCD monitors. |
So that TFT panel can be used as screen, inventors did some tricks arranged along “sandwich” that represents monitor. First layer is actually light. Special attention should be directed on choosing of adequate source of light that should be: handy for incorporating it into thin monitors, power efficient, to dissipate as less energy as possible and the most important to produce as whiter light as possible. The choice fell on little “white lamps” containing some gas and which gas will be used, depends from manufacturer. Their shape, number and disposition also depend from monitor’s size and quality, but as mush you stuff them in there, you won’t get nicely distributed light across whole necessary surface. And there we meet new layer – diffusion material. That is mainly piece of special plastic which inside refracts the light to all sides, distributing it and diffusing through whole surface. In the end we get what we need, nice, rectangular source of light which produces white light with uniform and adequate intensity. Or this is just in theory…
Lately has been experimented even with LED (Light Emitting Diode) backlight which provides wider colour spectrum and enables implementation of new tricks for improvements of other panel’s characteristics. For now there is no wider implementation for it.
Sunglasses
We can test liquid crystal up to smallest details, but for now it isn’t much matter why crystal should be liquid, or how the atoms are arranged in its molecular. What’s important, is that crystal spins the light that crosses through it if is in the right position. Also is important that if crystal gets into electrical field, it stops being “adjusted” and doesn’t spin light anymore, light now only passes through the crystal. Sunglasses are very helpful when “sun burns” because they polarize light, at least the better one does. Polarizer can be described as some sort of lattice for light waves. Instead of waving in all directions, light when passes through lattice is only in one planar directed. Now, all that should join in a useful wholeness. First we put polarization layer that will oriented the light into one planar, for example vertically. Then we need a layer of liquid crystals that will pass through polarized light or to spin it so that it won’t be vertical anymore. What scenario would happen depends only from current. In the end, we should put one more polarizer but this time horizontal one. This layer is also called analyzer, because if we direct into it already polarized light, light could pass through only if it is in adequate planar. And that’s it! Vertical light passes through liquid crystals that spin it and orient it in horizontal planar which enables crossing through second, horizontal polarizer. When the liquid crystal is under influent of electric field, it loses the structure, stops to spin the light that stays vertically oriented and as like that, light cannot pass through analyzer. So, with the help of current we can dosage if and how many of light will pass on the other side.
Liquid crystals are placing in proper position, suitable for physical light spinning. Surfaces of front and back glasses, which are in contact with other crystals, get “scratched” and therefore they have microscopic recesses that follow polarizer’s planar, i.e. analyzer’s. And that’s how crystal molecules oriented perpendicularly from one to the other side, creating spinning form by themselves.
And What About Colour Picture?
It’s common known fact that computer image is created from tiny dots, pixels. They are also called basic part of the picture. Colour is getting by combining three basic colours – red, green and blue in different proportions. Last, crucial layer in our TFT sandwich is colour filter that is located somewhere between back glass and analyzer. Screen is divided on wanted number of pixels and that will be in the end its physical resolution. Pixels are later divided on three subpixels, each for one basic colour. Every one of them will get one transistor and electrode which enables precise voltage control for attaining how much light will pass through analyzer on exactly determined tiny surface. Next electrode is mutual for whole screen and we will apply it in thin transparent layer on the opposite side. Light after analyzer is still white, but after filter on every little subpixel light will “pickup” some of basic colours, i.e. other wave lengths will be stopped. On pixel level that will seem as wanted nuance. And if we look at this process reversely, than everything becomes so clear. Light intensity is controlled on every single subpixel. That also means colour controlling for every pixel on the screen, and when the pixels are under our command than is also the whole picture. So, we have the computer screen!
Some of you will for certain notice that this screen is actually matrix of subpixels. Electric wirings crosses through whole surface, so the horizontal ones are attached on gate of all transistors in specific row and vertical ones are attached on every source in column. By releasing power sequentially on horizontal and vertical wirings, we “aim” at specific subpixel, i.e. its transistor. When that is happen, current flows from drain to electrode, electrical field is creating between them and liquid crystals start to “play”. Some sort of current condensation is also provided, which enables more precise voltage control keeping its level up to next refresh cycle.
When all layers of sandwich are in place, it isn’t hard to understand how everything works. You just need to follow the light. Lamp (A) produces light, and diffuse layer (B) is distributing it properly. Polarizer (C) is placing into vertical position providing the path to back glass (D).On this glass are directed also electrodes for subpixels, horizontal and vertical wirings and transistors. Liquid crystals (E) spinning the light before it passes through front glass (F). Earlier introduced recesses set liquid crystals into right position and on it is applied also mutual electrode. Light continues through colour filter (G) and finally comes to analyzer (H) which is actually horizontal polarizer. If everything went fine, light will cross analyzer (H) and we can finally see it. But if liquid crystals found themselves into electric field between mutual and subpixel electrodes, they won’t spin the light and it will be stopped, partially or entirely, on analyzer.
| Panel Types |
This box is related especially on TFT LCD TN+Film monitors, even though it can be applied on most panel types. We already learn about spinning and nematic (words that comes from Greek word for thread) is one of states of liquid crystals when molecules don’t have any certain planar disposition, but they are all oriented in the same direction. Word Film pointed on manufacturing process that means partially applying of elements in thin layers (films), not too differ from manufacturing process for integrated circuits. These panels are also called active matrix, because of active element – transistor that is included in manufacturing process. You can read more about their faults later in this text, but for manufacturers thorn in their flash was viewing angle. That was the motive for inventing some other types of panels, like for example widely spread IPS (In Plane Switching) and VA (Vertical Alignment). They are also TFT LCD but different from TN+Film.
IPS was made in Hitachi laboratories in 1996, and lately came S-IPS (Super IPS) and other variants from the same brand. Today, these panels produce only Hitachi and LG. Viewing angle is noticeably repaired and also colour quality comparing to TN panels, which isn’t much of a task. Here are also “hitched” liquid crystals but their molecules have different arrangement in sandwich. Instead of being twisted, they are rotating horizontally and parallel with glass under the command of two electrodes.
VA panels can be found in several productions, and first was started Fujitsu in 1996. Development includes MVA (Multidomain VA), P-MVA (Premium MVA) and AMVA (Advanced MVA) from AU Optronics, S-MVA (Super MVA) from Chi Mei Optoelectronics in cooperation with Fujitsu and PVA and S-PVA (Super Patterned VA), both from Samsung. This technology also increased viewing angles and improved colour quality and for position and for orientation of liquid crystals. They are in vertical position in relation to glass when they are in non- activated state, and if they are exposed to influence of electric field they are positioned parallel.
New technologies fulfilled their tasks, but along the way have been lost some of response time. After inventing of overdrive which is applicable also for VA and IPS, many predicted ending for TN panels. In the mean time, these panels become so cheap and quality stabilized, that VA and IPS left behind. Of course that they are still on the market but they are not concurrent anymore with price. For how long this state on the market will continue in this manner, we cannot predict, but certainly won’t be good thing if it last forever. Achieved quality is that high that even newer upcoming TN monitors will be satisfactory for most people, but certainly that shouldn’t be enough reason for stopping the improvement. Market of manufacturers of TFT panels hardly that counts handful of players, but we still do hope that open competition on the free market will keep forcing improvement forward. |
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