theory, special features, TN TFT vs IPS LCD comparison
We know already that TN (twisted nematic) displays, suffer from grayscale inversion, which means the display has one viewing side, where the image color suddenly changes. It is tricky, and you need to be careful. On the picture above there is a part of the LCD TFT specification of a TN (twisted nematic) display, that has grayscale inversion, and if we go to this table, we can see the viewing angles. They are defined at 70, 70, 60 and 70 degrees, that is the maximum viewing angle, at which the user can see the image. Normally we may think that 70 degrees is better, so we will choose left and right side to be 70 degrees, and then up and down, and if we do not know the grayscale inversion phenomena, we may put our user on the bottom side which is also 70 degrees. The viewing direction will be then like a 6 o’clock direction, so we call it a 6 o’clock display. But you need to be careful! Looking at the specification, we can see that this display was defined as a 12 o’clock display, so it is best for it to be seen from a 12 o’clock direction. But we can find that the 12 o’clock has a lower viewing angle – 60 degrees. What does it mean? It means that on this side there will be no grayscale inversion. If we go to 40, 50, 60 degrees and even a little bit more, probably we will still see the image properly. Maybe with lower contrast, but the colors will not change. If we go from the bottom, from a 6 o’clock direction where we have the grayscale inversion, after 70 degrees or lower we will see a sudden color change, and of course this is something we want to avoid.
To summarize, when you buy older technology like TN and displays, which are still very popular, and Riverdi is selling them as well, you need to be careful where you put your display. If it is a handheld device, you will see the display from the bottom, but if you put it on a wall, you will see the display from the top, so you need to define it during the design phase, because later it is usually impossible or expensive to change the direction.
What is a TFT LCD?
A TFT LCD , or a thin film transistor liquid crystal display, is one of the fastest growing forms of display technology today. The thin film transistor (TFT) is a type of semiconductor device used in display technology to enhance efficiency, compactness, and cost of the product. In conjunction with its semiconductor properties, the TFT LCD is an active matrix display, controlling pixels individually and actively rather than passively, furthering the benefits of this semiconductor device.
Since paired with flat panel technology, notably liquid crystal displays (LCD), TFT displays have grown extensively in popularity for display screens and LCD monitors like computer monitors and smartphones. With this development, the cathode-ray tube, otherwise known as a CRT, began to fall into the past as the lighter, less bulky LCD took over in the field of displays. Modern-day high-resolution and -quality displays primarily use TFT technology within the LCDs.
Structure of TFT LCD
The TFT LCD is built with three key layers. Two sandwiching layers consist of glass substrates, though one includes TFTs while the other has an RGB, or red green blue, color filter. The layer between the glass layers is a liquid crystal layer.
Fig. 1: A visual diagram of the different layers and components used in a TFT LCD display.
The TFT glass substrate layer is the deepest or back-most layer of a device’s circuit board. It is made of amorphous silicon, a type of silicon with a non-crystalline structure. This silicon is then deposited on the actual glass substrate. The TFTs in this layer are paired individually to each sub-pixel (refer to Architecture of a TFT Pixel below) from the other substrate layer of the device and control the amount of voltage applied to their respective sub-pixels. This layer also has pixel electrodes between the substrate and the liquid crystal layer. Electrodes are conductors that channel electricity into or out of something, in this case, pixels.
On the surface level is the other glass substrate. Just beneath this glass substrate is where the actual pixels and sub-pixels reside, forming the RGB color filter. In order to counteract the electrodes of the previously mentioned layer, this surface layer has counter (or common) electrodes on the side closer to the liquid crystals that close off the circuit that travels between the two layers. In both these substrate layers, the electrodes are most frequently made of indium tin oxide (ITO) because they allow for transparency and have good conductive properties.
The outer sides of the glass substrates (closest to the surface or closest to the back) have filter layers called polarizers. These filters allow only certain beams of light to pass through if they are polarized in a specific manner, meaning that the geometric waves of the light are appropriate for the filter. If not polarized correctly, the light does not pass through the polarizer which creates an opaque LCD screen.
Between the two substrate layers lie liquid crystals. Together, the liquid crystal molecules may behave as a liquid in terms of movement, but it holds its structure as a crystal. There are a variety of chemical formulas available for use in this layer. Typically, liquid crystals are aligned to position the molecules in a certain way to induce specific behaviors of passing light through the polarization of the light waves. To do this, either a magnetic or electric field must be used; however, with displays, for a magnetic field to be usable, it will be too strong for the display itself, and thus electric fields, using very low power and requiring no current, are used.
Before applying an electric field to the crystals between the electrodes, the alignment of the crystals is in a 90 degree twisted pattern, allowing a properly crystal-polarized light to pass through the surface polarizer in a display’s “normal white” mode. This state is caused by electrodes that are purposely coated in a material that orients the structure with this specific twist.
However, when the electric field is applied, the twist is broken as the crystals straighten out, otherwise known as re-aligning. The passing light can still pass through the back polarizer, but because the crystal layer does not polarize the lights to pass through the surface polarizer, light is not transmitted to the surface, thus an opaque display. If the voltage is lessened, only some crystals re-align, allowing for a partial amount of light to pass and creating different shades of grey (levels of light). This effect is called the twisted nematic effect.
Fig. 2: On the left is the twisted liquid crystal layer in which polarized light passes freely; on the right is after the electric field is charged into the layer, completely re-aligning the molecule orientations so that light is not polarized and cannot pass through the surface polarizer.
The twisted nematic effect is one of the cheapest options for LCD technology, and it also allows for fast pixel response time. There are still some limits, though; color reproduction quality may not be great, and viewing angles, or the direction at which the screen is looked at, are more limited.
A solution to these limits was given through in-plane switching (IPS) of the liquid crystals. Rather than aligning the crystals perpendicularly to the electrodes, IPS aligns them in a parallel fashion. Light is then more streamlined within the matrix. There were initial problems like slow response time, but recently, these problems have been mostly resolved, making the benefits of better viewing angles and color reproduction greater than the faults. It is, however, a more costly technology than the twisted nematic devices.
Fig. 3:The top row characterizes the nature of alignment in using IPS as well as the quality of viewing angles. The bottom row displays how the twisted nematic is used to align the crystals and how viewing angles are affected by it.
The light that passes through the device is sourced from the backlight which can shine light from the back or the side of the display. Because the LCD does not produce its own light, it needs to use the backlight in the LCD module. This light source most commonly comes in the form of light-emitting diodes, better known as LEDs. Recently, organic LEDs (OLED) have come into use as well. Typically white, this light, if polarized correctly, will pass through the RGB color filter of the surface substrate layer, displaying the color signaled for by the TFT device.
TFT LCD Driving
If you refer back to the first paragraph under “Evolution of TFTs” in the last article, “The History of Thin Film Transistor Displays,” there will be a basic explanation of the Field Effect Transistor (FET). The TFT is a form of FET, and so it also follows the driving principle of FETs as well. Essentially, if a voltage is applied to the gate of a TFT, the signal current can be controlled or altered. This current, called driving voltage, on the TFT-based LCD panel then flows from the source to drain and casts a signal to its sub-pixel, allowing light to pass through.
Architecture of a TFT Pixel
Within an LCD, each pixel can be characterized by its three sub-pixels. These three sub-pixels create the RGB colorization of that overall pixel. These sub-pixels act as capacitors, or electrical storage units within a device, each with their own independent structural and functional layers as described earlier. With the three sub-pixels per pixel, colors of almost any kind can be mixed from the light passing through the filters and polarizer at different brightness based on the liquid crystal alignment.
Beginners Ultimate Guide to TFT Display Modules with Melrose Vol. 1
If you’re wondering whether a TFT LCD display module is the right choice for your application or not, then you’re in the right place. We prepared a detailed guide to help you make the smartest choice for your business. The guide will help you in better understanding the choices you have in subassemblies of a TFT display module.
Read on to find out everything you need to know about TFT display modules before approaching a manufacturer.
What is a TFT LCD display?
Let’s start with the basics. What does TFT LCD actually mean? TFT stands for “Thin-Film Transistor” and LCD stands for “Liquid Crystal Display.” When put together, a TFT LCD display is a flat-panel display or screen that you may find in computer monitors, TV sets, and mobile devices like smartphones and tablets.
How does a TFT display work?
TFT displays are made of large sheets of transistors, where each transistor of which is controlled independently. In its essence, a TFT screen is an active-matrix screen – each pixel on display is illuminated individually.
Pros of TFT displays
Brightness and sharp images – Expect a TFT display to be sharper and brighter than a common LCD display. It also refreshes more quickly than a regular LCD display, showing motion more smoothly.
Less energy consumption – TFT displays use more power than regular LCD screens. They not only cost more upfront but are also more expensive to operate.
TFT displays module – key subassemblies
Layer 1a: Cover glass – rigid
The first layer we’re going to examine is the cover glass in its rigid variant. Most of the time, clients choose chemically strengthened glass or Gorilla Glass™ in different impact-resisting thicknesses. The cover glass may come with a gloss or matte anti-reflective finish. Adding static backlit images is an option as well. The cover glass may also have mechanical buttons – fixed capacitive or through-hole mechanical ones.
Let’s dive into the details:
Chemical treated glass – This glass is made via a process where the glass is dipped into an ion-exchange solution. As a result, smaller potassium molecules are replaced by larger potassium molecules, reducing surface flaws that might occur in untempered glass.
Gorilla Glass™ – A proven brand that provides top-notch protection against high-impact stress. Many major smartphones and wearables use this glass.
Final notes: Glass strength is mostly related to the question of equilibrium. As the market requires thinner and stronger glass options, the manufacturers of protective glass strive to innovate and develop new glass technologies. It’s also likely that the demand for flexible protection choices grows in the future.
Layer 1b: Cover glass – flexible
If you’re looking to develop a TFT LCD resistive touch screen that responds to touch pressure, you need a cover glass that is flexible. To accomplish that, the manufacturer may use the same material as the top layer of a membrane switch – a resistive touch screen can include membrane switch functions.
How does this glass layer work in a resistive touch screen? A resistive touch screen is an overlay that uses physical pressure to detect any touch input. This type of touch screen consists of two layers – a flexible outer layer and a glass inner layer. These two layers are separated by an air gap maintained by microdots. This layer configuration can process only single touch events, reducing the transmittance (brightness) of the display underneath by 20% to 30%.
Layer 2: Touch sensor
PCAP screens
The capacitive touch screen (PCAP) was invented many years before resistive touch screens, but they have become more popular recently with the rise of consumer electronics products like smartphones and tablets.
PCAP touch screens offer high sensitivity and respond immediately to touch input. These screens are made of transparent and conductive materials such as ITO that are coated onto the glass material.
Contrary to the resistive touch screen we mentioned in the previous point, PCAP screens don’t rely on mechanical pressure. Instead, they take advantage of the electroconductivity of the human body (which is naturally conductive). This is why PCAP screens work only with exposed human fingers or special styluses.
Resistive touch screens
As we mentioned in the preceding section, a resistive touch screen includes a glass panel and a film screen. Both are covered with a thin metallic layer, which is separated by a small gap. When the user touches the screen and applies pressure, the two metallic layers meet and create an electrical flow. This change in voltage detects the point of contact, converting the voltages into X and Y coordinates that are later sent to the controller.
Final notes: It’s important that you select a touch sensor appropriate that matches the requirements of your application – for example, exposure to moisture and humidity or use of gloved hands.
Thanks to their durability, resistive touch screens find broad use in manufacturing, ATMs, and kiosks, or medical devices. Since in many industries, users need to wear gloves when using touch screens, the resistive screen is a good solution since it doesn’t require contact with exposed human skin or a stylus with capacitive capabilities.
However, thanks to advancements in this technology, PCAP screens are replacing resistive solutions thanks to greater capabilities in handling moisture or gloved hand usage.
Layer 3: TFT display
When it comes to the TFT display itself, you can choose from two technologies: TFT IPS and TFT TN.
In general, many businesses choose IPS and consider it better than the traditional TFT TN display – especially when it comes to the viewing angle and color conversion. IPS offers higher contrast but also consumes slightly more power. This technology is also more costly – an IPS display costs around 30-50% more than a standard TFT TN screen.
TFT IPS display
Pros:
The superior clarity of images – they remain stable and clear, not sparkly,
Colors are more vibrant and clear,
Easy installation on walls thanks to the compact form and low depth,
Super IPS screens offer a higher angle (170˚) for improved clarity and wider viewing, especially at night,
Longer battery life and screen life (on smaller screens),
Lower release of heat is lower,
Variety of options.
Cons:
High cost,
Colors don’t always transcribe correctly or accurately,
High resolutions might not always be readily available for personal applications.
TFT NT display
Pros:
Lower energy consumption in bigger screens,
Lower upfront and operation costs,
Excellent visibility – no geometric distortion,
Good response time and physical screen design.
Cons:
Poor viewing angles that might create distortions,
Static resolution (the resolution can’t be changed – however, newer models deal with this issue efficiently),
The accuracy of the display colors might not be perfect (especially strong blacks and bright whites).
Stay tuned for the next part of this series, where we continue to explain all the different subassemblies that are part of TFT display modules.
And if you need expert advice, please get in touch with our consultants. At Melrose Systems, we have the know-how and expertise you need to make the best decisions for your application.
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