Liquid Crystal Displays (LCDs) have been long established as the number one display choice of many designers of electrical and electronic devices for enhancing the user experience of their finished products through the versatility of visual information. They are found in many electrical applications across many sectors and are available in many sizes and screen formats.
With special attention paid to an end product’s design requirements, an LCD can be realised for a broad range of machines suitable for use in their intended environments. Their versatility has meant that many technological forms of Liquid Crystal Displays each with their own unique properties, have been invented and this paper is designed to give a brief and hopefully comprehensible insight into the various types of Liquid Crystal Display technologies still in use today:
Optics of Liquid Crystal Displays
To begin, let’s explore in simple terms the optics of liquid crystal displays. LCDs are fundamentally optoelectrical display systems with a main component set that includes, a liquid crystal layer, two plates of glass substrates, two polarizing plates, two alignment layers, transparent separators, transparent electrodes, retardation films, a reflective or transflective layer, conductive medium for system connectivity, and in the case of colour displays, appropriately chosen colour filters.
Unlike CRT, plasma and OLED displays, liquid crystal displays are non-emissive display technologies meaning, they cannot emit their own light. Therefore, in order for the liquid crystal display system to operate and provide the user with useful information, an external light source is needed. This source of light can be incident light captured from surrounding ambient lighting or from a special design backlight unit positioned directly behind the rear glass of the LCD structure which can be mechanically integrated with the LCD to create an LCD module. For liquid crystal displays designed solely for use in high ambient light conditions, the LC display’s rear polarizer will require the addition of a reflective layer to be able to reflect light that has entered into the display system via the front polarizer and front glass substrate, back through and out of the display towards to viewer. This type of LCD display is known as a reflective display.
Liquid Crystal Displays that rely on their light source produced solely from a backlight unit is described as a transmissive display and requires the backlight unit to be powered on at all times when the display is in use. Partially reflective and partially transmissive displays utilize rear polarizers which include both reflective and transparent properties in its structure. The term given to displays utilizing both these properties is “Transflective” since essentially it is a combination of a transmissive display and a reflective display rolled into one. By combining the optical properties of both reflective and transmissive displays, the advantage of being readable in high ambient lighting conditions even when the backlight is turned off, and readable in low ambient lighting conditions simply by ensuring that the backlight unit is turned on, is gained. The most commonly used light emitting components used for producing the light source for backlit LCD displays are LEDs. They are available in various colours and the most commonly used are white. yellow-green and red.
But why do we need all these optical and conductive components, what do they do and what is the importance of light? To answer these questions, we require the aid of a schematic drawing showing the basic elements of an LCD.
Figure 1. Cross section schematic drawing showing the basic elements of a Liquid Crystal Display
As aforementioned, there are many types of LCD technologies that still grace the user interfaces of today and the technologies that will be featured in this paper are TN displays, STN displays, vertical alignment displays, and TFT displays. What follows is an explanation of the fundamental principles of these display technologies.
A TN Liquid Crystal Display (Twisted Nematics)) can be appreciated as the first LCD technology produced in mass production quantities for use in consumer-type products such as calculator, low-level control panels, digital clocks, counters and metering devices. Accepted for their ease of manufacture, the TN display is so called because of the way in which its rod-like LC molecules is arranged in the liquid crystal layer.
The display itself comprises a liquid crystal material sandwiched between two glass substrates coated with a transparent material called ITO (Indium Tin Oxide) that are separated by microscopic glass fibre or plastic spheres and an alignment layer deposited on the inner surfaces of the glass substrates. Two polarizing plates are added to the structure, one on the outside of each of the glass substrates. The alignment layer is treated by rubbing in order to create microscopic sized grooves into which the LC molecule at the boundaries between the two glass substrates become anchored. By treating the alignment layer so that grooves formed on one glass substrates are created orthogonal to the grooves formed on the opposing glass substrate, the liquid crystal molecules are made to arrange themselves into a twisted stack formation through 90 degrees in the gap created by the spherical separators. The twisted stack of LC molecules acts as wave guides for polarized light entering into the LCD structure. This structure is what is referred to as the liquid crystal cell, or the TN cell to give it it’s specific name. The gap created by the microscopic spherical separators is known as the cell gap and is approximately 5 -6 microns wide.
Figure 2. TN cell structure with crossed polarizers and 90-degree angle twist formation of LCD molecules for a normally white mode display
The example on the left of the schematic (Figure 1.) shows that a beam of unpolarized light transmitted by the backlight travels towards the LCD structure and encounters the first polarizer (Polarizer 1). Here, a portion of the light is absorbed by Polarizer 1, allowing only the portion of light which is in alignment with the plane of polarisation produced by the first polarizer to enter the display via the first glass plate.
At this point, the now polarised light enters the twisted formation of the rod-like LC molecules and propagates through the LC material with its plane of polarisation rotating towards the second polarizer, Polarizer 2. Note that the molecular rods of LC material at the boundary of the first glass plate and Polarizer 1, are themselves aligned with the plane of polarisation produced by Polarizer 1, and hence the beam of polarised light is allowed to continue its journey through and out of the display. Note that this beam of polarized light which twisted though 90-degress as it followed the direction of rotation of the LC molecules before it encountered the second polarizer, is now positioned at a right angle to the first polarizer, (since they are crossed), whereby allowing the beam of polarized light to exit the display system.
The display at this point, being a normally white mode display, will appear light and no change to the display is observed. The schematic on the right of figure 1, shows the orderly arrangement rod-like LC molecules being disrupted by the application of an electric field. This induced electrical causes the LC molecules to break formation. In this event, the beam of light polarised by the first polarizer can no longer propagate through the display since the LC twist arrangement between the two glass plates which allowed it to meet the second polarizer with the correct angle of polarisation, no longer exists. Thus, the display appears dark since light cannot pass. Removing the voltage source sees the return of the twisted order with the display and hence the display appears light once again. Note that the twisted order of the molecules is only disrupted in the location of an electrical field. It is this principle that has made it possible to produce displays to impart with useful information.
By creating designated areas of the display that can be made to go dark or light, in essence, a light valve/light shutter comes into effect and thus a visual display can be produced. Indium Tin Oxide (ITO) coated on the insides of both glass substrates of the display can be etched so as to form specific patterns of basic designs such as 7 segments, decimal points, colons, icons and other symbols.
Unlike TN displays, Super Twisted Nematic (STN) displays rely on their ability to re-orientate the angle of polarisation or polarised light as it propagates through a twisted stack of LC molecules. Known as STN, the molecules within the LC layer are twisted thru up to 270-degrees. This higher twist angle allows the display to perform far better than TN displays at higher multiplex rates. A display that will respond to higher multiplex rate led to the development of graphic displays. Again using ITO (Indium Tin Oxide) individual pixels can be created by etching ITO patterns of horizontal lines on one glass and vertical lines on the other thereby bringing about the realisation of full graphic displays. Where a horizontal line intersects with a vertical line, a pixel is formed. One need only to apply synchronised pulses of electrical energy to change the state of the brightness of a pixel at a x, y intersection by changing the internal state of polarised light within the specific regions of the display. d Nematic displays, Super Twisted Nematic displays rely on its ability to re-orientate the angle of polarisation of polarised light as it propagates through a twisted stack of LC molecules. Known as STN, the molecules within the LC layer are twisted thru up to 270-degrees. This higher twist angle allows the display to perform far better than TN displays at higher multiplex rates. A display that will respond to higher multiplex rate led to the development of graphic displays.
It is worth noting that all Liquid Crystal Displays rely on its ability to control light through polarisation, only some are designed to be better at performing this task than others.
Figure 3. STN twisted compared with TN.
A Vertical Alignment (VA) liquid crystal cell consists of liquid crystal material sandwiched between two glass substrates separated by microscopic transparent spheres with a sheet of a polarizer on the outer surface of each glass substrate. Unlike we saw with the TN and STN examples, the liquid crystal molecules are vertically aligned with the glass as appose to being aligned in parallel. Protrusion is made on the inner surfaces of the glass to facilitate vertical alignment and the tilting of the LC molecules.
Figure 4. Vertically aligned LC molecules between crossed polarizers in the off-state produce deeper black background and enable high light transmission in the on-state.
Once again, transparent electrodes called Indium Tin Oxide are deposited on the glass and etched into a display pattern and conductive traces. Because the LC molecules are vertically aligned between two crossed polarizers in the off-state, the display prevents light from passing and in doing so produces a deep black display background. When an electric field is introduced by applying a voltage across the glass, the molecules align themselves perpendicular to the electric field leading to a high transmission of light. By controlling the on-off voltage one can control the light transmission thru the V.A. cell.
Colour TFT Displays
The invention of TFT displays has revolutionalised the flat screen colour displays industry that we know today. So called because it employs tiny Thin Film Transistors which are embedded into each and every RGB sub-picture element (RGB sub-pixel) on the display’s glass substrate. Each sub-pixel can be individually addressed by the display drive system to switch pixels on and off as and when required. This technique has allowed display makers to develop and manufacture vibrant colour displays of high resolution with fast response times making them highly desirable to TV manufacturers and makers of computer monitors, notebooks, tablets, mobile and off course industrial, medical, marine and automotive applications.
Figure 5. Basic elements of a TN-TFT cell
Figure 5 shows a cross-sectional view of the basic elements of TN-TFT cell. Here, we see the cell consists of a liquid crystal layer twisted through 90-degrees, typical of a TN display, along with two glass substrates, crossed polarizers and ITO conductive layers. In addition, there are the thin film transistor semiconductor elements: drain, source and gate including a storage capacitor which is used to maintain an applied voltage across the cell during display updating. Red, Green and Blue colour filters are deposited on the inner side of the top glass substrate to enable colour information to be displayed.
The ability to control the intensity of light transmission thru each of these colour filters is what makes it possible to render millions of colours on the display screen. Note that light propagation within the TN-TFT cell follows the same principle as we’ve seen with TN displays but with the luxury of three active switching mechanisms built into each pixel.
When an electrical signal is applied to the selected pixel the arrangement of the liquid crystals in that the region of the display is aligned with the electrical field into an almost vertical position. In this position, no light can pass through the LCD cell and the result is a dark dot. The Liquid crystal springs back into its twist arrangement with when the voltage is removed. The pixels in TN TFT displays can be switched between the “ON” and “OFF” states quite quickly giving them excellent response times, making them ideal for applications where fast display updates are a necessity, typically when displaying video content or smooth animation.
Not only was the TN TFT display the first active matrix LCD display to be developed it is the cheapest to produce requiring simpler manufacturing processes and as a result of this has found its way in a variety of applications. From industrial to medical, gaming to media players, POS to vending. However, the twisted Nematic cell faces challenges when required to deliver very high contrast and extra wide viewing angles. This is because when the pixel is turned on, the transition of the liquid crystal from its twist position to vertical position doesn’t quite reach a state of complete perpendicularity to the glass. This means that black is not fully black and colours not so vibrant due to the birefringent of the LC molecules. In addition to this, the display contrast is dependent on the direction of viewing. In many cases, colour shift, display brightness and eventually colour inversion is observable if viewed from the wrong angle. A typical contrast ratio for a standard TN-TFT display is around 400:1 with viewing angles of L70/R70/T70/B60.
In-Plane Switching (IPS) Displays
IPS technology provides a solution to the limited contrast and viewing angles which are disadvantages of Twisted Nematic liquid crystal technology. In an IPS LCD display, the long axis of the crystals is always oriented parallel to the glass plates whether the pixel is ‘ON’ or ‘OFF’.
Unlike the TN glass cell structure highlighted earlier, the electrodes are placed on the same sheet of glass instead of on opposite sheets. When an electrical signal is applied, the crystals rotate horizontally, and therefore in the same plane.
Figure 6. Diagram showing in-plane electrodes (e1 and e2) and LC molecule alignment in the OFF state and alignment when in the ON state with voltage signal applied.
A major advantage of IPS is that it is a normally black mode display and therefore when an electrical signal is applied it passes the beam of polarised light only in the ON position and blocks light when in the OFF position. Thus if a transistor were to become defective the pixel will remain dark, which is less disturbing than a bright one.
MVA LCD technology
In 1998, Fujitsu introduced a technology called Multi-Domain Vertical Alignment. The VA system differs from both the TN and IPS systems. In the VA system, the LC molecules are aligned perpendicular to the substrates when no voltage is applied, thus producing a black image. When a voltage is applied, the molecules tilt to a horizontal position, producing a white image. With no voltage, all the LC molecules, including those at the boundaries with the substrates, are completely perpendicular. In this state, the polarised light passes through the cell without interruption from the LC molecules and is blocked by the front polarizer. Because the blockage is complete, the quality of black produced in this way is excellent and the viewer sees this black from all viewing angles. This system can achieve faster response speeds because there is no twisted structure and the LC molecules are simply switched between the vertical and horizontal alignments.
Figure 7. Light transmission levels when viewed from 3 different angles for (a) Mono-domain structure and (b) Multi-domain structure. Note that wider viewing angles are achieved with the Multi-domain structure.
With this technology, the molecules on the left and right sides of the cell are arranged so that they tilt in opposite directions (Figure 7). By combining areas of molecules oriented in one direction with areas
of molecules oriented in the opposite direction, and by making the areas very small, we can make the brightness of the cells appear uniform over a wide range of viewing angles.
Figure 8. LC molecule alignment in a basic MVA cell when in the OFF state and in the ON state.