What is Liquid Crystal Display technology, and how does an LCD panel work? Why is LCD flat-panel display technology the most common display technology in use today?
In this easy-to-follow how-it-works guide to LCD flat-panel display technology, we look at the underlying technology that makes LCD panels work in order to represent numbers, words, and high-resolution images.
We explain the physical setup of a liquid crystal display and how this impacts the production of LCD panels. We highlight the main differences between backlight and reflective displays, passive vs. active LCD panel technology, and see how liquid crystal display panels generate color information. We also discuss the issue of ‘bad’ pixels and see why manufacturers never guarantee that LCD panels are 100% free of bad, stuck or dead pixels.
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Liquid Crystal Display (LCD) devices have become an important part of everyday life. Their use range from wristwatches, calculators, and mobile phones, to more demanding high-resolution applications in test instrumentation, computer monitors and high definition CCFL and LED LCD HDTVs.
And the use of LCD panels is growing at an incredible fast rate. Suffice to say that during 2011, according to a DisplaySearch report, total LCD TV shipments reached a global 214 million units despite the very difficult year for the TV manufacturing industry. In addition, it is estimated that LCD TV sales amount to 89% of all flat-panel TV sales – making the LCD display the undisputable TV display of choice!
LCD displays have become so common because they offer a few advantages no other display technology has so far managed to achieve. LCD displays are slim; most LCD TVs hardly exceed 3″ depth while most of the latest LED LCD TVs are just 1-inch thin! In addition, LCD display panels do not emit harmful electromagnetic radiation. The image produced by an LCD TV is often much easier on the eye than that produced by both plasma and CRT TVs. Equally important in the home entertainment and in the industry, LCD display panels are much lighter than either plasma or CRT TVs. And in a world where the energy bill is on the increase, CCFL-based LCD panels use 20 to 30 percent less power than the latest energy-efficient plasma displays, while the edge-lit LED LCD TVs consume less than 50% power of similar size plasma TVs.
Add an expected panel lifetime of close to 100,000 hours in contrast to the typically 20,000 hours of CRTs, and there you have one of the most versatile and robust display panel technologies available today. Mind you, liquid crystal displays do not represent the perfect display technology.
Apart from the higher price in comparison to plasma displays—especially as one exceeds the 55-inch screen size—LCD displays have their drawbacks as well. In particular, LCD display panels still have a rather restricted viewing angle with respect to plasma and CRT TVs, and this is especially so with LED LCD TVs. In addition, pixel response time in LCD TVs is still a thousand times slower than that of plasma HDTVs. We are not saying that LCD and LED TVs are not capable of handling fast action content but there is a higher possibility of experiencing image lag issues when handling very fast action content on LCD TVs than on plasma HDTVs.
LCD displays consist primarily of two sheets of polarized glass plates with some liquid crystal solution trapped between them. The type of liquid crystals used in LCD panels have got very specific properties that enable them to serve as effective ‘shutters’ that close or open to block in a varying degree, the passage of light. This blocking—or partial blocking—action takes place in a perpendicular manner to the passage of light once an electric current flows through the liquid crystal solution.
This current through the liquid crystals is controlled by a voltage applied between the glass plates through the use of transparent electrodes that form a grid—with rows on one side of the panel and columns on the other—representing the picture elements or pixels.
What are Liquid Crystals?
Though the three most common states of matter are solid, liquid, and gaseous, yet some substances can exist in a totally odd state that is a sort of liquid and a sort of solid at the same time.
Equally odd is the behavior of their molecules when substances are in this state, since these tend to maintain their orientation, like the molecules in a solid, but at the same time, they also move around to different positions, like the molecules in a liquid.
This means that liquid crystals are neither a solid nor a liquid, even though from a behavior perspective, these are closer to a liquid than a solid.
Use of Liquid Crystals in LCD Display Panels
There is a variety of liquid crystals, each with different properties. The liquid crystals used in LCD panels are referred to as Nematic Phase liquid crystals. These have their molecules arranged in a definite pattern.
Basic Operational Principle
(Click on image to enlarge)
One type of nematic liquid crystal, called twisted nematic (TN), has its molecular structure naturally twisted. The orientation of the molecules in the nematic phase is based on the ‘director’; this can be anything from a magnetic field, say resulting from the application of an electric current due to an applied voltage across the glass plates holding the liquid crystal solution, to a surface that has microscopic grooves in it. In the latter, the molecules at the various layers of the liquid crystal will gradually align themselves till the molecules at the layer adjacent to the surface will be exactly in line with the direction of the microscopic grooves on the surface.
Microscopic grooves in LCD display panels are applied on the surface of the glass plate that does not have the polarizing film on it to help align the molecular structure of the liquid crystals as these approach the glass surface in line with the polarization filters on either side of the LCD panel.
Now, the polarization filters on either side of an LCD display are set at 90 degrees to each other (ref. to above diagram). This means that the crystal lineup will go through a 90 degrees twist from one panel surface to the other. When a light shines on the glass surface of the first polarization filter, the molecules in each layer of the liquid crystal solution will guide the light they receive to the next layer. In the process, they will also change the light’s plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.
When an electric current is passed through these liquid crystals, they will untwist to varying degrees, depending on the current’s voltage. This untwisting effect will change the polarization of the light passing through the LCD panel. As the polarization changes in response to the applied voltage across the glass plates, more or less light is able to pass through the polarized filter on the face of the LCD display.
Backlit versus Reflective
Unlike CRT, OLED or plasma displays, LCD displays require an external light source to display the picture. The least expensive LCD displays make use of a reflective process to reflect ambient light over to display the information. However, computer and LCD TV displays are lit with an external light source, which typically takes the form of built-in micro fluorescent tubes that are often just a few millimeters in diameter. These are placed above, besides, and sometimes behind the LCD. A white diffusion panel is used behind the LCD to redirect and scatter the light evenly to ensure uniform display brightness.
Latest developments in LCD backlight have also brought about the use of LED-based backlight systems. LED-based LCD displays can be either edge-lit or full array with local dimming; the latter are capable of exceptional picture quality while making use of less energy requirements. But LED backlights have their cons as well. We discuss the different LED-based LCD backlighting systems in an article on our site here; this article goes into details of the latest technological developments in LCD displays.
LCD Display Systems – Passive vs. Active Matrix Displays
There are two main types of LCD displays: Passive matrix and active matrix.
Passive Matrix: These are the type of LCD display panels that rely on the display persistence to maintain the state of each display element (pixel) between refresh scans. The use of such displays is very much limited to a certain extent by the ratio between the time to set a pixel and the time it takes to fade.
To operate, passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. The grid is made up of conductive transparent material(usually indium-tin oxide) placed over two glass layers, or substrates housing the liquid crystal solution, with one substrate taking the columns, and the other the rows. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The point of intersection of the row and column represents the designated pixel on the LCD panel to which a voltage is applied to untwist the liquid crystals at that pixel to control the passage of light.
A display can have more than one pixel ‘on’ at any point in time because of the response time of the liquid crystal material. Pixels have a short turn-on time during which the liquid crystal molecules will untwist to control the passage of light. Once the voltage between the respective electrodes addressing a pixel is removed, the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their twisted orientation.
Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed.
Passive matrix LCD displays are simple to manufacture, and therefore cheap, but they have a slow response time in the order of a few hundred milliseconds, and a relatively imprecise voltage control. These characteristics render images produce by passive matrix LCD displays somewhat fuzzy and lacking in contrast. Passive matrix LCD displays are therefore unsuitable for most of today’s high speed, high resolution video applications.
Active Matrix LCD display panels depend on thin film transistors (TFT) to maintain the state of each pixel between scans while improving response times.
TFTs are micro-switching transistors (and associated capacitors) that are arranged in a matrix on a glass substrate to control each picture element (or pixel). Switching on one of the TFTs will activate the associated pixel.
The use of an active switching device embedded onto the display panel itself to control each picture element helps reduce cross-talk between adjacent pixels while drastically improving the display response.
By carefully adjusting the amount of voltage applied in very small increments, it is possible to create a gray-scale effect. Most of today’s LCD displays support a minimum of 256 levels of brightness per pixel though high-end LCD panels used in HDTV LCD televisions support up to 1024 different levels of brightness. This results in improved gray scale performance and therefore improved picture detail in those areas of the image that are primarily all dark or all bright.
Color in LCD Display Panels
For an LCD display to show color, each individual pixel is divided into three sub-pixels with red, green and blue (RGB) color filters incorporated within the pixel, to create color information.
This is somewhat similar in principle to the way CRT and Plasma display technologies use different phosphors to glow red, green, or blue to create color.
With a combination of red, green and blue sub-pixels of various intensities, a pixel can be made to appear any number of different colors.
The range of colors that can be made by mixing red, green and blue sub pixels depend on the number of distinct gray scales (intensities) that can be achieved by the display. If each red, green and blue sub-pixel can display 256 different intensities of their respective color, then each pixel can produce a possible palette of up to 16.8 million (256x256x256) colors.
Color TFT LCD TV displays require as many controlling transistors as the number of sub-color pixels forming the display.
This means that the manufacturing process associated with the production of color LCD display panels involves also the production of an enormous number of thin film transistors etched onto the glass substrate to control each and every sub-pixel. Simple mathematics shows that a typical wide screen 1080p panel with a screen resolution of 1920 x 1080 pixels would require over 6.2 million transistors!
Any faulty transistor during the manufacturing process cannot be replaced; these faulty transistors lead to what are known as ‘bad pixels’ – mainly visible only during static displays. A bad pixel is referred to as ‘dead pixel‘ and will show up as a black spot if it remains always off; it is referred to as ‘hot pixel‘ and will show up as a white spot of light if it is permanently on; and it is referred to as ‘stuck pixel‘ and will show up as a colored spot of light if one of the sub-pixels is damaged. This also explains why some refer to bad pixels as ‘bad’, ‘hot’, or ‘stuck’ pixels.
If the number of ‘bad’ pixels is above normal, the whole LCD display panel will have to be rejected. LCD display manufacturing processes have a significantly lower yield than plasma displays; this is the primary reason behind the higher cost of LCD panels as the sale of ‘good’ panels will have to make up for the manufacturing costs of rejected screens as well.
The large number of TFT’s on LCD display panels also means that even brand new LCD panels may contain a few bad pixels. We are not saying that they will surely contain a few bad pixels but the possibility is there. It is unfortunate that most large manufactures of electronic gear do not have clear policies when it comes to replacing LCD display panels with bad pixels. Rather, the industry consider the presence of a few ‘bad’ pixels not as a sign of some malfunction, but rather, as something inherent within the production process itself. It is as if it is OK to buy a brand new car with a few dents on its sparkling paintwork!
Bad Pixel: ISO Guidelines
It is interesting to note that way back in 2001, the International Organization for Standards had come up with its set of guidelines – referred to as ISO 13406-2:2001. These represent a set of requirements for electronic visual displays, implying they also apply to LCD panels.
These were then revised in 2008 and again in 2011 by ISO 9241-303:2011; this standard established image-quality requirements, as well as providing guidelines for electronic visual displays to ensure effective and comfortable viewing conditions for users.
These standards list four pixel fault classes, while defining three types of defective pixels for each class:
Type 1: a hot pixel (always on)
Type 2: a dead pixel (always off)
Type 3: a stuck pixel as a result of one or more sub-pixels being always on or always off; the result is a colored pixel that is either red, or green, or blue, or one of the secondary colors.
The standard recommends a maximum number of defective pixels per million pixels for each class as further shown in the table below:
|Class||Type 1||Type 2||Type 3||Cluster with more than Type 1 or Type 2 faults||Cluster of Type 3 faults|
As of 2007, most LCD display panel manufactures started specifying their pixel fault rate as ISO 9241-303 Class II compliant. However, this standard ‘per se’ is nothing more than a guideline and is not mandatory. Worst still, the standard leaves too many loopholes on how a display manufacture may interpret the requirements specified by the standard, both with respect to the positioning of the faulty pixels in this case on the LCD display panel, and also in interpreting the bad pixel numbers.
Unfortunately, some manufactures are just using the ISO standard as an excuse. You see, a TV maker would still be abiding by the standard if he does replace a one million pixel resolution LCD display if it has say three Type 1 or Type 2 damaged pixels but then does not replace the same faulty one million pixel panel if it has two Type 1, two Type 2, and five Type 3 damages pixels, for a total of 9 damaged pixels!
Luckily, LCD display manufactures are realizing that what they may consider as an inherent aspect of the LCD display panel manufacturing process may eventually turn out to be of great concern to end customers. For this reason, since 2009, we started experiencing a shift by top flat-panel display manufacturers towards a ‘zero bad pixel‘ policy; Samsung and Viewsonic were among the first to have moved in this direction.