Liquid-crystal display
A liquid crystal display (English liquid crystal display, LCD) or a liquid crystal display is a display (English display) or a screen whose function is based on liquid crystals that influence the direction of polarization of light when a certain level is applied to an electric voltage.
LCDs consist of segments that can change its transparency independently. To the orientation of the liquid crystals is controlled by an electric voltage to each segment. Thus, the permeability of polarized light which is generated by a backlight and polarizing filters changes.
If a display can display any type of content, the segments are arranged in a uniform grid (see pixels). For devices that are intended to represent only certain characters, the segments often have a specially adapted form, especially in the seven-segment display for representation of numbers ( see also matrix display).
A further development is the active matrix display that includes for driving a matrix of thin film transistors ( engl. thin film transistor TFT). In flat panel technologies, this technique dominated since about 2005.
In advertising, there is often talk of LED televisions since about 2009. This is in most cases also to liquid crystal displays (LCDs) for image representation, which are used for backlight LEDs ( LED backlight ). Screens with organic light emitting diodes ( OLEDs) are not yet generally available for large televisions.
LCDs are used in many electronic devices, such as in consumer electronics of measurement devices, mobile phones, digital watches and calculators. Also, head-up displays and video projectors work with this technique.
- 7.1 formulas
- 7.2 motion blur
- 7.3 approaches to optimize
History
1904 by Otto Lehmann his main work liquid crystals. In 1911 Charles Mauguin described the structure and the properties of liquid crystals. 1936 patented the American Marconi Wireless Telegraph Company, the first practical application of the technology, the liquid crystal light valve. Published in 1962 the first major English language publication on Molecular Structure and properties of liquid crystals ( Original: Molecular Structure and Properties of Liquid Crystals ) by George William Gray.
Pioneering work on liquid crystals were made in the late 1960s by the British Radar Research Establishment at Malvern. The local team supported the ongoing work of George William Gray, who eventually synthesized liquid crystalline cyanobiphenyl compounds with his team at the University of Hull in Kingston upon Hull (England ), which met the requirements with respect to stability and thermal behavior for the LCD.
The first working LCD based on the dynamic scattering mode ( engl. dynamic scattering mode, DSM) and was introduced in the U.S. in 1968 by a group at the Radio Corporation of America (RCA) under the direction of George H. Meier salvation. Healing Meier founded the company OPTEL that developed some LCDs according to this principle.
On December 4, 1970 Martin Schadt and Wolfgang Helfrich reported, then the Central Research Laboratory of Hoffmann- LaRoche worked, the first patent on the " twisted nematic cell " (also TN cell, Schadt -Helfrich cell twisted nematic field effect ) in from Switzerland. The patent was granted in 21 countries, but not in Germany.
On April 22, 1971 filed James Fergason from Kent State University in the U.S. its patent application on the twisted nematic field effect in liquid crystals and presented in 1971 in his company ILIXCO, called today (2005 ) LXD Incorporated, LCD with this technique here. They quickly replaced the poorer DSM types.
Ideas, which are now used in active - matrix displays, arose in the design of liquid crystal bar displays ( engl. bar graph displays ). As an LC matrix display 1971, designed for bar graphs at the company Brown, Boveri & Cie., Baden, Switzerland, each LC - segment diode upstream ( nonlinear element ), and an additional capacitor is connected in parallel with the storage element.
On June 28, 1973, a arrangement to the bar chart has been filed for patent in the United States, in which interdigital electrodes are placed on one glass plate to produce an electric field parallel to the glass plate and re-orient the liquid crystals in a plane ( in-plane switching, IPS, see Display types).
On 7 July 1983 a supplement 28 October 1983 ranged H. Amstutz and his co-inventor of the Research Center of the firm of Brown, Boveri & Cie, Baden, Switzerland, a patent application, which formed the basis for Super - Twisted Nematic STN - LCD (see display types). With STN - LCD is left for the first time monochrome passive matrix displays with sufficient resolution for simple image representations realized (see illustration of a world map with Electronic Control ). This patent was granted in many countries. Especially Asian manufacturers were licensees (over 60 worldwide).
On January 9, 1990 reported G. Baur and his coauthor of the Fraunhofer-Gesellschaft in Freiburg for a patent in Germany, which formed the concrete basis for optimized in-plane switching in matrix displays ( IPS LCDs). This patent was granted in many countries, acquired by Merck KGaA of Darmstadt, the world's largest manufacturer of liquid crystal substances and licensed to many companies.
On September 18, 1992 Supplement of 20 January 1993 K. Kondo and his co-inventor of a patent application made at Hitachi in Japan, setting forth as another essential element of the In-Plane Switching a specifically suitable connection technique of thin film transistors in a matrix form. Later, another Hitachi patent application, which improved the viewing angle dependence of this type of LC displays took place.
Ad Types
Twisted Nematic (TN)
Liquid crystals are used in liquid crystal displays, organic compounds which have properties of both liquids and solids. You are the one more or less fluid as a liquid, on the other hand they show properties such as birefringence.
A simple liquid crystal display element can be combined with the Schadt -Helfrich cell ( twisted nematic cell, Eng. Twisted nematic, TN cell ) can be realized. The adjacent illustration, only the components of the lower half of such a turning cell are numbered. The reference numbers ( x) are incorporated in this specification. The inner sides of two very thin glass plates (substrates ) (4) are coated with a transparent electrode layer ( indium tin oxide, ITO ) (3), the middle is the liquid crystal layer (1 ) of less than 10 microns thickness. The liquid crystal molecules are arranged in a predetermined direction, ie parallel to the example with a polyimide ( 2) coated and brushed in a preferred direction surface. The preferred directions of the two substrate plates are rotated by 90 ° to each other. In the production of hand-made prototypes you can use to brush coated polystyrene foam or velvet fabrics rolls.
In addition, the two substrate plates (4 ) are coated with 90 ° to one another twisted polarization filters (5). On the back of this arrangement, a mirror (6) can be located ( reflector or transreflector ) which reflects the incident light ( reflective mode ). In the transmissive mode instead of the reflector is a lighting unit behind the display element.
From the mutually screwed substrate plates results in a helical structure in the liquid crystal; at a 90 ° rotated screw one speaks of TN. Incident light is linearly polarized in the liquid crystal layer so before entering. By the twist of the molecules followed by a rotation of the polarization direction of the light, whereby the light can pass through the second polarizer and the cell light-transmissive (transparent) is. Because the display in idle status is transparent, this mode is referred to as a normally- white mode. When electric voltage to the electrodes, as occurs under the influence of the electric field, a rotation of the liquid crystal molecules align parallel to the electric field. The twist is thus being waived, the polarization direction of the light is not rotated, and so that it can no longer pass through the second polarizing filter.
One arranges the polarization filters parallel to each other, then the cell is dark without power and is transparent only with increasing voltage. One then speaks of the normally black mode. The Schadt-Helfrich cells is therefore (along with other liquid crystal displays, as well), a voltage- controlled light valve. A screen may consist of any number of such cells (picture elements, pixels). When calculator is a simple 7 -segment display is a digit in a color-capable screen per picture element (pixel ), three field elements ( subpixels ) used for the primary colors red, green and blue.
Super - Twisted Nematic ( STN)
For STN displays (English super-twisted nematic) the twist angle of the molecules is increased to 180 ° to 270 °. This allows a steeper electro-optical characteristic and so improved Multiplexbarkeit than TN displays are achieved. Due to various technical limitations it is only possible with some effort ( double cell = DSTN cell compensation with birefringent retardation films - retarder sheets) to make the representation color-neutral (ie, produce only shades of gray between black and white). Instead, the light states are yellowish and dark states fall dark blue ( with violet color ) from.
Double Super - Twisted Nematic ( DSTN )
It has been variously attempted to achieve a black and white illustration on the passive - matrix display: with guest-host technique, the OMI method of Martin Schadt ( "Optical Mode Interference " ) and the Double -Super- Twisted technique. Enforced has the latter as DSTN technology.
The construction principle of this DSTN cell can be seen in the picture. There are now available two STN layers. In the active cell - that is the one to which an electric field can be set - of the liquid crystal is rotated by 240 ° counter-clockwise. The passive cell also contains nematic material that is present here but 240 ° rotated clockwise. Both cells are rotated each other such that the orientation of the strips at the inlet side is perpendicular to the to the output side. The polarizing films are also rotated by 90 ° to each other.
In the conventional TN or STN cell is obtained after the passage of linearly polarized light exactly considered not simply linearly polarized light with twisted vibrational level, but elliptical (or circular) polarized light. The tip of the electric field vector describes an ellipse or a circle. Such light passes through the polarizer passes, said color separation caused by the dichroism - depending on the polarization and the orientation film at the beam exit - leads to colored light.
The picture shows the operation of DSTN LCD is clear: White light falls on the rear polarizer (pictured below) and is linearly polarized there. Then it passes in the active STN cell which generates (not field ) is now circularly polarized light therefrom. This light - modified by dichroism - as in the conventional STN cell. The path through the subsequent passive cell ( containing the same liquid crystal material as the first - active - cell, but twisted in the opposite direction ) leads to compensation of color splitting ( the phase difference is equal to zero ). As a result, there is a linearly polarized light having the same level of vibration as above after passing through the rear polarization film. However, because of the front polarizer is rotated by 90 °, he does not let any light: The screen is black at this point.
It lies on the active cell to an electric field, then the linearly polarized light from the rear polarizer is there smooth through without being changed. Only in the passive cell will now be circular polarization. But because the circularly polarized light is not retained by the polarizers, the screen at this location is bright, by accurately adjusting both the material used and the cell sizes, the transmitted light is white.
In this way displays have been implemented, which provide a clean black and white with a contrast ratio of up to 15:1.
Triple Super Twisted Nematic ( TSTN; nowadays referred to as TN or film -TN )
The complex structure of a DSTN liquid crystal cell into a relatively high expenditure in their production. Therefore, there is a new method was developed which results in flatter displays with reduced weight. This new solution is called " Triple Super Twisted Nematic " LCD ( TSTN ). The picture shows the construction principle of such TSTN displays.
Here there is only an STN liquid crystal cell. The color interference with normal STN technology are offset by two special films before and after the cell - attached - between polarizer and glass. These films are responsible for another name of this technique: FST, which means "Film Super Twisted " (sometimes referred to displays where only a compensation film is used as FST, those with two or more films than TST LCD, also common is the name for FSTN film STN). The improved contrast ( up to 18:1 ), the lower weight and the flatter and less costly construction have TSTN -LCDs made to prevail. Such displays have been implemented for the first time as " VGA monitor " in notebook computers.
Vertical Alignment (PVA and MVA)
When Patterned Vertical Alignment technology, also called short- PVA, is a development of the multi-domain Vertical Alignment technique ( MVA) of a manufacturer. Both techniques are based on a similar method. The advantages of MVA-/PVA-Bildschirmen are in a higher contrast ( > 1000:1 typical) at a TN screen (< 800:1 ). In addition MVA-/PVA-Bildschirme offer a wide viewing angle independence. The disadvantage of MVA / PVA screens is that they are slower than TN screens and therefore for motion pictures - are less suitable - like games and videos. In addition, the price is higher than that of TN displays.
The in -plane switching technology ( IPS, English for switching in the plane ) the electrodes are located next to each other in a plane parallel to the display surface. When a voltage is applied, the molecules rotate in the plane of the screen; typical for TN displays helical form is not required. IPS reduces the viewing angle dependence of the contrast ( actually this is referred to as a function of the viewing direction or the viewing direction ).
At subtypes, a distinction between S -IPS (Super IPS), AS- IPS ( Advanced Super IPS), A- TW -IPS (Advanced True White IPS ), H -IPS ( Horizontal IPS) and since 2009 e- IPS (Enhanced IPS ) and since 2011 AH- IPS (Advanced high Performance IPS). Except for the H-IPS type, the other IPS - types can be seen from the fact that it considers the side and in contrast to VA panels that have a slight purple hue. The e- IPS technology, which came on the market in 2009, offers an even wider view angle and lower switching times of 5 milliseconds.
The further development of PLS ( Plane -to -Line Switching) was developed by Samsung and offers among other things a higher transparency ( lower power consumption at the same brightness ) and the disappearance of the typical IPS panels Glitter effect. The first screens with PLS panels came to market in 2011.
Ferroelectric LCD ( FLCDs )
Still in development are ferroelectric liquid crystals, FLCDs short. The so-called ferroelectrics have the property to store electric fields for very long. Conceivable are thus displays that show a once- loaded image over weeks, months or years without refresh. Only an erase pulse can disappear then. But even in the conventional applications provide ferroelectric liquid crystals Advantages: The "refresh cycle" to refresh the pixel fields would not so often be run through ( the fields are not so fast, " Lost " ), leading to reduced effort in the control electronics.
Electronic control
In the first prototype LCDs in the early 1970s it was recognized that the DC operating irreversible electrochemical processes in the liquid crystal layer and thus life limitations of the display may cause. Although it is unlike LCDs after the dynamic scattering mode (English, DSM) in TN cells is an electric field effect, ie no current flow is necessary, nevertheless passed the mentioned problems in the application of a DC voltage. Therefore, a long time were operated all LCDs commercially used with AC voltage. Essentially a liquid crystal display element is a small capacitance, which is periodically transferred by a pulse-like voltage. For this purpose, the already available at that time electronic complementary MOS circuits ( CMOS ICs) both with respect to adjustable voltage swing as well as the symmetry properties awarded suitable. One of the advantages of TN cells is the low operating voltage and thus the low power consumption. Liquid crystal materials were synthesized for TN - LCDs Early on, which gave a good contrast with 3- volt operation and had a useful operating temperature range.
The operation of the passive matrix display depends on how pronounced an electro-optical threshold voltage. Because the control of the individual picture elements (pixels ) have to be repeated periodically in order to control not only one line but all pixels of a matrix, such as the lines of a matrix are driven sequentially pulsed. In the columns, the image information is input in such a way that the largest possible potential difference at the line pulses and the non-activated pixels, a voltage difference is created at the activated picture elements below the threshold value. Crucial for the optimization was the realization by Peter J. Wild, that with pulse-like, periodically repeated activation of the RMS ( Root Mean Square English, RMS) is the voltage differences prevail. Detailed information on the best possible control techniques for passive - matrix displays can be found in other literature.
As already mentioned, an electro-optical characteristic curve with a pronounced threshold and then a steep rise at higher voltage requirement for the realization of passive - matrix displays with many rows and columns. Only then can a sufficient contrast can be achieved even with matrices with many pixels. The lengthy optimization of voltage - contrast characteristic of liquid crystal material selection and cell structure of TN cells did not produce the desired results. It took over 10 years to 1983, the breakthrough with the discovery of super twisted nematic LCDs (English super- twisted nematic LCDs, STN - LCDs) at Brown, Boveri & Cie, Baden (now Asea Brown Boveri, ABB) succeeded. The characteristic of STN cells is much better suited for the control of passive matrix displays than that of TN cells.
Relatively early attempt was made to each pixel individually upstream switching transistors to avoid such the characteristic limitations of a passive matrix display. These had to thin film transistors ( english thin- film transistor TFT) suitable dimension and connection technology can be applied in a matrix arrangement on the glass substrate of the liquid crystal display. This is called a display of this technique active matrix display, because the operation of each pixel is actively controlled by the associated transistors. Although the ideas were already to 1968-1973 at the Radio Corporation of America (RCA ) and Westinghouse, USA, formulated, it took a long time until the technological requirements for mass production were developed. In particular, experiments were conducted with different semiconductor materials, until finally a specific type of amorphous silicon ( see the thin film transistor ) as set by the most suitable for the field-effect transistors in thin-film technology. This material technical breakthrough in Europe. In the implementation of commercial products, Japanese companies were responsible. Without the mentioned progress in electronic triggering large-scale television Liquid crystal displays would not have been possible. However, active matrix displays are due to the numerous additional process steps for TFTs more expensive to manufacture and can therefore also have pixel errors, so for simple ads with lower information content of passive matrix displays are still used.
Contact
From the outset, the task of connecting the transparent conductive traces on both LCD glass substrates with the control electronics was. These novel compound techniques have been developed.
For ads with not too close contact grid so-called zebra stripes or conductive rubber are used, which consist of alternating insulating and conducting layers of elastomer. The adjacent recording with a standard of comparison in the cm frame of the dark zebra strip of 180 microns is therefore visible with magnification in rosy band only when clicking on the picture. By building installation tolerances are balanced in all three directions.
Breakfast also solutions with chip - on-glass were tested. Here, the chip solder bumps are applied to the contacts of the driving circuits, then positioned on the corresponding contacts of the display, and then soldered at elevated temperature.
An important progress meant the use of flexible thin PCB with corresponding connecting tracks to display, which allow a very close contact screen.
Passive matrix displays
In passive - matrix displays the picture elements ( a group or a symbol ) can be controlled in time multiplex operation, ie, each image element is connected directly and permanently connected to a drive circuit whose output has a low resistance. Therefore, at the time of addressing applied charge relatively quickly breaks down again and must be replaced in the following field period ( engl. frame ) again. This change in the electrical activation leads to pronounced modulations of the optical response of the display (so-called frame response).
For addressing and control of a matrix of active elements for active matrix addressing of displays at the time of a charge on the pixel is applied, the most, an additional capacitor is connected in parallel ( storage capacitor ). After the application of the charge, the amount corresponding to the data information, the active device (generally, a thin film transistor, TFT) is switched back to the high impedance state, whereby the charge and thus the control is maintained during one frame period substantially.
This type of control results in active matrix displays have a higher effective voltage across the picture element, so that a higher modulation of the liquid crystal, and thus, an improved contrast, and a reduced dependence of the contrast on the viewing direction.
Pros and Cons
The LC - screens have over the older cathode ray tube (CRT) screens some advantages.
Moreover, they have a flicker-free, distortion-free, sharp at ideal resolution, lower weight, and low installation depth.
Unlike CRT display devices Liquid crystal displays are in practice not by magnetic fields, such as the earth's magnetic field or the magnetic fields of overhead line, NMR devices, transformers, speakers, impaired.
During the development of the device, at least until the development of controlled with TFT LCD, were harmed by the low contrast and the long switching times. Meanwhile, the color reproduction of LCD ( the representable color space, Eng. Gamut color ) by adjusting the Hintergrundleuchtung even be extreme requirements (extended gamut, multi -primary display). Another problem was the limited range of viewing directions with a constant contrast and uniform color appearance; newer techniques such as in- plane switching ( IPS), Multi -domain Vertical Alignment (MVA ) and Patterned Vertical Alignment ( PVA) and the use of birefringent compensation films ( retarder sheets) managed to remedy this. These disadvantages still exist, but are not as serious as previously by far. Since each pixel represents its own little unit, it comes to production conditions to occasional errors (pixel error ): pixels in only one color shine through consistently or play the predefined color wrong. The displays are classified into different error classes depending on the number of defective pixels that can have an impact on the price.
During production, the physical picture resolution is set, the control signal with a different resolution can lead to loss of quality. A TFT -based LCD screen provides compared to a CRT screen a much sharper image - but only in its design-related physical resolution. Lower resolution signals must be interpolated and blurry. Alternatively, the image can be well centered with black borders as clearly represent ( a digital connection that can usually adjust the graphics card driver ).
The backlight by so-called cold-cathode tubes is filtered (usually red, green and blue) to get the basic colors of the pixels, but the trade-off between brightness and color rendering quality must be found. The colors of the LCD are far less saturated than the CRT or plasma screen technology. From the generated light is only about 4% penetrate through the panel ( in white image content ).
One reason why tube (CRT) monitors in tests often performed better than flat screens, is by no means the better black levels in dark space and the contrast with the bright areas when no ambient light falling on the screen, but the improved reproduction of moving image content (see below ). In the meantime, however, the LCD technology is so advanced that some ( depending on the type of panels) than can be achieved with CRT monitors even better results.
The fluorescent backlight have a limited lifespan ( around 100,000 hours). Influenced by the backlight quality of the representation of white space changes already apparent after only a few thousand hours of operation usually more yellowish, as luminous intensity of fluorescent tube decreases with time. However, we can also according to the brightness of CRT monitors during the operation. The backlit by LEDs is indeed more resistant to aging, but also shows depending on the type of light-emitting diodes and mode of operation used slow aging. In addition, lighting by LEDs allows a more compact design, homogeneous illumination and contrast enhancement by selective, dependent on the image content control ( LED backlight ).
Switching times, and techniques
The response time of modern LCDs currently between 1 ms and 5 ms. Here, the reaction time is the time which elapses for the change in the luminance (brightness) of an image area of 10 % after 90%; Here are 0% and 100 %, the luminance of the stationary ( steady ) states. The response time in accordance with ISO 13406-2 is the sum of the switching times from light to dark (or vice versa) and back again. Due to switching of the asymptotic curve but 13406-2 switching times of <3 ms is required in order to avoid visible streaking ISO.
Formulas
The time ( increasing tension ) and the off time ( decreasing voltage ) were calculated using the formulas of Jakeman and Raynes:
Herein, the rotational viscosity of the liquid crystal, which describes the " inertia " of the liquid crystal to a change in orientation; the distance between the glass plates ( = thickness of liquid crystal layer ); and the elastic constant, indicating the "power" (torque) of the provision of crystals in the original alignment position.
For example, a large accelerated the recovery of the crystal in the initial state, but also acts as the orientation of the crystal when a voltage counter ( by the correspondingly increased threshold voltage ). Also can be alleviated by reducing the layer thickness, increase the switching speeds. When the layer thickness is reduced, for example by 30% () go the switching times to about half ( because ).
Motion blur
In such as LCD and OLED screens hold-type displays the state of a pixel for the duration of one frame period continues until the applied voltage is changed in the course of image construction of a new image ( conservation representation). Since the eye in the pursuit of a moving image content (English smooth pursuit eye tracking) the " brightness " of a frame period integrated while the image content remains fixed but, it comes to blurring of the image on the retina of the viewer. This is particularly noticeable when fast moving scenes and is therefore also called motion blur (even Engl. Motion blur ) refers. In today's LCDs this motion blur is already considerably reduced. ( Gray german gray to ) the response time of " gray to gray " is on average 6 ms. It should be noted that even with negligible switching times, that is, at almost infinitely fast switching, because of the preservation of illustration, the motion blur would not be eliminated.
Approaches for optimizing
Purpose
In digital watches and calculators LCDs are used since the early 1970s. This monochrome display without backlight characterized by minimum energy consumption and very long life and find alternative used in applications where a long maintenance-free operation is required. Dissemination LCDs found on other portable or battery- powered devices such as mobile phones, laptops and the like.
Typical resolutions for computer flat screens range from 1024 × 768 pixels ( 38 cm/15 " ) of up to 2560 × 1600 pixels ( 76 cm/30 " ), in notebooks ranging from 800 × 480 pixels to 3200 × 1800 pixels. PDAs and portable DVD players have resolutions between 320 × 240 and 800 × 480 pixels, displays of still and video cameras between 160 × 176 pixels (84 thousand pixels) and 640 × 480 pixels ( 900 thousand pixels).
Meanwhile, LCD and plasma displays have largely replaced the cathode ray tube. This relates to computer monitors ( since 2006 ), and TVs with larger screens ( since 2008). Other application areas such as oscilloscopes are in the hands of computer- driven LCD for quite some time. 2003 than conventional CRT monitors for PCs and 2006 more flat screen TVs in Germany already more LCD - Sold as tube sets - ie LCD and plasma screens.
The Imagina 90 was the first big-screen video projector manufactured in series with liquid crystal screen, which was also suitable for continuous operation in the world.
Production
The LCD technology has experienced in recent years, particularly through the development of flat panel displays an enormous upswing. Large production for flat screens were initially built in Japan. It soon began the exodus of industry in the new Asian industrial nations where cheap labor and abundant state funding attracted. Currently, the focus of the flat panel display industry in Taiwan and South Korea in particular. In South Korea operate the world's largest flat panel manufacturer based there - Samsung, LG Display and Chi Mei Optoelectronics (CMO ) - which is currently (2008) the largest LC - screen production. The migration of the industry continues, however.
Looking for even more cost-effective production sites of the boom has reached China. Production facilities to manufacture high-quality flat panel displays are there currently (2008) under construction.
Conservation
From the perspective of climate change is the liquid crystal display manufacturing seen as problematic, as in the traditional production of very large amounts of air polluting substances would be used. GWP 22800 CO2e - - In the key " array process " in which the TFT control matrix is applied over a large area on thin sheets of glass, potent greenhouse gases such as sulfur hexafluoride ( SF6) are and nitrogen trifluoride ( NF3 ) - GWP 17200 CO2e - used in very large scale and in the liberated atmosphere, as a study from 2008 shows.