The screen with liquid crystals (posting with liquid crystals ACL or LCD, liquid hook display , in English) is the main component of the flat monitors for the Informatique and the Télévision and provides the function of posting of a great number of portable devices.
Presentation
It uses the polarization Lumière thanks to polarizing filters and with the Biréfringence of some Liquid crystals in nematic phase which one can vary the orientation according to the Electric field. From the optical point of view, the screen with liquid crystals is a passive device (it does not emit a light) whose transparency varies; it must thus be enlightened.
Initially available into monochromic and small size, it is used in the calculators and the watches because of its low electricity consumption; it currently makes it possible to post colors in dimensions exceeding the meter of diagonal. It supplanted the Cathode tube in the majority of the applications, except in very high-definition when the palette must be precise and faithful and in harsh environment (in particular of the temperatures lower than 5° C).
Technical details
Screen ACL (see also " Technologies ") consists of two polarizers whose directions of polarization form an angle of 90°, on each side of a formed sandwich of two glass plates enclosing of the Liquid crystals. Each interface with liquid crystals, a layer of polymer, generally a polyimide, grooved ensure the anchoring of the Molécule S at rest.The two inner faces of the glass plates comprise a matrix of transparent electrodes, one (black and white) or three (color) by Pixel. The thickness of the device and the nature of liquid crystals are selected so as to obtain the desired rotation of the plan of polarization in the absence of electric tension (90° in screens TN). In the screens of great dimension, one adds spacers, small transparent balls, in the space filled of liquid crystals to maintain the very low thickness (20 µm) constant and precise. The application of a Potential difference more or less high between the two electrodes of a pixel involves a change of orientation of the molecules, a variation of the plan of polarization and thus a variation of the transparency of the whole of the device.
This variation of transparency is exploited by a retro-lighting, reflection of the incidental light or by projection. The electrodes of the pixels are accessible only by whole line or column and the extinction or spark control must be done by a regular sweeping of the lines of points. Small monochromic bill-posters ACL rest on the same principle but often use front electrodes in the form of segments of character in order to simplify electronics (direct direct order in all or nothing) while obtaining a very good legibility (not sweeping).
Liquid crystals color
The basic principle is the same one. It requires three cells by pixels and the sandwich is supplemented by a coloured filter of red, green and blue reasons. Generally the filter is a succession of vertical bands alternating the three colors. There are however other distributions shifting the colors from one line to another.In order to improve the precision of returned colors, the elements of the filter RVB are separated by an opaque black band.
Technology TN does not allow the posting of more than 262 144 colors (3x6 bits), the posting of 16 million colors (3x8 bits) uses a technique of approximation is by flickering ( blinking ) which alternates the posting of 2 colors which frame the “true one”, that is to say by effect of diffusion ( dithering ) between adjacent cells. Many screens seem to use a combination of these two techniques.
Lighting
There exist several modes of lighting adapted to each context of use which must take account of the relative transparency of the devices with liquid crystals: 15% for the monochromic bill-posters and less than 5% for the color screens because of interposition of the coloured mask.transmissif Lighting : The screen functions with a retro-lighting (TV, data-processing Moniteur, Camera and Caméra) by one or of the gas-discharge lamps with cold cathode whose light is distributed by two networks of orthogonal prisms.
The main advantage is the facility of use with a weak or average ambient light. The disadvantages are:
- a limited luminosity (problematic in full sun);
- the electricity consumption of the source of light;
- limited lifespan of the lamps (what is not any more the using case for the recent screens of LEDs).
Projection : Transmissif lighting is also used in the projectors, where the image of a screen ACL color of small size (approximately 2 cm of diagonal) is projected by an optical device comparable with a projector of Diapositive using a halogenous lamp of strong power. The best results are obtained by using 3 monochromic screens and a whole of filters and prisms breaking up and recomposing the luminous spectrum.
reflective Lighting : The screen functions with the reflection of the incidental light, very much used for the personal digital assistants, the Calculatrice S, the wandering and the watch S. It is used especially with the monochromic screens, sufficiently transparent.
- the advantage: A luminosity naturally adapted to ambient lighting;
- the principal disadvantage: Illegible when ambient lighting is weak.
transflectif Lighting : It combines a reflective device and a transmissif retro-lighting. Available on many personal assistants (PDA) and certain cameras.
Characteristics of a screen ACL
Measurements are defined by the standard ISO 13406-2 of which the most known rule relates to the defective pixels which distributes the screens in 4 classes according to the number of defects par million pixels: Among other definite measurements:- Definition of many pixels: The number of points constituting the visible image.
- Dimensions : Generally it is the diagonal which is indicated and, in spite of the French law, in inches (2,54 cm).
- Angle of vision horizontal and vertical: Indicate until which angle one can observe the image with a contrast higher than 10:1 (what is very little compared to the contrast of face!). The performances generally indicated are not those defined by the standard ISO, less flattering.
- Contrast : Relationship of luminosity between a white pixel and a black pixel. Often obtained by pushing the luminosity beyond the usable one (for a data-processing screen, the value recommended is of approximately 100 cd/m ²)
- Luminosity : (in any rigor it is the Luminance) Mesurée in the axis, in cd/m ²
- Response time : The ISO defines total time to go it return blanc->noir->blanc. It is often better than that necessary to the transition blanc->gris->blanc, more representative of a current use.
- Dimensions : Generally it is the diagonal which is indicated and, in spite of the French law, in inches (2,54 cm).
Current prices in 2007: Certain screens assert a contrast " dynamique" 3000:1.
Chromaticity
The International commission of Lighting (Co) determined on a sample of the population the color range which the human eye can distinguish and distinguish. The majority of the means of restitution (screens, printers) are far from being able to reproduce the whole of this range of color.Screens ACL progressed much in the capacity as colors and the majority exceed the extent of color (Gamut) SRGB, reference under Windows, and certain model professional approach the gamut NTSC used by television.
A novel method of lighting is democratized into 2007 which replaces the gas-discharge lamp by a whole of white electroluminescent diodes. Those make it possible the ACL to cover 114% of space NTSC and offer an adjustment much more stable balance of the colors as well as a very good uniformity of lighting.
See also: data-processing Coding of the colors
Technologies
TN, DSTN
Basic technology, the TN ( Twisted nematic ) in spite of insufficiencies in was made most widespread colors and contrast as well as a strong hauling. It was improved in the screens DSTN ( Dual scan twisted nematic ) which improves the stability of the image while carrying out its formation by a double sweeping. In spite of successive improvements, these technologies called to matrix passivates have a contrast limited to 50:1, an average quality of the blacks in general. Screens with double-layer (Double Super Twisted nematic) were also produced to optimize the chromatic balance of the produced light.Screens TN and DSTN are transparent at rest.
TFT
Its alternative TFT, is used in the color screens for data processing and television. It replaces the grid of front electrodes by only one electrode in ITO (oxide of Indium - tin InSn2O3) and the back grid by a matrix of Transistor S out of thin film ( Thin-film transistor ), one by pixel (three per pixel color) which makes it possible to better control the maintenance of tension of each pixel to improve the response time and the stability of posting.The majority of the screens ACL color of quality use today this technology TFT called to matrix activates which made it possible to obtain Response time in 10 ms old lower part. Contrast remains however limited to the neighborhoods of 300:1 that only the screens of the type PVA exceed.
The thin film of Silicium is engraved with the Manufactoring process of the devices to semiconductors on an extremely thin deposit (a few hundreds of micrometer) of silicon. One cannot currently deposit single-crystal silicon because it is impossible to make grow this one on glass (at the temperature necessary, 1450 °C, glass is almost liquid). Silicon is deposited by gas diffusion (one obtains then a layer Amorphe) or by Recuit of a fine silicon wafer (silicon locally remains crystallized: polycrystalline). This annealing can be done:
- by stoving of the unit, which is not possible that with quartz because of temperature >1000° C necessary. This technique is used for panels ACL for projection whose low dimensions are compatible with those of the quartz blades.
- by spot heating by sweeping with a laser beam.
Screens TFT not under tension are black.
IPS and S-IPS
Technology IPS ( In-Plane Switching ) developed by Hitachi in 1996 improves technology TN-TFT by using liquid crystals whose axis is parallel to the plan of the screen. The angle of vision is very broad but the number of transistors doubles, decreasing the transparency.
Mva and PVA
An improvement, Mva ( Vertical Multi-domain Alignment ), was introduced in 1998 by Fujitsu and improves its technology IS integrating several fields of Réfraction by cell, increasing the quality of the black (<1 cd/m ²) thus allowing to strongly improve useful contrast and the angle of vision. The last development is the PVA ( Patterned Vertical Alignment ) by Samsung which reaches blacks of 0,15 cd/m ² allowing a contrast of 1000:1.Screens Mva are opaque at rest.
Manufacture
Process
The manufacturing process of flagstones ACL is very automated and uses a succession of machines of very utmost precision in controlled atmosphere. The starting point of each face is a flagstone of glass of great dimension (until 1,9 m by 2,2 m for the " 7" generation;) on which several screens are prepared simultaneously. They are cut out after the bonded assembly on the two sides.Glass used must be able to be produced in low thickness (lower than the millimetre) and to resist the various chemical and thermal treatments without deformation (temperature of vitreous transition higher than 600° C) nor loss of transparency (resistance to derived the fluorinated). For this purpose, one uses glasses with strong content of Silice, without addition of Baryum.
The front pane receives successively the pigments of the coloured mask, a layer of protection, a layer of ITO (front electrode) then of polyimide. This one is slightly grooved by friction with a special velvet. The rear window follows a more complex process: silicon deposits, of metals for the electrodes, the lines of data and condenser (Tantalum, Aluminum), oxidation, photolithography, then spacers and finally polyimide.
The bonded assembly must be extremely precise (about the micrometer) to ensure a perfect correspondence between the coloured mask and the under-pixels. Then only the unit is filled with the solution of liquid crystals. The last operation is the application of a polarizing film (polymer acetate) on each side of the assembly.
Orders of magnitude
For better realizing of certain problems of industrialization:- the glass plates have a thickness lower than 1 mm (usually 0,7 mm);
- the thickness of the electrodes in ITO, 100 to 150 µm, gives them a good transparency;
- the films polyimide are extremely fine: 10 with 20 µm;
- the layer of liquid crystals slips into a space of 10 with 20 µm, is less than 1/100 the total thickness, which makes very long the filling of the screens of big size;
- In screens TFT, the layer of silicon does not exceed 100 µm;
- Taking into account these dimensions, quantity of liquid crystals in a screen of 1 m of east coast about 20 cm ³ is 2 Cl!
Recent improvements
They aim at improving:- response time:
- Overdrive : Technique of order consisting in applying an impulse of tension higher than necessary to obtaining a level of gray during the beginning of the cycle. The response time blanc->gris approaches blanc->noir thus;
- the contrast and depth of the black by decreasing the proportion of surface occupied by the mask while rejecting the parasitic light as well as possible;
- Electrodes on resin : Electrodes ITO are not deposited any more on the substrate between the tracks but after filling by a fine layer of resin, on this one, making it possible the electrodes to have the effective maximum size,
- Masque on layer TFT : In complement of the mask between the paving stones of color of filter RVB, a masking is directly applied to layer TFT between the electrodes of each cell;
- quality:
- photogravés Spacers : The balls of spacing are dispersed by chance and can damage filter RVB or obstruct operation. They are replaced by cones cut out in photosensitive epoxy resin positioned on optimal sites.
- the quality and uniformity of the colors
- Retro-lighting by LED (electroluminescent diodes).
Other manufactoring processes
Among alternative technologies utlisant liquid crystals, Philips comes to present prototypes of ACL " peints" or paintable display produces by a simpler process (deposit of superimposed layers) ending in a photoengraving of the cells of liquid crystals ( photo-enforced stratification ).
Future and competition
Screen ACL zero energy
The device " zenith flip-flop device" (ZBD), developed by the British company QinetiQ preserves an image without power supply.The Nemoptic French company develops another technology zero-energy, the nematic technology bistable BiNem®, which makes it possible to post levels of gray and to even manufacture color screens and is from now on at the going stage of pre-industrialization for applications of the ultra-portable (E-delivers, E-dictionary) to the electronic label.
Concurrent emergent technologies
Screens
- the screens électrolumiscents with OLED ( Organic light-emitting diode );
- the screens électrochromes using the properties of the Viologène S (derivatives of the 4,4 ' - bipyridine);
- Beginning again by simplifying it and gearing down it the principle of the cathode tubes (impact of electrons accelerated on a " phosphore") SED ( Surface-conduction Electron-emitter Display ) seem more promising in the long term than the screens plasma. For the moment (August 2007), the date of at the end of 2007 is not confirmed for the first television sets.
Projection
- cells DLP ( DIGITAL Light Processing TM ) using microscopic oscillating mirrors, the DMD ( DIGITAL Micromirror Device );
- technology LCOS ( Liquid Hook One Silicon ), very recent, adds a reflective layer between the TFT and liquid crystals.
Notes and references of the article
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