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Display Monitors - Glossary of display technology
Category: Electronics and Engineering > display technology
Date & country: 20/09/2007, UK
Words: 44

Active Matrix
In Super Twisted Nematic LCs, we see how multiplexed addressing allows an array of pixels to be controlled in such a way as to allow a high definition display to be built with liquid crystals. The method used to switch individual pixels in multiplexed displays, called passive matrix addressing, has many drawbacks, such as: Current leakage among adjacent pixels causes ghosting, or 'crosstalk', and reduces contrast; Because of the high twist angles of the liquid crystal, viewing angles are limited; Response times for the high twist angles are also slow. This causes smearing of moving image elements, such as a mouse pointer. In the late 1960's, an alternative to passive addressing was developed which uses a thin film diode in the corner of each LC pixel on the rear backplane of the two glass substrates which make up the LCD sandwich. The thin film diode was later replaced by a thin film transistor (TFT), which proved more efficient. Although the presence of the opaque TFT within each cell boundary means that a brighter back light is required with this addressing method, current leakage between the top and bottom substrates is reduced because of the isolation provided by the transistor. Moreover, because the transistor controls the charge on the LC material, a conventional twisted nematic alignment can be used, reducing smearing and making the display of motion video possible for the first time. The finer control also allows a number of variations in LC material and cell configuration to address the problems of ensuring wider viewing angles. This method of driving an LC matrix is called Active Matrix (AM) addressing and AM LCD panels have become the most commonly used in notebook computers, desktop monitors and video projectors.

Developed by Fujitsu as an improvement to the Three Electrode Surface Discharge method of driving a PDP, which it invented in 1984, ALiS (Alternate Lighting of Surfaces) displays are not made up of enclosed pixel elements with the display electrodes fixed to both top and bottom substrates, as are conventional side discharge PDPs. Instead, the two substrates are separated only by spacer columns with the main display electrodes mounted on the same substrate with only the column address electrodes (also acting as pre-charge electrodes) mounted on the opposite substrate. This arrangement of electrodes makes it possible to control the top and bottom portions of each pixel element independently during alternate vertical refreshes, two scan lines being controlled using just three electrodes (see diagram). Using this technique, the vertical resolution of the panel is effectively doubled, making it suitable for displaying HDTV pictures of 1,000 or more lines.

Alpha-blending is a technique which can be used when computer graphics are laid on top of each other and one or more of the objects contain a transparent, or semi-transparent, portion. It ensures that the pixels of the graphic which are underneath a transparent area are visible through it and that their colour or brightness is adjusted according to the degree of transparency of the upper object. The alpha channel is really a form of mask, dictating what amount of information should be allowed to show through from lower-lying graphics.

Anisotropic Filtering
Anisotropic means 'non-uniform shape' and it is because this filtering technique works on non-uniform, or uneven, shaped areas that it is more powerful, and takes much more processing power, than point sampling, bi-linear or tri-linear filtering.

The control grid regulates the number of electrons which are allowed to leave the gun of a CRT by using a negative voltage to repel the electrons. Once free of the control grid's influence, the electrons then need to be accelerated down the tube to ensure that they strike the phosphor face-plate lining with sufficient energy to excite the phosphor molecules and produce light. Just as a negative voltage repels the electrons, a positive voltage will attract them and an electrode, called the anode, to which a positive voltage is applied is used to provide this acceleration. The process is similar to that used to accelerate space probes by making use of the gravity-well of a planet to provide a 'sling-shot' effect. The idea being that, by the time the electrodes arrive in the vicinity of the anode, they are going too fast to be sufficiently diverted from their path to strike it.

Antialiasing, as the name suggests, is a technique used to combat an effect known as aliasing.

Aperture Grille
Before the three-gun, shadow mask CRT was firmly established as a standard for producing colour CRTs, several other novel systems were developed. One of these was the PDF (Post Deflection Focusing) Chromatron developed at the University of California. This tube had a single electron gun addressing a pattern of continuous vertical phosphor stripes on its faceplate. As the name suggests, the beam of electrons in this CRT were focused on the required red, green or blue phosphor stripe by electrodes which the beam encountered after it had passed beyond the influence of the deflection yoke. These electrodes were mounted less than half an inch from the phosphor-coated faceplate and took the form of a series of vertically-oriented parallel wires. By altering the way current flowed through pairs of these electrodes, the electron beam could be made to pass directly between them or be deflected slightly to left or right and so made to strike the desired colour of phosphor stripe. The Chromatron was produced by Sony in the early 1960's but was dropped after a short while because of manufacturing difficulties.

There are parts of the CRT that control the production, flow, acceleration and direction of the tube's electron beam but leave its shape unaltered. As the electrons accelerate towards the screen, they tend to diverge, as they all have a similar negative charge. If this divergence were to continue, the beam's diameter when it reached the faceplate would be much wider than a single pixel, so some form of focusing device is needed to concentrate the beam. Three methods of achieving this beam-focusing have been used at various times: gas, where a small quantity of a gas, such as argon or mercury vapour, is introduced into the CRT; electromagnetic, where the beam is controlled in a similar manner to that employed by the deflection yoke ; and electrostatic, which uses the same principles as do the control grid and anode .

Bi-Tri-Linear Filtering
A simple form of texture filtering, point sampling, could be used to scale textures prior to their being mapped onto surfaces in a 3D scene. This technique could easily introduce errors by taking a too simple approach to the task.

Braun Tube
The development of the first CRT was the culmination of many years, or in this case, centuries, of research and discovery which put into place all the necessary elements. These include: the first manmade phosphor (1603); the first gas discharge tube (1751); the first production of light by exciting a phosphor with an electrical discharge (1768); and the first magnetic (1859) and electrostatic (1876) deflection of cathode-rays. Along the way, discoveries by such famous names as Bernoulli, Faraday and Hertz contributed to the sum of knowledge which allowed Karl Ferdinand Braun to combine them in what became known as the Braun tube (1897).

Control Grid
Although the Braun tube contained all the essential elements of the modern CRT, these existed in a fairly primitive form and it lacked many of the refinements which are now considered as basic components.

Deflection Circuit
The deflection yoke's electromagnets are used to alter the path of the electron beam in a CRT. The job of synchronising the influence exerted by each of the yoke's two magnets, and so controlling the directing of the beam towards specific parts of the screen, is handled by some electronics known as the deflection circuit.

Deflection Yoke
To make a beam of electrons do something useful, it has to be deflected from its course in some manner so as to make a trace or scan across the CRT's faceplate. There are two methods used to do this: with an electrostatic charge and an electro-magnetic field.

Originally, the term 'dongle' was applied to any external attachment to a PC or one of its components. It seems to have originated in the UK and may have been coined to describe something which 'dangled', or hung, from the back of a machine.

Dynamic Beam Forming
A CRTs electron beam focusing circuitry has to change the focal length of an electrostatic or electromagnetic 'lens' according to where on the screen the beam is being directed, as shown in Dynamic Focus . This ensures that the beam is consistently focused on a spot of the correct size on the inside of the faceplate. If the spot is too large, it may spill over into the next screen pixel location; too small and the pixel brightness is reduced. For similar reasons, the shape of the beam should also remain the same, whether it strikes the centre or the corner of the CRT faceplate.

Dynamic Focus
When looking at the operation of the deflection circuit , we see that it constantly adjusts the strength of the deflection yoke's influence on the electron beam as the length of its path to the faceplate, and the amount of time the beam spends under the influence of the deflection yoke and the anode , varies. The focusing circuitry, likewise, has to take these variables into account to ensure that the beam is correctly focused, no matter where on the screen it lands. To achieve this, additional electrodes are added to the basic focusing system illustrated in beam-focusing . Signals taken from the deflection amplifiers are superimposed on the voltage fed to these electrodes, altering the strength of their electromagnetic field in step with the adjustments made to the strength of the deflection yoke's electro-magnetic field. This process of constantly changing the CRT's focusing characteristics is known as dynamic focusing.

Elegant in their simiplicity, electro-luminescent displays offer the thinnest profile of any colour flat panel technology.

When Display Monitor began publication in 1994, one of the technologies being tipped as representing the future of flat panel displays was the FED or Field Emission Display. Similar in principle to a CRT, the FED uses a beam of electrons to excite phosphors which then emit visible light. Unlike a CRT, it uses multiple electron emitters for each pixel in the display. The basic concept therefore requires no beam steering circuitry and can be made as shallow as the depth of 10mm. Another bonus is that the electrons in a FED are not produced by heat, as they are in a CRT, so the display does not need to warm up and does not produce large amounts of unwanted heat. It has several advantages over LCDs as well: it requires no back light, is very light, has a very wide viewing angle, its response time is very short, it has a very high contrast ratio and has excellent colour properties. The first generation of FEDs uses tiny, conical electron emitters (known as a 'Spindt tip') but in the future there could be versions using carbon nanotubes which have been demonstrated by Samsung. Nanotubes are potentially a more efficient electron emitter than the Spindt tip.

Grating Light Valve
Sony's licensing of Silicon Light Machine's Grating Light Valve (GLV) technology means that it may not be too long before TI's DMD loses its place as the only Micro-Electro-Mechanical System (MEMS) to be used in projection. Like the DMD, the GLV is a digital, reflective technology in which parts of the device are physically moved to alter the path of light shining on the chip's surface. Unlike the DMD, however, pixels in a GLV reflect light only in their 'off' state. In their 'on' state, they diffract light in much the same way as does the recorded surface of a CD.

High-Field EL
The basic electro-luminescent (EL) display is a sandwich consisting of an upper and a lower electrode sandwiching a material which gives off light when excited by high frequency.

In-Plane Switching
If LCDs are ever to replace the CRT-based display, one of the major problems to be overcome is that of narrow viewing angles. Several solutions have been developed but the different approaches have met with varying success when other factors such as contrast, response times and power consumption are taken into account.

One group of electro-luminescent (EL) devices is known as high-field ELs. The other main group is known as the Light Emitting Diodes, which is itself split into two groups consisting of those devices using inorganic light emitting materials and those using organic light emitting materials. The devices using inorganic light emitting materials are known simply as LEDs, while those using organic materials are known as OLEDs. The reason that the inorganic group has the same name as the higher level group, rather than being known as ILEDs, is that they were developed before it was known that there were organic materials suitable for creating LEDs and the name has stuck.

The Universal Display Corporation's TOLED is based on small-molecule organic light emitting materials. Another of the high-profile OLED development companies, Cambridge Display Technology (CDT), is putting its faith in large-molecule, or polymer, organic light emitting materials.

Light Valve
A light valve (LV) is a form of optical switch which can be used to vary the amount of light which reaches a target (e.g. a screen) by deflecting the light from its path towards the target (a reflective LV) or by blocking the light-path (a transmissive LV). They were first used in projection TVs and are now used widely in flat panel displays, data and video projectors and, increasingly, in rear projection monitors.

Light Valve Amplifier
Light Valve Amplifier (LVA) is a term used to describe a specific type of light valve developed by Hughes-JVC (Image Light Amplifier (ILA)).

Liquid Crystal
Liquid crystal displays (LCDs) are now so common that we take it for granted that people know what we are talking about when we use the phrase. But how many of us know what liquid crystals are? The term itself seems to be a contradiction. Crystals, such as quartz, are formed when molecules form a three-dimensional matrix by attaching themselves firmly to each other in a regular pattern in which they all point in exactly the same direction. This is why crystals have perfectly flat surfaces. The bond between the molecules of some crystals, such as common salt or sugar, can be broken fairly easily by putting them into water but the result isn't a liquid crystal, just a solution. So what is a liquid crystal?

MIP map is the name given to a collection of images to be used as surface textures when building, or rendering, a 2D representation of a 3D scene. The acronym 'MIP' comes from the Latin phrase: multum in parvo which is translated as 'many things in a small place'.

The word 'moiré' comes from the French word for mohair and is defined as 'having a watered or wavelike pattern' (Collins English Dictionary). Moiré patterns can occur whenever similar, regular patterns are superimposed on each other and are offset or angled with respect to one another. They are commonly seen amongst the folds of net curtains and, more relevantly to Display Monitor readers, in CRT displays.

The word 'moiré' comes from the French word for mohair and is defined as 'having a watered or wavelike pattern' (Collins English Dictionary). Moiré patterns can occur whenever similar, regular patterns are superimposed on each other and are offset or angled with respect to one another. They are commonly seen amongst the folds of net curtains and, more relevantly to Display Monitor readers, in CRT displays.

Multi-Domain Vertical Alignment
While Hitachi was developing its In-Plane Switching (IPS) Super TFT, Fujitsu and the chemical giant Merck were taking a different approach to increasing viewing angles and, in 1996, announced the development of a new liquid crystal mode, known as Vertical Alignment (VA). In VA, as the name suggests, the molecules of liquid crystal are normally aligned at right angles to the substrates, swinging through 90° to lie parallel with the substrates in the presence of an electromagnetic field. This new mode produces a display which, like Hitachi's, has an ultra-wide viewing angle (140° in all directions) and high contrast but with the added bonus of higher brightness and a response time of 25 milliseconds, shorter than for IPS and STN LCDs. The display also consumes less power than IPS but, is still too power hungry for battery-power applications.

In light emitting diodes (LEDs) based on inorganic light emitting materials, some of the photons released by the inorganic materials are also reabsorbed by them. In organic light emitting diodes (OLEDs), which produce light using the same electron/hole recombination method as LEDs, the frequencies of light absorbed by the light emitting materials lie mostly outside the visible spectrum, making OLEDs transparent when switched off and highly efficient emitters, and transmitters, of light when switched on.

A hybrid of the LCD and a PDP: the Plasma Addressed Liquid Crystal (PALC) display.

Plasma Display
In the search for a large sized flat screen display technology, LCDs do not have the field all to themselves. Though late to join the commercial battle, the next flat large screen devices being commercialised are plasma display panels (PDPs), a technology developed from the principle of the fluorescent (or 'strip') light.

Point Sampling
MIP-maps are used to store several different copies of a texture, at different scales, for use in 3D scene rendering. One of the accompanying illustrations shows four versions of a texture applied to flat surfaces which were parallel to the screen, each surface covered by a single, unprocessed, copy of one of the textures stored in the MIP-map. In 3D programs which allow movement within a scene, however, textures of scales between those stored in the MIP-map have to be applied to surfaces as the viewer approaches or retreats from them. There are several methods which can be applied to achieve this scaling and they are known collectively as 'texture filtering'. The simplest, and least effective, of these is known as 'point sampling'.

Most silicon chips are made of single crystal silicon, which has a very low resistance to electron flow. However, silicon can exist in different forms (just as carbon can exist in the form of diamond, graphite, soot and buckminsterfullerine). As well as single crystal silicon, other forms are Poly-silicon and Amorphous silicon.

Ray tracing mimics the reflection and refraction of light rays in a 3D scene to produce image detail surpassing anything possible using Phong shading. Effective as it is, though, ray tracing by itself will not produce photo-realistic images. At the very minimum, some kind of ambient lighting must be applied to the scene to reduce the contrast between lit and shadowed areas, because ray tracing does not deal at all well with light reflected from matt surfaces, such as painted walls. To soften things up a bit, and to make matt surfaces look more life-like, radiosity is often used in conjunction with ray tracing.

Ray Tracing
Effective as they can be at producing recognisable 3D scenes, Gouraud and Phong shading will never produce anything that could be described as photo-realistic. To achieve this, it's necessary to use rendering techniques that mimic the physics of real-world lighting. The most commonly used of these are called 'ray tracing' and 'radiosity'. Ray tracing is especially useful for rendering images which contain reflective surfaces and transparent or translucent objects. As the name implies, it involves tracing the path taken by rays of light within a 3D scene but, contrary to expectations, it doesn't start working from the scene's light source but from the observer's eye.

Saddle-Saddle Yoke
The most common form of deflection yoke used in early colour monitors is known as a saddle-toroidal yoke. Its name comes from the shapes of the vertical and horizontal deflection electro-magnets' windings. The winding of the electro-magnet used for vertical beam deflection is saddle shaped, while the one responsible for horizontal deflection takes the form of a torus, i.e. is doughnut-shaped. The word saddle is used because the shape of the coil is similar to that of a horse's saddle.

FEDs have proved difficult to develop and many companies had dropped their development projects in favour of other technologies. One company, however, has been working on a variation of the FED which currently looks to be a good contender for commercial production due to its simplicity and lower production costs. The company is Canon and the FED-variant is the Surface-conduction Emission Display (SED).

Objects in 3D scenes are most commonly created by fitting together a large number of polygons (usually triangles), edge to edge, creating a 'wire-frame' which approximates the object's surface. By drawing only those triangles which lie on surfaces visible to the viewer (known as 'hidden line removal'), recognisable, though unrealistic, objects can be portrayed. There are a number of ways to make objects more realistic. One being texture mapping and another: shading. As the name suggests, shading means simulating light falling on a scene and the effect this has on the brightness and colour of different areas of objects in the scene. There are many different ways to shade a scene but we will look at the three most commonly used:

Shadow Mask
Before the stream of electrons produced by the CRT's cathode reach the phosphor coated faceplate, it encounters the shadow mask, a sheet of metal etched with a pattern of holes. The mask is positioned in the glass funnel of the CRT during manufacture and the phosphor is coated onto the screen so that electrons coming from the red, green and blue gun positions only land on the appropriate phosphor.

The Universal Display Corporation (UDC) is one of the most prominent developers of Organic Light Emitting Devices (OLEDs). Working with researchers at Princeton University and the University of Southern California, it has produced several variations of the OLED designed to help it achieve its goal of developing multi-purpose flat screen monitors which can be wall, or window, mounted. The company's technology is based on 'small-molecule' OLEDs, in which the layers of the display are built up using vacuum deposition. It claims brightness levels of over 100,000 cd/m² and a lifetime of over 10,000 hours at 100 cd/m².

Super Twisted Nematic LCs
The development of the twisted nematic liquid crystal (TNLC), with its 90° twist, made the production of commercially viable LCDs possible. Though relatively cheap to make and requiring very little power (hence their use in watches, calculators, etc.), there are problems associated with using TNLCDs in larger, more complex displays.

Twisted Nematic LCs
For nearly a hundred years after Friedrich Reinitzer observed cholesteryl benzoate changing into its liquid crystal state in 1888, nine years before the invention of the CRT, liquid crystals remained little more than a chemical curiosity. Then in 1971, Drs. Schadt and Helfrich of Roche developed the twisted nematic liquid crystal and opened the door to the first commercial use of LCs.