Monochrome display     LCD     OLED     STN     CSTN     QVGA     

Monochrome display

In physics, the word is used more generally to refer to electromagnetic radiation of a single wavelength. In the physical sense, no real source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration. Even sources such as lasers have some narrow range of wavelengths (known as the linewidth or bandwidth of the source) within which they operate.

For an image, the term monochrome is usually taken to mean the same as black-and-white or, more likely, grayscale, but may also be used to refer to other combinations containing only two colors, such as green-and-white, green-and-black. It may also refer to sepia or cyanotype images. In computing, monochrome has two meanings:

it may mean having only one color which is either on or off, allowing shades of that color, although the latter is more correctly known as greyscale. A monochrome computer display is capable of displaying only a single color, often green, amber, red or white, and often also shades of that color.

The monochromatic scheme should be used with caution when designing a space. Certain monochromatic color concepts will appear rather monotonous, and some variety in the intensities, textures and forms should be used to give life to the interior.

Liquid Crystal Display(LCD)

Each pixel consists of a layer of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other.

Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves with microscopic grooves on the electrodes. The grooves on the two electrodes are perpendicular, so the molecules arrange themselves in a helical structure, or twist (the "crystal"). Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. Half of the light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.

When an electrical charge is applied to the electrodes, the molecules of the liquid crystal are pulled parallel to the electric field, thus reducing the rotation of the entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked. The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel.

It is normal to align the polarizing filters so that pixels are transparent when relaxed and become opaque in the presence of an electric field, however the opposite is sometimes done for special effect.

The electric field necessary to align the liquid crystal molecules rapidly is also enough to pull them out of position, damaging the display. This is solved by using an alternating current to rapidly pull the molecules in alternate directions.

To save cost in the electronics, LCDs are often multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (say, in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (say, in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

Important factors to consider when evaluating an LCD monitor include resolution, viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, aspect ratio, and input ports (e.g. DVI or VGA).

Transmissive and reflective displays

Reflective LCDs, often found in digital watches and calculators, are illuminated by external light reflected by a (sometimes) diffusing reflector behind the display. This type of LCD can produce darker 'blacks' than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated twice. Because the reflected light is also attenuated twice in the translucent parts of the display image, however, contrast is usually poorer than in a transmissive display. The absence of a lamp significantly reduces power consumption, allowing for longer battery life in battery-powered devices; small reflective LCDs consume so little power that they can rely on a photovoltaic cell, as often found in pocket calculators.

Transflective LCDs work as either transmissive or reflective LCDs, depending on the ambient light. They work reflectively when external light levels are high, and transmissively in darker environments via a low-power backlight.

Color Displays

The first operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced in 1968 by a group at RCA in the USA headed by George Heilmeier. Heilmeier founded Optel, which introduced a number of LCDs based on this technology.

In December 1970, the twisted nematic field effect in liquid crystals was filed for patent by M. Schadt and W. Helfrich, then working for the Central Research Laboratories of Hoffmann-LaRoche in Switzerland (Swiss patent No. 532 261). James Fergason at Kent State University filed an identical patent in the USA in February 1971. In 1971 the company of Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on the TN-effect, which soon superseded the poor-quality DSM types due improvements of lower operating voltages and lower power consumption.

In 1972, the first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.(1) for displaying color. Color LCDs initially were used only for handheld video games, but thanks to improvements in quality and price they are now becoming the dominant form of computer display.

Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through subpixel rendering. This technique is especially useful for text anti-aliasing.

Passive-matrix and active-matrix

Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing supertwist nematic (STN) or double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with STN). Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called a passive matrix because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix LCDs.

High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times, producing much better images.

Organic light-emitting diode(OLED)

One of the great benefits of an OLED display over the traditional LCD displays is that OLEDs do not require a backlight to function. This means that they draw far less power and, when powered from a battery, can operate longer on the same charge. It is also known that OLED based display devices can be more effectively manufactured than liquid-crystal and plasma displays. However, degradation of OLED materials (see Drawbacks) have limited the use of these materials.

History

The low electrical conductivity of such materials limited light output and thus commercial development until more conductive organic materials became available. One important example is the linear-backbone polyacetylene-based "Blacks" (AKA "Melanins"). These first emerged in 1963 when BA Bolto et al reported [2] [3] high conductivity in iodine-"doped" oxidized polypyrrole. They achieved a conductivity of 1S/cm. Unfortunately, this discovery was "lost", as was the subsequent report [4] by John McGinness and coworkers of a melanin-based bistable switch with a high conductivity "ON" state. This material emited a flash of light when it switched.

In a subsequent 1977 paper Shirakawa et al, reported high conductivity in a similar oxidized iodine-doped polyacetylene. The latter researchers received the 2000 Noble prize in Chemistry for this and its subsequent development. The Nobel citation made no reference to the earlier discoveries.

Modern work with electroluminescence in such polymers culminated with Burroughs et al's 1990 paper in the journal Nature [6] reporting a very high efficiency green-light-emiting polymer.

Functionality

An exciton can be in one of two states, singlet or triplet. Only one in four excitons is a singlet. The materials currently employed in the emissive layer are typically fluorophors, which can only emit light when a singlet exciton forms, which reduces the OLED's efficiency.

Luckily, by incorporating transition metals into a small-molecule OLED, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from the triplet state. However, this emission is always redshifted, making blue light more difficult to achieve from a triplet excited state. It is pointed out that triplet emitters can be four times more efficient than OLED technology [3].

To create the excitons, a thin film of the luminophore is sandwiched between electrodes of differing work functions. Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode. The electron and hole move into the emissive layer and can meet to form an exciton. Mechanisms and details of exciton formation are discussed in [3] and [4].

Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer luminophores in OLEDs. Indium tin oxide is a common transparent anode, while aluminium or calcium are common cathode materials. Other materials[5] are added between the emissive layer and the cathode or the anode to facilitate or hinder hole or electron injection, thereby enhancing the OLED efficiency.

Advantages

The range of colors, brightness, and viewing angle possible with OLEDs are greater than that of LCDs because OLED pixels directly emit light. Because of this, OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from the axis perpendicular to the display. LCDs employ a backlight and are incapable of showing true black, while an "off" OLED element produces no light and consumes no power. In LCDs, energy is also wasted because the liquid crystal acts as a Polarizer which filters out about half of the light emitted by the backlight.

Drawbacks

Also, the intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

Commercial development of the technology is also restrained by patents held by Eastman Kodak and other firms, requiring other companies to acquire a license. In the past, many display technologies have become widespread only once the patents had expired; aperture grille CRT is a classic example.

Commercial Uses

OLEDs could also be used as solid state light sources. As by now the OLED efficacies and lifetime already go beyond those of tungsten bulbs, white OLEDs are under worldwide investigation as source for general illumination

Quarter Video Graphics Array(QVGA)

The name is derived from the fact that it offers 1/4 of the 640 × 480 maximum resolution of the original IBM VGA display technology, which became a de facto industry standard in the late 1980s. QVGA implementations are not compatible with, nor directly derived from, standard VGA chipsets or interfaces; the term refers only to the display's resolution and thus the abbreviated term QVGA or Quarter VGA is more appropriate to use.

The QVGA term is also seen in digital video recording equipment as a space-efficient mode, typically in multi-function devices that are also still digital cameras (such as the Fujifilm FinePix S602) or mobile phones (such as the Pantech PH-L4000V, Samsung SGH-D600). Each frame is an image of 320 × 240 pixels. QVGA video is typically 15 or 30 frames per second. QVGA mode refers just to the resolution and is not a video file format.

Prior to Version 7, iTunes distributed television programs in QVGA for watching on the computer or syncing to the fifth-generation iPod, which is capable of playing QVGA resolution videos at 30 frames per second. The service now distributes VGA resolution television programs and movies.

At higher resolutions the "Q" prefix sometimes means "Quad" or four times the display resolution (e.g. QXGA which is 2048 × 1536).

STN

CSTN stands for "color super-twist nematic" a form of passive matrix LCD (Liquid Crystal Display) for electronic display screens. Originally developed by Sharp Electronics Corporation. Unlike TFT, CSTN is based on a passive matrix, which is less expensive to produce. The original CSTN displays developed in the early 90's suffered from slow response times and ghosting (where lit pixels in a row can affect the unlit pixels). Recent advances in the technology, however, have made CSTN a viable alternative to active matrix displays. New CSTN displays offer 100ms response times (TFT 8ms), a 140 degree viewing angle, and high-quality color rivaling TFT displays - all at about half the cost. A newer passive-matrix technology called High-Performance Addressing (HPA) offers even better response times and contrast than CSTN.

There are four different kinds of STN displays:

• STN (Monochrome)
• CSTN (Color)
• DSTN (Dual-scan STN)
• FRSTN (Fast Response STN)

Originally developed by Sharp Electronics Corporation. Unlike TFT, CSTN is based on a passive matrix, which is less expensive to produce. The original CSTN displays developed in the early 90's suffered from slow response times and ghosting (where lit pixels in a row can affect the unlit pixels). Recent advances in the technology, however, have made CSTN a viable alternative to active matrix displays. New CSTN displays offer 100ms response times, a 140 degree viewing angle, and high-quality color rivaling TFT displays - all at about half the cost. A newer passive-matrix technology called High-Performance Addressing (HPA) offers even better response times and contrast than CSTN.