FLAT PANEL DISPLAY

Flat panel displays are utilized in a multitude of products and applications. However, each one has different technical requirements and specifications for an electronic display. A cash terminal (ATM machine) for example requires a display with a very narrow viewing angle to protect privacy, while a television set should ideally have a wide viewing angle in order to allow multiple users to enjoy the program. A single display technology meeting all technical requirements does not exist – each one has its pros and cons. One can roughly divide displays into self-emissive displays, which act as a light source themselves, and non-emissive displays which need an external light source to function. Moreover, a display is characterized by a light modulation or -generation technique and a driving scheme (active or passive matrix driving; AM and PM). AM and PM can therefore be used in conjunction with different light modulation or -generation techniques.

Active-matrix liquid-crystal display (AMLCD)

Electronic paper: E Ink, Gyricon

Electroluminescent display (ELD)

Digital Light Processing (DLP)

Field emission display (FED), also named nano-emissive display (NED)

Interferometric modulator display (IMOD)

Light-emitting diode display (LED)

Liquid-crystal display (LCD)

Organic light-emitting diode (OLED)

Plasma display panel (PDP)

Quantum dot display (QLED)

Surface-conduction electron-emitter display (SED, SED-TV)

Flat panel displays encompass a growing number of electronic visual display technologies. They are far lighter and thinner than traditional television sets and video displays that use cathode ray tubes (CRTs), and are usually less than 10 centimeters (3.9 in) thick.

Flat panel displays can be divided into two general display technology categories: volatile and static.

The first engineering proposal for a flat panel TV was by General Electric as a result of its work on radar monitors. Their publication of their findings gave all the basics of future flat panel TVs and monitors. But GE did not continue with the R&D required and never built a working flat panel at that time.

The first production flat panel display was the Aiken tube, developed in the early 1950s and produced in limited numbers in 1958. This saw some use in military systems as aheads up display, but conventional technologies overtook its development. Attempts to commercialize the system for home television use ran into continued problems and the system was never released commercially.

The plasma display panel was invented at the University of Illinois in 1964 at the University of Illinois, according to The History of Plasma Display Panels 

PLASMA DISPLAY PANEL

A plasma display panel (PDP) is a type of flat panel display common to large TV displays 30 inches (76 cm) or larger. They are called "plasma" displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent lamps.

Plasma TV create a picture from a gas (plasma) filled with xenon and neon atoms and millions of electrically charged atoms and electrons, that collide when you turn the power on. The energy the collision releases increases the energy level in the plasma and the neon and xenon release photons of light (similar to the way neon lights work). While they offer excellent picture quality, they are quite expensive and are fast becoming the popular choice for HDTV.

HOW PLASMA DISPLAY PANEL WORKS

The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell, along the front glass plate.

Both sets of electrodes extend across the entire screen. The display electrodes are arranged in horizontal rows along the screen and the address electrodes are arranged in vertical columns. As you can see in the diagram below, the vertical and horizontal electrodes form a basic grid.

To ionize the gas in a particular cell, the plasma display's computer charges the electrodes that intersect at that cell. It does this thousands of times in a small fraction of a second, charging each cell in turn.

When the intersecting electrodes are charged (with a voltage difference between them), electric current flows through the gas in the cell. As we saw in the last section, the current creates a rapid flow of charged particles, which stimulates the gas atoms to release ultraviolet photons.

The released ultraviolet photons interact with phosphor material coated on the inside wall of the cell. Phosphors are substances that give off light when they are exposed to other light. When an ultraviolet photon hits a phosphor atom in the cell, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of a visible light photon.

The phosphors in a plasma display give off colored light when they are excited. Every pixel is made up of three separate subpixel cells, each with different colored phosphors. One subpixel has a red light phosphor, one subpixel has a green light phosphor and one subpixel has a blue light phosphor. These colors blend together to create the overall color of the pixel.

By varying the pulses of current flowing through the different cells, the control system can increase or decrease the intensity of each subpixel color to create hundreds of different combinations of red, green and blue. In this way, the control system can produce colors across the entire spectrum.

ADVANTAGES OF PDP

Picture quality

Capable of producing deeper blacks allowing for superior contrast ratio

Wider viewing angles than those of LCD; images do not suffer from degradation at high angles like LCDs

Less visible motion blur, thanks in large part to very high refresh rates and a faster response time, contributing to superior performance when displaying content with significant amounts of rapid motion.

Undeniable Qualities

Plasma technology has numerous advantages over LCDs and CRTs. First of all, the choice of scintillators for plasma TVs allows richer colors in a wider range. The chromatic range of plasma screens is much broader than for CRT television sets.

Source NEC-Mitsubishi

Next, the viewing angles are very wide, especially compared to LCD displays. The main reason is that the light is generated in the pixels themselves, unlike LCD technology, as we'll see. And plasma screens need no polarizer.

Finally, the contrast is equivalent to the best CRT TVs. The main reason for that is the high quality of the blacks: A pixel that's switched off emits no light at all, contrary to the way an LCD pixel works. Plasma TVs also have better brightness performance than CRT monitors, achieving values of 900 to 1000 nits.

Also note that plasma displays can have very large diagonal measurements (32 to 50 inches) with minimal thickness. That's a decisive advantage over CRTs, which as you know get very bulky in larger sizes.

DISADVANTAGES OF PDP

The size of the pixels themselves is a big problem in plasma displays. It's difficult, if not impossible, to reduce the size of plasma pixels to less than 0.5 or 0.6 mm. Consequently, plasma TVs don't exist in sizes under 32" (82 cm) diagonal. To achieve a resolution that was competitive, plasma had no other choice than to increase the size of the displays, to 32 to 50 inches (82 to 127 cm).

The method is simple. To light a pixel brightly, it's lit very frequently. To get a darker shade, it's lit less often; the user's eye calculates a kind of temporal average. This method is functional, but several problems are associated with it. Most significantly, while it's effective for medium and bright colors, darker colors suffer from the reduced quantification, making it more difficult to distinguish between two dark shades.

While the technology results in a uniform image when the viewer is far enough from the panel, it causes visual discomfort at close distances. It's generally accepted that the human eye isn't capable of distinguishing flickering if the frequency is above about 85 Hz, but that's not exactly true. In fact, the eye is perfectly capable of doing so, but the brain can't "render" the images that fast. Consequently, an image at 85 Hz can cause eye fatigue without the viewer even being aware of the flickering.

That's unfortunately the case with plasma pixels. The flickering can be a cause of discomfort if you're too close to the panel. So, the image on a plasma display is bigger, but you have to be that much farther away from it. Consequently, the immersion experience is no more intense.

Plasma pixels are also subject to burn-in. On a CRT monitor, when the same image is projected for a very long time, it becomes permanently imprinted on the phosphor. After too long an exposure, when the image changes, the preceding one remains visible, as if it were engraved into the monitor. This phenomenon is due to premature aging of the scintillators. When they're used continuously, they age and become less efficient. Since plasma displays use scintillators, they're also subject to burn-in just like CRT monitors.

Due to the stable nature of the colour and intensity generating method, some people will notice that plasma displays have a shimmering or flickering effect with a number of hues, intensities and dither patterns.

Earlier generation displays (circa 2006 and prior) had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness (newer models may be less susceptible to this, having advertised lifespans exceeding 100 000 hours, far longer than older CRT technology)

Screen-door effects (black lines between rows of pixels) become noticeable on screen sizes larger than 127 cm (50 in); the effect is more visible at shorter viewing distances.

Uses more electrical power, on average, than an LCD TV.

Does not work as well at high altitudes above 2 km due to pressure differential between the gases inside the screen and the air pressure at altitude. It may cause a buzzing noise. Manufacturers rate their screens to indicate the altitude parameters.

For those who wish to listen to AM radio, or are amateur radio operators (hams) or shortwave listeners (SWL), the radio frequency interference (RFI) from these devices can be irritating or disabling.

LIQUID CRYSTAL DISPLAY (LCD)

The term "liquid crystal" dates back not just to the last century, but the one before that - the phrase originated in 1889! And it comes to us not via electronics, but botany. However, it wasn't until 1968 that RCA became interested in the phenomenon and invented the first liquid-crystal display. In 1969, James Fergason discovered the twisted nematic (TN) effect. This was a fundamental discovery, since all the LCD displays we're familiar with are based on this principle of rotation of the plane of polarization. In 1973, George Gray invented the biphenyl liquid crystal, which made it possible to implement liquid-crystal solutions that were stable under normal pressure and temperature conditions. And as early as 1986, NEC produced the first portable computer with a Liquid Crystal Display (LCD). In 1995, LCD panels with large diagonal measurements - over 28" (71 cm) - began to be produced.

LCDs are used in digital clocks, cellular phones, desktop and laptop computers, and some televisions and other electronic systems. They offer a decided advantage over other display technologies, such as cathode ray tubes, in that they are much lighter and thinner and consume a lot less power to operate.

Physical Principle

Liquid crystals are neither a pure solid nor a pure liquid, but rather a hybrid of both. One particular variety of interest is the twisted nematic liquid crystal whose molecules have a natural tendency to assume a twisted spiral structure when the is sandwiched between finely grooved glass substrates with orthogonal orientations (A). Note that the molecules in contact with the grooved surfaces align themselves in parallel along the grooves. The molecular spiral causes the crystal to behave like a wave polarizer; unpolarized light incident upon the entrance substrate follows the orientation of the spiral, emerging through the exit substrate with its polarization (direction of electric field) parallel to the groove’s direction.

LCD Structure

A single-pixel LCD structure is shown in (B1) and (B2) for the OFF and ON states, with OFF corresponding to a bright-looking pixel and ON to a dark-looking pixel. The sandwiched liquid-

crystal layer (typically on the order of 5 microns in thickness, or 1/20 of the width of a human hair) is straddled by a pair of optical filters with orthogonal polarizations. When no voltage is applied across the crystal layer (B1), incoming unpolarized light gets polarized as it passes through the entrance polarizer, then rotates by 90º as it follows the molecular spiral, and finally emerges from the exit polarizer, giving the exited surface a bright appearance. A useful feature of nematic liquid crystals is that their spiral untwists (B2) under the influence of an electric field (induced by a voltage difference across the layer). The degree of untwisting depends on the strength of the electric field. With no spiral to rotate the wave polarization as the light travels through the crystal, the light polarization will be orthogonal to that of the exit polarizer, allowing no light to pass through it. Hence, the pixel will exhibit a dark appearance.

2D Array

By extending the concept to a two-dimensional array of pixels and devising a scheme to control the voltage across each pixel individually (usually using a thinfilm transistor), a complete image can be displayed as illustrated in (C). For color displays, each pixel is made up of three sub-pixels with complementary color filters (red, green, and blue).

HOW LCD WORKS

The main difference between plasma and LCD technology is that LCD pixels don't emit any light. All the qualities but also all the faults of the technology stem from that key characteristic.

As with other technologies, an LCD pixel is made up of three sub-pixels in elementary colors. The operating principle is interesting though: the LCD doesn't emit light, but rather acts as a switch, which is why LCD displays need white backlighting. The light emitted by the

backlighting passes through the liquid crystal and is then colored by a filter. Each sub-pixel has the same architecture - only the color of the filter changes depending on the pixel. The liquid crystal of each sub-pixel can be controlled electrically like a valve. More or less light is allowed to pass through the crystal to control how much red, green and blue is emitted for each pixel.

The backlighting emits natural, non-polarized, white light. The polarization of light is determined by the orientation of its electric field vector. Without going into too much detail, light is an electromagnetic wave. Its electric and magnetic field vectors are perpendicular to the direction of its movement. A lamp emits non-polarized light, so the electrical field can travel in any direction perpendicular to the axis of propagation of the light. When light passes through a polarizer, the light that comes out the other side has an electric field vector oriented in a known direction (vertical in our example). If the light is then passed through a second polarizer, perpendicular to the first (horizontal in this example), no light can pass through. But if a liquid crystal is placed between the two polarizers, the crystal turns the plane of polarization of the light to align with the second polarizer, and the light can then pass through. This natural property of liquid crystals is what accounts for their success in display technologies.

Now, if a direct current is introduced at each end of the liquid crystal, the crystals orient themselves with the difference in potential, a little like the way a magnet orients itself to the Earth's magnetic field. By preventing rotation of the plane of polarization in this way, the crystals prevent the light from passing through the horizontal polarizer, since it remains vertically polarized. The beam of light is interrupted.

By varying the voltage on the terminals of the liquid crystal, the closure of the "switch" can be modulated more finely to produce intermediate states.

The LCD panel is lit either by two cold cathode fluorescent lamps placed at opposite edges of the display or an array of parallel CCFLs behind larger displays. A diffuser then spreads the light out evenly across the whole display. For many years, this technology had been used almost exclusively. Unlike white LEDs, most CCFLs have an even-white spectral output resulting in better color gamut for the display. However, CCFLs are less energy efficient than LEDs and require a somewhat costly inverter to convert whatever DC voltage the device uses (usually 5 or 12 V) to ~1000 V needed to light a CCFL.

Addressing LCD Matrices

Addressing passive-matrix LCDs is done very similarly to the way it's done in plasma displays. A front electrode, common to the entire column, conducts the voltage. The rear electrode, common to the entire row, serves as ground.

The disadvantages of old-fashioned passive matrixes are numerous and well known: the panels are slow, and the display is not sharp. There are two reasons for that. The first has to do with the fact that the pixel, once it has been addressed, slowly begins returning to its normal state, creating a blurred image. The second is due to the capacitive coupling between the control lines.

This coupling makes the propagation of voltage imprecise and contaminates the neighboring pixels.

To remedy these problems, manufacturers have adopted active matrix technologies.

This technology, known as "TFT" for the thin film transistors it uses, has become so popular now that its name has come to be associated with all LCD monitors.

The voltages used are much lower than for plasma displays. To operate a TFT pixel, voltages of around -5 to +20 volts are needed, which is a far cry from the hundreds of volts plasma panels require.

In general, LCD-based solutions are less expensive than plasma TVs, but naturally, market factors have to be taken into account. When there's a shortage of panels, even lower-cost technologies can reach exorbitant prices. We saw that happen a little over a year ago.

In terms of image quality, LCDs offer better brightness than CRT displays. And LCD pixels don't flicker, which means they can be used close up, creating a better immersion experience.

LCD TVs have exceptional image stability, meaning you can sit close and not experience eye fatigue. In addition, the brightness is excellent and the image is perfectly sharp. Add to that the reasonable production price - barring shortages - and reduced footprint, and you can see that LCD has a lot going for it.

Advantages of LCD TV:1. Sharpness Image is perfectly sharp at the native resolution of the panel. By using analog input require careful adjustment of pixel tracking/phase in LCDs

2. Geometric Distortion At the native resolution of the panel there is zero geometric distortion. Minor distortion for other resolutions because the images must be rescaled.

3. Brightness By using very bright images high peak intensity is produced. Best for brightly lit environments.

4. Screen Shape Screens are perfectly flat.

5. Physical There is a small foot print and Consume little electricity and produce little heat.

Disadvantages of LCD TV;1. Resolution Each panel has a fixed pixel resolution format determined at the time of manufacture that cannot be changed. All other image resolutions require rescaling, which generally results in significant image degradation, particularly for fine text and graphics. For most applications should only be used at the native resolution of the panel. If you need fine text and graphics at more than one resolution do not get an LCD display.

2. Interference LCDs using an analog input require careful adjustment of pixel tracking/phase in order to reduce or eliminate digital noise in the image. Automatic pixel tracking/phase controls seldom produce the optimum setting. Timing drift and jitter may require frequent readjustments during the day. For some displays and video boards you may not be able to entirely eliminate the digital noise.

3. Viewing Angle Limited viewing angle. Brightness, contrast, gamma and color mixtures vary with the viewing angle. Can lead to contrast and color reversal at large angles. Need to be viewed as close to straight ahead as possible.

4. Black-Level, Contrast and Color Saturation LCDs have difficulty producing black and very dark grays. As a result they generally have lower contrast than CRTs and the color saturation for low intensity colors is also reduced. Not suitable for use in dimly lit and dark environments.

5. White Saturation The bright-end of the LCD intensity scale is easily overloaded, which leads to saturation and compression. When this happens the maximum brightness occurs before reaching the peak of the gray-scale or the brightness increases slowly near the maximum. Requires careful adjustment of the Contrast control.

6. Color and Gray-Scale Accuracy The internal Gamma and gray-scale of an LCD is very irregular. Special circuitry attempts to fix it, often with only limited success. LCDs typically produce fewer than 256 discrete intensity levels. For some LCDs portions of the gray-scale may be dithered. Images are pleasing but not accurate because of problems with black-level, gray-scale and Gamma, which affects the accuracy of the gray-scale and color mixtures. Generally not suitable for professional image color balancing.

7. Bad Pixels and Screen Uniformity LCDs can have many weak or stuck pixels, which are permanently on or off. Some pixels may be improperly connected to adjoining pixels, rows or columns. Also, the panel may not be uniformly illuminated by the backlight resulting in uneven intensity and shading over the screen.

8. Motion Artifacts Slow response times and scan rate conversion result in severe motion artifacts and image degradation for moving or rapidly changing images.

9. Aspect Ratio LCDs have a fixed resolution and aspect ratio. For panels with a resolution of 1280x1024 the aspect ratio is 5:4=1.25, which is noticeably smaller than the 4:3=1.33 aspect ratio for almost all other standard display modes. For some applications may require switching to a letterboxed 1280x960, which has a 4:3 aspect ratio.

LED

A light-emitting diode (LED) is an electronic light source. The LED was first invented in Russia in the 1920s, and introduced in America as a practical electronic component in 1962.

LEDs are based on the semiconductor diode.C

When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called ELECTROLUMINESCENCE and the color of the light is determined by the energy gap of the semiconductor.

Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field band gap energy is the span of energies that lie between the valence and conduction bands for insulators and semiconductors.

The diagram schematically shows the path of electrons moving through a circuit containing a p-n junction. Electrons flow from the negative pole to the n-type semiconductor, where they occupy the higher-energy (conduction) band. The electrons then move into the conduction band of the p-type semiconductor and fall into the empty orbitals of the valence band, which releases energy in the form of light

A LED display, or light emitting diode display, is a flat panel display that uses light emitting diodes as the video display. They are typically used outdoors in store signs and billboards . An LED panel is a small display, or a component of a larger display. LED panels are sometimes used as form of lighting, for the purpose of general illumination, task lighting, or even stage lighting rather than display. LED panel consists of several LEDs, whereas an LED display consists of several LED panels

The first recorded flat panel LED television screen developed was by J. P. Mitchell in 1977

LEDs present many advantages over traditional light sources including lower energy

consumption, longer lifetime, improved robustness, smaller size and faster switching. The

compact size of LEDs has allowed new text and video displays and sensors to be developed

How Flat Screen Display Works

Back Lighting

Types of LED TVs

Full- or Direct-Lit LED TV

In a full-array LED TV, many clusters of LEDs

are arrayed across the back of the screen to light

the image; this allows for full-array sets that can

also feature local-dimming, whereby individual

or small groups of LEDs can be separately dimmed or even completely shut off in dark areas of

the picture. This results in more accurate contrast and brightness, deeper black levels, and richer

color saturation over standard. Full-Array LED sets can rival plasma displays in terms of overall

picture quality.

Edge-Lit LED TV

LED televisions have a series of LED backlights that run along the outside edges of the screen, which is dispersed across the entire screen by light guides. By placing the LEDs along the edges only, manufacturers are able to make screens very thin and further reduce costs. This configuration sometimes creates a brightness around the edge of the screen and decreases the depth of blacks, but this is not often noticeable when viewing in brightly lit rooms. Spotlighting in the corners or white blotches across the screen may also be noticeable when viewing in a dark room.

Advantages of LED TVsLess power than LCD televisions and plasma setsWeight less and take up less space. Display more realistic colors than CCFL-backlit LCD modelsFor local dimming as this feature produces wider viewing angles, more contrast in black levels, and increased depth in the picture. Have very long lives, They do not have the same tendency as CCFL-backlit LCD TVs to develop white-balance color changes as they age.

Disadvantages of LED TVs

LED technology is also more expensive than CCFL

Uniformity of picture suffers with the thinner screens found on LED-backlit televisions, and off-angle viewing is not very good, although LED-backlit LCD models with local dimming do give a much better picture. However, models with local dimming consume more power than models without it. In some instances, models with local dimming consume as much energy as power-hungry plasma televisions.