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Large-screen television technology

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Large-screen television technology developed rapidly in the late 1990’s and 2000’s, and currently the most popular technologies are liquid crystal display (LCD), plasma display, and projection television. These technologies have quickly displaced cathode ray tubes (CRT) in the television space, mainly because of the bulkiness of CRT televisions and increasing popularity of flatter, large-screen televisions. The diagonal screen size of a CRT television is limited to about 40 inches because of the size requirements of the cathode ray tube, which fires a beam of electrons onto the screen, creating a viewable image. A larger screen size requires a longer tube, making a CRT television with a large screen (50 to 80 inches) unrealistic because of size. The aforementioned technologies are much more suitable for large-screen televisions because they yield television sets that are much thinner in terms of depth.

Display Specifications

The following are important factors for evaluating television displays:

  • Display Size: This refers to the diagonal length of the display.
  • Display resolution: This refers to the number of pixels in each dimension on a display. In general a higher resolution will yield a clearer, sharper image.
  • Dot pitch: This measures the size of an individual pixel, which includes the length of the subpixels and distances between subpixels. It can be measured as the horizontal or diagonal length of a pixel. A smaller dot pitch generally results in sharper images because there are more pixels in a given area. In the case of CRT based displays, pixels are not equivalent to the phospor dots, as they are to the pixel triads in LCD displays. Projection displays that use 3 monochrome CRTs do not have a dot structure, so this specification does not apply.
  • Response Time: This is the time it takes for the display to respond to a given input. For an LCD display it is defined as the total time it takes for a pixel to transition from black to white, and then back to black. A display with slow response times displaying moving pictures may result in blurring and distortion. Displays with fast response times can make better transitions in displaying moving objects without unwanted image artifacts.
  • Brightness: This is the amount of light emitted from the display. It is sometimes synonymous with the term "luminance", which is defined as the amount of light emitted in a given area and is measured in SI units as candela per square meter.
  • Contrast ratio: This is defined as the ratio of the luminance of the brightest color to the luminance of the darkest color on the display. High contrast ratios are desirable but the method of measurement varies greatly. It can be measured with the display isolated from its environment or with the lighting of the room being accounted for. Static contrast ratio is measured on a static image at some instant in time. Dynamic contrast ratio is measured on the image over a period of time. Manufacturers can market either static or dynamic contrast ratio depending on which one is higher.
  • Aspect Ratio: This is the ratio of the display width to the display height. The aspect ratio of a traditional television is 4:3, but there is an increasing trend towards the 16:9 ratio typically used by large-screen, high-definition televisions.
  • Viewing Angle: This is the maximum angle at which the display can be viewed with acceptable quality. The angle is measured from one direction to the opposite direction of the display, such that the maximum viewing angle is 180 degrees. Outside of this angle the viewer will see a distorted version of the image being displayed. The definition of what is acceptable quality for the image can be different among manufacturers and display types. Many manufacturers define this as the point at which the luminance is half of the maximum luminance. Some manufacturers define it based on contrast ratio and look at the angle at which a certain contrast ratio is realized.
  • Color Reproduction/Gamut: This is the range of colors that the display can accurately represent.

Display Technologies

LCD

A pixel on an LCD consists of multiple layers of components: two polarizing filters, two glass plates with electrodes, and liquid crystal molecules. The liquid crystals are sandwiched between the glass plates and are in direct contact with the electrodes. The two polarizing filters are the outer layers in this structure. The polarity of one of these filters is oriented horizontally, while the polarity of the other filter is oriented vertically. The electrodes are treated with a layer of polymer to control the alignment of liquid crystal molecules in a particular direction. These rod-like molecules are arranged to match the horizontal orientation on one side and the vertical orientation on the other, giving the molecules a twisted, helical structure. Twisted nematic liquid crystals are naturally twisted, and are commonly used for LCD’s because they react predictably to temperature variation and electric current.

When the liquid crystals are in its natural state, light passing through the first filter will be rotated (in terms of polarity) by the twisted molecule structure, which allows the light to pass through the second filter. When voltage is applied across the electrodes, the liquid crystal structure is untwisted to an extent determined by the amount of voltage. An extremely large voltage will cause the molecules to untwist completely, such that the polarity of any light passing through will not be rotated and will instead be perpendicular to the filter polarity. This filter will block the passage of light because of the difference in polarity orientation, and the resulting pixel will be black. The amount of light allowed to pass through at each pixel can be controlled the varying the corresponding voltage accordingly. In a color LCD each pixel consists of a red, green, and blue subpixel, which requires appropriate color filters in addition to the components mentioned previously. Each subpixel can be controlled individually to display a large range of possible colors for a particular pixel.

The electrodes on one side of the LCD are arranged in columns, while the electrodes on the other side are arranged in rows, forming a large matrix that controls every pixel. Each pixel is designated a unique row-column combination, and the pixel can be accessed by the control circuits using this combination. These circuits send charge down the appropriate row and column, effectively applying a voltage across the electrodes at a given pixel. Simple LCD’s such as those on digital watches can operate on what is called a passive-matrix structure, in which each pixel is addressed one at a time. This results in extremely slow response times and poor voltage control. A voltage applied to one pixel can cause the liquid crystals at surrounding pixels to untwist undesirably, resulting in fuzziness and poor contrast in this area of the image. LCD’s with high resolutions, such as large-screen LCD televisions, require an active-matrix structure. This structure is a matrix of thin-film transistors, each corresponding to one pixel on the display. The switching ability of the transistors allows each pixel to be accessed individually and precisely, without affecting nearby pixels. Each transistor also acts as a capacitor while leaking very little current, so it can effectively store the charge while the display is being refreshed.

The following are types of LCD display technologies:

  • Twisted Nematic (TN): This type of display is the most common and makes use of twisted nematic-phase crystals, which have a natural helical structure and can be untwisted by an applied voltage to allow light to pass through. These displays have low production costs and fast response times but also limited viewing angles, and many have a limited color gamut that cannot take full advantage of advanced graphics cards. These limitations are due to variation in the angles of the liquid crystal molecules at different depths, restricting the angles at which light can leave the pixel.
  • In-Plane Switching (IPS): Unlike the electrode arrangement in traditional TN displays, the two electrodes corresponding to a pixel are both on the same glass plate and are parallel to each other. The liquid crystal molecules do not form a helical structure and instead are also parallel to each other. In its natural or "off" state, the molecule structure is arranged parallel to the glass plates and electrodes. Because the twisted molecule structure is not used in an IPS display, the angle at which light leaves a pixel is not as restricted, and therefore viewing angles and color reproduction are much improved compared to those of TN displays. However, IPS displays have slower response times. IPS displays also initially suffered from poor contrast ratios but has been significantly improved with the development of Advanced Super IPS (AS - IPS).
  • Multi-Domain Vertical Alignment (MVA): In this type of display the liquid crystals are naturally arranged perpendicular to the glass plates but can be rotated to control light passing through. There are also pyramid-like protrusions in the glass substrates to control the rotation of the liquid crystals such that the light is channeled at an angle with the glass plate. This technology results in wide viewing angles while boasting good contrast ratios and faster response times than those of TN and IPS displays. The major drawback is a reduction in brightness.
  • Patterned Vertical Alignment (PVA): This type of display is a variation of MVA and performs very similarly, but with much higher contrast ratios

Plasma Display

A plasma display is made up of many thousands of gas-filled cells that are sandwiched in between two glass plates, two sets of electrodes, dielectric material, and protective layers. The address electrodes are arranged vertically between the rear glass plate and a protective layer. This structure sits behind the cells in the rear of the display, with the protective layer in direct contact with the cells. On the front side of the display there are horizontal display electrodes that sit in between a magnesium-oxide (MgO) protective layer and an insulating dielectric layer. The MgO layer is in direct contact with the cells and the dielectric layer is in direct contact with the front glass plate. The horizontal and vertical electrodes form a grid from which each individual cell can be accessed. Each individual cell is walled off from surrounding cells so that activity in one cell does not affect another. The cell structure is similar to a honeycomb structure except with rectangular cells.

To illuminate a particular cell, the electrodes that intersect at the cell are charged by control circuitry and electric current flows through the cell, stimulating the gas (typically xenon and neon) atoms inside the cell. These ionized gas atoms, or plasmas, then release ultraviolet photons that interact with a phosphor material on the inside wall of the cell. The phosphor atoms are stimulated and electrons jump to higher energy levels. When these electrons return to is natural state energy is released in the form of visible light. Every pixel on the display is made up of three subpixel cells. One subpixel cell is coated with red phosphor, another is coated with green phosphor, and the third cell is coated with blue phosphor. Light emitted from the subpixel cells is blended together to create an overall color for the pixel. The control circuitry can manipulate the intensity of light emitted from each cell, and therefore can produce a large spectrum of colors. Light from each cell can be controlled and changed rapidly to produce a high-quality moving picture.

Projection Television

A projection television uses a projector to create a small image from a video signal and magnify this image onto a viewable screen. The projector uses a bright beam of light and a lens system to project the image to a much larger size. A front-projection television uses a projector that is separate from the screen, and the projector is placed in front of the screen. The setup of a rear-projection television is in some ways similar to that of a traditional television. The projector is contained inside the television box and projects the image from behind the screen.

The following are different types of projection televisions, which differ based on the type of projector and how the image (before projection) is created:

  • CRT projector: Small CRT's create the image in the same manner that a traditional CRT television does, which is by firing a beam of electrons onto a phosphor-coated screen. The CRT's can be arranged in various ways. One arrangement is to use one tube and three phosphor (red, green, blue) coatings. Alternatively, one black-and-white tube can be used with a spinning color wheel. A third option is to use three CRT's, one for red, green, and blue.
  • LCD projector: A lamp transmits light through a small LCD chip made up of individual pixels to create an image. The LCD projector uses mirrors to take the light and create three separate red, green, and blue beams, which are then passed through three separate LCD panels. The liquid crystals are manipulated using electric current to control the amount of light passing through. The lens system takes the three color beams and projects the image.
  • Digital Light Processing (DLP) Projector: A DLP projector creates an image using a digital micromirror device (DMD chip), which on its surface contains a large matrix of microscopic mirrors, each corresponding to one pixel in an image. Each mirror can be rotated to reflect light such that the pixel appears bright, or the mirror can be rotated to direct light elsewhere and make the pixel appear dark. The mirror is made of aluminum and is rotated on an axle hinge. There are electrodes on both sides of the hinge controlling the rotation of the mirror using electrostatic attraction. The electrodes are connected to an SRAM cell located under each pixel, and charges from the SRAM cell drive the movement of the mirrors. Color is added to the image-creation process either through a spinning color wheel (used with a single-chip projector) or a three-chip (red, green, blue) projector. The color wheel is placed between the lamp light source and the DMD chip such that the light passing through is colored and then reflected off a mirror to determine the level of darkness. A color wheel consists of a red, green, and blue sector, as well as a fourth sector to either control brightness or include a fourth color. This spinning color wheel in the single-chip arrangement can be replaced by red, green, and blue light-emitting diodes (LED). The three-chip projector uses a prism to split up the light into three beams (red, green, blue), each directed towards its own DMD chip. The outputs of the three DMD chips are recombined and then projected.

Comparison of Television Display Technologies

LCD

Advantages

  • Slim profile
  • Lighter and less bulky than projection televisions
  • Is not susceptible to burn-in: Burn-in refers to the television displaying a permanent ghost-like image due to constant, prolonged display of the image. Light-emitting phosphors lose their luminosity over time and when frequently used, the low-luminosity areas become permanently visible.
  • Does not suffer from glare in bright rooms
  • Can be mounted on walls

Disadvantages

  • Poor black level: Some light passes through even when liquid crystals completely untwist, so the best black color that can be achieved is a very dark gray, resulting in worse contrast ratios and detail in the image.
  • Generally have smaller viewing angles but this is improving due to advanced technologies like MVA and PVA
  • More difficult, and therefore more expensive, to make LCDs with large screen sizes: LCDs rely heavily on thin-film transistors, which are often defective, resulting in a defective pixel. The number of defective pixels at which the LCD is determined to be unusable varies. LCDs currently have a rejection rate of about 50% but this is improving. A larger screen size requires more transistors, which increases the chances of yielding a defective LCD. This contributes heavily to a large LCD costing significantly more than its plasma counterpart of equivalent size. Technology advancements are slowly easing this problem.
  • Typically have slower response times, which can cause ghosting and blurring during the display of fast-moving images.

Plasma Display

Advantages

  • Slim profile
  • Lighter and less bulky than projection televisions
  • Easier to manufacture and cheaper at large screen sizes than LCDs
  • Can achieve a true black because pixel can be completely turned off, resulting in better contrast, detail, and naturalness
  • Better viewing angles than those of LCDs but this is quickly becoming a non-issue

Disadvantages

  • Shorter lifespan than a LCD television.
  • Susceptible to burn-in: Burn-in refers to the television displaying a permanent ghost-like image due to constant, prolonged display of the image. Light-emitting phosphors lose their luminosity over time and when frequently used, the low-luminosity areas become permanently visible.
  • Phosphors lose luminosity over time, resulting in gradual decline of image quality
  • Generally do not come in smaller sizes than 40 inch.
  • Susceptible to reflection glare in bright rooms.
  • Plasma screens run a lot hotter than LCD or projection, because of the need to electrically charge the gas into a plasma which generates an excess amount of heat. For this reason, it is not recommended plasmas be mounted over fireplaces or similar hot areas.

Projection Television

Advantages

  • Significantly cheaper than flat-panel counterparts
  • Front-projection picture quality resembles that of movie theater
  • Front-projection takes up very little space because a projector screen is extremely slim, or alternatively the wall could be used as the display medium
  • Display size can be extremely large, up to hundreds of inches
  • Projectors that are not phosphor-based (LCD/DLP) are not susceptible to burn-in: Burn-in refers to the television displaying a permanent ghost-like image due to constant, prolonged display of the image. Light-emitting phosphors lose their luminosity over time and when frequently used, the low-luminosity areas become permanently visible.

Disadvantages

  • Front-projection more difficult to set up because projector is separate and must be placed in front of the screen, typically on the ceiling
  • Rear-projection televisions are much bulkier than flat-panel televisions
  • Lamp may need to be replaced after heavy usage
  • Rear-projection has smaller viewing angles than those of flat-panel displays
  • Rear-projection is susceptible to glare

Comparison of Different Types of Projection Televisions

CRT Projector
Advantages
  • Not restricted to fixed pixel resolutions, able to display varying resolutions
  • Achieves the best black level and contrast ratio
  • Achieves the best color reproduction
Disadvantages
  • Heavy and large, especially depth-wise
  • If one CRT fails the other two have to be replaced as well to maintain color and brightness balance
  • Susceptible to burn-in because CRT is phosphor-based
  • Limited viewing angles
LCD Projector
Advantages
  • Smaller than CRT projector because LCD chip is very small
  • LCD chip can be easily repaired or replaced
  • Is not susceptible to burn-in
  • Better viewing angles than those of CRT projector
Disadvantages
  • The Screen-door effect: Individual pixels may be visible on the large screen, giving the appearance that the viewer is looking through a screen door.
  • Defective pixels
  • Poor black level and contrast ratio
  • Not as slim as DLP projection television
  • Uses lamps for light, lamps may need to be replaced
  • Fixed number of pixels, other resolutions need to be scaled to fit this
DLP Projector
Advantages
  • Slimmest of all types of projection televisions
  • Achieves the best black level and contrast ratio
  • DMD chip can be easily repaired or replaced
  • Is not susceptible to burn-in
  • Better viewing angles than those of CRT projectors
  • Image quality will not fade with time, unlike phosphor-based projectors
  • Defective pixels are rare
  • Does not experience the screen-door effect
Disadvantages
  • Uses lamps for light, lamps may need to be replaced
  • Fixed number of pixels, other resolutions need to be scaled to fit this
  • The Rainbow Effect: This is an unwanted visual artifact that is described as flashes of colored light seen when the viewer looks across the display from one side to the other. This artifact is unique to single-chip DLP projectors.

Major Manufacturers

See also

References

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