The CRT technology although quite old is fast being replaced by newer technologies in display science. Thus new systems like LCD, PDP, OLED, PLED, etc are making their presence felt, particularly the first two, viz, LCD and PDP in the domestic TV industry. Both PDP and LCD are similar in technique. But while the LCD uses liquid crystals in producing better TV screen displays, the PDP screens display better images based on the production and technology of plasmas. Each LCD and PDP offer distinct advantages as compared with each other. Also, the consumer trend is towards, lighter, slimmer, flatter, and advanced TVs having better features like better viewing angles, more brightness, and contrast adjustability, etc. While technology is still evolving, and it cannot be predicted as to which will prevail over the year, at least in the present time, LCD is better placed in the market owing to cost and energy efficiency. However, it may not be long before other and better technologies overtake both LCD and PDP display technologies in the near or far future.
Cathode Ray Tubes or CRT have had long innings as display technology and have formed an essential part of television through the years. However, newer technologies like Liquid Display Displays (LCD), Organic Light-Emitting Diodes (OLED), Small Molecule OLED (SMOLED), Polymer LED (PLED), Passive Matrix OLED (PMOLED), Digital Light Processing (DLP), Plasma Display Panels (PDP), Field Emission Displays (FED), Electronic Ink Displays or EID, and Carbon Nanotube FED are emerging as alternative display mechanisms owing to their advantages in picture size, image quality and power consumption (Gurski and Quach, 2005, p. 3). Many of these are increasingly being used in applications like television, mobiles, etc. Particularly in the television industry, LCD and PDP have emerged as better display options and also offer many advantages in respect of each other as also as compared to conventional and other emergent alternative technology. While the use of one technology or the other in television display panels is driven by such considerations and also by consumer preferences, the technology behind the same can best be understood and compared based on factors that characterize display systems like color gamut and perception, field and angle of viewing, perception of motion, visual quality, refresh rate, CFF, etc. Hence, this paper essentially attempts to compare LCD and PDP display systems used in television screens based on these factors and as per available literature on the subject. However, a brief overview of what are actually LCD and PDP technologies, and their evolution are examined first.
LCD: Origin and what it is
Gurski and Quach (2005, p. 4) observe that liquid crystals are an intermediate phase between solids and liquids and may be classified into nematic, smectic and cholesteric of which, the nematic liquid crystals are generally used in LCD s. Using electric or magnetic field, the molecules of the crystals are manipulated predictably so as to find application in LCD s (p. 4). LCD s are non-self-emissive, flat panel displays (Takiguchi, 2007, p. 1). LCD Televisions utilize LCD s made from special liquid crystal mixtures and avail advanced display techniques like in-plane switching or IPS and multi-domain vertical alignment or MVA (Becker and Lemp, 2004, p. 13). Simple LCDs are made from a combination of innumerable tiny liquid crystals sandwiched between glass panels, an alignment layer, electrodes, and polarizers. LCDs can be either passive display ones or active type ones. However, essentially in all types of LCDs, the molecules which are loosely facing with their long axes parallel are transformed to a configuration parallel to the alignment layer or finely grooved surface. The light that passes through these crystals also is made to be transformed in direction similarly. Electrodes are provided to pass current through the crystal cells when light is aligned to pass directly through these liquid crystals. The role of the polarizers is simply in confining the electric and magnetic fields around the passing light in a single plane. Various colors are displayed on LCD screens when the second polarizer filters out the light rays that pass through due to the first polarizer and creates regions of darkness and light on the screen in the process.
Reinitz and later on Lehmann first studied liquid crystals. Lehmann later on could establish such crystals as a separate, distinct phase of matter. Liquid crystals were classified into three types by Friedel in 1923, after which, the scientific community appeared to go slow on research on these materials. It was only during the 1960s that scientists in the US tried to apply the liquid crystals and their properties in applications in the field of metrology, non-destructive materials testing, cancer diagnosis, etc. Heilmeier was the first to publish research on dynamic scattering mode (DSM) in nematic LCs. He even made the first flat display using such an effect in LCs. And in 1968 was born the first novel idea of a flat-screen for televisions using the LCs at a conference held at Kent State University. This idea was then extensively researched by Merck
While the initial research focused on “dynamic scattering, deformation of aligned phases, and, dichroism” (Becker and Lemp, p. 12), it was only during the 1980s and 1990s that LCD s came to be utilized increasingly in large or commercial applications as in mobile phones, PCs, etc. With the development of new technology by Merck, particularly the IPS and MVA, flat televisions using LCDs on commercial-scale also became a reality. With the trend in decreasing prices of LCD s and owing to the distinct advantages in using LCD s in television screens, no wonder that, increasingly, the commercial television industry is seeing the adoption of these technological innovations like never before. And among comparable technologies that are gradually adopted across the world in television display engineering, perhaps the most challenge to LCD s and promise to consumers are from the so-called plasma technology or PDP, whose origin and basics may be provided below for better understanding of the two technologies.
Plasma Technology, PDP Origin and Fundamentals
Anderson (2005, p. 49) defines a Plasma Display Panel (PDP) as an emissive type of flat panel display (FPD) and perceives a plasma as an ionized gas. He further observes that numerous tiny cells (or pixels) in the PDP which contain xenon and neon are placed between two glass plates and the gases are ionized by passing electric current when stream of ultraviolet photons is released. These photons then illuminate the scintillators or sub-pixels when they reach the front of each cell.
Variations in current strength flowing through the cells contribute to the variations in the color of the resultant displays. Plasma display technology is the display technology generally preferred in the case of high-definition television or HDTV s. Similar to LCD s, plasma screens to consist of millions of tiny cells which are sandwiched between two glass plates. A combination of display and address electrodes forms a grid across the entire screen. Electric current passing through the grid excites the xenon and neon gases, which are present in the cells. This in turn creates the plasma that then helps release ultra-violet light which excites the phosphor electrons, emitting light in the process. Altering the current passing through helps generate variations in color on the screen.
Factors for comparing PDP and LCD: Human Visual System or HVS PDP and LCD are both display technologies used in television screens and technical comparison of the two would necessarily involve the human visual system or HVS. In fact, Wisnieff and Ritsko (2000) state, “The grand goal of displays research and development is, therefore, to try to match the information output of the display to the input and processing capacity of the human visual system”. Accordingly, it would be in order to know and understand the various factors applicable in the context of HVS to better compare the capabilities of PDP and LCD systems. Such factors are detailed in the following paragraphs.
Anderson (2005) defines brightness as “the level of light intensity perceived by the user” (p. 5). The amount of brightness actually depends on the intensity of light reflected or emitted as also on the “section of dynamic range that the absolute ambient brightness falls into” (Anderson, 2005, p. 5). Anderson also notes that brightness may also be defined as “luminance of white color in the center of the screen and is measured in candela per square meter”. He also says that “dynamic range is the ratio between maximum and minimum intensities that can be generated” (p. 5).
Differences in luminance levels in an image contribute to ‘contrast” in them. Contrast considerations are a significant issue in the design of television display screens. Woods and Woods (1995) state, “Higher contrast is required to detect or ‘resolve’ smaller objects and this resolution limit is related to contrast and visual acuity. Also Poynton (1998) notes that, “The minimum contrast sensitivity the HVS can detect is an intensity difference between two patches where the ratio of their intensities differs by at least about one percent”.
One of the most important factors involved in TV display designing, is the degree of detail that the human eye can discern from the pixel patterns on the screen. Wisnieff and Ritsko (2000) state, “This is related to the ‘information content’ of a display (which is defined as the total number of pixels, the size of the pixels (resolution) and the size of the display) and the eye’s ability to discern detail – our visual acuity”. Kolb, Fernandez and Nelson (2005) also aver, “The large number and density of photoreceptors present in the HVS mean we have a considerable ability to discern detail at a distance and this is affected by the degree of luminance and the contrast present in a scene or image and hence these are particularly important factors in display design”. Obviously, resolution of an image on the TV screen is based upon the distance of the viewer from the screen or object. Usually, visual acuity is measured in terms of pixels per inch (ppl) of display.
Field of View & Angle of Viewing
Lantz and Spitz (1997) observe that it is only from a small part of the visual area that the human eye derives visual information. Hence, the eye attempts to constantly bring visual objects into its foveal area (a central part of the eye) and typically, a human eye can visualize vertically up to an angle of 155 degrees and horizontally up to an angle of 1858m degrees (Rheingold, 1991). This is the field of view that the human eye is able to derive information from. It also obviously assumes significance in designing TV screens for displaying images so that a wider angle of viewing is possible, in the entire angular area, where the resolution, brightness and contrast are uniforms. Here, the angle of view is the angle of the human eye in relation to the TV screen so as to clearly view the images displayed on the TV screen.
Anderson says that “color is a function of the various wavelengths of light that an object emits or reflects” (p. 7). He also perceives color to be fully defined by means of properties like hue, brightness/lightness, and saturation. While the hue is dependant on the dominant wavelength of the light, the brightness or lightness is determined by the color luminosity. Lightness is related to reflected light whereas brightness is related to the emitted light. Again, saturation is caused by the paleness or strength of a color and is related to the mixing of wavelengths of light rays. Thus, narrow concentration of wavelengths of light would create a highly saturated color. Foley (1990) observes that “the human eye can perceive 128 fully saturated color hues”.
Also termed color gamut, this is the ability of the TV screen to produce and display numerous colors. Wisnieff and Ritsko (2000) observe that, “The whole gamut of colors from a display depends on the mechanism of color generation and is limited by the degree of saturation for the three primary colors”. But one important fact observed from TV images is that actual objects are always richer in real life than they appear in their TV images. This is because the “displays provide color gamut smaller than what range the HVS can process” (Anderson, 2005, p. 8).
Perception of Motion, Refresh Rate and CFF
Burr and Morgan (1997) state that, “the time taken by the human eye to detect and visually process an object and its movement is about 120 milliseconds”. And, “this is related to persistence of vision and the human eye can absorb 20 distinct images per second before the images get blurred” (Anderson, 2005, p. 8). Also, flicker needs to be prevented and continuity of image displayed on-screen needs to ensure considering that the rate at which a TV display redraws image on the TV screen (refresh rate) is fast enough. Another factor is the CFF or critical fusion frequency is related to this refresh rate, ambient light, brightness, and angle of viewing (Sidebottom, 1997). This CFF is the “point at which the eye stops seeing the individual pictures as they are refreshed and ‘fuses’ it into a single image” (Anderson, 2005, p. 9).
Other Non-HVS Factors
Other than the HVS factors described above, Anderson also outlines some non-HVS factors that determine the state of advances made by the display technologies over the years. These are also provided here for a better understanding of the issues involved.
This assumes significance particularly in the case of PDP screens. This is because the size of each cell in a PDP is large so that Plasma TVs have to be made to a minimum size of 32 inches to date. However, smaller and flatter TVs are the trend. In contrast, manufacturers are trying to increase the small size of LCD screens although both LCD and PDP screens are much smaller compared to traditional CRT display screens.
This is the ratio of screen width to screen height in a TV.
Consumers perceive risks from X–rays and other harmful rays from PDP and LCD screens to be minimal which could be influencing their buying decisions (PC World, 2003)
Power Consumption/Environmental Conservation
In addition to saving energy, the modern consumer also wants to save his expenses, Also, the technology must be invariable by environment friendly in the modern conscious world as also to comply with laws existing in various parts of the world.
Comparison of LCD and PDP Screens in Televisions
Based on the HVS factors described before, and as per Dupont (2005) and Mann (2004), the following advantages and disadvantages of LCD as compared to PDP can be outlined here.
Based on non-HVS factors, following are the comparisons of PDP with LCD
From the above discussion, it is apparent that LCD s are fast replacing traditional CRT display technology in screens for televisions. LCD s have obvious advantages like flat and thin screens, less costly, and lighter in weight as also ensure better viewing due to improved angle of views, brightness, contrast, etc. PDP screens are still better in many aspects, although they are highly expensive. While larger displays can be achieved using PDP screens, it is still suitable for LCD systems in the case of smaller sets, and LCD s in combination with TFT s are getting to be the most preferred customer choice of digital television systems. While the technology behind LCD s and PDP s are similar, in the case of LCD, liquid crystals are used to enhance displays on the television screen while in the case of PDP displays, a plasma is produced which helps in making for a better display screen offering better viewing quality, etc., However, only the future can determine if either of these two technologies will dominate the domestic TV market or whether other technologies will replace both in course of time
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