Saturday, August 29, 2009
Viewing Angles
LCD's produce their image by having a film that when a current runs through the pixel, it turns on that shade of color. The problem with the LCD film is that this color can only be accurately represented when viewed straight on. The further away from a perpendicular viewing angle, the color will tend to wash out. The LCD monitors are generally rated for their visible viewing angle for both horizontal and vertical. This is rated in degrees and is the arc of a semicircle whose center is at the perpendicular to the screen. A theoretical viewing angle of 180 degrees would mean that it is fully visible from any angle in front of the screen. A higher viewing angle is preferred over a lower angle unless you happen to want some security with your screen.
Response Times
In order to achieve the color on a pixel in an LCD panel, a current is applied to the crystals at that pixel to change the state of the crystals. Response times refer to the amount of time it takes for the crystals in the panel to move from an on to off state. A rising response time refers to the amount of time it takes to turn on the crystals and the falling time is the amount of time it takes for the crystals to move from an on to off state. Rising times tend to be very fast on LCDs, but the falling time tends to be much slower. This tends to cause a slight blurring effect on bright moving images on black backgrounds. The lower the response time, the less of a blurring effect there will be on the screen. Most response times now refer to a gray to gray rating that doesn't do the full on off state that generates a lower time than the traditional response times
Color Gamut
Each LCD panel will vary slightly in how well they can reproduce color. When an LCD is being used for tasks that require a high level of color accuracy, it is important to find out what the panels color gamut is. This is a description that lets you know how wide a range of color the screen can display. The larger the percentage of NTSC, the greater level of color a monitor can display. It is somewhat complex and best described in my article on Color Gamuts
Contrast Ratio
Contrast ratios are a big marketing tool by the manufacturers and one that is not easy for consumers to grasp. Essentially, this is the measurement of the difference in brightness from the darkest to brightest portion on the screen. The problem is that this measurement will vary throughout the screen. This is due to the slight variations in the lighting behind the panel. Manufacturers will use the highest contrast ratio they can find on a screen, so it is somewhat deceptive. Basically a higher contrast ratio will mean that the screen will tend to have deeper blacks and brighter whites.
Native Resolutions
All LCD screens can actually display only a single given resolution referred to as the native resolution. This is the physical number of horizontal and vertical pixels that make up the LCD matrix of the display. Setting a computer display to a resolution lower than this resolution will either cause the monitor to use a reduced visible area of the screen or it will have to do extrapolation. This extrapolation attempts to blend multiple pixels together to produce a similar image to what you would see if the monitor were to display it at the given resolution but it can result in fuzzy images.
Here are some of the common native resolutions found in LCD monitors:
17-19": 1280x1024 (SXGA)
20"+: 1600x1200 (UXGA)
17" (Widescreen): 1280x800 (WXGA)
19" (Widescreen): 1440x900 (WXGA+)
22" (Widescreen): 1680x1050 (WSXGA+)
23.6" (Widescreen): 1920x1080 (WUXGA)
23" (Ultra-Widescreen): 2048x1152 (QWXGA)
24" (Widescreen): 1920x1200 (WUXGA)
30" (Widescreen): 2560x1600
Here are some of the common native resolutions found in LCD monitors:
17-19": 1280x1024 (SXGA)
20"+: 1600x1200 (UXGA)
17" (Widescreen): 1280x800 (WXGA)
19" (Widescreen): 1440x900 (WXGA+)
22" (Widescreen): 1680x1050 (WSXGA+)
23.6" (Widescreen): 1920x1080 (WUXGA)
23" (Ultra-Widescreen): 2048x1152 (QWXGA)
24" (Widescreen): 1920x1200 (WUXGA)
30" (Widescreen): 2560x1600
Service and support
The $599 price includes a three-year limited warranty that covers defects in the display and its peripherals. This also includes a 24-7 toll-free phone technical support as well as technical support through live Web chat. We could not find the drivers for this display on Dell's site. Under the support/drivers area on their Web site, this display was not listed at the time this review was written.
Features
The Dell UltraSharp 2408WFP includes an abundance of connection options. For video connections, you'll find a VGA, two DVI, an HDMI, a DisplayPort, component, and composite ports. There's also a speaker port, four USB ports (plus one upstream USB port), and a media card reader for Compact Flash and SD formats. That's certainly a long list of connections, but if we're being greedy, we would have liked to have seen an optical audio out connection. Compared with the lower priced Gateway FHD2400 we reviewed recently and praised for its connection options, the Dell surpasses it in the variety and volume of connection options. In particular, the extra DVI port and the DisplayPort are valuable extras if you want to connect the display to a media center or high-end PC.
The Dell has a Dynamic Contrast of 3000:1. This means--according to Dell--that the blacks the display outputs are three times darker than the whites are when viewing dark scenes. To get that kind of contrast ratio the display powers down its backlight in dark scenes, so that the blacks are very dark. This also means that if the dark scene in question contains areas of bright light, the light may be sacrificed as the backlight does not have the power to represent it accurately. Basically, Dynamic Contrast is just a marketing term and, for now, there is no independent standard for measuring it, so it should not be considered when making a buying decision. We felt the display was capable of dark blacks, but they could have been a bit darker. The whites were as bright as any we've seen in a recent display.
The Dell has a Dynamic Contrast of 3000:1. This means--according to Dell--that the blacks the display outputs are three times darker than the whites are when viewing dark scenes. To get that kind of contrast ratio the display powers down its backlight in dark scenes, so that the blacks are very dark. This also means that if the dark scene in question contains areas of bright light, the light may be sacrificed as the backlight does not have the power to represent it accurately. Basically, Dynamic Contrast is just a marketing term and, for now, there is no independent standard for measuring it, so it should not be considered when making a buying decision. We felt the display was capable of dark blacks, but they could have been a bit darker. The whites were as bright as any we've seen in a recent display.
Design
The Dell 2408WFP shares the same basic design as its predecessor, the UltraSharp 2407WFP. This includes the relatively thin bezel around the edge of the screen with the Dell logo along the bottom.
The onscreen display is easy to navigate and includes the usual options of brightness, contrast, color, and so on. We also liked that the OSD stays on the screen long enough to evaluate any changes you make while calibrating the display. There are also six included preset modes for activities such as playing games, watching movies, and graphics work that affect color temperature, contrast, and brightness.
From the back, you're looking at a mostly silver enclosure, which runs along the foot and neck of the stand with a large silver Dell logo at the top. The screen rotates 45 degrees to the left and right, and about 30 degrees back. The screen also pivots 90 degrees to the left into portrait mode, but you'll have to rotate the screen back first before you can actually pivot it as the stand is in the way normally. This is a minor gripe, but it's something we hope Dell will consider when they choose to redesign this chassis.
The foot of the stand is the same Y-shaped, or "bird-foot" as it is sometimes called, design as found on last year's model. The width of the stand is about 15.5 inches at its widest and really helps to make the display feel very sturdy, even when placed on a narrow stand and the screen is raised to the top of its 4.5-inch range. This display has many connection options, and Dell continues to make it easy to find them all. Each connection has a very clear illustration beneath it that makes it a cinch to find and connect
The onscreen display is easy to navigate and includes the usual options of brightness, contrast, color, and so on. We also liked that the OSD stays on the screen long enough to evaluate any changes you make while calibrating the display. There are also six included preset modes for activities such as playing games, watching movies, and graphics work that affect color temperature, contrast, and brightness.
From the back, you're looking at a mostly silver enclosure, which runs along the foot and neck of the stand with a large silver Dell logo at the top. The screen rotates 45 degrees to the left and right, and about 30 degrees back. The screen also pivots 90 degrees to the left into portrait mode, but you'll have to rotate the screen back first before you can actually pivot it as the stand is in the way normally. This is a minor gripe, but it's something we hope Dell will consider when they choose to redesign this chassis.
The foot of the stand is the same Y-shaped, or "bird-foot" as it is sometimes called, design as found on last year's model. The width of the stand is about 15.5 inches at its widest and really helps to make the display feel very sturdy, even when placed on a narrow stand and the screen is raised to the top of its 4.5-inch range. This display has many connection options, and Dell continues to make it easy to find them all. Each connection has a very clear illustration beneath it that makes it a cinch to find and connect
CNET editors' review
Reviewed by:
Eric Franklin
Edited by:
Matthew Elliott
Reviewed on: 04/17/2008
Released on: 04/15/2008
The Gateway FHD2400 may be cheaper, but the Dell UltraSharp 2408WPF display is the better value of the two 24-inch LCD monitors. It performed outstandingly in our labs-based DisplayMate tests, delivering quite possibly the best image we've seen for DVD playback. The 2408WPF packs all of its stellar performance inside a practical and aesthetically pleasing design while delivering an embarrassment of connection riches. Moreover, it carries a fair price of $599; we'd wager any 24-inch display you find for less will come with trade-offs in terms of features or performance or both. The UltraSharp 2408WFP is a great entertainment display, whether it is games, DVDs, or--making use of its 1,920x1,200 resolution--HD movies. Rest assured, it does not drop the ball on the basics; we give it a strong recommendation should you seek a large productivity monitor.
Eric Franklin
Edited by:
Matthew Elliott
Reviewed on: 04/17/2008
Released on: 04/15/2008
The Gateway FHD2400 may be cheaper, but the Dell UltraSharp 2408WPF display is the better value of the two 24-inch LCD monitors. It performed outstandingly in our labs-based DisplayMate tests, delivering quite possibly the best image we've seen for DVD playback. The 2408WPF packs all of its stellar performance inside a practical and aesthetically pleasing design while delivering an embarrassment of connection riches. Moreover, it carries a fair price of $599; we'd wager any 24-inch display you find for less will come with trade-offs in terms of features or performance or both. The UltraSharp 2408WFP is a great entertainment display, whether it is games, DVDs, or--making use of its 1,920x1,200 resolution--HD movies. Rest assured, it does not drop the ball on the basics; we give it a strong recommendation should you seek a large productivity monitor.
RGB -vs- DVI Interface
CRT monitors are generally connected to the computer through an Analog-RGB interface. Most of the early LCD monitors supported that interface only, but an increasing number of new ones support the DVI interface only, or both. DVI interfaces generally produce images of higher quality. For Analog-RGB, the computer converts the image data from digital form to an analog signal and transmits it to the LCD monitor. The monitor converts the analog signal back to digital data and displayed it on the screen. In case of DVI, the computer transmits digital data that is displayed directly on the screen. Not all computers have a DVI output, but if your computer does, buy an LCD monitor that works on DVI. In case your computer does not have a DVI output, you can upgrade it with a graphics card – most graphics cards come with a built-in DVI output.
As far as manufacturers are concerned, ViewSonic seems to be a good trade-off between quality and price. Acer appears to be the value-price leader. Apple and Sony are the winners when looks are a factor.
As far as manufacturers are concerned, ViewSonic seems to be a good trade-off between quality and price. Acer appears to be the value-price leader. Apple and Sony are the winners when looks are a factor.
Shapes and Sizes
LCD monitors come in numerous shapes and sizes. Height-to-width ratio of 3:4 is dominating these days, but those with 9:16 ratios are gaining fast. These wider displays make the best use of the screen in case you want to watch DVD on your computer or would like to display two pages at a time while word-processing. Business folks like them as they can see more columns of their spreadsheets on these wider displays.
The size of the monitor is specified in terms of the diagonal measure of the display area. 15” diagonal displays were popular until recently, then 17” ones became cost-effective, and now 19” monitors are selling rapidly. Their popularity is, generally, a measure of the cost: the costs are coming down fast due to the increasing efficiency of the manufacturing process, and will continue to do so for the foreseeable future. Here is my advice on which one to buy: spend around $300 on a monitor. A year back that bought you a 15” size; a year from now it will buy you a 21” one.The maximum resolution of the display increases with diagonal size. Most 19” displays (with a 3:4 height-to-width ratio) now support a resolution of 1,280x1,024. Try to find a monitor having a resolution at least that much.
The size of the monitor is specified in terms of the diagonal measure of the display area. 15” diagonal displays were popular until recently, then 17” ones became cost-effective, and now 19” monitors are selling rapidly. Their popularity is, generally, a measure of the cost: the costs are coming down fast due to the increasing efficiency of the manufacturing process, and will continue to do so for the foreseeable future. Here is my advice on which one to buy: spend around $300 on a monitor. A year back that bought you a 15” size; a year from now it will buy you a 21” one.The maximum resolution of the display increases with diagonal size. Most 19” displays (with a 3:4 height-to-width ratio) now support a resolution of 1,280x1,024. Try to find a monitor having a resolution at least that much.
Summary
LCD monitors started outselling CRT monitors starting in 2005. Their many advantages over the conventional CRT’s include smaller size, lower weight, reduced power consumption, lower heat and radiation emission. Due to ease of fabrication, manufacturers are able to build them in an increasingly large number of sizes and form factors. Moreover, favorable economies of scale are driving there costs lower and lower with every passing month.
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LCD monitors have become more popular as compared with CRT monitors not only due to their space-saving advantage, but also because they typically consume 65% less power and emit little of it as heat or radiation. Their prices have been spiraling downwards for a while now, making even the 19" models quite affordable. All these factors have made CRT’s extinct in all but the most high-end of the high-end environments where precise reproduction of colors is essential
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LCD monitors have become more popular as compared with CRT monitors not only due to their space-saving advantage, but also because they typically consume 65% less power and emit little of it as heat or radiation. Their prices have been spiraling downwards for a while now, making even the 19" models quite affordable. All these factors have made CRT’s extinct in all but the most high-end of the high-end environments where precise reproduction of colors is essential
LCD monitor
A monitor that uses LCD technologies rather than the conventional CRT technologies used by most desktop monitors. Until recently, LCD panels were used exclusively on notebook computers and other portable devices. In 1997, however, several manufacturers began offering full-size LCD monitors as alternatives to CRT monitors. The main advantage of LCD displays is that they take up less desk space and are lighter. Currently, however, they are also much more expensive.
Overview
Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.
The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).
Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.
LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are parallel.
The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.
Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).
When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically 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.
The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).
Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.
LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are parallel.
The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.
Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).
When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically 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.
A liquid crystal display (LCD)
(LCD) is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures. Its uses include monitors for computers, televisions, instrument panels, and other devices ranging from aircraft cockpit displays, to every-day consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones. Among its major features are its lightweight construction, its portability, and its ability to be produced in much larger screen sizes than are practical for the construction of cathode ray tube (CRT) display technology. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome. The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888.[1] By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.
Packaging
In a CRT the electron beam is produced by heating a metal filament, which "boils" electrons off its surface. The electrons are then accelerated and focused in an electron gun, and aimed at the proper location on the screen using electromagnets. The majority of the power budget of a CRT goes into heating the filament, which is why the back of a CRT-based television is hot. Since the electrons are easily deflected by gas molecules, the entire tube has to be held in vacuum. The atmospheric force on the front face of the tube grows with the area, which requires ever-thicker glass. This limits practical CRTs to sizes around 30 inches; displays up to 40 inches were produced but weighed several hundred pounds, and televisions larger than this had to turn to other technologies like rear-projection.
The lack of vacuum in an LCD television is one of its advantages; there is a small amount of vacuum in sets using CCFL backlights, but this is arranged in cylinders which are naturally stronger than large flat plates. Removing the need for heavy glass faces allows LCDs to be much lighter than other technologies. For instance, the Sharp LC-42D65, a fairly typical 42-inch LCD television, weighs 55 lbs including a stand,[1] while the late-model Sony KV-40XBR800, a 40" 4:3 CRT weighs a massive 304 lbs without a stand, almost six times the weight.[2]
LCD panels, like other flat panel displays, are also much thinner than CRTs. Since the CRT can only bend the electron beam through a critical angle while still maintaining focus, the electron gun has to be located some distance from the front face of the television. In early sets from the 1950s the angle was often as small as 35 degrees off-axis, but improvements, especially computer assisted convergence, allowed that to be dramatically improved and, late in their evolution, folded. Nevertheless, even the best CRTs are much deeper than an LCD; the KV-40XBR800 is 26 inches deep,[2] while the LC-42D65U is less than 4 inches thick[1] – its stand is much deeper than the screen in order to provide stability.
LCDs can, in theory, be built at any size, with production yields being the primary constraint. As yields increased, common LCD screen sizes grew, from 14 to 30", to 42", then 52", and 65" sets are now widely available. This allowed LCDs to compete directly with most in-home projection television sets, and in comparison to those technologies direct-view LCDs have a better image quality. Experimental and limited run sets are available with sizes over 100 inches.
The lack of vacuum in an LCD television is one of its advantages; there is a small amount of vacuum in sets using CCFL backlights, but this is arranged in cylinders which are naturally stronger than large flat plates. Removing the need for heavy glass faces allows LCDs to be much lighter than other technologies. For instance, the Sharp LC-42D65, a fairly typical 42-inch LCD television, weighs 55 lbs including a stand,[1] while the late-model Sony KV-40XBR800, a 40" 4:3 CRT weighs a massive 304 lbs without a stand, almost six times the weight.[2]
LCD panels, like other flat panel displays, are also much thinner than CRTs. Since the CRT can only bend the electron beam through a critical angle while still maintaining focus, the electron gun has to be located some distance from the front face of the television. In early sets from the 1950s the angle was often as small as 35 degrees off-axis, but improvements, especially computer assisted convergence, allowed that to be dramatically improved and, late in their evolution, folded. Nevertheless, even the best CRTs are much deeper than an LCD; the KV-40XBR800 is 26 inches deep,[2] while the LC-42D65U is less than 4 inches thick[1] – its stand is much deeper than the screen in order to provide stability.
LCDs can, in theory, be built at any size, with production yields being the primary constraint. As yields increased, common LCD screen sizes grew, from 14 to 30", to 42", then 52", and 65" sets are now widely available. This allowed LCDs to compete directly with most in-home projection television sets, and in comparison to those technologies direct-view LCDs have a better image quality. Experimental and limited run sets are available with sizes over 100 inches.
Building a display
A typical shutter assembly consists of a sandwich of several layers deposited on two thin glass sheets forming the front and back of the display. For smaller display sizes (under 30 inches), the glass sheets can be replaced with plastic.
The rear sheet starts with a polarizing film, the glass sheet, the active matrix components and addressing electrodes, and then the director. The front sheet is similar, but lacks the active matrix components, replacing those with the patterned color filters. Using a multi-step construction process, both sheets can be produced on the same assembly line. The liquid crystal is placed between the two sheets in a patterned plastic sheet that divides the liquid into individual shutters and keeps the sheets at a precise distance from each other.
The critical step in the manufacturing process is the deposition of the active matrix components. These have a relatively high failure rate, which renders those pixels on the screen "always on". If there are enough broken pixels, the screen has to be discarded. The number of discarded panels has a strong effect on the price of the resulting television sets, and the major downward fall in pricing between 2006 and 2008 was due mostly to improved processes.
To produce a complete television, the shutter assembly is combined with control electronics and backlight. The backlight for small sets can be provided by a single lamp using a diffuser or frosted mirror to spread out the light, but for larger displays a single lamp is not bright enough and the rear surface is instead covered with a number of separate lamps. Achieving even lighting over the front of an entire display remains a challenge, and bright and dark spots are not uncommon.
The rear sheet starts with a polarizing film, the glass sheet, the active matrix components and addressing electrodes, and then the director. The front sheet is similar, but lacks the active matrix components, replacing those with the patterned color filters. Using a multi-step construction process, both sheets can be produced on the same assembly line. The liquid crystal is placed between the two sheets in a patterned plastic sheet that divides the liquid into individual shutters and keeps the sheets at a precise distance from each other.
The critical step in the manufacturing process is the deposition of the active matrix components. These have a relatively high failure rate, which renders those pixels on the screen "always on". If there are enough broken pixels, the screen has to be discarded. The number of discarded panels has a strong effect on the price of the resulting television sets, and the major downward fall in pricing between 2006 and 2008 was due mostly to improved processes.
To produce a complete television, the shutter assembly is combined with control electronics and backlight. The backlight for small sets can be provided by a single lamp using a diffuser or frosted mirror to spread out the light, but for larger displays a single lamp is not bright enough and the rear surface is instead covered with a number of separate lamps. Achieving even lighting over the front of an entire display remains a challenge, and bright and dark spots are not uncommon.
Addressing sub-pixels
In order to address a single shutter on the display, a series of electrodes is deposited on the plates on either side of the liquid crystal. One side has horizontal stripes that form rows, the other has vertical stripes that form columns. By supplying voltage to one row and one column, a field will be generated at the point where they cross. Since a metal electrode would be opaque, LCDs use electrodes made of a transparent conductor, typically indium tin oxide.
Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters to a particular twist was very low, but that voltage was too low to make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.
In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line, is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well.
Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters to a particular twist was very low, but that voltage was too low to make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.
In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line, is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well.
Basic LCD concepts
LCD televisions produce a colored image by selectively filtering a white light. The light is typically provided by a series of cold cathode fluorescent lamps (CCFLs) at the back of the screen, although some displays use white or colored LEDs instead. Millions of individual LCD shutters, arranged in a grid, open and close to allow a metered amount of the white light through. Each shutter is paired with a colored filter to remove all but the red, green or blue (RGB) portion of the light from the original white source. Each shutter–filter pair forms a single sub-pixel. The sub-pixels are so small that when the display is viewed from even a short distance, the individual colors blend together to produce a single spot of color, a pixel. The shade of color is controlled by changing the relative intensity of the light passing through the sub-pixels.
Liquid crystals encompass a wide range of (typically) rod-shaped polymers that naturally form into thin layers, as opposed to the more random alignment of a normal liquid. Some of these, the nematic liquid crystals, also show an alignment effect between the layers. The particular direction of the alignment of a nematic liquid crystal can be set by placing it in contact with an alignment layer or director, which is essentially a material with microscopic groves in it. When placed on a director, the layer in contact will align itself with the grooves, and the layers above will subsequently align themselves with the layers below, the bulk material taking on the director's alignment. In the case of an LCD, this effect is utilized by using two directors arranged at right angles and placed close together with the liquid crystal between them. This forces the layers to align themselves in two directions, creating a twisted structure with each layer aligned at a slightly different angle to the ones on either side.
LCD shutters consist of a stack of three primary elements. On the bottom and top of the shutter are polarizer plates set at (typically) right angles. Normally light cannot travel through a pair of polarizers arranged in this fashion, and the display would be black. The polarizers also carry the directors to create the twisted structure aligned with the polarizers on either side. As the light flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front of the liquid crystal having been rotated through the correct angle that allows it to pass through the front polarizer. LCDs are normally transparent.
To turn a shutter off, an electrical voltage is applied across it from front to back. When this happens, the rod-shaped molecules align themselves with the electric field instead of the directors, destroying the twisted structure. The light no longer changes polarization as it flows through the liquid crystal, and can no longer pass through the front polarizer. By controlling the voltage applied across the crystal, the amount of remaining twist can be finely selected. This allows the transparency or opacity of the shutter to be accurately controlled. In order to improve switching time, the cells are placed under pressure, which increases the force to re-align themselves with the directors when the field is turned off
Liquid crystals encompass a wide range of (typically) rod-shaped polymers that naturally form into thin layers, as opposed to the more random alignment of a normal liquid. Some of these, the nematic liquid crystals, also show an alignment effect between the layers. The particular direction of the alignment of a nematic liquid crystal can be set by placing it in contact with an alignment layer or director, which is essentially a material with microscopic groves in it. When placed on a director, the layer in contact will align itself with the grooves, and the layers above will subsequently align themselves with the layers below, the bulk material taking on the director's alignment. In the case of an LCD, this effect is utilized by using two directors arranged at right angles and placed close together with the liquid crystal between them. This forces the layers to align themselves in two directions, creating a twisted structure with each layer aligned at a slightly different angle to the ones on either side.
LCD shutters consist of a stack of three primary elements. On the bottom and top of the shutter are polarizer plates set at (typically) right angles. Normally light cannot travel through a pair of polarizers arranged in this fashion, and the display would be black. The polarizers also carry the directors to create the twisted structure aligned with the polarizers on either side. As the light flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front of the liquid crystal having been rotated through the correct angle that allows it to pass through the front polarizer. LCDs are normally transparent.
To turn a shutter off, an electrical voltage is applied across it from front to back. When this happens, the rod-shaped molecules align themselves with the electric field instead of the directors, destroying the twisted structure. The light no longer changes polarization as it flows through the liquid crystal, and can no longer pass through the front polarizer. By controlling the voltage applied across the crystal, the amount of remaining twist can be finely selected. This allows the transparency or opacity of the shutter to be accurately controlled. In order to improve switching time, the cells are placed under pressure, which increases the force to re-align themselves with the directors when the field is turned off
Liquid-crystal display televisions (LCD TV)
LCD TV) are color television sets that use LCD technology to produce images. LCD televisions are thinner and lighter than CRTs of similar display size, and are available in much larger sizes as well. This combination of features made LCDs more practical than CRTs for many roles, and as manufacturing costs fell their eventual dominance of the television market was all but guaranteed.
In 2007, LCD televisions surpassed sales of CRT-based televisions worldwide for the first time, and its sales figures relative to other technologies is accelerating. LCD TVs are quickly displacing the only major competitors in the large-screen market, the plasma display panel and rear-projection television. LCDs are, by far, the most widely produced and sold television technology today, pushing all other technologies into niche roles.
In spite of the LCD's many advantages over the CRT technology they displaced, LCDs also have a variety of disadvantages as well. A number of other technologies are vying to enter the large-screen television market by taking advantage of these weaknesses, including OLEDs, FED and SED, but none of these have entered widespread production
In 2007, LCD televisions surpassed sales of CRT-based televisions worldwide for the first time, and its sales figures relative to other technologies is accelerating. LCD TVs are quickly displacing the only major competitors in the large-screen market, the plasma display panel and rear-projection television. LCDs are, by far, the most widely produced and sold television technology today, pushing all other technologies into niche roles.
In spite of the LCD's many advantages over the CRT technology they displaced, LCDs also have a variety of disadvantages as well. A number of other technologies are vying to enter the large-screen television market by taking advantage of these weaknesses, including OLEDs, FED and SED, but none of these have entered widespread production
Tuesday, July 28, 2009
Liquid crystal display
A liquid crystal display (LCD) is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures. Its uses include monitors for computers, televisions, instrument panels, and other devices ranging from aircraft cockpit displays, to every-day consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones. Among its major features are its lightweight construction, its portability, and its ability to be produced in much larger screen sizes than are practical for the construction of cathode ray tube (CRT) display technology. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome. The earliest discoveries leading to the development of LCD technology date from 1888. By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.
Brief history
1888: Friedrich Reinitzer (1858-1927) discovers the liquid crystalline nature of cholesterol extracted from carrots (that is, two melting points and generation of colors) and published his findings at a meeting of the Vienna Chemical Society on May 3, 1888 (F. Reinitzer: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)).[5]
1904: Otto Lehmann publishes his work "Flüssige Kristalle" (Liquid Crystals).
1911: Charles Mauguin first experiments of liquids crystals confined between plates in thin layers.
1922: George Friedel describes the structure and properties of liquid crystals and classified them in 3 types (nematics, smectics and cholesterics).
1904: Otto Lehmann publishes his work "Flüssige Kristalle" (Liquid Crystals).
1911: Charles Mauguin first experiments of liquids crystals confined between plates in thin layers.
1922: George Friedel describes the structure and properties of liquid crystals and classified them in 3 types (nematics, smectics and cholesterics).
Color displays
subpixels, which are colored red, green, and blue, respectively, by additional filters (pigment filters, dye filters and metal oxide filters). Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. CRT monitors employ a similar 'subpixel' structures via phosphors, although the electron beam employed in CRTs do not hit exact 'subpixels'.
Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If the 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.
To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used
Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If the 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.
To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used
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