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.
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.
LCDs are relatively inefficient in terms of power use per display size, because the vast majority of light that is being produced at the back of the screen is blocked before it reaches the viewer. To start with, the rear polarizer filters out over half of the original un-polarized light. Examining the image above, you can see that a good portion of the screen area is covered by the cell structure around the shutters, which removes another portion. After that, each sub-pixel's color filter removes the majority of what is left to leave only the desired color. Finally, to control the color and luminance of a pixel as a whole, the light has to be further absorbed in the shutters.
For these reasons the backlighting system has to be extremely powerful. In spite of using highly efficient CCFLs, most sets use several hundred watts of power, more than would be required to light an entire house with the same technology. As a result, LCD televisions end up with overall power usage similar to a CRT of the same size. Using the same examples, the KV-40XBR800 dissipates 245 W,[2] while the LC-42D65 dissipates 235 W.[1] Plasma displays are worse; the best are on par with LCDs, but typical sets draw much more.[3]
Modern LCD sets have attempted to address the power use through a process known as "dynamic lighting" (originally introduced for other reasons, see below). This system examines the image to find areas that are darker, and reduces the backlighting in those areas. CCFLs are long cylinders that run the length of the screen, so this change can only be used to control the brightness of the screen as a whole, or at least wide horizontal bands of it. This makes the technique suitable only for particular types of images, like the credits at the end of a movie. In 2009 some manufacturers[4] made some TVs using HCFL (more power efficient than CCFL). Sets using LEDs are more distributed, with each LED lighting only a small number of pixels, typically a 16 by 16 patch. This allows them to dynamically adjust brightness of much smaller areas, which is suitable for a much wider set of images.
Another ongoing area of research is to use materials that optically route light in order to re-use as much of the signal as possible. One potential improvement is to use microprisms or dichromic mirrors to split the light into R, G and B, instead of absorbing the unwanted colors in a filter. A successful system would improve efficiency by three times. Another would be to direct the light that would normally fall on opaque elements back into the transparent portion of the shutters.
Several newer technologies, OLED, FED and SED, have lower power use as one of their primary advantages. All of these technologies directly produce light on a sub-pixel basis, and use only as much power as that light level requires. Sony has demonstrated 36" FED units displaying very bright images drawing only 14 W, less than 1/10 as much as a similarly sized LCD. OLEDs and SEDs are similar to FEDs in power terms. The dramatically lower power requirements make these technologies particularly interesting in low-power uses like laptop computers and mobile phones. These sorts of devices were the market that originally bootstrapped LCD technology, due to its light weight and thinness.
Early LCD sets were widely derided for their poor overall image quality, most notably the ghosting on fast-moving images, poor contrast ratio, and muddy colors. In spite of many predictions that other technologies would always beat LCDs, massive investment in LCD production, manufacturing, and electronic image processing has addressed many of these concerns.
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For 60 frames per second video, common in North America, each pixel is lit for 17 ms before it has to be re-drawn (20 ms in Europe). Early LCD displays had response times on the order of hundreds of milliseconds, which made them useless for television. A combination of improvements in materials technology since the 1970s greatly improved this, as did the active matrix techniques. By 2000, LCD panels with response times around 20 ms were relatively common in computer roles. This was still not fast enough for television use.
A major improvement, pioneered by NEC, led to the first practical LCD televisions. NEC noticed that liquid crystals take some time to start moving into their new orientation, but stop rapidly. If the initial movement could be accelerated, the overall performance would be increased. NEC's solution was to boost the voltage during the "spin up period" when the capacitor is initially being charged, and then dropping back to normal levels to fill it to the required voltage. A common method is to double the voltage, but halve the pulse width, delivering the same total amount of power. Named "Overdrive" by NEC, the technique is now widely used on almost all LCDs.
Another major improvement in response time was achieved by adding memory to hold the contents of the display – something that a television needs to do anyway, but was not originally required in the computer monitor role that bootstrapped the LCD industry. In older displays the active matrix capacitors were first drained, and then recharged to the new value with every refresh. But in most cases, the vast majority of the screen's image does not change from frame to frame. By holding the before and after values in computer memory, comparing them, and only resetting those sub-pixels that actually changed, the amount of time spent charging and discharging the capacitors was reduced. Moreover the capacitors are not drained completely; instead, their existing charge level is either increased or decreased to match the new value, which typically requires fewer charging pulses. This change, which was isolated to the driver electronics and inexpensive to implement, improved response times by about two times.
Together, along with continued improvements in the liquid crystals themselves, and by increasing refresh rates from 60 Hz to 120 and 240 Hz, response times fell from 20 ms in 2000 to about 2 ms in the best modern displays. But even this is not really fast enough because the pixel will still be switching while the frame is being displayed. Conventional CRTs are well under 1 ms, and plasma and OLED displays boast times on the order of 0.001 ms.
One way to further improve the effective refresh rate is to use "super-sampling", and it is becoming increasingly common on high-end sets. Since the blurring of the motion occurs during the transition from one state to another, this can be reduced by doubling the refresh rate of the LCD panel, and building intermediate frames using various motion compensation techniques. This smooths out the transitions, and means the backlighting is turned on only when the transitions are settled. A number of high-end sets offer 120 Hz (in North America) or 100 Hz (in Europe) refresh rates using this technique. Another solution is to only turn the backlighting on once the shutter has fully switched. In order to ensure that the display does not flicker, these systems fire the backlighting several times per refresh, in a fashion similar to movie projection where the shutter opens and closes several times per frame.
Even in a fully switched-off state, liquid crystals allow some light to leak through the shutters. This limits their contrast ratios to about 1600:1 on the best modern sets, when measured using the ANSI measurement (ANSI IT7.215-1992). Manufacturers often quote the "Full On/Off" contrast ratio instead, which is about 25% greater for any given set.[5]
This lack of contrast is most noticeable in darker scenes. To display a color close to black, the LCD shutters have to be turned to almost full opacity, limiting the number of discrete colors they can display. This leads to "posterizing" effects and bands of discrete colors that become visible in shadows, which is why many reviews of LCD TVs mention the "shadow detail".[6] In comparison, the highest-end LED TVs offer regular contrast ratios of 5,000,000:1.
Since the total amount of light reaching the viewer is a combination of the backlighting and shuttering, modern sets can use "dynamic backlighting" to improve the contrast ratio and shadow detail. If a particular area of the screen is dark, a conventional set will have to set its shutters close to opaque to cut down the light. However, if the backlighting is reduced by half in that area, the shuttering can be reduced by half, and the number of available shuttering levels in the sub-pixels doubles. This is the main reason high-end sets offer dynamic lighting (as opposed to power savings, mentioned earlier), allowing the contrast ratio across the screen to be dramatically improved. While the LCD shutters are capable of producing about 1000:1 contrast ratio, by adding 30 levels of dynamic backlighting this is improved to 30,000:1.
However, the area of the screen that can be dynamically adjusted is a function of the backlighting source. CCFLs are thin tubes that light up many rows (or columns) across the entire screen at once, and that light is spread out with diffusers. The CCFL must be driven with enough power to light the brightest area of the portion of the image in front of it, so if the image is light on one side and dark on the other, this technique cannot be used successfully. Displays backlit by full arrays of LEDs have an advantage, because each LED lights only a small patch of the screen. This allows the dynamic backlighting to be used on a much wider variety of images. Edge-lit displays do not enjoy this advantage. These displays have LEDs only along the edges and use a light guide plate covered with thousands of convex bumps that reflect light from the side-firing LEDs out through the LCD matrix and filters. LEDs on edge-lit displays can be dimmed only globally, not individually.
The massive on-paper boost this method provides is the reason many sets now place the "dynamic contrast ratio" in their specifications sheets. There is widespread debate in the audio-visual world as to whether or not dynamic contrast ratios are real, or simply marketing speak.[7][8] Reviewers commonly note that even the best LCD displays cannot match the contrast ratios or deep blacks of plasma displays, in spite of being rated, on paper, as having much higher ratios.
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Color on an LCD television is produced by filtering down a white source and then selectively shuttering the three primary colors relative to each other. The accuracy and quality of the resulting colors are thus dependent on the backlighting source and its ability to evenly produce white light. The CCFLs used in early LCD televisions were not particularly white, and tended to be strongest in greens. Modern backlighting has improved this, and sets commonly quote a color space covering about 75% of the NTSC 1953 color gamut. Using white LEDs as the backlight improves this further.
In September 2009 Nanoco Group announced that it had signed a joint development agreement with a major Japanese electronics company under which it will design and develop quantum dots for use in LED backlights in LCD televisions.[9] Quantum dots are valued for displays, because they emit light in very specific Gaussian distributions. This can result in a display that more accurately renders the colors that the human eye can perceive.Quantum dots also require very little power since they are not color filtered. To generate white light best suited as an LCD backlight, parts of the light of a blue-emitting LED are transformed by quantum dots into small-bandwidth green and red light such that the combined white light allows for a nearly ideal color gamut generated by the color filters of the LCD panel. US company QD Vision worked with Sony to launch LCD TVs using this technique under the marketing label Triluminos in 2013.
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Passive matrix LCDs first became common in the 1980s for various portable computer roles. At the time they competed with plasma displays in the same market space. The LCDs had very slow refresh rates that blurred the screen even with scrolling text, but their light weight and low cost were major benefits. Screens using reflective LCDs required no internal light source, making them particularly well suited to laptop computers.
Refresh rates of early devices were too slow to be useful for television. Portable televisions were a target application for LCDs. LCDs consumed far less battery power then even the miniature tubes used in portable televisions of the era. The earliest commercially made LCD TV was the Casio TV-10 made in 1983.[10] Resolutions were limited to standard definition, although a number of technologies were pushing displays towards the limits of that standard; Super VHS offered improved color saturation, and DVDs added higher resolutions as well. Even with these advances, screen sizes over 30" were rare as these formats would start to appear blocky at normal seating distances when viewed on larger screens. Projection systems were generally limited to situations where the image had to be viewed by a larger audience.
Nevertheless, some experimentation with LCD televisions took place during this period. In 1988, Sharp Corporation introduced the first commercial LCD television, a 14" model with active matrix addressing using thin-film transistors (TFT). These were offered primarily as boutique items for discerning customers, and were not aimed at the general market. At the same time, plasma displays could easily offer the performance needed to make a high quality display, but suffered from low brightness and very high power consumption. However, a series of advances led to plasma displays outpacing LCDs in performance improvements, starting with Fujitsu's improved construction techniques in 1979, Hitachi's improved phosphors in 1984, and AT&T's elimination of the black areas between the sub-pixels in the mid-1980s. By the late 1980s, plasma displays were far in advance of LCDs.
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It was the slow standardization of high definition television that first produced a market for new television technologies. In particular, the wider 16:9aspect ratio of the new material was difficult to build using CRTs; ideally a CRT should be perfectly circular in order to best contain its internal vacuum, and as the aspect ratio becomes more rectangular it becomes more difficult to make the tubes. At the same time, the much higher resolutions these new formats offered were lost at smaller screen sizes, so CRTs faced the twin problems of becoming larger and more rectangular at the same time. LCDs of the era were still not able to cope with fast-moving images, especially at higher resolutions, and from the mid-1990s the plasma display was the only real offering in the high resolution space.
Through the halting introduction of HDTV in the mid-1990s into the early 2000s, plasma displays were the primary high-definition display technology. However, their high cost, both manufacturing and on the street, meant that older technologies like CRTs maintained a footprint in spite of their disadvantages. LCD, however, was widely considered to be unable to scale into the same space, and it was widely believed that the move to high-definition would push it from the market entirely.
This situation changed rapidly. Contrary to early optimism, plasma displays never saw the massive economies of scale that were expected, and remained expensive. Meanwhile, LCD technologies like Overdrive started to address their ability to work at television speeds. Initially produced at smaller sizes, fitting into the low-end space that plasmas could not fill, LCDs started to experience the economies of scale that plasmas failed to achieve. By 2004, 32" models were widely available, 42" sets were becoming common, and much larger prototypes were being demonstrated.
Although plasmas continued to hold an arguable picture quality edge over LCDs, and even a price advantage for sets at the critical 42" size and larger, LCD prices started falling rapidly in 2006 while their screen sizes were increasing at a similarly rapid rate. By late 2006, several vendors were offering 42" LCDs, albeit at a price premium, encroaching on plasma's only stronghold. More critically, LCDs offer higher resolutions and true1080p support, while plasmas were stuck at 720p, which made up for the price difference.[11]
Predictions that prices for LCDs would drop rapidly through 2007 led to a "wait and see" attitude in the market, and sales of all large-screen televisions stagnated while customers watched to see if this would happen.[11]Plasmas and LCDs reached price parity in 2007, at which point the LCD's higher resolution was a winning point for many sales.[11] By late 2007, it was clear that LCDs were going to outsell plasmas during the critical Christmas sales season.
