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Improvements in TFT-LCD Performance: Better Picture, Thinner, and Lower Power

Large-area TFT-LCDs have made great strides in terms of improved image quality and form factors, and developments in LED backlights will lead to additional improvements. The next wave of TFT-LCD development will focus on 3-D capability and advanced formats.

by Paul Semenza

MANUFACTURERS of large-area TFT-LCD panels face an ongoing dilemma – they need to continuously invest in advanced-generation manufacturing facilities, or fabs – which are increasingly expensive, but at the same time, they face strong competition, which drives down prices. Given the need to quickly amortize the high up-front costs of building a fab, and given the high material costs of making each panel, panel makers are focused on developing features that will support prices. Over the past several years, TFT-LCD manufacturers have focused on several areas of improvement. Primarily, these have involved better image quality, particularly for TVs, and thinner, more power-efficient panels.

Image Quality

Perhaps the most important improvement has been image quality. It is difficult to characterize image quality in a simple specification, but it was clear to consumers that, in many cases, LCDs lagged behind that of the CRTs they were replacing, and often behind competing technologies such as plasma, particularly in video performance. The differences became much more noticeable as panel sizes above 40 in. became widely available. Many of the challenges were related to the unique nature of LCDs – the speed depends on the liquid-crystal materials as well as the manner in which they are driven, and the color and contrast ratio depend on optimizing the liquid-crystal material, color filters, optical films, and backlight. Key metrics in this regard include response time, frame rate, color gamut, and contrast ratio. These metrics are often subjects of debate, in terms of their relationship to perceived performance, and also in that they can be subject to overuse or misuse when being used to market these products.

Response time has been a key enabler for improved video performance; current panel performance is in the range of 2–6 msec. This is significantly faster than a decade ago and is related to new formulations of liquid-crystal material, as well as to the use of "over-driving," which involves temporarily driving the liquid-crystal mixture with a voltage higher than needed to maintain the desired optical state in order to reach that state more quickly.

However, this metric only measures the time to switch the liquid crystal from one gray level to another, typically from full white to full black and/or back again. Because LCDs typically produce images by holding informa-tion for each frame, as opposed to the "impulse" method of CRTs, viewers can perceive blurring of images even with fast switching.

Panel makers have addressed this issue through several techniques. One approach is to increase the frame rate from 60 to 120 Hz or higher and insert interpolated frames (created by analyzing two adjacent frames and estimating what an intervening frame would look like), which results in a smoother motion appearance. This approach is called ME/MC (motion estimation/motion compensation).

A simpler approach is to insert black frames, or frames with partial data, which simulates the impulse method in terms of leading to sharper images, but results in lowered brightness and flicker. Another approach is to scan the backlight in a synchronized fashion with the row scan of the display. This was first achieved with CCFL backlights and is now accomplished through scanning LED backlights. Combined with 120- or 240-Hz refresh rates and ME/MC, very high-quality motion reproduction has been achieved. In order to describe the effect of these moving-picture improvements, MPRT (moving picture response time) is used instead of refresh rate.

Color performance is generally measured in two ways. One describes the percentage of the color space that the display can show, as defined by the NTSC standard. (This standard, created in the early days of color-TV broadcasts, is considered obsolete by many, but is still widely used as a specification.) The other is the number of colors, which is typically indicated by the term "bits," which defines the number of realizable gray levels in the red, green, and blue primaries. The standard reference for "full color" is 8 bits, which translates into 256 levels of gray per color and 16.7 million total colors. Recent displays can address 10 bits, which results in over 1 billion colors, and even 12 bits or more, but it is not clear that there is a perceptible difference at such high numbers of colors. Additional bit depth can be achieved through dynamic backlight control.

Another attribute of image quality is contrast ratio, which in its simplest form is the ratio of the brightness of a "white" pixel to that of a "black" pixel. This is another area in which the method of an LCD is at a disadvantage; in emissive displays, individual pixels can be turned completely off – no light is emitted. In a typical LCD, all pixels are constantly illuminated by the backlight, so turning off a pixel relies on the combination of polarization rotation in the liquid-crystal material and the extinction of crossed polarizers, neither of which is complete. One way to improve the contrast ratio is to actively control the backlight, through the scanning mentioned above or through local dimming, which provides varying levels of control over the backlight. In 0-D local dimming, the entire backlight is dimmed, or turned off, during dark image sequences. In 1-D local dimming, a horizontal band or bands can be dimmed separately. 2-D dimming breaks up the backlight into blocks of multiple pixels, giving a very high level of control. So-called 3-D dimming adds color control, using RGB LEDs.

Local dimming of the backlight, combined with the ability to analyze the content of each frame to determine the optimal backlight brightness level, has enabled much higher contrast ratios in LCDs. Similar to the situation with MPRT, a new metric has been developed to try to capture this improvement; "dynamic contrast ratio" (DCR) is a term that has been used to describe the presence of local dimming and also to differentiate the specification from "static" contrast ratio, the typical metric. Figures for DCR specifications are arrived at by comparing the brightest pixels in any given sequence of frames to the darkest one in the sequence, as opposed to static contrast ratio, which compares bright to dark in a given image. The DCR values can thus be very high, in excess of 10,000:1. Again, it is not clear how perceptible such high levels of DCR are, though the fact that TV is often viewed under low ambient light levels means that there is a greater degree of sensitivity than for other types of display viewing.

Physical Attributes

Another aspect of the rapid growth in panel size has been increasing concern about the size and power consumption of LCD panels. When LCDs first began competing with CRTs, the benefit in size was obvious – no longer was the display roughly as deep as the screen diagonal. However, there have been increasingly significant declines in the thickness of panels, driven by weight and form-factor considerations in notebooks and design considerations in TVs. These two applications have also demanded reductions in power consumption – in notebooks to extend battery life and in TVs to comply with environmental regulations.

The reduction in thickness has been achieved through a combination of techniques: thinner glass and components such as light-guide plates, reduction in optical components, use of edge-lit backlights, and reduced thickness of LED packages. Even large screen sizes are now available that are thinner than 10 mm: Samsung's 55-in. C9000 model uses a panel that is 7.98 mm thick. Given the weight and volume savings from thinner panels, there is perhaps even greater benefit to using them in mobile PC applications. Since these displays are made on smaller substrates, thinner glass can be used – 0.5 mm instead of 0.7 mm; for smaller displays (less than 15 in.), 0.4-mm substrates can be used, and for ultra-portable notebooks, the display cell can be thinned even more through the use of mechanical or chemical treatments. These techniques have enabled the production of displays as thin as 3 mm or less.

Given the increasing level of concern over global energy usage, regions around the world are implementing power-consumption regulations that cover flat-panel TV. While less power hungry than the CRTs they have replaced and many of the plasma TVs they compete against, the sheer number and growing screen sizes of LCD TVs have put their power consumption in the spotlight. Since nearly all of the power consumption is due to the backlight in the LC module, LCD makers have been working on reducing power consumption through a variety of means. One avenue is to improve the optical transmission of the LCD cell, for which there are multiple approaches.¹ The other way is to improve the efficiency of the backlight through the use of more-efficient LED chips, as well as better backlight optical design.

Where to Next?

With higher-quality, thinner, and lower-power-consuming panels becoming mainstream, what are the next steps in LCD technology development?

The rapid improvements in image quality, display thickness, and power consumption described earlier owe a great deal to developments in LED backlighting. The first LED TV backlights were direct configurations – the LEDs were placed in an array directly behind the panel. But the high cost of the LEDs and the desire to create very thin form factors caused a quick shift to edge-lit configurations. Such backlights couple the light from arrays of LEDs into light-guide plates, which distribute the light across the display and extract the light through optical structures that use reflection or refraction to turn the light 90¼. By addressing individual "bars" of LEDs, edge-lit LED backlights have been able to implement both 1-D and 2-D local dimming; the latter originally thought to require direct backlighting. However, with the large declines in LED prices and the desire for ever-higher performance, a new crop of direct-lit LED backlit panels is emerging. The emphasis will likely be on large (40 in. and larger) high-end panels that can command premiums.

Continued improvement in LED brightness, efficiency, and package designs are likely, and this will enable continued display improvements. Most LED backlights use white LEDs, and there are ongoing improvements in phosphor design as well as developments such as quantum dots that could enable greater efficiencies. It is also possible that RGB LEDs could be utilized, which could eventually enable implementation of field-sequential color.

The year 2010 marked the beginning of mass-production of large 3-D LCD panels for TV. Most of these panels are for "active," or frame-sequential-type 3-D sets, which can use standard 120/240 Hz or higher panels – the set maker adds an additional video channel and a transmitter/receiver circuit to communicate with the shutter glasses. However, panel makers are developing "passive," or polarization-based 3-D panels, in which the left and right frames are presented simultaneously and presented to the left and right eyes through the use of polarizing glasses. (See, "Evolving Technologies for LCD-Based 3-D Entertainment" in this issue.) This involves the integration of a polarizing retardation layer or other type of film that is built into the panel. This could mean lower costs for the consumer because the polarizing glasses are much cheaper; more importantly, it could enable panel makers to capture a greater share of any 3-D premium. However, the performance of passive 3-D displays has not yet reached the level of the active systems. Autostereoscopic 3-D displays, for which no glasses are required, are farther behind in development for large panels, though mass production is now starting in small sizes for mobile games, cameras, and mobile phones.

In 2009, panel makers started promoting what is being called cinema displays – 21:9-aspect-ratio panels, with pixel formats of 2560 x 1080. As with most transitions to widescreen panels, part of the rationale for this format is "panelization" – the ability to use a greater fraction of the substrate, particularly in Gen 8 and higher fabs, which lowers manufacturing cost. Some argue that an aspect ratio of 21:9 more closely simulates the feeling of cinema and that Blu-ray DVD supports Cinemascope HDTV, a 2.35:1 format, without the letter-box effect. Finally, with the growth in connected TV, some sort of tool bar is often required, and a 21:9 widescreen allows space for this along with a full-HD image. It is not clear if this format will succeed because there is little to no content available and the format means that consumers will have to purchase even larger displays to maintain the same screen height. Most likely, this format will be most effective in very large (greater than 60-in. diagonal) screen sizes used in home theaters. Other formats have been proposed, most notably quad-HD (3840 x 2160 pixels), but given the gradual transition to full-HD (1920 x 1080) it is not clear when the demand for such panels will become significant.

TFT-LCD Development in Perspective

With improvements in performance, particularly in video image quality, TFT-LCDs have come a long way toward matching CRT performance across the board, and surpassing it in several aspects. At the same time, available screen sizes have expanded tremendously and the physical extent of these devices has been reduced significantly. With the exception of power consumption, the rate of improvement in these areas is likely to slow, and the emphasis is shifting to advanced capabilities such as 3-D, higher resolution, and new formats. (See the article, "Two New Technology Developments for the LC Display Industry" in this issue.)

In the future, it is likely that developments in large-area TFT-LCDs will shift toward embedding more intelligence on the panel. This could include increased integration of existing functions (for example, communications or memory), as well as the development of panels that can sense and react to their environments. Integration of touch, ambient light sensing, imaging, and other functions could enable TFT-LCDs to serve as communication portals (for example, videoconferencing) and increase the capability for inter-activity (for example, gesture recognition). These types of functions will provide added value and enable revenue streams that are needed to justify ongoing investments in research and manufacturing.

References

¹C. Annis and P. Semenza, "Better Transmission: TFT-LCD Manufacturing Advances Reduce Cost and Energy Consumption," Information Display 17, No. 12 (December 2009). •