Liquid Crystal Displays in LCD flat panel TV Applications

Developments in LCD Panel Technology

LCD Panel Technology is constantly evolving in all areas from developments in the use of different liquid crystals to manufacturing processes that help improve the yield factor and therefore, reduce costs.

In this short article, we discuss the latest developments as applicable to LCD television. We also explore what major LCD flat-panel manufactures are doing to come up with LCD TV sets that are better, sharper, larger, and cheaper!

New Technological Developments in LCD HDTV Applications

LCD panel technology is probably one of the fastest growing video display technologies with advancements taking place in all areas.

This is not to say that technological advancements are not also taking place in other flat-panel display technologies - in particular plasma, which in itself is the real major flat-panel competitor to LCD. But in contrast, major developments in plasma displays as applicable to present-day television applications, are moving at a slower pace and are taking place mainly to consolidate the technology.

In other words, the real technological break-through in flat-panel displays is taking place in the area of liquid crystal displays.

LCD technology is fast evolving in all areas from developments in the use of different liquid crystals to manufacturing process that help improve the yield factor and therefore, reduce costs.

Liquid Crystals

LCD panels today employ several variations of liquid crystal technology, including super twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC) and surface stabilized ferroelectric liquid crystal (SSFLC).

Use of special plastics instead of the glass substrates and flexible backplanes for flexible displays for use in specialized applications are also becoming a reality.

Price and LCD Display Size

In the meantime, developments in the manufacturing process aiming at improving the yield factor are helping to push the price down while increasing the display size - rendering LCD TV sets bigger and more affordable than ever.

In this respect, LCD TV sets in the 40-inch screen size range are already selling at practically the same price as similar size plasma televisions. LCD panel manufactures are now targeting the 50-inch TV size segment, which is expected to become the real favorite TV screen size in the multi-billion dollar home entertainment market.

For this purpose, LCD panel makers are addressing their efforts towards next-generation LCD production techniques. These should lower production costs - thus rendering 50-inch LCD televisions more competitive against same-sized plasma display panel TVs.

In the meantime, some really big screens LCD display panels have also started to emerge, with Samsung and Sony boosting the world's largest TFT-LCD TV 82-inch prototypes at the IFA 2005 and CES 2006 shows respectively.
 

It must be noted that 'Price' and 'Display Size' are both limited by the quality-control problems faced by manufacturers. Increasing the display size implies adding more pixels and therefore more transistors - thus increasing the chance of including a few bad transistors in a display. This in turn will further increase the rejection level.

In other words, advancement in present day LCD manufacturing technology is a pre-requisite for manufacturers to come up with more affordable displays in bigger sizes.

Developments in LCD Back Light Technology

Advancement in back light technology aimed mainly at improved picture quality of LCD panel displays is another area where we are seeing a lot of investment, with different manufacturers proposing different backlight solutions.

Philips are employing intelligence in the control of the multiple high output florescent lamps operated in scanning mode on some of their high-end LCD displays; this helps cancel out the sample-and-hold effect, which is characteristic of LCD technology. The end result is improved motion sharpness.

Similarly, actively controlling the backlight brightness level in synchronization with the picture content helps improve gray scale performance and deeper black levels.

Samsung is making use of a flat fluorescent lamp (FFL) instead of the standard cold cathode fluorescent tubular lights (CCFL), to power some of its flat-panel LCD HDTVs. The main advantage is that FFLs have a paper-thin form factor that produces light from its entire surface, offering the potential for greater picture uniformity, better brightness, and a higher contrast ratio.

Sharp is also making use of improved CCFL in their LCD panels - making use of their proprietary 4-color florescent backlight - through the addition of a 'crimson' emitter to the traditional red, green, and blue (RGB) cold cathode florescent lamp light sources. Sharp has added this fourth color to add more expression in the red range as research shows that the eye is extremely sensitive to this color. This has substantially increased the color gamut from the typical 70% of the NTSC to close to 80%. Still far from the almost 100% possible with the use of LED based backlight technology, but Sharp's 4-color florescent backlight is substantially cheaper.

Samsung and Sony have also exhibited the first LCD panel prototypes to use backlights running off 3-color RGB LED light sources on some of their high-end prototypes displayed in recent technology shows.

LED backlighting enjoys a number of advantages over LCD panels using cold-cathode florescent lamps (CCFL), with the most significant being:

• improved color rendering that is more true to life since the white light is produced from frequencies that are closer to those of the three primary colors,

• improved video performance that is similar to CRTs,

• reduced power consumption, and

• longer life that is typical twice that of CCFL based models.

Color Field Sequential Technology

Directly related with LED backlighting, is 'color sequential'. Color sequential is becoming popular as a method of eliminating the color filters in LCDs. In 30 to 40 inch LCD panels, the color filter accounts for some 25% of the total cost of the LCD display panel. Thus, eliminating the color filters represents a significant way of reducing costs and simplifying the production process.

Color sequential requires that a backlight emits red, green and blue light in sequence with one color at a time as required by the display content.  This is somewhat similar to the color wheel process used in DLP-based projection systems (more info on the use of the color wheel in projection TV based systems is available in our Projection Television: How-it-Works guide).

The LCD controller in a color sequential system is synchronized with the backlight so that when a given color backlight is on, only the matching color sub-pixels in the LCD are turned on.  This would result in improved color gamut that is close to 100% of the NTSC gamut; this contrasts with the typical less than 80% of NTSC gamut achievable through the use of standard tubular CCFL backlight LCD displays. The end result is color that is much richer and more realistic.

Eliminating the color filters has a further advantage. Red, green, and blue color filters used on sub-pixels absorb up to some 70% of light output from the backlight - thus eliminating the color filters implies that it is possible to use a lower brightness light source to achieve the same display brightness level. The end result is a significant reduction in power consumption.

Yet, there is more in favor of color sequential backlighting. The use of color sequential technology eliminates the need for color sub-pixels; instead, the LCD panel can be redesigned so that it has three times as many regular pixels. In effect, each sub-pixel will become a pixel, which means that the resolution can be increased by a factor of 3.

The LCD controller is still synchronized with the backlight; when a given color backlight is on, each pixel is set to the correct value for that color in the image. The eyes will see a rapidly changing sequence of red-green-blue images - (again, similar to a single chip DLP projection set-up); the brain will then combines these red, green, and blue monochrome images into a full-color image.

Interesting to note here that this increase in display resolution is achieved without increasing the risk of a higher level of bad pixels from that of present display technology. This is possible since the increase in display resolution is being achieved not through an increase in the number of transistors on the display surface, but rather through re-design of the sub-pixel elements for these to operate as main pixels.

Optically Compensated Bend (OCB)

Toshiba has been working on Optically Compensated Bend (OCB) liquid crystals for a number of years, with work mainly directed towards the development of small LCD panels for automotive applications.

Up to very recent, a typical problem associated with standard LCD panels was the deterioration of the image as one moves away from the supported field of view. A lot of improvement has been going on in this direction. Most of today's LCD TV set support 165 - 170 degrees both horizontally and vertically. There are various ways that can help support a wider angle of view - Optically Compensated Bend is one of them.

One side-benefit of OCB is that it makes the LCD inherently faster - Toshiba indicated that they measured a 3.3msec response time on their prototypes as against the typical 7 to 8 msec of most of today's displays.  Another interesting side-benefit of OCB is that it makes the LCD work much better in cold temperatures (-20C).

The inherently faster response of OCB LCD panels makes these the ideal candidate for use with color field sequential technology. If color sequential is used to increase the resolution of the LCD, then a faster-responding LCD is needed because each pixel on the LCD panel is changing three times as often to display a rapid sequence of red, green, and blue images on the screen.

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