LCD is winning the race to be the dominant display technology for HDTV. According to DisplaySearch, in 2007, the market for LCD (liquid crystal diode) TVs was estimated at $27.4 billion. LCD TV shipments rose a stronger than expected 52% year to year in the second quarter of 2008 to nearly 7.5 million units (79.8% of total HDTV sales).
The LCD screen on a LCD HDTV is a selective light filter. It does not in itself emit light, but selectively filters light passing through the display from the back. Most HDTVs include either a single sheet of luminescent plastic or sidelighting with fluorescent lamps to provide the backlight. Both techniques are difficult to control.
Many consumers focus on screen resolution when selecting a High Def (HDTV) system. Is this HDTV 720p, 1020i, or 1020p? However, when an expert group, the SMPTE (Society of Motion Pictures and Television Engineers), recently ranked the importance of screen resolution, it ranked fourth. The SMPTE ranked contrast ratio, color saturation, color temperature and grayscale ahead of screen resolution in importance. The Imaging Science Foundation reached a similar conclusion. Backlighting is the key component to improvement in contrast ratio and grayscale.
It is important that an LCD system allows for adjustment of the level of backlighting, and preferably, performs this function automatically. This will improve the contrast of the picture. Also, a reduced backlight level will also reduce power consumption and extend lamp life, as the power used and lamp lifetime is directly related to the backlight level. A lamp with a 50,000 hour lifetime at 6 milliamps may only have a 30,000 hour lifetime if the current is 7 milliamps, just 1 milliamp higher (Example: Sharp Electronics LCD Display).
Another important factor in the choice of backlighting is the consideration of "blur". Blur occurs when there is rapid motion in the programming, such as sports. When the picture changes quickly, the pixels can lag in response. Effective backlight engineering can reduce this significantly by strobing the backlighting, so that the individual pixels do not remain lit longer than needed. Examples of this technique include the Philips "Aptura" and the Samsung "LED Motion Plus".
Sidelighting
With sidelighting, the backlight is located in the back of the LCD panel, and to each side to create panel illumination. To make the light more uniform, a light diffuser is located between the lamps and the rear of the optical filter module. Most LCD systems that use sidelighting use cold cathode fluorescents (CCF) lamps. This technology provides a very bright white light. The lifetime of the lamps is usually between 10,000 to 20,000 hours (6-12 years at a rate of 5 hours of use per day).
The most important disadvantage is that the intensity of light cannot be varied locally, and sidelighting can look non-uniform. Some CCF lamps contain mercury. This is an ecological disadvantage.
The major drawbacks:
- The intensity of light cannot be varied locally. This results in a lower contrast picture.
- The light can be non-uniform.
- Some CCF lamps contain mercury.
Conventional Backlighting
Most backlighting is performed with a single sheet of electroluminescent plastic. These panels can age (though improvements in this area have reduced this problem). Electroluminescent Panel backlighting uses colored phosphors to generate light. Displays using this technique can be thin and lightweight, and provide even light distribution.
The major drawbacks:
- Limited life of 3,000 to 5,000 hours to half brightness (2-3 years at a rate of 5 hours of use per day).
- The intensity of light cannot be varied locally.
LED Backlighting
LED backlighting uses a matrix of LEDs that can be controlled individually to offer very good control. In this configuration, a large number of LEDs are mounted uniformly behind the display. Brightness improvements in LEDs have made them more practical for backlighting. NEC pioneered this technique with their monitor, the 2180WG. Other companies, such as Samsung, have recently introduced versions of this technology. This should push prices down (the Samsung model cost less than 1/3 the price of a comparable NEC monitor).
This system offers local dimming technology and can achieve 1,000,000:1 dynamic contrast ratio. True actual black levels (the "Holy Grail" of LCD) are attainable by shutting off a pixel's light source. This can eliminate the grayish black picture typical of many LCD HDTVs. Groups of LEDs can be locally controlled to produce more light, increasing the brightness when necessary.
The problem with conventional white LEDs is that their spectrum is not ideal for photographic reproduction because they are basically blue LEDs with a yellow phosphor on top. Their color spectrum has two peaks, one at blue and another at yellow. White LEDs that are based on a group of red, green and blue LEDs are ideal for the RGB color filters of an LCD and can result in the most saturated colors. Sony's "Triluminos" LED backlight system is an example of this technique.
Sometimes, these HDTVs have a row of LEDs across the top and bottom of the display. This improves the brightness and color reproduction, and enables almost complete uniformity across the display.
LED backlights are much more efficient that alternative techniques. And since the LED backlight waste less energy, less heat is generated, that must be managed with air conditioning. LED backlights offer a long operating lifetime of more than 50,000 hours (more than 25 years of use at a rate of 5 hours a day).
LED backlit systems may not show well in a typical retail environment. Typically, the screens are somewhat reflective in order to achieve the high contrast ratio, and subsequently, a brightly lit showroom may not flatter this technology. In a more typical less-lit environment, this problem should not be as noticeable.
Engineering Concerns
The electrical engineering is simpler with LED backlights. Cold cathode fluorescent lamps (CCF) require high AC voltages (1,500 VAC) at startup, and operating voltages of 700 VAC or more. When the lamp is new it may take much less voltage to start, but with age the voltage requirements usually increase, complicating a designer's problem. An electroluminescent panel requires a voltage of about 100 VAC @ 400 Hertz.
A power inverter (to generate VAC) is needed for both CCF and electroluminescent panels to operate with a DC power supply. This is a significant expense, in cost, size, and weight of the system. The best inverter for this application permits the output voltage to automatically increase as the lamps age, extending the useful life of the lamps.
LCD systems with LED backlighting utilize low voltage DC, usually 12 or 24 VDC. This results in a smaller, simpler, and more reliable system. LED backlit systems must be well cooled or their performance can suffer. The LEDs generate less heat than CCF lamps, but the heat is very concentrated. Operating at high temperatures reduces the LED lifetime and shifts their light spectrum, while reducing output luminance.
OLED
A new technology that you will be hearing about in the future is Organic LED (OLED). This technology has a completely different backlight technology, it doesn't need one. Because of this, they draw much less power. And because there is no backlight, an OLED system has a larger viewing angle than an LED system. An OLED display is much thinner than an LCD display. The response time for OLED is faster than normal LCD screens. An average of 8 to 12 milliseconds in response time is normal for a LCD compared to 0.01 milliseconds in response time for an OLED.
Sony introduced an 11" HDTV at the Consumer Electronics Show in Vegas. The new 11" Sony TV is called the "XEL-1". It's very thin (1/10") and currently sells for $2500. Samsung demonstrated a 31" OLED HDTV with 1080P resolution at a tradeshow in Asia in November. LG, Toshiba, Matsushita (Panasonic), and others are investing hundreds of millions of dollars, developing this extremely promising technology. Toshiba has plans to sell a 30" OLED display in 2009. Samsung has announced they will sell large screen OLED HDTVs in 2010.
OLED displays have already been used for some time in digital cameras, cell phones and other devices with relatively small panels, because they are very energy efficient, which is very important in portable devices.
In theory, OLED displays can be more inexpensively manufactured than LCD or plasma displays, meaning that they should not be as expensive. Remember that the first large LCD and Plasma displays were much more expensive when they were first introduced.
Summary
LCD HDTVs and monitors with LED array backlighting are now the approach of choice because they exhibit better image quality while saving power. The ability to locally control light output is key to achieving high contrast ratios.
About the Author: Brian Bradshaw is a Certified Technical Specialist (InfoComm CTS). Areas of expertise include Video, Audio, Computation, HDTV, Satellite Systems, and Communications. He has a communications technology business in Plano, Texas (Dallas). More information can be found at his Website
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