LCD is winning the race to be the dominant display technology for HDTV. According to DisplaySearch, in 2007, the LCD (Liquid Crystal Diode) TV market was estimated at $27.4 billion. LCD TV shipments rose a stronger-than-expected 52% year over year in the second quarter of 2008 to almost 7.5 million units (79.8% of total HDTV sales).

The LCD screen of an LCD HDTV is a selective light filter. It does not emit light itself, but rather selectively filters the light that passes through the screen from the back. Most HDTVs include a single sheet of luminescent plastic or side lighting with fluorescent lamps to provide backlight. Both techniques are difficult to control.

Many consumers focus on screen resolution when selecting a high-definition (HDTV) system. Is this HDTV 720p, 1020i, or 1020p? However, when a group of experts, the SMPTE (Society of Motion Picture and Television Engineers), recently rated the importance of screen resolution, it ranked fourth. The SMPTE ranked contrast ratio, color saturation, color temperature, and gray scale above screen resolution in importance. The Imaging Science Foundation reached a similar conclusion. The backlight is the key component to improve contrast ratio and gray scale.

It is important that an LCD system allow adjustment of the backlight level and preferably perform this function automatically. This will improve the contrast of the image. In addition, a reduced backlight level will also reduce power consumption and extend lamp life, as the power used and lamp life are directly related to the backlight level. A lamp with a lifespan of 50,000 hours at 6 milliamps can only have a lifespan of 30,000 hours if the current is 7 milliamps, just 1 milliamp more (Example: Sharp Electronics LCD).

Another important factor in backlight choice is consideration of “defocus”. Blur occurs when there is fast motion in the programming, such as sports. When the image changes rapidly, the pixels may lag in response. Effective backlight engineering can reduce this significantly by strobing the backlight so that individual pixels don’t stay lit longer than necessary. Examples of this technique include Philips “Aptura” and Samsung “LED Motion Plus”.

side lighting

With side lighting, the backlight is on the back of the LCD panel and on each side to create panel lighting. To make the light more even, a light diffuser is located between the lamps and the back of the optical filter module. Most LCD systems that use side lighting use cold cathode fluorescent (CCF) lamps. This technology provides a very bright white light. The useful life 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 the light cannot be varied locally and the lateral illumination can appear non-uniform. Some CCF lamps contain mercury. This is an ecological disadvantage.

The main drawbacks:

  • The light intensity cannot be varied locally. This results in a lower contrast image.
  • The light may be non-uniform.
  • Some CCF lamps contain mercury.

conventional backlight

Most of the backlighting is done with a single sheet of electroluminescent plastic. These panels can age (although improvements in this area have reduced this problem). The electroluminescent panel backlight uses colored phosphors to generate light. Screens using this technique can be thin and light and provide an even distribution of light.

The main drawbacks:

  • Limited life of 3,000 to 5,000 hours at medium brightness (2-3 years at a rate of 5 hours of use per day).
  • The light intensity cannot be varied locally.

LED backlight

The LED backlight uses an array of individually controllable LEDs to offer very good control. In this configuration, a large number of LEDs are evenly mounted behind the screen. Brightness improvements to LEDs have made them more practical for backlighting. NEC pioneered this technique with its monitor, the 2180WG. Other companies, such as Samsung, have recently introduced versions of this technology. This should drive prices down (Samsung’s model is less than 1/3 the price of a comparable NEC monitor).

This system offers local dimming technology and can achieve a dynamic contrast ratio of 1,000,000:1. True true black levels (the “Holy Grail” of the LCD screen) can be achieved by turning off the light source of one pixel. This can eliminate the grayish-black image typical of many high-definition LCD TVs. Groups of LEDs can be controlled locally to produce more light, increasing brightness when required.

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. Its color spectrum has two peaks, one in blue and one in yellow. White LEDs that are based on a group of red, green, and blue LEDs are ideal for RGB color filters on an LCD screen and can produce the most saturated colors. Sony’s “Triluminos” LED backlight system is an example of this technique.
Sometimes these HDTVs have a row of LEDs at the top and bottom of the screen. This improves brightness and color reproduction, and allows almost complete uniformity across the screen.

LED backlights are much more efficient than alternative techniques. And since LED backlighting wastes less energy, less heat is generated, that has to be managed with air conditioning. The LED backlight offers a long service life of more than 50,000 hours (more than 25 years of use at a rate of 5 hours per day).

Systems with LED backlighting may not display well in a typical retail environment. Screens are typically a bit reflective to achieve the high contrast ratio, and therefore a well-lit showroom may not favor this technology. In a more typical less lit environment, this problem should not be as noticeable.

engineering concerns

Electrical engineering is easier with LED backlighting. Cold Cathode Fluorescent Lamps (CCF) require high AC voltages (1500 VAC) at startup and operating voltages of 700 VAC or more. When the lamp is new, it may require much less voltage to turn on, but over time the voltage requirements generally increase, complicating the designer’s problem. An electroluminescent panel requires a voltage of around 100 VAC @ 400 Hertz.

A power inverter (to generate VAC) is required for CCF and electroluminescent panels to operate on a DC power supply. This is a significant expense, in cost, size and weight of the system. The best inverter for this application allows the output voltage to automatically increase as the lamps age, extending the life of the lamps.

LED-backlit LCD systems use low-voltage DC, typically 12 or 24 V DC. This results in a smaller, simpler and more reliable system. Systems with LED backlights must be well cooled or their performance may suffer. LEDs generate less heat than CCF lamps, but the heat is highly concentrated. Operating at high temperatures reduces the life of the LED and changes its light spectrum, while reducing the output luminance.

OLED

A new technology you will hear about in the future is organic LED (OLED). This technology has a completely different backlight technology, you don’t need it. Due to this, they consume much less energy. And because there is no backlight, an OLED system has a larger viewing angle than an LED system. An OLED screen is much thinner than an LCD screen. The response time of OLED is faster than that of normal LCD screens. An average of 8 to 12 milliseconds in response time is normal for an LCD compared to 0.01 milliseconds in response time for an OLED.

Sony introduced an 11″ HDTV at the Consumer Electronics Show in Las Vegas. Sony’s new 11″ TV is called “XEL-1”. It is very thin (1/10″) and currently retails for $2,500. Samsung demonstrated a 31″ OLED HDTV with 1080P resolution at a trade show in Asia in November. LG, Toshiba, Matsushita (Panasonic) and others are investing hundreds of millions of dollars in developing this extremely promising technology. Toshiba plans to sell a 30″ OLED screen in 2009. Samsung has announced that it will sell large screen OLED HDTVs in 2010.

OLED screens have already been used for some time in digital cameras, mobile phones and other devices with relatively small panels, because they are very energy efficient, which is very important in portable devices.

In theory, OLED screens can be made more cheaply than LCD or plasma screens, which means they shouldn’t be as expensive. Remember that the first large LCD and plasma screens were much more expensive when they were first introduced.

Summary

LCD HDTVs and LED-backlit monitors are now the preferred approach because they exhibit better picture quality and save energy. The ability to locally control light output is key to achieving high contrast ratios.

LCD backlight for HDTV

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