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HOME                                  REVIEWS                                CONTACT                             ABOUT                         March 14, 2009

 

      

LCD Backlighting for HDTV

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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).

 

An LCD as used in HDTV is basically 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.

 

Most 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 primary 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 Drawbacks:

o       The intensity of light cannot be varied locally.  This results in a lower contrast picture.

o       The light can be non-uniform.

o       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 Drawbacks:

o       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).

o       The intensity of light cannot be varied locally.

 

LED Backlighting

 

LED backlighting uses a matrix of white 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 white 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.

 

At one time, a significant disadvantage of LED backlighting was the consumption of energy, especially with large displays.  But advances in recent years have improved the efficiency to the point that energy efficiency is now an advantage.  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. 

 

LED systems 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.  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, Toshiba, Matsushita (Panasonic), and others are investing hundreds of millions of dollars, also trying to develop this extremely promising technology.  Toshiba has plans to sell a 30" OLED display in 2009.

 

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 is now the approach of choice because it exhibits better image quality while saving power.  The ability to locally control light output is key to achieving high contrast ratios.  Long term, OLED has the potential to rival LED systems in price and performance. 

 

 

 

 

 

 

 

 

 About the Author:

 

Brian Bradshaw is an InfoComm Certified Technical Specialist (CTS), Microsoft Certified Technology Specialist (MCTS), and CompTIA A+ Computer Technologist.  Areas of expertise include Video, HDTV, Audio, Computation, SATCOM Systems, and Communications.


Website:  http://bradshaw-vacuum-technology.com/