Flat Panel Displays Go Green

Dr. Marko Slusarczuk, Optoelectronics Industry Development Association

The LCD is now the dominant flat panel display and continues to take market share from other technologies. These displays span from small units for handheld devices to large panels for wall-mounted televisions. Although they are not yet available to the consumer, 108-in.-diagonal prototypes are being shown by leading manufacturers.

LED backlighting can help lower power consumption. Backlighting for this full-high-definition LCD is provided by LEDs. Courtesy of Samsung.

The LCD is a light valve device that modulates the transmission of light at individual pixels to generate an image. Manufacturers have developed very small, highly efficient cold-cathode fluorescent lamps to serve as backlights. Typically, an LCD has a cold-cathode fluorescent lamp light source at one or more edges of the panel and light pipes to distribute the light uniformly across the back of the panel. The problem is that even if the LCD displays only one pixel, the backlight has to operate at full brightness. A backlight is the largest consumer of power of all the components in a display panel, and it accounts for most of its thickness.

The power consumption of a cold-cathode fluorescent lamp does not scale linearly with size. The two ends that generate the ions that ultimately stimulate the phosphor consume most of the power. Thus, for small portable devices, the fluorescent backlight can be the dominant drain on the power.

Phosphor-based white LEDs can be used as backlights in LCDs. Courtesy of Philips Lumileds.

To reduce power consumption and the form factor of small handheld devices such as cell phones, many manufacturers now use LEDs as backlights. Because the power consumption of the backlight is directly proportional to the number of LEDs incorporated, LEDs have a distinct advantage over cold-cathode fluorescent lamps in these applications. In 2005, Sony introduced a laptop with an LED backlight, which allowed a much thinner screen than was possible with a fluorescent one.

LED advantages

LED backlights can use one of two configurations: clusters of individual red, green and blue LEDs that combine to create white light, or blue indium-gallium-nitride (InGaN) LEDs that use a phosphor coating to convert the blue light to white. Individual LEDs are much smaller than a fluorescent lamp, and they put out less light. For small displays, a few LEDs provide adequate illumination, whereas large displays require many LEDs. The relatively low efficiency and associated thermal management issues have limited the size of LED-backlit displays.

Recently, however, LED manufacturers have improved the brightness and efficiency of LEDs significantly, enabling several manufacturers to introduce large-screen LED-backlit LCDs. The LED backlights consist of a matrix of individual emitters. The light sources are set back from the panel to achieve color mixing, and the light then passes through diffusers. This eliminates hot spots and color shift, producing a uniform backlight.

LEDs have several advantages over cold-cathode fluorescent lamps. First, they deliver a higher quality color gamut — the image shows more deeply saturated colors and higher fidelity. Second, the LEDs allow for “smart” backlights where software control dims certain parts of the backlight to correspond with bright and dark areas of the image. This, in turn, results in higher contrast and deeper blacks and reduces power consumption. Third, LEDs are a green technology — they do not contain mercury as cold-cathode fluorescent lamps do. This consideration can become a technology driver as governments increasingly regulate the use of toxins and as consumers become more environmentally conscious. Lastly, unlike fluorescents, LEDs are low-voltage devices that operate at thousands of volts. As a result, they produce lower electromagnetic interference emissions.

The trend toward LED backlights is set in motion, but they still face several technical and cost challenges that may influence their rate of adoption. Presently, the light output of an individual LED is such that large displays (40- to 60-in.-diagonal) require a relatively large number of LEDs in the backlight. This adds a level of complexity to the assembly process along with the associated cost and reliability concerns. LED manufacturers, however, have been continuously improving the devices, developing more efficient and brighter LEDs.

With so many individual light sources, uniform brightness can become an issue. Small variations in light output of individual LEDs can cause a shift in white-light color temperature. Manufacturers address this problem by testing and binning individual LEDs according to their light output characteristics.

This way, a backlight consists of closely matched devices. As LED manufacturers improve their ability to fabricate uniform devices, this issue will diminish.

Another issue for LEDs is brightness uniformity over time. As phosphor-based LEDs age, their light output decreases. Although the same is true for cold-cathode fluorescent lamps, the effect is less noticeable. Because there is usually one fluorescent lamp for the entire screen, the screen dims uniformly. But LED backlights consist of many individual LEDs, which may not age uniformly, possibly resulting in dimmer and brighter areas on the display. Improvements in phosphor technology and feedback control systems could ameliorate this effect.

If any of the individual components in an LED backlight fail because of a faulty electrical connection or other mechanism, the screen will develop a dark area. If, in screens that use the red-green-blue approach, one of the three LEDs in a cluster should fail, that region would experience a color shift.

The technical challenges limiting LED adoption are all solvable, and LED manufacturers are constantly announcing improved products. As such, nontechnical factors likely will drive the adoption of LED backlights. These include government regulation of toxic materials and consumer demand for green technology and for better performing, more aesthetically pleasing products. Already the superior performance of LEDs is widely recognized. At its annual meeting in May, the Society for Information Display recognized Samsung’s LED-backlit 40-in. LCD TV with the Display of the Year award.

At this time, large-screen displays with LED backlights are limited to the premium product market. As LED technology improves and production volumes drive costs down, LED backlights will penetrate more and more display market segments. The question is no longer “if” but rather “when” the LED backlight will become the technology of choice.

Meet the author

Marko Slusarczuk is director of government programs at the Optoelectronics Industry Development Association in Washington; e-mail: marko@oida.org.