Guide To LED Technology
LEDs have risen from their original occupation as humble indicator lamps to serving as the light source for some of today's most advanced TVs. Electronics engineers prize the LED for its brightness and cool-running efficiency. Environmentalists and utility companies tout its low power consumption. Videophiles are warming to it for the performance enhancements it facilitates. And average consumers love the way it has slimmed their new TVs. In this article, we'll explain how the LED works; how it's used in current-model TVs and in the latest video projectors; and how it's likely to be used in future displays. Of course, we can't say exactly what the LED's future in video will be, but we can say with a great deal of confidence that within the next 10 years, you'll own at least one LEDbased TV - if you don't have one already.
LED is the acronym for light-emitting diode. A diode is the simplest type of semiconductor. Rather than control the flow of electrons, as a transistor does, a diode just conducts electricity in one direction and blocks it in the other. It's made from a semiconducting material such as gallium arsenide or indium gallium nitride, combined with another substance that changes its electrical properties to suit the task at hand.
About a century ago, scientists discovered that diodes emit infrared light as an electrical current passes through them. In the 1960s, several companies developed diodes that produced visible light, and the LED was born. Early LEDs were dim and mostly limited in color to red, green, and amber. Despite these restraints, LEDs quickly replaced incandescent light bulbs for use as indicator lights, primarily because they last so much longer. Almost all LEDs have lifetimes specified in tens of thousands of hours, and some are even rated to last 100,000 hours or longer. In the 1980s and 1990s, brighter LEDs emerged, along with LEDs in white, blue, and other colors. These breakthroughs caught the attention of video engineers. As display technologies evolved away from light-emitting cathode-ray tubes toward "light valve" technologies such as LCD and DLP, engineers needed a cool-running, efficient, reliable light source. LED delivers on all three.
Interestingly, most white LEDs are actually blue LEDs coated with a yellow phosphor. Some of the photons emerging from the blue LED excite the yellow phosphor, thus producing yellow photons, which combine with the blue photons to produce white light. By fine-tuning the underlying blue LEDs' color and the phosphor formulation, LED makers are able to deliver white light pure enough to drive high-quality video displays. In fact, white LED light is even broader in spectrum than the light from the coldcathode fluorescent lamps (CCFLs) used as backlights in most LCD TVs. LEDs can therefore produce a wider range of colors.
In LCD TVs that use a fullarray LED backlight, individual LEDs are arranged in rows, spaced 1 to 3 inches apart. A diffuser panel spreads the light, ensuring an even distribution to the display's LCD panel.
LED MEETS TV
In video displays, LEDs are now used in several different ways. For the most part, they serve merely as a light source rather than as a way to reproduce individual pixels of video. But they can also produce images directly, rather than just working in tandem with other display technologies such as LCD.
For now, the video industry uses LEDs primarily as a CCFL backlight alternative for LCD panels. LED-driven LCD sets first appeared about 2 years ago, and they have since taken over much of the high-end LCD TV market. However, the cost is still high and overall market penetration is low; they currently account for only about 3 percent of total LCD TV sales.
LEDs have three general advantages over CCFLs: They're more energy-efficient, they allow for a slimmer chassis, and they deliver a wider color gamut (or range of available colors). They can have other advantages, too, depending on how the TV is designed.
Some confusion has occurred in the labeling of these LED-driven TVs, which could fairly be called LED/LCD TVs. Samsung has heightened the confusion by labeling these displays LED TV, which most video experts consider a misleading moniker. Generally, a true LED TV is defined as one in which the pixels are formed from individual LEDs. Each pixel is self-illuminating and requires no backlight.
True LED TVs do exist - most of the large-format displays you see in ballparks and used as digital signage are made from arrays of thousands of LEDs, which are often similar to the 5-mm LEDs your local RadioShack stocks in the "dork drawers" at the back of the store. The new Organic Light-Emitting Diode (OLED) TVs are also true LED TVs.
The "Triluminos" full-array LED backlight found in certain Sony LCD models uses separate red, green, and blue LEDs - an arrangement that can deliver an increased color gamut over a white LED array.
FULL-ARRAY LED: THE STATE OF THE ART?
LED/LCD TVs come in two basic varieties: fullarray (which some manufacturers simply refer to as backlit) and edge-lit.
In a full-array TV, LEDs are positioned directly behind the LCD panel in rows, with the LEDs typically spaced 1 to 3 inches apart. A diffuser panel between the LED array and the LCD panel spreads out the light so that the screen gets a smooth, consistent field of illumination.
This arrangement offers the potential for gigantic performance advantages. The LEDs can be dimmed individually or in small groups. This process, called "local dimming," allows for LEDs behind the dark parts of a picture to be run at a lower intensity, so the blacks and dark grays look darker while the brighter parts of the picture stay the same. The effect is a huge increase in contrast, which has historically been a weak point for LCD TVs. (Local dimming is impossible with CCFLs because they run the entire length of the screen.) Consequently, the newest full-array LED/ LCD models match or even surpass the contrast of plasma TVs.
However, local dimming has some limitations. A large LED/LCD TV might have an array of roughly 1,000 LEDs. That means each LED backlights about 2,000 pixels in the LCD display panel on a 1080p-rez TV. Furthermore, many sets control the LEDs not individually but in blocks of perhaps five or 10 LEDs. Obviously, with so many pixels being illuminated by so few LEDs, it's impossible to achieve precise transitions between high-brightness and low-brightness areas. This imprecision can result in an artifact called "blooming": white halos that appear around the edges of bright onscreen objects silhouetted against a dark background - a white rocket floating through black space, for example. Manufacturers can combat blooming by increasing the number of LEDs in their sets' backlight array, decreasing the number of LEDs in each control block, refining the drive electronics for the LEDs, and increasing the native contrast of their LCD panels. Newer full-array TVs show less of this artifact, but it still exists.
The above pictures illustrate the local dimming process at work in an LCD TV with a full-array LED backlight. Various "zones" in the LED backlight can be either dimmed or shut off completely to track brightness variations in the image. The process, which unfolds on a continuous, dynamic basis, goes far to enhance picture contrast.
Another advantage of full-array LED/LCD TVs is improved picture uniformity. Because the screen is lit by hundreds or thousands of LEDs instead of 20 or so CCFLs, you don't see the gaps that are sometimes visible between CCFLs. (This artifact, which is often referred to as screen "clouding," crops up regularly in our reviews of standard, non- LED-based LCD TVs.)
Most manufacturers use white LEDs in their arrays. However, for some of its LCD TVs Sony instead uses groups of four closely spaced color LEDs: two green, one red, one blue. (The green, red, and blue light combine to make white.) Sony has trademarked this technology Triluminos. The advantage is that the exact colors of red, green, and blue can be chosen independently to give a potentially wider color gamut than a TV using white LEDs. However, Triluminos is more expensive to implement than a white LED array, and other manufacturers have been able to meet or exceed the HDTV color gamut specifications using just white LEDs.