Picture This: How BrightLED backlighting enhances LCD panel color performance. 10/01/2005 8:00 AM Eastern
Picture This: How Bright
Oct 1, 2005 12:00 PM, By Jeff Sauer
LED backlighting enhances LCD panel color performance.
NEC's LCD 3210 (reviewed on p. 66 of this issue) offers a pretty good marker as to the current state of LCD color. That LCD panel does about as well as one could expect in reproducing solid primary (red, green, and blue) and even secondary (yellow, cyan, and magenta). But, as is generally the case with LCD panels, it doesn't achieve the color variation of something like a plasma screen. In the continuing plasma vs. LCD battle for flat panel supremacy, color reproduction is invariably tallied on the side of plasma.
That shortcoming, however, is not necessarily a factor of LCD technology per se, but rather current flat panel implementations, specifically current backlight technology. And NEC has another product, the 21in. MultiSync LCD 2180WG-LED, that offers a peek at an alternative that might bring “color reproduction” out of the plasma column of pros.
In place of the CCFL (Cold Cathode Fluorescent Lamp) backlight typical of most LCD panels and monitors, the LCD 2180 uses an array of red, green, and blue LEDs as the backlight. It still uses the same basic LCD technology: liquid crystals twisting to let the light through red, green, and blue filters. But it starts with a different light source, and at least in terms of color reproduction, LEDs make a better light source.
Why is a lamp not just a lamp? Simply put, brightness and color temperature vary between different light sources. The comparison between fluorescent lamps and traditional tungsten light bulbs for home use is the classic example. Which one is white? Or whiter?
We often talk about tungsten light bulbs producing a “warmer” or “softer” light in our living rooms, but does that really mean they're whiter? They actually tend to produce a white that is more orange or red than something like a fluorescent bulb, and they have a much lower color temperature.
Those color temperatures become critical when they are the light source for LCD panels because all of the colors that an LCD can produce start with that light's version of white. LCDs produce colors by letting all or some of the backlight brightness through color filters, thus literally subtracting colors from the white light. In theory, subtracting portions of red, green, and blue from pure white light should yield a full spectrum of color. In practice with CCFLs, however, you're rarely starting with “pure” white. And if you start with less than 100 percent of white, you can never get 100 percent of any other color.
EVERYTHING OLD IS NEW AGAIN
The LCD 2180 uses two arrays of red, green, and blue LEDs that blend together to create something a lot closer to 100 percent white. In a very real way, that combination of light elements is a return to the red, green, and blue light guns of old — the CRT projectors that many experts still believe offer the best color. That's because together they can produce a fuller range of saturated colors.
The LEDs in the LCD 2180 can, for example, produce a color range that exceeds both Adobe RGB (by 9 percent) and the NTSC color (by 4 percent). My tests on a sample unit showed green and red both outside the RGB color gamut reference triangle, although (harder to produce) blue was inside. Also like those old light guns, an LED array can be tuned to achieve a specific color temperature, or corrected to accommodate the color variations that are an inevitable part of technology's aging.
When a CCFL backlight fades over time, there's not much you can do about it. But even if an LED array dims over time, you could at least maintain a specific color temperature by adjusting the amount of each of three individual primary colors.
While the LCD 2180 is currently NEC's only venture into LED backlit LCD panels — it is heavily targeted toward color critical desktop applications like prepress and color artists — a small handful of other companies are also experimenting with LEDs. Both Sony and Samsung, for example, demonstrated video entertainment panels at the CES show in January.
And a company called BrightSide Technologies demonstrated what it calls an Extended Dynamic Range display this past summer at the graphics and animation-oriented Siggraph show. That Brightside panel is even more technologically intriguing than the others because it takes advantage of another potential LED benefit: the ability to individually modulate the LEDs.
LEDs are solid-state devices that can go on and off in as little as 20 nanoseconds. They can also be programmed to change brightness very quickly, including between frames of motion video. By literally changing color temperature or lamp brightness between frames, an LED-backlit panel could theoretically maintain a wider range of colors even in overly dark or bright scenes. Not only can a panel like Brightside's DR37-P achieve a remarkable 200,000:1 contrast ratio, but it can also more importantly achieve a wider range of colors and grayscales in between.
Of course, as with any new technology, LED-backlit panels are going to carry a premium. It's likely to be a significant premium until manufacturers can achieve a greater economy of scale. For example, NEC's 1600×1200 resolution LCD 2180 currently lists for $6,995, compared to NEC's other 21in. monitors at about $1,500. Similar-sized desktop LCDs are available online for less than $1,000. Brightside's full HD 1920×1080 resolution, 37in. display carries an even greater premium at almost $50,000, compared to between $1,500 and $4,000 for a more traditional 37in. televisions.
However, both NEC and Brightside are still likely to have plenty of interest from users for whom accurate color is critical. This is a technology that holds a significant advantage for graphics artists, animators, product designers, and high-end home theater consumers willing to pay for the best, most accurate image. And as the technology matures, it is likely to become more affordable and more prevalent in a broad range of LCD product lines.
LED-backlit LCDs are likely to remain premium products, both LCD-TV and desktop monitors, because CCFLs and other fluorescent lamps will likely have a cost advantage for years to come. Therefore, that technology is likely to remain in price-sensitive markets.
On the other hand, as LED technology continues to mature and more companies leverage the benefits of the programmable solid-state devices, the premium is likely to fall into a much more familiar good-better-best pricing model, and it could have broad appeal.