The pros and cons of LEDs for backlighting LCD screens
The pros and cons of LEDs for backlighting LCD screens
"The screen is lit by LEDs instead of by traditional lamps.
That makes for more brightness and saves power."
Raise your hand if you are familiar with the use of LEDs as a light source in laptop computers.
I don't see many hands. Mine isn't raised either.
They seem to be getting popular, just last month Apple started selling their first laptop computer with LED backlighting. The Sony VAIO TX line of laptops uses LEDs as does their TZ line, due to be released very soon. Sony too, claims that LEDs offer increased brightness and decreased power consumption. In addition, they claim that their LED lit screen offers better colors.
Can LEDs really make laptop screens brighter, consume less power and offer better colors?
For those of us who didn't raise our hands, I turned to screen and monitor expert Alfred Poor for advice. For more than 20 years Alfred wrote for PC Magazine, and was their first Lead Analyst for Business Displays. He is a member of the Society for Information Display and the editor and publisher of HDTV Almanac, a web site with news and commentary about HDTV and related topics.
Starting at the beginning, Alfred explained that the liquid crystals in an LCD panel/monitor don't emit light themselves [insert your own dilithium joke here]. Rather "the molecules move in response to electrical fields, and are used as a shutter to block the light." I was surprised how inefficient the technology is. An LCD screen blocks 95% of the backlight, even when it's showing a full white screen.
Traditionally, Alfred said, the backlight source behind the crystals have been cold-cathode fluorescent lamps (CCFL). The use of LEDs in laptop screens is relatively new. According to Alfred, LEDs "already are commonplace in mobile devices such as GPS receivers, cell phones, and PDAs ... the first desktop monitors probably appeared within the past couple of years. Sony had an LCD TV with LED backlights a couple of years ago. I expect that laptops were the last to get the technology."
Since none of the companies offering LED backlit screens said anything about cost, it's reasonable to assume that LEDs are more expensive than CCFLs.
The M1330 comes with either a CCFL or LED lit screen, so it makes for a handy comparison of the two technologies. The M1330 costs $150 more with the LED lit screen.
With Sony, Toshiba and Apple, the cost of the LED screen is a hidden component of the total price. But these machines aren't cheap. As of July 22nd, the least expensive pre-configured Toshiba Portege R500 was $1,999 and the Sony TZ line started at $2,199.99 (think of it as $2,200). The 15.4 inch Macbook Pro started at $1,999.
Thin and Light
Toshiba claims that in one configuration the Portege R500 is "...the world's thinnest widescreen 12.1 inch notebook PC with an integrated DVD-SuperMulti drive..." Dell claims their M1330 laptop with the optional LED screen is the thinnest laptop computer equipped with a 13.3 inch screen. The Sony VAIO TZ machines are less than an inch thick, but only if measured at the narrowest point. At the highest point, they are 1.17 inches.
Alfred confirmed that LEDs are indeed thinner and therefore the screens can be made thinner. And, they weigh less than cold-cathode fluorescent lamps.
We can see this in the M1330. According to Dell, the LED display "starts at 3.97lbs and is 0.87 inches thick compared to the standard display which starts at 4.28lbs and is 0.97 inches thick." The difference in weight and thickness seem, to me, to be small, but, I suppose if you frequently carry a laptop computer, then perhaps every little bit helps.
Mr. Mossberg gives the impression that by their very nature LEDs save power. Not true, according to our expert. Alfred pointed out that "At present LEDs generally draw more power and produce more heat than CCFL designs." Heat is a problem for all personal computers. It's more of an issue with laptops and still more important in ultraportable models where everything is so closely packed together.
So what is the basis for the claimed power savings? It turns out that the number of LEDs in a screen varies. If the number is low enough, less power is needed and less heat is generated. With a small enough number of LEDs, Alfred said you can "probably save power compared with a CCFL design. This can be used to give either a longer battery life, or to reduce the battery weight and thus get a lighter weight design overall."
I couldn't find anything from Sony, Toshiba or Apple about the number of LEDs in their screens. But in describing the M1330 Dell says "Our optional LED display uses 32 tiny, white LEDs ..." According to Alfred, "32 is a relatively high number for a small screen. Some large HDTV panels using high brightness LEDs could use that count or less for a panel with 8 or 10 times the surface area."
So, if the relatively high number of LEDs means increased heat and no power savings, why does Dell use so many? Alfred explains that LED screens "need a sophisticated lightpipe and diffuser to spread the light evenly behind the LCD panel. The fewer LEDs you use, the more difficult the diffusion process becomes."
As to whether LEDs are brighter, Toshiba claims this is true, but offers no specific numbers. Sony claims "incredibly high brightness levels" and the specs for the screen list it at 11.1 candelas (trust me, you don't want to know the exact definition of a candela). The point is that Sony does not offer the candela ratings for their CCFL screens as a point of comparison.
The owners manual for the Dell M1330 shows the LED panel to be 36% brighter than the CCFL panel. Specifically the luminance of the LED screen is 300 cd/m² vs. 220 cd/m² for CCFL (and no, I can't explain what cd/m² means).
Sony is the most aggressive in making claims about the better colors in their LED screens, using the terms "brilliant", "amplified" and "true-to-life" to describe them. Toshiba says that indoors, "the LED backlit display produces rich color saturation." I couldn't find anything from Dell that mentioned better colors. Alfred said it is possible that "LEDs can offer better color than CCFL, though advances in CCFL phosphor technology are rapidly diminishing this advantage."
Glossy vs. Matte finish
LED backlighting, being in the back, can be used with screens whose front has either a glossy or matte finish. A glossy screen suffers from glare, but produces more vibrant colors. Each laptop vendor has their own marketing term for glossy screens, Apple is the only company I've seen that actually uses the word glossy. A matte finish may be described as anti-glare or anti-reflective.
The Sony TX and TZ laptops have a matte finish. At the Apple online store you can chose either a glossy or matte finish when you order the 15 inch LED backlit Macbook Pro. I can't be sure about the other laptops because the claims of better colors could be either based on the LED backlighting or the glossy screen or both.
I didn't see any marketing material from a laptop manufacturer that mentioned the expected lifespan of LEDs vs. CCFLs. But, a company that manufacturers LEDs did claim they last longer than CCFLs. When I ran this by Alfred, he said:
The difference is probably not important, but yes, CCFLs don't last as long. Even more significant is that their output decreases over time. End of life is when they are half as bright. LEDs are solid state devices, and "fall off the cliff" in failure mode; in other words, they keep working like when they were new until they stop working. Most people aren't going to keep their notebooks long enough for the CCFL aging to show any difference.
Alfred estimates the market share of LEDs at less than five percent, but he expects them to become more common as costs come down. DigiTimes reports that laptop and panel vendors expect that LEDs will be used in about 7% of laptop screens next year (See Nearly 100% of 10-inch-and-smaller LCD panels using LED backlight by Susie Pan and Emily Chuang, July 23, 2007). They estimate that LEDs will be used in 3-5% of laptop computers this year.
To date, LEDs have been popular mostly in smaller displays. In part this is because smaller screens use fewer LEDs which lowers the price differential over CCFL. The DigiTimes article reports that most LCD screens 10 inches and under use LED backlighting. The Sony TX and TZ screens are 11.1 inches, the Toshiba R500 screen is 12.1 inches and the Dell M1330 LED screen is 13.3 inches. The Apple Macbook Pro has the only available 15 inch screen using LEDs, but Apple appears to be having supply problems with them.
Alfred also mentioned that "environmental concerns about heavy metals in the CCFLs" may help to popularize LEDs. Apple seems to be the only laptop vendor using environmental concerns in their marketing. They tout their LED lit screens as being "mercury-free" and the company has long term plans to eliminate mercury from all their products.
Finally, I wondered why Dell and Sony mentioned that the LEDs they use are white. Alfred pointed out that some LED backlights use red, green, and blue, and mix the colors in the diffuser. I didn't bother asking what a diffuser is.