The pros and cons of LEDs for backlighting LCD screens
In the July 12th edition of the Wall Street Journal, Walter Mossberg reviewed two new laptop computers, the Dell M1330 and the Toshiba Portege R500. In describing the R500 he said:
"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."
Cost
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.
Power
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."
Brightness
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/m? vs. 220 cd/m? for CCFL (and no, I can't explain what cd/m? means).
Better Color
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.
Lifespan
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.
Market Share
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.
Note: You can hear both Alfred Poor and myself, weekly, on the Personal Computer Radio Show. The show hosts, Joe King and Hank Kee, have been broadcasting out of WBAI in New York City for the last 23 years.
Update July 29, 2007: Updated with a direct comparison between the brightness rating of the two screens offered by Dell for the M1330 (from the Owners Manual).
Michael Horowitz is an independent computer consultant and the author of several classes on Defensive Computing. He is a member of the CNET Blog Network, and is not an employee of CNET. Disclosure.
> cost, it's reasonable to assume that LEDs are more expensive than CCFLs.
That is true for the consumer as it is being sold as a premium feature but you are offering no evidence that it is true for the manufacturer. I suspect the manufacturing cost is less.
In time the cost may come down as more manufacturers adopt them.
You write:
> Dell says the M1330 with the LED panel is "36% brighter than its predecessor (M1330 with 300
> nits v. M1210 with 220 nits)." Fascinating. Why doesn't Dell compare the M1330 LED panel with
> the M1330 CCFL panel? Why instead, the comparison to an older machine? They offered a direct
> comparison of the two M1330 screens with regard to weight and thickness. Perhaps such a
> comparison would not be flattering to the LED screen.
The Dell Owner's Manual for the M1330 does make a direct comparison with the CCFL screen and reaches the same figure - so it sounds like unwarranted speculation that the comparison to the M1210 is to hide a weakness.
See: http://support.dell.com/support/edocs/systems/xpsm1330/en/index.htm
> Luminance:
> 250 min. 300 typ. cd/m² (LED 5 point avg)
> 200 min. 220 typ. cd/m² (CCFL 5 point avg)
However, note the range of brightness listed for the WLED screen - they have left quite a lot of leeway for some machines to have a lesser difference. Also we cannot be sure whether these CCFL screens are representative across the industry.
Also you don't mention viewing angles. The same Dell Owner's Manual indicates a much better vertical viewing angle for the LED backlit screen:
> Viewing angles:
> Horizontal:
> ±40° (CCFL) min.
> ±50° (LED) min.
> Vertical:
> +15°/?30° (CCFL) min.
> +40°/-50° (LED) min.
Note - that is a doubling of vertical viewing angle which seems very significant to me (and means a lot more light must be coming through the LCD to be both brighter and covering a wider angle).
It might have a useful side effect of illuminating the keyboard in dark rooms.
You skip over the light diffusion - but that sounds to me one of the most difficult engineering challenges - I really wonder how the light is so evenly distributed.
Question for the expert - if the user reduces the screen brightness of the LED backlit screens to comparable levels to the CCFL screens is there then a power saving - or does that just change the opacity of the LCD? (reason for asking is I have seen some other models where the brightness of the 'hardware' cursor does not decrease when the screen brightness is decreased).
Michael Horowitz
It is said that reducing the screen brightness reduces power consumption (presumably by lowering the power used by the backlight). However, On one notebook with ATI display card I reduced the brightness using the ATI display properties and noticed that the white part of the cursor (which is a graphics card overlay) did not dim when the rest of the screen did. That showed me that the backlight was not really being dimmed by adjusting that control. Instead it was the opacity of the display that was changing. That would presumably not save power (unless it takes more current to turn a pixel white than to leave it as black - in which case we would be better off viewing text as white on black when trying to optimize battery life).
Now I realize that one can dim the backlight - by using the function key combination to do that. That does also dim the cursor so presumably is saving power.
So if you want to extend your battery life - dim the screen via BIOS settings or function keys rather than via Display Properties.
Once I started using my new laptop for long periods of time, I noticed severe eye strain and minor symptoms almost similar to motion sickness. After 20 or 30 minutes of use, I felt like I had been looking at the screen all day. Much longer and I would get headaches. If I used the old white MacBook (with its CCFL display), I had no eye troubles at all. Moreover, I could detect a distinct flicker on the MacBook Pro display when I moved my eyes across it - especially over high contract areas of the screen. White text on a black background was virtually impossible for me to read without feeling sick to my stomach because of all the flickering from moving my eyes over the text.
The strangest thing about all of this was that nobody else I showed the screen to could see these flickers I was seeing. I began to question my sanity until I did a little research. Discovering that the MacBook Pro introduced a new LED backlit display started to shed some light (so to speak) on what might be going on. I had long known that I could see LED flicker in things like car taillights and christmas lights that most of my friends could not see. I also knew that I could easily see the "rainbow effect" in DLP televisions that many other people don't see.
My research into LED technology turned up the fact that it is a bit of a technological challenge to dim an LED. Varying the voltage generally doesn't work as they are essentially designed to be either on or off with a fixed brightness. To work around this limitation, designers use a technique called pulse width modulation to mimic the appearance of lower intensity light coming out of the LED. I don't claim to fully understand the concept, but it essentially seems to involve very briefly turning off the LED several times over a given time span. The dimmer the LED needs to appear, the more time it spends in the off state.
Because this all happens so very quickly, the human brain does not interpret the flickers as flickers, rather as simply dimmer light. For most people that is. Some people (myself included) are much more sensitive to these flickers. From what I can tell, the concept is called the "flicker fusion threshold" and is the frequency at which sometime that is actually flickering is interpreted by the human brain as being continuously lit. While the vast majority of people have a threshold that doesn't allow them to see the flicker in dimmed LEDs, some people have a higher threshold that causes them to see the flickering in things like LED car tail lights and, unfortunately, LED backlit displays - leading to this terrible eye strain.
The solution? I now keep my screen turned up to full brightness to eliminate the need for the flicker-inducing pulse width modulation. The screen is very bright, but there are no more flickers and I love my MacBook Pro too much to exchange it for a plain MacBook with CCFL backlighting (which will also supposedly be switching to LED backlighting in 2009 anyway.) Sitting in a brightly-lit room tends to help alleviate how blinding the full brightness of the screen can be. In a dimly-lit room I guess I just wear sunglasses. Either way, the extreme brightness is worlds better than the sickening flicker I saw with a lower brightness setting
I would caution anybody considering buying a product with an LED backlit display to pay careful attention to make sure you don't have this same sensitivity. Turn the screen brightness down, find a high contract area of the screen, and quickly move your eyes back and forth over the screen. If you can detect the flicker, you may end up with this same problem.
I have no idea what percentage of the population has this sensitivity. I imagine we will hear more about it as more and more displays start using this technology. Hopefully the designers will come up with a way to eliminate this flicker some of us can see.
Russ Martin
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by cjfuka
November 13, 2008 1:11 AM PST
- I can't use LCD displays I thought it may have something to do with the backlite being florescent
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(5 Comments)so I was semi convinced that LED display might make it more usable. I've tried 3 different LCD Monitors after using CRTs up to 2005 I had to sell every LCD monitor due to eyestrain. I'm convinced they are only designed to be harsh on the human eye the designers of these machines don't take in account the biological effects and harms on the human body or the organ such as the retina or eye.
I realy doubt stress tests are done on electronics except for Frame rates in Quake and Crysis.
I'm still using CRT technology waiting around for SED not really OLEDs because those look way too bright to be useable on this planet unless Neon is your thing. The amazon Kindle should be a model for all future display tec.