Comments on: The pros and cons of LEDs for backlighting LCD screens

The pros and cons of LEDs for backlighting LCD screens

[Coli]

Feedback on the article

> 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.

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).

[mhinnewyork]

Good points Coli

Coli raises some good questions, I'll ask Alfred Poor his opinion. In particular, the restricted vertical viewing angle for LCD screens is a problem that annoys me no end. If LEDs doubled the vertical viewing angle it would, for me personally, justify the extra cost. I hadn't seen the direct comparison Dell offered for the two types of screens on the M1330. I'll update the posting to remove my speculation.

Michael Horowitz

[Coli]

Reducing screen brightness to save battery power

I think realised a mistake I was making and I will mention it as others may be doing the same.

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.

[RMartin111]

There is one relatively serious con of LED backlit displays that seems to not get too much mention in the media. About a month ago I bought a new MacBook Pro to replace my standard white MacBook. One feature of the MacBook Pro that I was unaware of was the introduction of the LED backlit display to replace the CCFL backlight.

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

[cjfuka]

I can't use LCD displays I thought it may have something to do with the backlite being florescent
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.