Latest models of computer and TV LCD screens use LED (light-emitting diode) backlight. But is it really better than older CCFL (cold cathode fluorescent lamp)?
But before I tell you this story, first I want to explain one common misconception – LED is not a new (nor better) form of LCD display. They are all LCD screens (apart from Plasma displays of course) but it is the backlight that makes a difference. To put it in plain terms – all LCD screens use a LCD matrix which is semi-transparent. Backlight is on the backside of the matrix. When pixels move to non-transparent position, the light is not passing through and you get black pixels. When they are in transparent position the light goes through and white pixels are born (as well as other colors). So that’s how all LCSs work – in a nutshell.
Now onto a backlight and why LED might not be better choice. When you get a new screen you obviously want to reduce brightness as factory settings are usually too bright.
Usually, there are 2 controls to do this – Brightness and Contrast. Brightness setting is the one that controls the brightness level of the backlight (LED or CCFL), while contrast makes it also dimmer but more by reducing the range how much pixels can move back and forth between two phases (transparent and non-transparent). So where is the problem?
The problem is that to reduce brightness, manufacturers usually employ a technique called PWM (pulse width modulation). Essentially, this means the backlight has an ON-time and OFF-time and it quickly flickers between those two states. So for more brightness you have more ON states and less OFF states and vice versa. CCFL does the same thing but with one difference – it does not react as quickly as LED to state change and it can have maybe about 2-3 ms lag, while LEDs do it almost instantly. So CCFL might have a small “fade out” time while LED does not – therefore it might be easier on the eyes.
Here are some images how you can test this by simply waving your hand in front of a monitor – the less strobo effect as on first image – the better.
The problem is that PWM is set to too-low value of a couple hundreds of Hz (changes per second) so to more sensitive eyes it may be visible as flickering. Though you may not perceive it visually it is there and you might get a headache or eye-strain after prolonged period of staring into such screen. If it would be a couple of thousands Hz it might be better for the eyes (obviously the more continuous the light – the better). You cannot see this visually but if you record a screen through a cell-phone camera (or any other type of camera) you will see it as lines. You may also move finger or pencil quickly in front of a screen or place a rotating fan in front of it to see the effect. If you get a continuous line then the light is continuous but if instead you get a sort of flickering “frames” of fan or pencil movement then you know there is a stroboscopic effect in place which you are staring at.
The effect is illustrated in these videos (note this is NOT visible to the naked eye but most likely IS perceived by eyes and may as well have effects on the human nervous system). I am not convinced by the “experts” that claim that 100 or 200 Hz flickering is harmless for people.
But you might say – older cathode tube screens flickered at very low rate like 60 or 70 Hz. True but not entire screen was flickering at once like in the case of modern LCDs – only the part where electron beam was passing by and the rest was slowly fading to black (phosphorous used on the screen also has a delay and fade out time).
This may not be an issue for some people and can be a great deal of issue for others. In any case I suspect it will cause you more headaches and eye strain. There might be also other consequences related to flickering lights (increased seizures maybe?). All in all – not good.
So how to remedy this problem? This is a tough question.
One solution I personally use is to put brightness level to 100% and to choose CCFL monitor if possible. There is no visible flickering on my DELL U2311H when I use a fan or pencil or camera test and brightness is set to 100%. Of course, the screen is then awfully bright – but I simply reduce contrast to about 50% which does the effect of reducing brightness. This is not a perfect solution as it probably messes up monitor color-range but works quite fine for me (I am looking at text most of the time anyway). This particular model has CCFL backlight. The idea is to get a continuous light rather than stroboscope effect. If brightness is set to 100% the more backlight will bleed-through the LCD screen (black is almost never a perfect black) but at least the light is continuous. This is the reason why I choose DELL U2311H over its newer replacement DELL U2312HM (which uses LED as backlight).
Another solution might be choosing a monitor that does not employ PWM as a method of reducing brightness. These are quite rare though. One model I found (but haven’t tested myself) is HP ZR2740w. They may be hard to find but there are others too (look at links at the bottom of this post).
Some LED monitors may also not flicker at 100% – any case where the light is continuous is better than a pulse width modulated version. The easiest way to test this is to quickly move a pencil or put a fan or camera in front of the white screen and change brightness levels. Additional problem may be that some manufacturers also flicker even on 100% brightness settings.
Perhaps some future technology might give an answer to this – like OLED which might emit light continuously. But for now you should take more care when choosing your next screen or selecting its brightness settings.
And one more thing – if you plan on reading eBooks – you should prefer eBook reader like Kindle over the iPad because the former has E-Ink based display and the latter has LCD screen. It is still better to look at reflected light (like from a paper or eBook reader based on E-Ink) than directly from a light source (like from a screen) – the eyes are more used to watching reflected light than a light source directly which all monitor or TV screens are. Not to mention that E-Ink reader doesn’t flicker at all and displays images/text continuously. Of course iPad is more versatile for various other tasks like games, web etc. (not to mention eBook readers may not have color support) so this applies only for reading text and looking at static pictures.
Regardless of all of the above, your eyes still need occasional rest from screen or paper viewing (or reading) otherwise eye-strain and headaches will occur regardless.
Here are some links for further reading into this subject if you are interested:
Informing About LCDs And Luminescence
Fortschritt durch LED (German) – English translation by Google
Discussion on this topic as well as several monitors that don’t use PWM
2015 update:
It appears that more manufacturers are taking PWM issue into account – which is a good thing – here is what I found recently:
EIZO explains a bit about PWM flickering here
Acer Aspire S7 screen might not have screen flickering above 27% brighness level
2017 update:
Flicker Free Monitor Database – TFT central now maintains a regular list of non-flickering (PWM-free) monitors
PC Monitors – Recommendations – Recommended monitors on pcmonitors.info also feature PWM information
I found your post very helpful. It’s 2015 and it’s very difficult to find a laptop that uses the CCFL backlighting. I get terrible eye strain with LED backlit displays. Do you have any new information on alternatives?
LED backlight can emit constant light as long as the brightness setting is set to 100% and it does not have any PWM circuitry attached to it to strobe it. This is not always the case though because some manufacturers still use PWM strobing even when you set the brightness to the max. The best way might be to go into the laptop-store and max out the brightness and simply move pencil or finger in front of it to see if it would produce “strobes” or continual blur as explained in the above post. So LED can work but you have to consider other factors which affect eye strain such as color-temperature (LEDs can have colder color temperatures so you might want to use software to adjust desktop color temperature on your graphics card), strobing using PWM and dot-pitch (the smaller the dot the harder is to read the text, unless the font is enlarged which is still a problem for Windows even though Windows 10 addressed this at least for a bit). Finding a good laptop screen without strobing effect would be probably much harder than finding desktop monitor with the same functionality. This post looks promising though – http://laptopmedia.com/news/acer-aspire-s7s-screen-is-just-astonishing-also-no-pwm-above-27-screen-brightness/ – it appears that Acer Aspire S7 might not use PWM at certain brighness levels. Google search for more, there should be more manufacturers not utilizing PWM – because essentially this is what you are looking for – non PWM laptop display.
totally agree to external display. Use a external display – it is always better than internal. Buy an old high priced display like eizo, nec, hp, dell etc and save money with result of better quality than a new low cost procut for 100 or 150€/24 inches. PWM Problem is a controller problem. Conclusion of all is: do not use new technology without reasons like “this is new and must be better”. In no way – new products mostly are drilled on price bonus/benefit. Not sure new products are better than the old one. Trust facts and data sheets – no slogans. Marketing is one way to promote products nobody would buy without promotion
Another option with much less eye strain is to use a led backlighting strip
does ccfl cost much more than led? so far I know led is much expensive
what is different of CCFL and LED scrren backlight?
Recently bought a mid-range laptop (Inspiron 15 5000 series, 1080p non-IPS) and after almost 10 years on an old Dell Inspiron from 2009. Instantly something about the display felt wrong. It felt like it was burning my eyes even at full lighting in a well lit room. What was going on? It was so horrible that on merit of display alone I returned the item, determined to do more research.
I now know my antique 2009 Inspiron 1525 uses CCFL. At all settings it passes the finger/pencil wag test marvelously. I use f.lux to lower the color temperature at low lighting and this thing feels like a nice warm blanket on my eyes compared to the LED screen, even after changing its temperature setting in the card driver settings. Research in January 2020 shows that only laptops in the $1,000-1,500+ range have 0 PWM rating. How can this be? I am tempted to upgrade the CPU/RAM/HDD on this antique laptop rather than deal with the impact those horrible displays have on the eyes.
Thank you for this post and keeping it updated over the years, it’s a shame this hasn’t been addressed.
The more users care about PWM, the more will be done by manufacturers as well and that will drive the prices down with the mass production. PWM is/was a cheap way to dim the backlight. It is not the only way – LED can be dimmed by changing the voltage as well. The reason why PWM is used is because it doesn’t mess with the colors like changing the voltage to the LED does. When you change the voltage, the LED may change color temperature and that of course introduces a problem with displays where the color temperature is supposed to be uniform across all brightness levels, so you need a special circuit to compensate, which costs a bit more. So yes, there are laptops with PWM-free displays now but you have to research (or measure if possible). There are web sites which do these tests, simply google search “pwm free laptop” or “pwm free laptop displays”, or something of the sort. The situation is much better today than it used to be when this post was originally written and more attention is given to this.