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

Pulse Width Modulation

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