The title is a statement but an appropriate question could be: Do you feel eye strain, irritation, discomfort or maybe even headache when you work in front of a modern computer monitor? In this article we examine why modern PC LCD monitors with LED can still cause eyestrain, and how you can test and avoid it. Although flickering can also occur on TVs we will focus entirely on PC monitors and the so-called “PWM method”, as the problem is often called.
An Introduction to LED and CCFL based LCD monitorsMonitors have been flickering for many years. A CRT monitor basically created its picture with flicker and if you found it uncomfortable to sit in front of a CRT monitor, you were not alone. But it can also happen on modern PC monitors. We are not talking about visible flicker as in a defective monitor, but “invisible” flicker that can affect your experience in front of the monitor.
do you get a headache when working in front of a PC monitor? You are not aloneIt is an “invisible” flicker effect that can affect your experience in front of the monitorModern PC LCD monitors are not flicker-free but in order to understand why we need to include a short introduction. LCD monitors started out by using CCFL (cold cathode fluorescent lamps) as a backlight source but in recent years manufacturers have shifted to LEDs (light emitting diodes). If you have a thin monitor then you have a LCD monitor with LED. If you are unsure check the model number on the backside and Google it.
As you might have noticed you can adjust the backlight on pretty much any monitor today via the buttons on the front. In a brightly lit room a higher monitor brightness level is preferable and in a dimly lit room, for example a cellar or studio, a much dimmer monitor is preferable. Some monitors adjust automatically based on surroundings but often you have to do it manually. Whether it is the one or other is irrelevant. The important aspect here is the actual method used to reduce brightness on a monitor with LED.
When a monitor is set to maximum brightness the LEDs are typically glowing at full strength: 100 %. If you reduce the brightness setting in the menu to, for example, 50 % the LEDs need to omit less light. This is done by inserting small “breaks” – or pauses – in which the LEDs turn off for a very short time. When reducing the brightness setting in the menu further the breaks become longer, basically.
This happens with CCFL based backlight units and LED based backlight units but CCFLs have a much longer afterglow than LEDs that basically turn off instantly. Therefore, the breaks when using CCFLs are much easier on the eyes than when using LEDs. Thus; a greater risk of experiencing eyestrain, tired eyes or in the worst-case scenario headache when working in front of a LED based monitor.
The use of LED obviously has numerous benefits, including much lower power consumption, far fewer toxic substances and some obvious picture quality advantages but here we are only focusing on the potential eyestrain issues as it is a separate issue that can be avoided on new monitors and even reduced on the monitor you own right now.
Notice: We need to emphasize that all eyes are different. Those who are affected never see the actual flicker; it is “indirect”. Studies have shown that approximately 10 % of people experience discomfort. The rest experience either mild discomfort or no discomfort at all.
PWM (Pulse Width Modulation)The method of introducing "breaks" to reduce the brightness level is called PWM (pulse width modulation). It is a cheap and effective way of controlling the light output of a LCD monitor with LEDs as it gives a huge span of brightness levels. But, as you can see, it also has drawbacks.
How PWM works on a LCD monitor with CCFL and on a LCD monitor with LED. Notice the significant light fluctuations. These fluctuations can cause eye strainPWM is not the problem. The problem is the way PWM has been implemented in modern LCD monitors with LEDs, as it can cause eyestrainPWM is not the actual problem. It is not necessarily a bad thing. The problem occurs if the blinking/flickering is indirectly perceptible by the human eye when the PWM is running at too low frequencies. The most common scenario is that you buy a new monitor that is far too bright out-of-the-box and reduce brightness to maybe 20-30 %. Brightness is typically measured in cd/m2 and the brightest monitors hit around 450-500 cd/m2 but this is not necessarily an advantage because no one can sit in front of a monitor this bright. The recommended brightness level is approximately 120 cd/m2 in a bright room without direct sunbeams coming in. That is why we calibrate to 120 cd/m2 in our reviews.
The alternative to the PWM method is to lower/raise the electrical voltage for the LEDs. The method can also be used for CCFLs but they are not nearly as flexible as LEDs. The disadvantage to this method is that it is more expensive and that it can be very hard to control the color temperature of the backlight. There is also a risk of burning out the LEDs very fast.
The number of cycles (on / off periods) used in LEDs can obviously be measured and as so many other things it is measured in Hz. 100 Hz means that it updates 100 times per second.
But what frequency do LEDs run at then? Well, most monitors use PWM with a frequency around 90 to over 400 Hz. Those with 90 Hz PWM are worst, obviously. PC monitors with CCFL backlights all run at 175 Hz (but as mentioned, the afterglow is very different). For comparison, ceiling fluorescent lamps found in many offices used to operate at around 100-120 Hz and they have been proven to cause headache many times in the past. Newer installations are better but we need to reach much higher frequencies for a perfect result. Again; it depends on the individual and some will experience problems where others do not. We probably have to surpass 2000-3000 Hz before we can call it a safe zone.
Most thin LCD monitors with LCD backlights can output a very high level of brightness – but that is not necessarily an advantage even though you can reduce the backlight setting
If manufacturers want to use the PWM method in LCDs with LED there are two ways to eliminate the issues. The first option is to run the LEDs as a much higher frequency (combined with a decent brightness level). The second option is to use a combination of PWM and modulation of the electrical voltage.
But you can also help yourself. In just a moment we will tell you how to examine the PWM issue on your existing monitor but when you are looking to buy a new one you can often leave out the cheapest PC monitors (the same is true for displays in smartphones and tablets) as these have proven to be most affected by the issues. Many of us work in front a monitor for many many hours daily, so it is really worth saving a few bucks?
Look at the maximum brightness (the cd/m2 number). It is around 400-500 cd/m2 then you can be sure that you need to lower the brightness level considerably – and then you risk increasing flicker. Many graphics monitors run at much lower brightness levels – and it can actually be an advantage. Higher is – ironically – not always better.
How to test for PWM yourselfThe simple method:
Picture taken with a DSLR with a shutter time of 1/25 second with a motion from left to right
The latest news, in-depth articles, reviews, and exclusives in your inbox.