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PWM on LED monitors
LED Monitors can cause headaches due to flicker

By Rasmus Larsen (@flatpanels)
05 Mar 2013

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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 monitors

Monitors 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.

Headache in front of PC monitor
do you get a headache when working in front of a PC monitor? You are not alone

It is an “invisible” flicker effect that can affect your experience in front of the monitor
Modern 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 strain

PWM is not the problem. The problem is the way PWM has been implemented in modern LCD monitors with LEDs, as it can cause eyestrain
PWM 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.

LED monitors
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 yourself

The simple method:
  • 1. Do you have a fan in the office? Try to put it in front of the screen and see if you can spot the flickering when looking through the rotating fan. If you see flicker then you know that the LEDs are running at a low frequency.
  • 2. Use the camera on your smartphone. Start the camera app and point down towards the keyboard, then quickly up towards the monitor. Do you see flickering lines running from left to right? Then the LEDs are running at low frequency. Note that most smartphone automatically adjust the camera to avoid showing the flicker so it is important to start by pointing down towards the keyboard (or another place without that is not too bright).

    The more accurate method:
  • You do not need advanced testing equipment to determine the PWM frequency. Most display phenomena can be captured with a camera if you can manually adjust the shutter speed. This is possible on all DSLRs and on an increasing number of compact cameras. Do the following:

    1. Start off by downloading this test image and show it in full screen on your monitor (let the monitor operate at the same brightness level you usually work at. Or try to go with for example 100 % or 20 % to see the changes).
    2. Set your camera’s shutter speed to 1/25 and focus on the white line in the middle. On most cameras you can focus by holding the shutter button down halfway.
    3. Before taking the photo, move the camera to the left of the white line (while still holding the focus). Start to move the camera from left to right in a quick and smooth motion while taking the picture. You may need to repeat this a few times to master the technique.

    You now get a picture like this.

    Example of PWM
    Picture taken with a DSLR with a shutter time of 1/25 second with a motion from left to right

    Zoom in and count the number of lines (representing frequency cycles). We remember that we used a shutter speed of 1/25 of a second so we can easily calculate the frequency of the PWM used for the LEDs. We had 9 lines, so 25 * 9 = 225 Hz. The result is not 100% accurate, so you can try with other shutter speeds (just remember to multiply with the right shutter speed, for example 50 for a 1/50 shutter).

    If you do not get a pattern your monitor is not using PWM (if you experience PWM on your monitor try to set the brightness level to 100 % and try the test again. You might find that PWM is no longer used).

    How to avoid flicker on your existing monitor

    So, what if I already own one or more monitors, you probably ask yourself by now. You can actually do something to the PWM issue. Start by setting the brightness setting to 100 % and perform the camera test. If you see no lines PWM is not used at maximum brightness. If you still see lines at 100 % brightness you cannot use the method described below.

    You monitor is probably too bright at 100 % brightness but you can “cheat” it and have it run at lower brightness without changing anything in the actual monitor menus. Instead, you into your graphic card settings menu and proceed to the advanced options (typically be right clicking on your desktop and selecting screen settings). In the advanced settings menu you can often lower the brightness level, telling to monitor to display less intense colors. In practice the monitor translates this into a procedure where it uses the liquid crystals in the panel to block more backlight. However, it has a negative effect on contrast so is not a long-term solution.

    How it is done varies from PC to PC so we cannot offer a complete guide. You might also need to download the advanced color settings module from either Nvidia or AMD (depending on the graphic card in your PC).

    Everything in moderation

    Eat healthy, exercise, stimulate your brain; and you will live a long life. Something like that, right? Everything can be unhealthy in large quantities and it is not healthy to sit in front of a computer screen all day, every day. We obviously do not want you to stop working behind the desk but merely intends to share an explanation of why some people still feel discomfort when working in front of a modern computer LCD monitor with LED.

    Share the article with colleagues and friends. They might suffer from this without knowing that a monitor can cause it. You can help them

    There are some methods that you can use to reduce the problem and if you experience eyestrain, discomfort and headaches you might want to consider if it could be caused by PWM flicker (and you can even test it). It can obviously also be caused by other factors such as fluorescent ceiling lamps or stress, and if you headache starts around your neck or shoulders it is more likely to stem from a wrong working position (ergonomics). But in the end you should take care, and we think manufacturers should do the same. They have a responsibility and we hope that this is something they will take steps to eliminate in the near future.

    - Sources: Wikipedia, Eizo, TFTCentral

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