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Light flicker - what is it and why is it a problem?

Updated: May 14, 2021

Light flicker is a phenomena of quick repeated changes in the brightness of the light. Flicker is probably most known from computer monitors, especially old ones, which were blinking and flickering, causing many people annoyance and even headaches. Flickering lights can also cause many different negative health effects. Flicker is often caused by AC (alternating current) electrical system where the electricity is turning on and off 100 or 120 times a second (50 Hz or 60 Hz system). This constant on and off in the electric system causes also the lights to turn on and off as the electricity turns on and off. Basically all electricity that is produced by electrical companies is provided in alternating current.

Figure 1. Alternating current (AC) waveform.

As seen in the picture above, in one cycle of alternating current, the light turns on and off two times. This means that in 50 Hz electrical system (used for example in North America) the light turns on and off 100 times a second and in 60 Hz system (used for example in Europe) the light turns on and off 120 times. However, different lighting technologies produce flicker in different frequencies so many lights flicker much more faster than just 100 / 120 times a second.

All lights, that are connected to AC electric grid, flicker more or less. However, there are many aspects in flicker that vary between different types of lights and affect the severity and the detectibility of the flicker.

The different aspects affecting the flicker and the severity of it are:

  • The frequency of the flicker (Hz)

  • The modulation of the flicker (%)

  • The ambient light (natural light) levels in the space


The frequency is the number of times the light turns on and off in a second. It is measured in Herz (Hz). The lowest frequency flicker (50 - 120 Hz) is produced by incandescent, halogen, most LED and old fluorescent lights with magnetic ballasts. Some LEDs and new fluorescent lights with electronic ballasts flicker in much more higher frequency; fluorescent lights can flicker in up to 40,000 Hz frequency whereas LEDs flicker around 100 - 400 Hz.


The modulation is the change in the intensity of the light as the light turns on and off.

In different bulbs the change of the intensity that follows the voltage drops vary. This is due to the differencies in the way the bulbs themselves work.

Incandescent lights work by heating a filament that lights up as the filament's temperature rises. The filament in the bulb, however, reacts fairly slowly. This means that, as it takes a little bit of time for the filament to reach full light output, it also takes time for the filament to cool down completely. As the electricity turns on and off 50 or 60 times in a second, the filament doesn't have enough time to cool down so the drop in the light output is very small, only about 5 - 13 %.

Halogen lights work very much the same way as incandescent lights and thus produce flicker of only about 10%.

In LEDs there's a lot of different technologies and drivers that are used so the flicker modulation varies a lot between different LEDs. Basically, the flicker modulation-% of LEDs can vary anywhere from 0 to 100. Old and cheap LEDs can have flicker close to 100% but many modern LEDs have drivers that causes flicker of only 0 - 20 %.

Old fluorescent lights that used magnetic ballasts have very strong flicker, as they flicker in 50 - 60 Hz and have also high flicker modulation. Modern fluorescent lights use high frequency electronic ballasts (20,000 Hz or higher) which have been shown to cause 50 % less negative health effects.

Figure 2. Flickering of different lamp bulbs. The values are not absolute and can differ between different light bulbs.

Figure 3. Flickering frequencies and modulations.


Ambient light refers to the amount of natural light in the room. The more there is natural non-flickering light in the space, the less effect the flicker of the lighting has. It has also been noted that people on higher floors tend to have fewer headaches than people in lower floors, probably due to higher levels of natural light in the space.

Other causes of flicker

Modern dimmer switches that use phase-cut controls cause flicker. This is due to the phase-cut system where the electricity is turned on and off repeatedly which causes the light to be on less time and is perceived by the human eye as a dimmer light. In reality, the light is just made to flicker (in a lower frequency). Dimmable and smart LEDs also flicker if the driver uses pulse-width modulation (PWM) to reduce the average light output.

The health Effects of Flicker

The negative health effects that can occur due to flicker vary from small annoyances to epilectic seizures. These different health effects can occur even if the eyes are closed.

The flicker can be annoying and cause irritation when the frequency is so low that it can be detectable by eye. Flicker cannot usually be seen when it's above the critical flicker fusion frequency (CFF), which sets around the frequencies between 60 and 90 Hz, depending on the individual.

The flicker can cause eyestrain and discomfort. It can increase the incidence of headaches by 50% with nausea and visual disturbances and also trigger migranes in some individuals.

The biggest health issues come with flicker at frequencies between 3 and 70 Hz. Flicker at these frequencies have been shown to cause epilectic seizures in small percentage of people, the biggest risk produced by frequencies in the range of 15 Hz to 20 Hz. The severity also depends on the area of retina exposed, and the frequency and modulation of the flicker. Luckily, most lights, except old fluorescent lights with magnetic ballasts, flicker in frequencies higher than that.

Flicker can also cause blurryness and light trails in the vision. This can be noticed often in car (LED) taillights at dark. When you look directly to the taillights, the lights appear normal, but when you are moving your eyes rapidly (saccade) and glancing at other objects, a blurry trail of lights can appear. This phenomena is called a phantom array.

Fluorescent lighting with a flicker of 100Hz has been observed to affect the plasma corticosterone levels in birds and to affect the choice of their mating partners.

Flicker has also been shown to disrupt the natural eye movements across text and to impair visual performance in visual tasks.

Low frequency flicker can cause a stroboscopic effect that is associated with apparent slowing or stoppage of rotating machinery. This can lead to dangerous situations when operating heavy machinery.

Flicker has also been showed to increase repetitive behavior in autistic individuals.

Below you can see an illustrative chart on which flicker frequencies and modulations are known to cause health effects:

Figure 4. The health effects of different flicker frequencies and modulations.

How to know if your lighting flickers?

An easy way to test your lighting for flicker is to use your phone camera. Put the camera on, point it at a lamp and look at the phone screen; flicker will cause dark fringes to appear around the lamp. The frequency of the fringes is related to the flicker frequency and the frame rate of the camera. The contrast of the fringes can also give a clue on the flicker modulation: the bigger the contrast between the fringes, the higher the flicker modulation. If fringes appear, the modulation is likely higher than 20%.

Another way to spot flicker with your phone is to record a video pointing at a light and then using a video editor to slow the video down. The possible flicker can be seen as rapid blinking in the slowed down video. However, some cameras come with automatic flicker reducing technology, which will automatically correct the flicker, so before using your camera for this, check your camera instructions or your manufacturer if the camera uses such a technology.

The most accurate result you get when you hire a building biologist or other lighting professional to measure the flicker. There's also a lot of different flicker meters on the market, prices ranging from 50€ to thousands of euros. As the cheaper meters may not be the most accurate compared to expensive professional models, they can still give you more accurate results than your camera.

Some lower frequency flicker can also be detected with bare eyes. Focus your glance slightly to the side of the light. If the light flickers in low frequency you might see the light blinking rapidly. This can also be seen in TV:s, monitors and other screens.

How to avoid flicker and mitigate its negative effects?

All lighting should be designed so that as much natural light as possible is used. Even though glazing blocks certain frequencies of light, natural light is usually still superior to any artificial light sources. Natural light will also mitigate the negative health effects of possible flicker produced by the lighting.

Use lights that have higher flicker frequency and/or lower flicker modulation. These kind of lights include incandescent, halogen and some LED lights. As incandescent and halogen lights have naturally fairly high flicker frequency and low flicker modulation, LED lights can differ from catastrophic headache-causing lights to almost perfect zero-flicker lights, so always check the flicker levels from the package/website of the light or ask directly from the manufacturer before buying.

Take breaks from artificial light sources and screens and relax your eyes outdoors in natural light. This will calm down your nervous system in many ways and mitigate the negative health effects of flicker.

Use direct current (DC) lighting. DC electricity doesn't fluctuate as alternating current (AC) does, meaning that it provides a steady flow of electricity that doesn't turn on and off multiple times a second. This provides a steady, non-flickering light. DC lighting systems usually operate in low-voltage (12-24V) and are powered by solar panels, windmills or other renewable energy sources.

So what kind of lights do you use? Do you notice negative effects when spending time under flickering light? Is there something that the post failed to answer to? Leave a comment below or contact us here!

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1 Comment

I just found out about how most energy efficient lights flicker and then used my slow mo camera to see it. It’s crazy to think what it might be doing to everyone’s nervous systems. Having the lights go crazy all day can’t be good for us. Makes sense why I always got headaches in school, but I would have had the old fluorescents then . Thanks for the good information on the subject though. It’s organized so it’s easier to digest the technical aspects of it. Enjoyed the read.

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