Can one see flickering of a light bulb at 50 Hz?

Can one see flickering of a light bulb at 50 Hz?

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Yesterday I had a BBQ with some friends. The sun had already set and the only light source left (besides some ambient light from the world around) was a low energy light bulb.

After a while I started to see lighting changes in the faces of my friends and the number plates of their cars. It felt like someone toggled the light very fast. When looking at the wall or the light directly I didn't notice any flickering.

In my country the power grid is running at 50 Hz. Is it possible that I actually saw the flickering caused by the alterations in the power grid or am I just going insane?

Short answer
Yes, the flickering of a light bulb may be noticeable, and yes, that's directly related to the mains frequency. However, since the flickering of a bulb is about two times higher than the temporal limits of our visual system, it is unlikely to be perceivable.

The temporal resolution of the visual system can be quantified in a number of ways. As you are referring to a relatively simple flickering stimulus, the critical flicker fusion frequency is probably the most relevant. At a certain critical frequency, a flickering stimulus will appear as a continuous stimulus. This critical flicker fusion frequency limit is around 50 Hz, but variable between 5 - 50 Hz, dependent on the lighting conditions (Kalloniatis & Luu), see Fig. 1 below.

For example, the turn signal of a car is obviously flickering (flickering in the 1 Hz region). But an object displayed on a standard flat-screen computer seems steady. A monitor's refresh rate is typically 60 Hz, which is indeed above the critical flicker fusion frequency (Holcomb, 2009).

However, the good old CRT screens can sometimes seem to be flickering. The mains, as you indicate, is indeed 50 Hz (Europe, Australia) or 60 Hz (US), and indeed the flickering is at this frequency. Similarly, well functioning fluorescent tubes seem to flicker on occasion (when they are reaching their end they start to flicker too, but that's because of a failure of the device rather than the mains frequency peaking through). Due to a similar effect, light bulbs may seem to flicker too. However, because of the sine wave characteristic of the mains alternative current, featuring two peaks per wavelength (a negative and positive peak, the flickering of a light bulb is actually two times the mains frequency, or 100 - 120 Hz. This is quite far above the critical flicker fusion limit and hence will likely not be noticeable.

It's interesting to see that you mention that it was around sunset. Scotopic vision (night vision) is mediated mainly by the rod photoreceptors. The rod visual system mediates gray scale vision at low-lighting. While spatial resolution is poor, it's very well adapted to process fast-moving stimuli. Hence, the flicker fusion frequency under scotopic viewing conditions may indeed be higher; i.e., flickering of light bulbs may not be perceived during the day (Federov & Mkrticheva, 1938). Nice add-on there.

To add to this as alluded to in the comments, whether the flicker of mains-grid powered appliances are actually visible depends on a lot of factors other than flicker frequency. CRT screens, for instance, may have improved phosphors that have delayed response times, 'smearing' out the flickering into invisibility. In other words, it's not a simple matter of 'ON' and 'OFF'. Likewise, light bulbs heat up and hence the temperature difference might not be noticeable to us, as the temporal flickering depends on heating and cooling of the wire.

Fig. 1. Flicker fusion as a function of stimulus intensity. Note that the shape of this graph means that photopic vision is less sensitive to temporal changes; the intensity scale relates to the stimulus intensity, as alluded to in the other answer. Scotopic vision to promote the temporal resolution of vision in the sense mentioned in this answer alludes to the ambient lighting conditions conditions. source: Kalloniatis & Luu (2007)

- Federov & Mkrticheva, Nature; 142: 750-1
- Holcomb, Trends Cog Sci 2009; 13(5): 216-21
- Kalloniatis & Luu, WebVision, chapter "Temporal Resolution" 2007

A lamp flickers at 2x the mains frequency, i.e. 100 or 120 Hz, and that is typically not noticeable to human eyes. It is visible to chicken and insects.

That being said, a low quality lamp or a lamp at end-of-life may also flicker at 50 or 60 Hz, and you will notice. It depends on the brightness, so an area illuminated by the lamp may not seem to flicker.

A simple way to suppress the flicker of a 60 Hz CRT is to put on sunglasses. The chemistry in your eyes is slower at low brightness, this makes the flicker less visible. The invention of 100 Hz CRT TV (I was involved) was necessary for allowing higher brightness.

Let's say there is a point source of light (it could be a lamp or a highly reflective object) that undergoes large, rapid intensity changes, say 50-100 times a second.

If you quickly move your eyes while it's in your field of vision, it will trace out a path across your retina. Some sections of this path will receive little light, while others will receive a great amount. What you see will look like a dashed line. (The flicker fusion frequency is irrelevant because it refers to fixed points in your field of vision. In this case we are dealing with many spatially distant photoreceptors.)

Let's say it takes 0.15 seconds to "flick" your eyes from right to left. This means a light flickering at 100Hz would be broken up into 30 "off" sections and 30 "on" sections during this time. So in fact you would be able to detect frequencies much higher than 100Hz. (This could make an interesting Arduino experiment.) I have noticed the effect when my laptop varies the brightness of its charging light using PWM. As it "darkens", the dashes in the dashed line get shorter, and vice versa.

But let's put this in context. The conditions you describe mean that:

  • There is a big contrast between reflective objects and the background
  • There is a big contrast on some objects when the lamp is at its brightest and at its darkest (assuming a LED lamp without smoothing capacitors, the objects could receive virtually no light from the lamp for certain periods)
  • The objects are not "point sources"

This means that whenever you move your eyes around, a quick succession of bright "ghost" objects will additively combine in your field of vision. The effect will probably look like a sped-up version of strobe lighting. The same will happen with moving objects (e.g. someone waving a hand). But if you fix your gaze on a stationary object you will probably not see flickering.

Flickering light at outlet tester

Scratching my head after trying to troubleshoot some failed outdoor lights. It is a long story.

The issue was with the outdoor lights outside my garage door. There are 3 sconce lights, and one overhead light that is controlled by a motion detector. All 4 lights are controlled by a single switch. There are other lights inside the garage (including garage door opener) that shares the same circuit breaker, and they are working fine. This is a relatively new house, originally built is 1996, but significantly upgraded in 2006.

Wife complained that the overhead light always stays on (i.e. not controlled by motion detector), and the sconce lights never turned on.

I first tackled the overhead light. The motion detector was bad so I changed the motion detector, and it started working correctly. So far so good!

Then I started checking if any power is coming to the bulb holders of the sconce lights. Verified that all 3 bulbs are good. Also put a "Lamp Holder to Outlet Adapter" in the bulb holder, checked the voltage using a multimeter, and got

120V between hot and neutral.

However, when I connected a LED light to that adapter, I got a very dim output. Also tested it using a "Klein Tools Digital Circuit Breaker Finder/Receptacle Outlet Tester", making sure to connect the ground wire to it as well. It showed correct wiring (2nd and 3rd yellow lights were on), but these lights were dim and flickering. I saw the same issue at all three sconce lights.

In addition, if I connect a light bulb to the bulb holder, then the outlet voltage drops down to almost zero, even though the light bulb does not turn on. If I use a CFL bulb instead of filament bulb, I see a faint flickering.

At this time, I found that the overhead light has stopped working altogether. I removed the overhead light, and saw the same issue there as sconce lights i.e. 120v but outlet tester was showing dim/flickering lights.

To isolate any issue with the devices, I completely removed all the 4 lights, leaving only the wires coming out of the outside wall. Still seeing the same issue.

As the next common point was the switch, I removed the switch completely, and connected the black wires together. Still no change. Also, if I use the outlet tester at the switch, then the two yellow lights are bright and steady. I also checked that the neutral wire at switch box is securely attached with the other neutral wires (there is another switch in the same box).

At this point I am at a loss. I hate to think that this is a wire issue, because the wires go above the wall of the finished garage, and not easily accessible.

Is there any other experiment that I can do?

PS: Adding more datapoints based on the suggestions below.

(a) Connected an Edison bulb to one of the sconces, and on 2nd scone bulb-socket, attached an extension cord to bring the point near the switch. With the light-bulb connected, the voltage between the hot and neutral of extension cord drops to 1 volt. Now, I checked the voltage between neutral of extension cord and that of the switch. It was 0 volt. Next, checked the voltage between hot line of extension cord and that of this switch. It was 120volt.

(b) With bulb removed and power turned off, I measured resistance. Between neutral of switch and extension cord, it was 0 ohm. Between hot of extension cord and that of switch, it was about 6 M-ohm. This should also ideally be close to 0, right?

How to Test for LED Flicker at Home

While spending time indoors, at one point or another you’ve likely encountered a flickering light. Flashing on and off like a strobe or disco ball, it can be supremely annoying, but it can also even be dangerous.

Why does flicker occur? To some degree, flicker happens in all light sources with an AC power supply delivered at a frequency of 50 to 60 hertz, meaning the electrical current travels backward and forward 50 to 60 times per second. Flicker happens due to rapid voltage variations, or a “ripple” in a current which then leads to a ripple in light output— a flicker.

All types of lights are susceptible to flicker, including incandescent, halogen, and, yes, even LED bulbs. But the effects aren’t the same for every type of light. In halogen lamps, for example, the filament temperature reacts slowly to changes in electrical currents, so you won’t notice as much of a flicker effect. LEDs respond almost instantly to changes in current, so flicker is much more noticeable.

Besides being a nuisance, the effects of a flickering light can trigger some serious ailments such as headaches, visual impairment, or in extreme cases, epileptic seizures. Even if the flicker is slight, at higher frequencies of 100 to 150 Hz for example, your eye may not consciously register it, but the brain can still detect and react to it, potentially with negative consequences. In particular, athletes in stadiums and warehouse workers could be more susceptible to accidents under poor lighting conditions that could cause a stroboscopic effect.

At home, there are a couple of simple ways to test for LED flicker to prevent bothersome lighting and potential health hazards. First, you can perform a simple flicker test with your smartphone camera. Turn it on and aim it at the light source in question while looking at the image captured on the screen. If you see a series of dark and light bands traveling slowly across the screen, then your light has flicker. If the bands are not that noticeable, you’re in the clear. Smartphone cameras can capture images at a discrete frequency, so they’re reliable tools that clearly register when there is no light.

One more way to use your smartphone to test for flicker is to download a flicker test app. VISO Flicker Tester uses your phone’s camera to measure a light’s flicker index, flicker percentage, and frequency values to give you a bigger picture of your lighting quality.

What kind of things can cause light flicker or dimming?

Voltage changes can be caused by dimmer switches or when electrical equipment drawing heavy currents are turned on or when being used (e.g., resistance welding machines motors in refrigerators, air conditioners arc furnaces medical imaging machines (x-ray, CAT scan, MRI) motors subject to variable loads large capacity photocopiers). Resistance welding machines that repeats welding at a rate of one or more per second can cause repetitive voltage fluctuations and may result in a noticeable light flicker.

Usually voltage fluctuations are small and do not have adverse effects on electrical equipment. However, in offices, for example, voltage fluctuations of just a few tenths of one percent can produce very annoying flickers in the lighting, especially if they are regular and repetitive in the 5-15 Hz range.

4 Answers 4

The isolating it to when both switches were in the same position was as far as we could get for diagnostics. All connections were inspected and seemed fine. I finally gave in and just bought 2 new switches. Upon trying to install them I noticed mismatches in wiring colors so I brought in an electrician who was able to sort things out (and tagged the wiring for me). He then removed the oldest looking of the 2 switches and replaced it with one of my new switches. The flickering problem is gone and I have a spare switch now.

For old fixtures with a screw-in lamp base, I think the cause of flickering is often that the center hot contact is not making firm contact with the bulb. I pry the center contact up slightly with a screwdriver or a crochet hook. I also scrub oxide off the contact with a pencil eraser, but I'm not sure that does anything. Naturally be sure the switch is off and check the socket with a non contact or contact voltage tester before inserting anything in the socket.

I will tell you what I found with some flickering lights.

I had two light bulbs flickering on the same circuit. I inserted a voltmeter into one of the sockets (removed the bulb), and when the other light would flicker, I would see the voltage drop momentarily in the socket I was looking at. Which in my mind would indicate it's not the bulb (the fact that I had two bulbs do it at the same time is also a good sign there, heh).

Next step is to isolate the problem. You can either start "close" or "far"

If you're starting close, go to the switch, pull it out from the wall, put your voltmeter lead on the incoming power, and one on "common" or "ground" it should read 120. When the lights flicker does it change? If not then the problem is somewhere between the incoming lines and those bulbs (hint: might be the switch itself, or maybe where it connects to one of the fixtures). You could also try putting your voltmeter leads on the wires leading out of the switch, do they drop?

If voltage is still dropping there, go to the power box, identify the breaker it's on, and put one voltmeter lead on the breaker output, one on common. Does the voltage drop? You can also try the same thing with the main large incoming line (though that's pretty scary, might want to have a pro or a friend help). Does the voltage drop? If it's still dropping from the main coming in, time to call the power company and ask if they can look into it (in my neck of the woods they can install some kind of "meter" to monitor it for you temporarily).

In my case I tested the "main" and it didn't drop in voltage. I was then about to test the breaker when I noticed "that's odd, one of those breaker looks like it's sticking out slightly more than the rest" and sure enough, it was the one feeding my "periodically flickering" lights, so plugging it in more firmly I believe solved it for me.

If it's LED's (especially on a dimmer) then there are other things that could be at play, but still worthwhile to check if voltage is a problem.

100Hz flicker in lighting is hazardous to humans?

Please can we de-nonsense the “100Hz flicker problem” in lighting?

The web is full of conflicting stories about whether or not 100Hz flicker is actually harmful to humans.

I believe that 100Hz flicker in lighting is no problem whatsoever…..i lived under the old magnetic ballast fluorescent lighting for decades…and had no problem with its 100Hz flicker. Also, driving down motorways with sodium vapour lamp streetlighting which flickers at 100Hz, caused me no problem whatsoever…..and no problem to other drivers either.

Headaches in today’s world are more likely to be caused by Hangovers, too much caffeine, too much sugar, too little exercise, too little sleep due to watching Netflix all night, etc etc…..

The 100Hz flicker problem in lighting is simply a nonsense story pervaded by large electronics corporations so that they can sell their integrated circuits and lamps which get rid of this 100Hz flicker.

Also, most of the large electronics corporations have decided to volume manufacture LED power supplies with large-ish electrolytic capacitor banks, (eg after the PFC stage) which makes them cheaper and easier to design……..and means the 100Hz flicker is easily avoided…….having committed their resources to this in vast quantity, they then are looking to create nonsense stories about smaller manufacturers who make lamps which do flicker at 100Hz….in other words, yet again, all we are seeing, is the large globalist corporations lobbying Governments so as to increase their profits and destroy their smaller rivals.

There is not one single piece of conclusive evidence to state that 100Hz lighting flicker is actually a problem……..nonsense?


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The human eye can sense way above 60Hz - it all depends on how you look at the flashing object.
For example, GM has taken to using PWM modulation on their taillight/brake light assembly on Cadillac and Corvettes with LED rear lighting.

GM claims they are pulsing the lights way above human perception but that is only when looking directly at the light. If you are behind a GM car and scan left and right (rotate head PLUS change directly of eye scan at the same time) an high scan rate across the field of vision can be achieved (degrees per second). Thus even GMs 90 pulses per second of high-intensity LEDs are easily discerned through persistence of vision by a distracting chain of red taillight across the eye).

Minimum PWM frequency selectiunder review at several standards-setting committees in SAE and DOT.

Nigel Goodwin

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Not at all, movie film is 24 frames per second - hence the problems converting films to TV - particularly in the US, it's not bad here as it's already very close.

Flat screen TV's don't work in the same way as CRT's, so don't really flicker - but the signal is essentially the same, 50Hz over here, and 60Hz in the USA.

Most sets process the picture to supposedly improve it, although I'm not really sure it does, so a 100/120Hz set would generate an extra picture in-between the two actual frames of the video. This obviously takes a LOT of processing power, and is probably the most important difference between cheap sets and good quality ones - the same system also scales the picture to fit the screen. There are also higher frame rate sets, which generate extra pictures again, and again, between the original frames - my Sony Android set is apparently 800Hz.

The old Plasma sets weren't capable of more than 100Hz, as is obvious by the way they work - although the marketing guys invented a totally fictitious measurement that was intended to fool people in to believing that they had faster frame rates equal to LCD.

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I can think of two instances where I have been "affected" by lighting.

25+ years ago in a hotel, one of the restaurant/lounge areas was illuminated by the (then) new compact florescent lamps.
I was glad to eat up and get out of that room, it felt quite unpleasant.

30+ years ago in a factory, one room had several light fittings of the type where three or four short (about 2 foot long) florescent tubes were grouped together.
This room gave me a headache and hurt my eyes. This may have even given me a touch of migraine, I felt awful that week at that place.

So, other than those two instances, which may or may not have been directly attributable to the lighting, no problems.

Experience is directly proportional to the value of the equipment ruined.

Happily retired and playing with my big boys toys every day.


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Do you otherwise get epileptic seizures?

The issue with flashing lights is not that they cause non epileptics to have seizures. It's that they can trigger seizures in those who already get them. And even then, I doubt that it affects all epileptics the same.


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Interesting question. Regarding whether or not we can perceive such flickering, I've noticed an interesting thing with LED taillights on newer cars. I don't notice any flicker in them if I look straight at them, but if I scan them--move my eyes rapidly across them--I can definitely see "dashes" of light, indicating low-frequency flashing. Dunno of this affects anyone adversely, but I wouldn't be surprised if it did.

What I have a problem with is people who claim that the "flicker" in them newfangled light bulbs (CFLs and LEDs) affects them. Those bulbs do pulsate,, but the rate is way up in the multiples of kHz, which I don't think anyone can actually sense.


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I can think of two instances where I have been "affected" by lighting.

25+ years ago in a hotel, one of the restaurant/lounge areas was illuminated by the (then) new compact florescent lamps.
I was glad to eat up and get out of that room, it felt quite unpleasant.

30+ years ago in a factory, one room had several light fittings of the type where three or four short (about 2 foot long) florescent tubes were grouped together.
This room gave me a headache and hurt my eyes. This may have even given me a touch of migraine, I felt awful that week at that place.



I'll add nobody has mentioned the beat frequency of a fluorescent at 120Hz of ambient light on a monitor at 60-75Hz.
In the early 80s, I put a hood over my monitor to keep headaches at bay.

I recommend cool white for high visual acuity in shop floor areas by blue content, warm white for office areas to lower stress by red content.

Previous post of white text / black back ok, But black text over white back bad is due to amount of area "flickering" that they're concentrating on.
I sympathize. <<<)))


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An LCD monitor will basically stay at constant brightness between frames, so even if it is updating slowly, there's no flicker. A moving image on screen will be moving in a series of jumps, so the slower the frame rate, the larger the jumps.

Sensitivity to flicker varies enormously between people. I appear to be very sensitive to it, and I find the 100 Hz lighting on cars highly irritating, even though I can't see it flickering unless there is movement. Of course, at night, there is a lot of contrast which makes it worse, and the flicker becomes visible as I look away or towards a 100 Hz light, and in normal driving, there's a lot of looking at different things.

The 100 Hz flicker on car lights is often worse than the 100 Hz flicker on some 50 Hz lightbulbs because the duty cycle is different. Some car tail lights are only on for 10% of the time, so there are 9 ms gaps in the lighting. Some 50 Hz lightbulbs dim as the voltage reduces and and so they are on for the majority of the time and the actual dark period is probably only about 2 ms.

There are many ways that LED lightbulb circuits are arranged, and one method that I have seen used is a series capacitor as a current limiting device, followed by a bridge rectifier, the smoothing capacitor and the LEDs. On several of these the smoothing capacitor is far too small so it does little to reduce flicker, and I have virtually eliminated flicker by adding a significantly larger capacitor.

Flicker can be photographed by deliberately moving the camera during the exposure so that the image becomes a series of dots, like this:- That shows 100 Hz car lighting as dots, 50 Hz street lighting (so modulated at 100 Hz) as the orange strip with gaps, while a steady light, in this case an incandescent one, is a uniform strip. The photo gives an idea of what I see as I move my eyes.

I've been able to detect higher frequencies. I've used the moving camera technique to estimate the flashing frequency of one set of car lights at about 2 kHz. I had noticed the flicker with the naked eye before taking the photo. Beyond about 500 Hz I don't find the flickering distracting, even though I can see it.

There was TV documentary many years ago where the effect of fluorescent lighting with 100 Hz modulation was measured by experiments on volunteers. Even when the volunteers couldn't say whether there was modulation, when they moved their eyes to look in a different place, they were less accurate when there was lighting modulation, with their eyes often overshooting the correct position at first.

I don't think that 100 Hz lighting is harmful, but I know that it can be intensely irritating to me, even when other people are unable to detect flicker. It seems to me that allowing flicker in lighting systems just because only 5 - 10% of the population can tell the difference is simply bad engineering.

Why does LED Flicker?

The problem with LEDs is that they differ a lot from older lighting technologies. Tero wrote a post about LEDs and the basics behind this, so I won’t delve into that with much detail.

To be short, if LED is supplied with a constant current, it won’t flicker. But the current will have to be really constant.

The most common reason for LED flicker is a bad LED Driver. If the driver fails to provide constant current, the led that it powers will flicker.

As the driver converts the AC to DC, there will be some ripple, which will cause the frequency to jump typically to the double (as it was with fluorescent lamps). This means that the LEDs waveform will follow the driver’s waveform.

Flicker on drivers is called ripple. Basically it is a synonym for flicker.

Other reason for LED Flicker is dimming. If the dimmer controls the LED with lower frequency than 200Hz, it will cause significant flicker. This is due to the fact that some dimmers alternate the current that the LED is supplied with.

To correct flicker-related health problems:

  • Replace bulbs on a scheduled basis. Old bulbs tend to flicker more and they are not as bright.
  • Ensure that all parts of the light fixture, especially the ballast, are functioning properly.
  • When replacements are needed, upgrade to fluorescent lighting that uses electronic ballasts, such as compact spiral fluorescent lights.
  • Add fluorescent light filters to existing fluorescent lights to create a more natural lighting

What can you do about the LED flicker?

The reason we stressed the importance of determining the source of the flicker (AC power or PWM?) is that we need to know which components of the lighting system need to be upgraded or replaced.

If the source of the flicker is in the LED bulb, you will need to replace the bulb with a new bulb that is specifically designed to smooth out the AC power and convert it to a truly stable DC current that in turn is used to drive the LEDs. When searching for an LED bulb, specifically look for "flicker free" designations as well as flicker metrics:

Flicker Percent - a percentage score that describes the relative difference between the maximum and minimum brightness levels (amplitude) within a flicker cycle. An incandescent bulb typically exhibits between 10%-20% flicker (because its filament retains some of its heat during the "valleys" in an AC signal).

Flicker Index - a measure that describes the magnitude and time interval during which an LED emits more light than its average light output during a flicker cycle. An incandescent bub has a flicker index of 0.04.

Flicker Frequency - the number of times that a flicker cycle repeats itself per second, measured in hertz (Hz). Most LED bulbs will run at 100-120 Hz due to the input AC signal's frequency. For bulbs that have a higher frequency, the effects of similar flicker % and flicker index values would be less pronounced because of their faster switching times.

Most LED bulbs flicker at 100-120 Hz. At this frequency, the IEEE 1789 recommended safe ("low risk") flicker percentage is 8%, and 3% to completely eliminate the effects of flicker.

If the source of the flicker is in the PWM dimmer or controller, likewise, you will need to replace the PWM dimmer unit. The good news is that the flicker is most likely not caused by the LED strips or other components, so your replacement costs will be limited to the PWM dimmer / controller.

The only meaningful PWM flicker metric is its frequency (because it is almost invariably a signal with 100% flicker), so when searching for a flicker free PWM solution, make sure that there is an explicit frequency rating. For a truly flicker-free PWM solution, we recommend a PWM frequency of 25 kHz (25,000 Hz) or higher.

In reality, standards such as IEEE 1789 indicate that PWM light sources with 3000 Hz is a sufficiently high frequency to completely mitigate the effects of flicker. However, one of the advantages of increasing the frequency to above 20 kHz is that it eliminates the possibility of audible buzzing or whining sounds emitted from power supply units. That's because the range of audible frequencies maxes out at 20,000 Hz for most people, and by specifying something at 25,000 Hz, for example, you can eliminate the possibility of annoying buzzing or whining sounds, which can be particularly problematic if you are particularly sensitive, or if your application is very sound-sensitive (e.g. recording studio).

See here for Waveform Lighting's line of flicker-free PWM dimmers and DMX dimmers.

Watch the video: Burning lamp on 50 Hz frequency (May 2022).