Some LED lights do not flicker at all. If a particular kind of circuit is integrated into the bulb, the light will not have any flicker  (See Background: LED Lights for references).

Most LED lights sold in the U.S. in 2021 flicker. Currently sold household LED bulbs almost always flicker at twice the AC mains frequency, so they flicker at 120 Hz in the Americas or 100 Hz in the rest of the world (See Background: LED Lights for references).

There is no data to support this statement. If flicker is fast enough, an optical illusion is created that fools people into thinking that they can't see the flicker, even though they can. We witness this optical illusion when we view most forms of artificial light. The flicker frequency at which the optical illusion occurs to make the flickering light seem continuous was the focus of considerable early research that tried to distinguish between visible and "invisible" flicker. Recent research has greatly expanded the range of flicker known to be visible to at least 11,000 Hz, so the historic borderline between visible and "invisible" flicker is now known not to exist. Additionally, the nervous system can sense seemingly "invisible" flicker, even if people don't know that they are seeing it, and such flicker can harm human health (see Background: Health Effects, Testing LEDs and Screens and Survey of LED Sensitivity). 

This estimate came from early 1990s electroretinogram studies that showed that it was possible to use an ERG to detect a person's response to flicker up to about 160 Hz (Berman et al., 1991). An issue trying to detect ERG signal at higher frequencies is that the ERG signal decreases as flicker frequency increases and that signal isn't high enough to detect after about 160 Hz. Some have cited this study to suggest that human eyes can't detect flicker faster than 160 Hz. However, later research greatly expanded the range of flicker known to be visible to at least 11,000 Hz (see Background: Health Effects).

Berman SM, Greenhouse DS, Bailey IL, Clear RD, Raasch TW. Human electroretinogram responses to video displays, fluorescent lighting, and other high frequency sources. Optom Vis Sci. 1991 Aug;68(8):645-62. https://eta-publications.lbl.gov/sites/default/files/24666.pdf

While some research has been done on the short-term health effects of fluorescent light flicker, no research has been done yet to characterize the health effects of the flicker from LED lights or screens. While some doctors may have heard anecdotal reports of symptoms from other patients, there is nothing in the medical literature yet to define the condition, to suggest an appropriate research-based management of the condition, or to evaluate either short-term or long-term health effects. There is nothing in the medical literature yet to suggest that the symptoms caused by LED flicker are a manifestation of a previously-characterized health condition. (See Background: Health Effects and the Survey Discussion for a thorough discussion and references).

LED bulbs that do have flicker are sometimes marketed as "flicker-free" even though they are only free of <90 Hz flicker that is slow enough to be obviously visible and annoying to anyone. A company that is actually manufacturing completely flicker-free bulbs should provide evidence, including metrics measuring the level of ≥100 Hz flicker (see Background: LED Lights for references). Der LichtPeter lists many LED bulbs that are falsely marketed as being flicker-free on Amazon. I have also recently noticed other LED bulbs falsely marketed as "flicker-free" in the US on Amazon.com. When I have contacted the manufacturers, they admit that the bulbs are not flicker-free. This confusion may arise from a 2016 decision by the lighting industry to redefine "flicker" within the lighting industry to only refer to noticeably visible flicker (CIE TN 006: 2016). This means that lighting manufacturers may feel justified in labeling flickering lights as "flicker-free" if they think that the flicker is fast enough to create the optical illusion that fools people into thinking the flickering light looks continuous. Any light bulb that has typical 100 Hz or 120 Hz flicker due to the alternating current from the electrical grid, could be falsely labeled "flicker-free" using this reasoning. Needless to say, this practice is incredibly misleading to the average consumer and dangerous to the health of individuals sensitive to light flicker. [Note: this website calls any flicker "flicker," whether or not people know that they can see it].

Even if screen backlights don't flicker, there are many other potential sources of screen flicker (see Background: LED Screens for references).

While it may intuitively seem like a good idea to dim a light that is bothering someone, this will often make the situation worse. Usually, the dimming mechanism for LED lights or screens is to increase the amount of time that it is in the dark phase of flicker, thus making it more likely that the nervous system will sense the flicker when the light is dimmed. Screens and some industrial-style LED lights might have continuous light output at full brightness, but start to flicker when dimmed, so a dimmed LED light or screen can be much worse for sensitive individuals than a light or screen at full brightness. It's possible to engineer both LED lights and screens so that they do not flicker either at full brightness or when dimmed, but this is not typically done (see Background: LED Lights and Background: LED Screens). 

Caveat: It hasn't been studied, but it is possible, and would not be surprising, that those sensitive to light flicker might also be more sensitive to light brightness than normal control individuals, as has been suggested for migraine patients, so full light brightness may also be more likely to seem too bright to individuals sensitive to flicker. Personally, my sensitivity to light flicker is much more problematic than any enhanced sensitivity to light brightness that I might have because I also experience common migraine without aura. I have no way of knowing whether I'm more sensitive to light brightness than others (if anything, it doesn't seem particularly significant), but I haven't noticed any change in my sensitivity to light brightness (between migraine episodes) since the onset of symptoms due to LED flicker in 2018, although I have recently started to experience more frequent common migraine during which I become extremely sensitive to light brightness. The light brightness photophobia that I have experienced during common migraine episodes since 1990 is much less painful and much more transient than the sensitivity to light flicker that I experience regardless of whether I am experiencing a common migraine.

There are not regulations for LED screens. The California limit of 30% flicker for LED light bulbs is one of the few regulations on LED lights, although 30% is still a very high amount of flicker in my opinion. They are not regulations, but the IEEE NOEL recommendations for ≥90 Hz flicker are somewhat more restrictive than the California limit, but even when proposed, the panel of experts emphasized that future scientific research would be necessary to begin to actually evaluate the health consequences of LED lights. The IEEE recommendations were simply a best guess that was made in the absence of any directly relevant scientific data. My personal experience is that I experience severe health consequences even from LED lights with flicker below the IEEE No Effect Level (see Testing LEDs and Screens). Newer flicker metrics are less stringent than the IEEE recommendations. 

Although based on other kinds of light than LEDs, prior research showed that light flicker could have health consequences and produce neurological effects. Rather than first conducting controlled scientific research on LED lighting products before releasing them to the public, lighting designers and screen and software engineers have essentially been conducting unregulated medical experiments on the public. Eventual medical research by others and regulation by politicians may be long in coming (see Background: Health Effects and ARC: Lighting in Architecture: Human Centric Lighting - The New X Factor?, December 12, 2018). 

First, this is false because the shape of the waveform for most LED lights doesn't approximate the smooth, gentle sine curve shape of incandescent flicker even if other flicker parameters might be approximated. Therefore, the effect of LED flicker on the nervous system cannot be assumed to be the same as the effect of incandescent flicker. 

Second, there are more differences between incandescent light and LED light than simply the overall flicker waveform. It isn't known why sensitive people tend to tolerate incandescents much better than LEDs with comparable waveforms, but possibilities are that (1) LEDs often have significant color-to-color flicker while incandescents do not and (2) incandescent light is predominantly red and near-IR light while LED light not (see Testing LEDs and Screens).

Third, some people may become more sensitive to flicker with more flicker exposure, as shown for migraine patients exposed to visible flicker (Karanovic et al., 2011). Exposure to LED flicker might make people sensitive to forms of flicker that hadn't been bothersome prior to the introduction of LEDs, including incandescent flicker. 

Personally, after having experienced LED flicker sensitivity since 2018, I have only now in 2021 also become sensitive to the flicker of many incandescent lights. I was never bothered by incandescent lights prior to experiencing LED flicker and I also didn't notice any sensitivity to incandescent lights during my first significant period of LED flicker exposure in 2018. I have only become sensitive to incandescent flicker after a 3-hour exposure to the flicker of ambient LED lights in April 2021 that triggered severe months-long symptoms (see Testing LEDs and Screens). I am also more sensitive to fluorescent light flicker than I used to be. The only kinds of lights that have never triggered my symptoms are sunlight and candlelight.

The overall results in the LED Sensitivity Survey also show that most respondents are becoming increasingly sensitive over time.

In my experience, this does not seem to currently be common knowledge, at least among the lighting specialists that I have encountered in the United States. There is very little readily-available information about the health effects of LED lights and lighting product literature usually doesn't provide sufficient information to predict the amount of flicker prior to installation or to identify whether and how it might be possible to use products in a manner that produces completely flicker-free light output. Too often, lighting products are advertised as "flicker-free" when they are not actually completely free of all flicker (temporal light modulation). Modern lighting projects are also quite complicated in terms of having not only the light fixtures themselves, but also often drivers to deliver the current to the fixtures, computerized systems to control the drivers, and wall switches to communicate with the computerized system. Extra flicker can be introduced if any of these systems are incompatible with each other. There might not have been prior testing of all of the components (potentially from different manufacturers) in combination with each other prior to installation. Even if combinations of equipment are found that should theoretically produce completely-flicker free light, testing may need to be done on-site to verify this once they are actually assembled.

If a lighting specialist without expertise in this area is asked to take on such a job where a client requires completely flicker-free LED light in a building project, their options could be: 

While the option of taking on the job without actually installing completely-flicker free light is a possible option if making money in the short term is a lighting specialist's primary goal, it puts the health and cognitive function of people who will need to use the facility at risk and could cost the client considerably in terms of time, money, and the health and efficiency of their employees and others that use their facility. The outcome would be the same if a lighting specialist without expertise doesn't know what they don't know.

I hope that more lighting specialists will become educated about the very real health concerns surrounding LED light flicker and that lighting manufacturers will also start to create more completely flicker-free products and begin to clearly communicate about the flicker in their products.

This misconception is repeated in a "flicker manual" created to educate lighting professionals by a lighting meter company. In fact, fluorescent lights with electronic ballasts also have ~7% flicker at 120 Hz (see Figure 1 in Wilkins et al. (1989) and Poplawski and Miller, 2011). This means that it's incorrect to assume that the flicker of electronic ballast fluorescent lights can't be sensed by people. 

LED light fixtures controlled by drivers, such as commercial LED strip lights, often are engineered to have flicker that is faster than 120 Hz. Depending on how they are engineered, they may also have residual 120 Hz flicker from the AC mains power supply. Using a smart phone slow-motion video, both types of flicker may be noticeable, with higher-frequency flicker creating banding and 120Hz flicker creating alternating bright and dim sections on frames (see Testing LEDs and Screens). However, professional flicker meters may only report one flicker frequency in a list of data. If that frequency is the higher frequency, the lower frequency flicker, which might produce greater health effects, might be unreported. This is particularly problematic that some flicker may not be measured by these meters because extra flicker might be introduced in commercial lighting installations if incompatible lighting components are combined. Note that a savvy user of such a meter might be able to discern multiple significant flicker frequencies by examining graphs of flicker and adjusting the settings for sampling the light to create graphs with different resolutions, but doing so could require significant effort when assaying each light.