Testing LEDs and Screens

Correlating flicker with my "LED" symptoms

How do my "LED" symptoms correlate with light and screen flicker and is there any safe flicker? 

During 2023, I bought a professional flicker meter and other instruments to measure flicker. I'm a scientist, so tried to apply basic scientific practice as I measured the flicker of lights that do and do not hurt me. I measured the flicker of hundreds of lights. I also measured the flicker of screens that trigger neurological symptoms for me slowly to quickly. This testing is described in detail in the following Powerpoints and the data was shared with the FDA Office of Radiological Health in November 2023, to my knowledge this is the first report correlating measurements of LED light and screen flicker with health impacts. In biological science, we must be skeptical of the import and universality of findings when n=1, but my hope is that this study will provide a roadmap for what should be done to analyze the relationship between flicker and health for more people. The links below are Google Slides versions of the PowerPoints to view online. These files as original PowerPoints (better quality) and the underlying raw data are also available here.

Lights_HealthData_Pt1_LightFlicker_Public

Lights_HealthData_Pt2_LowFlickerBulbTesting

Lights_HealthData_Pt3_Flicker_ThroughMyWindows_Public

Lights_HealthData_Pt4_ColorFlicker

Lights_HealthData_Pt5_FailureFlicker

IssuesWhenMeasuringLightFlicker_Public

Screens_HealthData_Pt1_Dell_CCFL

Screens_HealthData_Pt2_MacbookAir

Screens_HealthData_Pt3_WhiteMacBook

Screens_HealthData_Pt4_iPhoneSE2022

Screens_HealthData_Pt5_OtherScreensAndSummary

A few highlights are integrated below, but please view the PowerPoints for the complete analysis. I apologize for any typos and roughness in presentation - I'm choosing not to focus on editing since all screen use injures my brain.

Discussion of results

Before purchasing a flicker meter, my hypothesis was that the few LED lights that didn't seem to injure me were completely flicker-free and thus had lower % flicker than LED lights that do injure me. This hypothesis was based on my not being able to detect the flicker of LEDs that didn't seem to injure me with a slow-motion phone video, claims of lights being flicker-free on the Waveform website, and the apparent absence of flicker for a Philips bulb in a waveform posted on the LEDBenchmark website. There were also many LED lights that injured my brain that did have noticeable flicker in slow-motion videos.

My hypothesis was wrong. The LED lights that don't seem to injure me do in fact flicker. Not much, but they clearly do. Their % flicker (0.8%-1.1%) is higher than that of several LED lights that injure my brain. The story is more complicated than I had initially thought.

The graph below shows 0.05 seconds of the 1.1% flicker of the Philips LEDs I have used since 2013 and that have never noticeably harmed me.

In contrast, the light below (IKEA Lunnom Globe) has flicker that is 10 times lower (0.11%), but it caused concussion-like symptoms with brain fog during 2 hour of testing in 2023 (see .Lights_HealthData_Pt2_LowFlickerBulbTesting). Although it seems implausible that such ow flicker can hurt me, it does. There are several LED bulbs with flicker similar to this that also trigger concussion-like symptoms for me.

Before purchasing the meter, I already knew that the workplace LED lights that first caused me noticeable serious brain injury in the fall of 2018 (beginning with intense pain within seconds of first exposure) had flicker that I could barely detect on slow-motion video, but other LED lights that I had at home for years without consciously connecting subtle ill effects to the lights had very high flicker that was easy to detect on video. My measurement data suggest that it is too simplistic to assume that seemingly subtle flicker patterns can at most cause subtle harm and that extreme harm only results from extreme flicker. It is quite possible that different kinds of flicker might affect different biological targets or do so with different intensity. As a molecular biologist, I know through experience that one can never assume you understand how a biological system will behave - you have to test it. And if you make a subtle change, you have to test it again. We can't assume that the only possible biological effects of flicker will be visual and based on the brain's image formation. Often, reports from people like me that subtle flicker is causing harm are dismissed because it's assumed that we can't see it. I almost never notice seeing flicker, but I definitely feel its effects as pain and brain injury.

I suspect that there was something specific about the workplace LED flicker or the combination of flicker patterns to which I had been exposed by that point that caused the beginning of my extreme flicker sensitivity in the fall of 2018. Interestingly, the high-flicker LED light at home was on a side table shining on my right temple for 5 years, as I sat on the couch at night and lost the ability to concentrate while grading papers and increasingly got minor headaches. Now, the feeling of swelling in my head that is my most persistent LED symptom is concentrated in my right temple and all of my LED symptoms are on the right side. Could the high-flicker LED lamp have caused initial injury in my right temple that was then primed for an even more extreme response to the 2018 workplace LED lights?

The graph below shows 0.05 seconds of  the 36% flicker of the Cree LED that was in the lamp near the right side of my head for 5 years.After my exposure to the 2018 workplace LED lights, this Cree bulb started to exacerbate my LED symptoms and it was noticeably painful, so I removed it.

My data indicate that there are probably several factors contributing to the biological effects of flicker:

• Whether the flicker is random or periodic

• Flicker waveform shape, including the exact nature of the periodic flicker pattern. Many LEDs have very complex flicker patterns in which many peaks of light form a pattern and then that complex pattern repeats 120 or more times a second. Sometimes flicker peaks have amplitudes that vary in overlapping patterns so that the overall waveform looks like overlapping mountains or overlapping feathers. Such overlapping patterns can create low frequency bursts of brightness even though the total number of peaks may be in the tens of thousands per second.

  • % flicker

  • Total number of flicker peaks per second

  • Frequency with which a periodic flicker pattern repeats

  • Pattern of peaks within a periodic pattern, including their amplitude and spacing

  • Nature of any overlapping flicker patterns and their lower frequency light bursts

• Whether there is color-to-color flicker. This depends on:

  • Whether or not there is more than one source of visible light within a single lumiaire, such as a single LED diode or a single fluorescent tube light, In fluorescent lights, invisible UV light excites multiple colored phosphors to produce visible light. In LED lights, either UV or visible purple/blue light excites phosphors to produce more colors.

  •  The rates of excitation and emission of light from any secondary light sources, such as the phosphors in LEDs or fluorescent lights that emit specific colors If these rates differ between phosphors or compared to any primary visible light source, there will be color-to-color flicker.

  •  If there is more than one luminaire (such as multiple LED diode chips or the many subpixels on screens) within a device. If these luminaires produce different colors and their flicker is asynchronous, there will be color-to-color flicker. In screens, this can involve complex 2-dimensional flicker patterns.\

Randomness

Random flicker might be better than regularly pulsed flicker. Candlelight flickers slowly, but it doesn't harm me and even makes my brain feel a little better when I'm recovering from LED injury. 

I have very little data about random flicker for LED lights, but it turns out that that the Philips LED bulbs I've used without harm for 10 years at home have a (1.1%) flicker pattern with semi-randomness within the periodic pattern. And perhaps more significantly, the fixture where they're installed has a very slightly unstable electrical supply that creates subtle random flicker at a slower rate than the flicker from the bulb. Perhaps this instability makes the light seem more like candlelight to the molecules in my brain.

Color-to-color flicker

Color-to-color flicker seems especially harmful. Many white LED lights have slightly red to slightly green flicker as shown in the consecutive 240 fps slow motion video frames below (notice the green-red-green-red change on the left side of the image). These lights quickly caused concussion-like symptoms for me.

I notice my neighbor's rainbow LED porch light more and find it more painful when it's programmed to alternately pulse its colors than when it pulses its colors concurrently.

I'm also harmed by screens faster and more severely when there's more color-to-color flicker. There is a lot of screen data indicating this, including that the least harmful screen that I can use in full color happens to have the most synchronous color flicker. On any screen, using Nightshift hurts me more. It turns out that on every screen I've tested, Nightshift works by flickering green subpixels somewhat more and flickering blue subpixels a lot more, thus reducing the average amount of green and blue light. The overall effect is to create much more color-to-color flicker, since the screen flickers between very red and somewhat more blue/green.

Any flicker might cause injury

Even if there isn't color-to-color flicker and even if there isn't LED light, flicker can cause injury. For example, an eInk screen used as a computer monitor has no backlight, the frontlight is off, and the pixels visibly flicker. This screen gives me "LED" symptoms. Data in Screens_HealthData_Pt5_OtherScreensAndSummary.

Why is incandescent light flicker so much safer than LED flicker?

Like me, many people with LED sensitivity report being able to tolerate incandescent lights much better than LEDs. Often, our claims are dismissed since there are LED lights with less flicker than incandescents that we claim are hurting us. It isn't known why sensitive people tend to tolerate incandescents much better than LEDs with comparable waveforms, but testing indicates that there are significant differences between incandescent lights and LED lights beyond flicker waveform shapes.   (1) LEDs often have significant color-to-color flicker while incandescent lights do not have any color-to-color flicker. All of the light from an incandescent bulb comes from the glow of the filament as it is heated. Thus all of the colors rise and fall in brightness concurrently as the filament heats and cools. Flicker data for incandescent lights using different color filters over the flicker meter supports this - all of the colors seem to have flicker waveforms that are the same. However, this is not the case for many of the LED lights tested. The LEDs can have different flicker patterns for different colors, producing color-to-color flicker. Sometimes the amount of color-to-color flicker is even obvious in slow-motion video frames. (2) Incandescent light is predominantly red and near-IR light while LED light not. It's quite possible that evolution might have supported some tolerance of red to IR flicker considering humans' use of firelight over many thousands of years. In contrast, the flicker of LED lights has little red and near-IR light and has a character that is completely novel. Research in photobiomodulation therapy (see below) indicates that there may be beneficial biological effects specifically of red and near-IR light. For example, red and near-IR light has been shown to speed wound healing, reduce inflammation, and has even been used to reduce long-term effects of brain injury.

The graphs below compare the flicker and light spectra for a GE 60 W A19 incandescent light bulb and a Par20 LED bulb sent to me from Signify as their choice for a potentially safe LED (it triggered concussion-like symptoms for me quickly). Flicker and light spectra were measured normally and also with blue, green, or red filters over the light meter's sensor.

Insights from photobiomodulation therapy: Are there non-visual biological effects of flickering light?

There is an increasing body of research from the field of photobiomodulation therapy, in which LED light shines through the skin as a medical treatment, that identifies many non-visual effects of light on cells, tissues, and complex systems. The wavelength (color) of light matters and whether the light is pulsed or constant (and how it is pulsed) matters in terms of which proteins are stimulated and the types of biological effects. Some of the biological effects last for a long time - at least weeks following several minutes of light treatment, suggesting that the light treatments might alter gene expression patterns.Insights from photobiomodulation therapy lend support to the hypothesis that the biological effects of flickering light might not all be strictly based on visual processing. It's pure speculation, but I think it's possible that certain flicker frequencies might create some kind of resonance in light-absorbing proteins that leads to abnormal signaling.Perhaps flickering light alters gene expression patterns (also a speculative hypothesis), explaining how a brief LED exposure can initiate weeks to months of concussion-like symptoms for people like me. The reviews below provide an overview of relevant research.

de Freitas LF, Hamblin MR. Proposed Mechanisms of Photobiomodulation or Low-Level Light Therapy. IEEE J Sel Top Quantum Electron. 2016 May-Jun;22(3):7000417. https://doi.org/10.1109/jstqe.2016.2561201  https://www.ncbi.nlm.nih.gov/pmc/articles/pmid/28070154/

Hamblin MR. Shining light on the head: Photobiomodulation for brain disorders. BBA Clin. 2016 Oct 1;6:113-124. https://doi.org/10.1016/j.bbacli.2016.09.002

Hashmi JT, Huang YY, Sharma SK, Kurup DB, De Taboada L, Carroll JD, Hamblin MR. Effect of pulsing in low-level light therapy. Lasers Surg Med. 2010 Aug;42(6):450-66. https://doi.org/10.1002/lsm.20950  https://www.ncbi.nlm.nih.gov/pmc/articles/pmid/20662021/

Salehpour F, Mahmoudi J, Kamari F, Sadigh-Eteghad S, Rasta SH, Hamblin MR. Brain Photobiomodulation Therapy: a Narrative Review. Mol Neurobiol. 2018 Aug;55(8):6601-6636. Epub 2018 Jan 11. https://doi.org/10.1007/s12035-017-0852-4  https://www.ncbi.nlm.nih.gov/pmc/articles/pmid/29327206/

Zein R, Selting W, Hamblin MR. Review of light parameters and photobiomodulation efficacy: dive into complexity. J Biomed Opt. 2018 Dec;23(12):1-17. https://doi.org/10.1117/1.jbo.23.12.120901  https://www.ncbi.nlm.nih.gov/pmc/articles/pmid/30550048/

LEDs flickering more as they fail

Since the 1920s, lighting manufacturers have been specifically engineering light bulbs to fail. This planned obsolescence is for the purpose of selling more light bulbs. (This sounds like just a conspiracy theory, but was due to a real collaboration by major lighting companies, known as the Phoebus Cartel to limit the lifespan of light bulbs to 1000 hours: https://www.newyorker.com/business/currency/the-l-e-d-quandary-why-theres-no-such-thing-as-built-to-last). The Waveform Centric Home bulbs that I used to the point of failure flickered more over time, with flicker increasing from 0.9% to 100% (at 120 Hz) as they reached failure. Eventually some of them were visibly blinking on and off a couple times a second. The failing bulbs rapidly triggered brain injury for me, even before they were visibly blinking. I have noticed other LED bulbs visibly blinking in my neighborhood in street lights and at construction sites, seemingly because they had failed. I don't know whether all LEDs fail in this way or whether it's only some LEDs.

I don't know whether invisibly flickering more as bulbs age or visibly blinking slowly on and off are purposely engineered effects or accidents of engineering. I don't know what the planned mechanism for LED failure might be, but the US Dept. of Energy website (https://www.energy.gov/eere/ssl/led-basics) says that LED bulbs decrease in brightness over time as they fail and 70% of original brightness is considered to be failure. Is this reduced brightness due to more flicker? A March 2023 article by Tom Scocca in New York Magazine discussed the problem of LED failure that included flickering or color changes (https://nymag.com/strategist/article/led-light-bulbs-investigation.html).

Summary of light flicker testing:

• Flicker is harmful, with higher flicker generally being worse.

• Color-to-color flicker is particularly harmful, but isn’t necessary for injury to occur if flicker is significant.

• Color-to-color flicker is associated with harmful health effects for me even if the total brightness % flicker is quite low.

• Aperiodic/random flicker might not be harmful, but more study is needed.

• Different flicker waveforms cause somewhat different health effects.

• A speculative hypothesis is that flicker might trigger signaling that leads to changes in gene expression, reprogramming cells/inflammatory processes/nervous signaling in ways that result in long-lasting biological effects. It’s possible that different flicker waveforms/wavelengths/color flicker might impact such processes in different ways. Perhaps being exposed to certain combinations of flicker waveforms is needed to produces particularly bad biological effects or perhaps some people have some kind of genetic or environmental predisposition to sensitivity.

• The data correlating flicker with health effects for me do not support the hypothesis that the IEEE levels are sufficient to prevent adverse health effects. Rather, these data suggest the IEEE levels may be multiple orders of magnitude higher than would be necessary to prevent debilitating health effects.

• All of the LED lights that are OK for me have some special quality. Some are DC or have good AC/DC conversion and have low, non-periodic flicker. Some have low flicker that lacks color flicker and that has either a very random pattern or slight randomness to its pattern. This suggests that specific engineering for qualities not yet described by the IEEE report and not yet clearly defined may be required to create safe LEDs.

• Since I almost never notice seeing flicker for lights that trigger concussion-like symptoms for me, the hypothesis that one needs to consciously notice seeing flicker in order for it to trigger adverse health effects is also not supported. There is not evidence to support the assumption that “If you can’t ‘see’ it, it can’t hurt you.”

• It would be interesting to test the hypothesis that very low, aperiodic flicker without color-to-color flicker might be required for flicker to be safe if there is flicker. Additionally, there may be a biological benefit to some wavelengths of light being predominant (red/IR?), such that otherwise harmful flicker effects might be mitigated.

• Data like this should be collected for more affected people. N=1 is not enough and could lead to biased assumptions.

Summary of screen flicker testing:

• Every screen I've tried triggers concussion-like symptoms for me. All of them have flicker.

• My health is harmed more when there is more flicker on the screen.

• Dimming tends to create more screen flicker

• Some of the tested screens partially or fully pulse the backlight during dimming. It isn’t clear that any of these actually use “Pulse Width Modification” (PWM) as the dimming mechanism. They flicker the screen more upon dimming, reducing average brightness, but it doesn’t necessarily occur by PWM which is turning the backlight completely off and varying the off time to achieve different dimming levels.

• In general, screen flicker, either measured by UPRTek meter or microscopic subpixel flicker, depends on which color is being displayed.

• Which colors have lower flicker, as measured by UPRTek meter, vary from screen to screen. For most screens, absolute black (0,0,0) tends to have very low and/or non-periodic flicker as measured by UPRTek, but even this is not a universal trend (black has high 60 Hz flicker for the 2011 MacBook Air).

• Subpixel flicker occurs for every tested color screen (microscope data):

• Subpixel flicker can have very high modulation depth (% flicker). Subpixel flicker tends to be higher when the color saturation RGB setting is a lower non-zero value in a particular R, G, or B channel (when the color is less bright/saturated).

• Subpixel flicker tends to be slow: 15 Hz or 30 Hz among tested LCD screens. It is 60 Hz for the only OLED screen tested.

• Since subpixel flicker is slow and has high % flicker, it is very likely to be a significant cause of the adverse health effects that people experience due to screen use.

• Subpixel flicker tends to occur in an alternating pattern in two adjacent subpixel groups among tested LCD screens, but the layout of the two subpixel groups varies. Text and presumably photos/complex graphics can create locally concurrent subpixel flicker if subpixels in the same group happen to be flickering more than subpixels in the other group. Subpixel flicker is locally concurrrent for the only OLED screen tested.

• Screens have a lot of color-to-color flicker:

  • Differences in flicker pattern for the different R, G, or B subpixels,in a given area 

    • In one kind of example, the difference in R, G, and B flicker with the Nightshift setting might be why using Nightshift tends to be much worse for triggering symptoms for me than without Nightshift.

  • Differences in flicker pattern associated with backlight color illumination vs. specific color phosphor excitation/decay patterns

• There are likely other significant kinds of screen flicker not assayed here, such as temporal dithering (a form of color-to-color flicker)..

• More screen testing and data for many more people is needed.

More Information

Pre 2023 data

For more data and analysis from before I obtained a flicker meter see Testing LEDs and Screens Before 2023.

Demographic statistics and relevant medical history:

Female, living in New York City

Highest level of education: PhD in Human Genetics and Molecular Biology.

Employed full-time in science education. My current job is low-stress.

No history of concussion. 

No history of sleep issues.

No anxiety or any mental health issue. 

No history of eye problems other than nearsightedness.

History of common migraine without aura.

Following the onset of LED symptoms in 2018, I consulted a neurologist who did an MRI, a neuro-ophthalmologist who did extensive tests of my eyes, and an optometrist. None of these experts detected any problem, except that the neuro-ophthalmologist detected peripheral blindness in my right eye in a visual field test during my first visit when the clinic LED lights triggered my symptoms, but did not detect it on a followup visit where I protected my eyes from the clinic LED lights. The neurologist did various tests of neurological function that did not detect any issues, but I had to wait so many months for an appointment, that I wasn't experiencing LED symptoms (except enhanced sensitivity to flicker) by the time I had the appointment.

Comparison of my common migraine symptoms and my LED-caused symptoms

Onset and frequency of occurrence

• Common migraine without aura: First onset at age 13 and at that time occurring repeatedly for several weeks. Since then occurring a few times a year (with unknown triggers) for less than a day each time, but can usually be stopped early with ibuprofen taken at symptom onset. Typically lasts about 6 hours if untreated. Often feel tired for about another 24 hours.

• LED symptoms: First onset at age 42 in 2018. Repeatedly re-exposed to flickering LED lights (overhead LED strip lights covered by a diffuser that had ~1000 Hz flicker with a very subtle flicker percent, 0.7%) that triggered symptoms at my place of employment over multiple months (Sept. - Nov.) with symptoms occurring every day of exposure. Symptoms increased in severity over this time period. There was a slight reduction in symptoms over the course of each weekend, with relapse in severity upon returning to work each Monday. Some symptoms lasted for days, some for weeks, and some for months after I became able to significantly limit exposure to the triggering lights in late November 2018 (ongoing building construction reached the point that the lights could be controlled by wall switches rather than constantly being on). Symptoms had mostly abated by the late spring of 2019, with the exception of enhanced sensitivity to flickering LED lights and LED screen use compared to prior to symptom onset. A 45 minute exposure to triggering LED light flicker at the newly-remodeled office of a neuro-ophthalmologist in January 2019 triggered new symptoms with headache lasting 8 days. Since then, minor exposures to flickering LED lights and screens would mostly trigger symptoms lasting from hours to days, depending on the severity of exposure. "Minor" exposures include any time I had to use a computer or device screen for more than a few minutes and any time I looked at a Zoom video feed for a few seconds. A major exposure to flickering LED lights occurred in April 2021 when I spent 3 hours in a newly-remodeled building with triggering LED lights. This exposure has triggered ongoing symptoms, some of which have lasted months (Timeline of 2021 "LED" Symptoms). I am still, in Decmber 2021, not back to the baseline sensitivity I had prior to this exposure. I now experience at least new minor symptoms almost every day, as it is now impossible to avoid exposure to triggering LED flicker in most NYC environments. I don't have a way to use a computer without triggering symptoms, so I keep getting new symptoms every time I work on this survey or website. 

Location of headache:

• Common migraine without aura: Left side only: top of forehead.

• LED symptoms: Right side only: behind eye, in tissue near the eye toward the temple, and temple.

Quality of headache:

• Common migraine without aura: Pulsing in time with heart beat.

• LED symptoms: Feeling of constant pressure and dull, constant pain behind right eye, near eye, and in temple. It feels like the tissue inside my head is swollen and the brain tissue is pressing outward. There is not a pulsing quality and It does not feel like there is a band pressing around the head. 

In addition to the above headache pain/pressure that can last days to months after a significant LED exposure, there is also a sharper, more localized pain behind my right eye that is only present when I can actually see LED light or other flickering light. It stops immediately if the light is turned off. The intensity of this pain and the time until this pain starts once I am exposed to flickering light depends on my sensitivity level and on the intensity of the flicker, with time until pain onset ranging from immediate up to at most 20 minutes. I'm more sensitive and the pain starts faster and is more intense if I've recently been exposed to flickering light or have ongoing symptoms from a previous exposure. This pain was the most intense in the fall of 2018 - it kept getting more intense each work day while working under the flickering LED lights. When very intense, it had a very high-frequency vibrating quality on the order of hundreds to thousands of hertz. It felt like a dentist's drill was creating vibrating pain/pressure behind my eye. At that time, the sharp vibrating pain would start immediately when I entered the flickering LED light and would stop immediately if I left the flickering LED light. This pain was sharp like the feeling of lemon juice in a paper cut, and seemed somewhat distracting, but wasn't debilitating in the way that common migraine headache pain is debilitating for me. 

Headache pain intensity

• Common migraine without aura:  Severe - debilitating; can't do any normal activities.

• LED symptoms: Varies from mild to moderate, depending on severity of LED exposure. Can almost always push through the pain, disorientation, and nausea to attempt to do normal activities, but with significantly compromised short-term memory, concentration, spatial orientation, appetite, and sleep.

Is the headache aggravated by routine physical activity?

• Common migraine without aura: Yes. To the point that I need to sit or lie down and not move at all while I wait for ibuprofen to take effect or for the migraine to end on its own. Movement intensifies the headache and movement significantly intensifies the nausea and feeling of vertigo.

• LED symptoms: No. Moderate exercise has absolutely no effect on the headache or other symptoms. I walk multiple miles each day, even when experiencing LED symptoms.

Is there nausea or vomiting?

• Common migraine without aura: If untreated, or if treatment with ibuprofen doesn't happen quickly enough, there is always nausea, usually with a spinning vertigo sensation, that builds in intensity until vomiting. Nose runs and headache pain becomes very severe prior to vomiting. The nausea and the migraine headache tend to end immediately after vomiting, after which fatigue is intense and I fall asleep.

• LED symptoms: There is nausea, but it never leads to vomiting. Almost always, flickering LEDs trigger a mild feeling of spatial disorientation only when actually looking at the LEDs. Sometimes flickering LEDs or repetitive patterns trigger a more intense feeling of spatial disorientation and moderate nausea only when looking at the LED light or at the patterns. If an LED exposure has been significant enough to trigger symptoms lasting hours or longer, ongoing symptoms that are still present even after the end of the LED exposure often include nausea and spatial disorientation. The nausea can vary from mild to moderate and tends to include a loss of appetite. Considering a single day in isolation, this isn't very severe, but when this nausea and loss of appetite lasts multiple weeks at a time, it can have a significant health impact. Additionally, a sense of spatial disorientation tends to coincide with the nausea. This disorientation is not of a spinning vertigo variety. Rather, it is a vague feeling that the environment isn't stable around my body. I don't seem to have problems with balance in the course of normal activities when my eyes are open, but my instinct is that I don't feel aware enough of my orientation relative to the ground to safely stand on a step ladder and stretch with both hands to manipulate a ceiling light fixture. When I have this spatial disorientation symptom, if I try to stand still with my eyes closed, my upper body tends to sway and I might take a step (Romberg's test). I can slightly, but not completely, limit the amount of sway if I very deliberately focus on how my leg muscles feel as my body starts to shift position. I don't sway or need to take a step when I don't have LED symptoms.

Is there photophobia (sensitivity to light) or phonophobia (sensitivity to sound)?

• Common migraine without aura: Yes, there is photophobia. The normal brightness of any light source hurts both of my eyes a little and tends to significantly exacerbate my migraine headache. Every kind of light (sunlight, incandescent, fluorescent, flickering LEDs, and completely flicker-free LEDs) are a little painful in both eyes and strongly intensify the left side forehead headache when I have a migraine. I prefer to be in a room that is as dark as possible. Incandescent light on the dimmest possible setting still feels too bright. The intensification of the migraine headache happens while I see the light and the headache tends to immediately feel slightly better if I turn off the light. I do not experience light brightness photophobia when I do not have a migraine. I do not experience phonophobia.

• LED symptoms: In some ways I have photophobia and in some ways I do not. When I have LED symptoms, I prefer to be in the sunlight - it not only doesn't hurt, but might make my head feel slightly better. Completely flicker-free LEDs also do not bother me at all. However, I am extremely sensitive to any flickering light. Flickering LEDs are very painful. They cause sharp pain behind my right eye while I am in the light and they exacerbate the dull headache pain and pressure surrounding my right eye and in my right temple and the effects are long-lasting. The headache tends to build in intensity fairly slowly. The headache might not start for about half an hour after the flicker exposure begins and the headache tends to intensify further for hours to days after the flicker exposure ends. The headache intensity correlates with my cumulative flicker exposure, but unlike the sharp pain behind my right eye that begins immediately in flickering light and ends immediately when the light goes off, the headache pain builds fairly slowly and tapers much more slowly. This type of photophobia occurs virtually any time I am exposed to ambient LED light flicker or screen flicker. 

I have become more sensitive to flickering light since the 2018 symptom onset. Beginning with a 3-hour exposure to flickering LEDs in April 2021, I have become sensitive to the flicker of some incandescent lights when I already have LED symptoms (causing mild pain and/or nausea) and have become more sensitive to the flicker of fluorescent lights. Sunlight and completely flicker-free LEDs have never caused or exacerbated my LED symptoms. I do not experience phonophobia.

• I usually experience these two types of photophobia (from common migraine or from LEDs) at different times, but also sometimes experience them concurrently if a common migraine starts while I am already experiencing LED symptoms. Successful medicating of a common migraine with ibuprofen also stops the light brightness photophobia in both eyes, but has no effect on my LED symptoms or flicker-associated photophobia pain in my right eye.

Other secondary neurological effects?

• Common migraine with aura: None.

• LED symptoms: I have concentration problems and severe short-term memory impairment for days to weeks after exposure to flickering LEDs, depending on the severity of exposure. For example, I've walked to the bathroom to brush my teeth in the morning and forgotten for a few minutes why I went there. I can't do simple Sudoku puzzles (on paper) because I can't even briefly keep the positions of any of the numbers in my short-term memory (these puzzles are easy for me when I don't have LED symptoms). The clarity of my writing deteriorates because I can't remember the previous sentence or hold all of the ideas for a single sentence in my head at once. I can't proofread for similar reasons. I can't add a series of single digit numbers in my head (which as a teacher since 2006, I did on a routine basis when grading papers, never having trouble with the task before 2018 and rarely making errors). I have trouble remembering new aural information and have trouble constructing articulate sentences when having conversations where the topic is new and of any complexity. My long-term memory is unaffected. I can recall and teach previously-designed lessons for fairly complex subjects - chemistry and molecular biology, including remembering previous strategies for guiding students to an understanding of these subjects.  My concentration problems do not seem to be an issue taking in visual information, per se. Rather, I seem to have trouble holding new information in my short-term (working) memory.

I have significant alterations to my sleep patterns with hypersomnia in the first day or two following a serious flicker exposure and insomnia for weeks to months following that. For example, I'll tend to fall asleep very early (~6pm) in the first couple of days following a serious exposure. Then a few days later, I'll start to wake up in the middle of the night or early morning and be unable to fall back asleep for hours. This happens every night for weeks to months. I'm not feeling anxious and my mind isn't racing. I just can't fall back asleep. I'm not waking up due to pain - the pain is usually lessened when I wake up. The inability to get enough sleep at night, along with my inability to eat properly due to ongoing nausea, contributes to extra daytime fatigue. When I don't have LED symptoms, it's quite rare for me to have any trouble sleeping at night. 

In addition to the symptoms already described, another common symptom is a slightly swollen right upper eyelid. It is slightly difficult to open completely - the eyelid seems very slightly puffy and the space into which it needs to fold feels too tight. When the eyelid feels most swollen, wearing gas-permeable contacts tends to irritate it and sticky mucus collects in the eye if wearing the contacts (I wore contacts one such day then stopped because of the irritation to the inner surface of the eyelid). 

On about half a dozen occasions, I've experienced transient, but severe central vision blurriness in my right eye that lasts about a minute. It seems like there's a film partially obscuring my vision, but there isn't any mucus obscuring my vision - rubbing the eye doesn't change anything and the blurriness just goes away on its own within about a minute. 

The neuro-ophthalmologist who had flickering LEDs in the new office detected peripheral blindness in my right eye in a visual field test. This was before he dilated my eyes. When I returned the next week to repeat the test, but protected my eyes from the clinic light, I did not have any blindness. 

Out of the many months that I've experienced LED symptoms, there have been a few days that the symptoms have been most intense and that additional symptoms have occurred. These additional symptoms include my right temple becoming painful to the touch, the pain in the temple taking on a pounding quality, specific points on the right side of my scalp becoming painful to the touch, and an intensification of the feeling of tissue swelling around my right eye increasing to the point that it felt like there was pressure on the muscles controlling the movement of my right eye in its socket. In this case, these muscles hurt when reading a paper source and even when moving the eyes with my eyes closed. This is the only time I've experienced "eyestrain," and it only happened on about 2 days overall. 

Since the significant flickering LED exposure in April 2021, I've experienced more common migraine episodes than usual. These have tended to occur around the time I've had a moderate re-exposure to flicker that's restarted some LED symptoms. I can't say for sure, but it's possible that LED symptoms might increase the likelihood of having common migraine symptoms too. Taking ibuprofen still stops the common migraine symptoms, but has no effect on the LED symptoms. I know common migraine symptoms are beginning on top of my LED symptoms when I feel a little pain in my left eye due to light in addition to the right eye pain, when any light source (including sunlight or flicker-free LED light) causes both left and right side eye pain, and when the headache begins to include the left side of my forehead.

I suspect that flicker causes neuroinflammation around my eye, leading to the eyelid puffiness and feeling of pressure around my eye. I suspect that it was only when this inflammation was most severe that moving my eye became painful. I suspect that flicker causes neuroinflammation in the trigeminal ganglion, leading to the pain and feeling of pressure in the temple. I suspect that sensitization in the thalamus leads to the allodynia and other symptoms that might involve the cortex or other parts of the brain.

Impact:

• Common migraine without aura: Although completely debilitating when in progress, these migraines have little impact on my quality of life and have not yet impacted my work. They only impacted my school attendance at age 13. These migraines are fairly infrequent and usually easily controlled with ibuprofen.

• LED symptoms: Although the symptoms on a given day are not particularly debilitating, the duration of symptoms and their long-term effects in the aggregate have a significant impact on my quality of life and ability to do my job effectively (Timeline of 2021 "LED" Symptoms). No over-the-counter pain medication helps, so I don't take anything. At work, I cannot enter facilities that use flickering LED lights. I cannot use a computer for longer than a few minutes at a time, and even that is a risk. I cannot use some applications with inherent flicker, like Zoom. I cannot spend much time on websites because they too often include flicker. Because flickering LEDs have recently become increasingly ubiquitous in NYC, with businesses swapping out fluorescent lights for flickering LEDs, I no longer have a way to shop for groceries that doesn't involve exposure to LED flicker, either from the local stores, from a screen, or from building lights and lights on public transportation if I travel to a distant store.

Is there a connection between LED symptoms, flicker sensitivity, and "Long-Covid"-like postviral symptoms?

• In August 2022, December 2022, and November 2023, I had respiratory infections with fever, for all of which I tested negative for Covid-19. In each case, my scren sensitivity went away during the fever and came back after the fever. In each case, I experienced many days of neurological symptoms similar to what some have described during long Covid. The August 2022 infection was the most severe, with 8 days of 100-101F fever. It seemed like something very similar to Covid-19, although I tested negative with 2 rapid and 2 PCR tests. Although I was very ill, including a scary period of not being able to breathe if I laid down, my brain felt better (clearer) than it had in a long time (I was in the process of recovering from a period of LED injury a few weeks before). During the fever, I could surprisingly use my phone screen without symptoms. I noticed posts on the LEDStrain.org forum where others mentioned their screen sensitivity going away and their brain function being better when they had Covid. After the fever, I had extremely severe brain fog and a feeling that there was something very wrong (maybe chemically) in my brain. The brain fog and other strange sensations could be reduced if I raised my feet over my head or moved around a little - maybe low brain blood pressure? If I moved more, such as slowly taking a walk around the block, there were jabs of pain in the "flicker spot" in my right temple. It felt like someone was jabbing something the size of a carrot into my brain. I hadn't ever experienced this before. I was noticeably very sensitive to screens. I stayed off of screens completely and propped up my feet, gradually building in a bit more exercise until I got better many days later. This happened again for the next two illnesses with fever, although the fevers did not last as long and post-illness effects weren't as severe. 

• Although the fall of 2018 was the first time it was obvious to me that LED lights were seriously harming my health, in retrospect, it's very possible that bizarre health issues in the summer of 2016 might have been a first instance and perhaps connected to postviral effects. I had been chaperoning a school trip to China and, along with others, got a severe case of what seemed like food poisoning, which for me included fever. A few days into my recovery, I was still feeling slightly weak, but was fairly OK when we arrived at the Beijing airport which was newly-built with LED lights. While waiting in the airport, I started to feel what I assumed at the time was the beginning of a common migraine, as my head was beginning to hurt and I was becoming sensitive to light. What was atypical, was that Advil didn't help and I continued to be light/screen sensitive and my head didn't go back to normal during the flight, but I also didn't progress to the worse vomiting stage of a common migraine. I remember needing to keep the screen off for the entire flight back because it hurt too much. What was especially strange was that I didn't feel like I could really get out of bead much for 3-4 weeks after this. In retrospect, the way I felt was having brain fog, fatigue, nausea, and lack of appetite, and disorientation, perhaps with a posture-linked component similar to how I can now feel following an LED injury or in a post-viral  state. At the time, I rationalized my experience as an extreme combination of jet lag and after effects of food poisoning, but now I suspect that the airport lights may have caused LED injury. My being in a post-viral period might have made me more susceptible to LED injury.

• Many people with long-Covid report light sensitivity as well as brain fog.fatigue, sleep abnormalities. headache, dizziness/balance problems, and other symptoms overlapping with common "LED: symptoms (Davis et al.) and the development of screen sensitivity creating head pressure has anecdotally been reported on LED strain.org. Could light and screen flicker be a factor in long-Covid symptoms? At least in NYC, the prevalence of LED lights greatly increased during the pandemic. Before Covid, I could go almost anywhere because most commercial lights were still fluorescent. After Covid, I can go almost nowhere because flickering LEDs are almost everywhere and there are many more LED exterior lights that have high flicker and high brightness.It has become extremely difficult to avoid new LED injury even though I know what to avoid. 

Davis, H.E., McCorkell, L., Vogel, J.M. et al. Long COVID: major findings, mechanisms and recommendations. Nat Rev Microbiol 21, 133–146 (2023). https://doi.org/10.1038/s41579-022-00846-2

How to detect flicker with a slow-motion phone video

Smartphone slow motion 240 frames per second video can detect 120 Hz flicker in the Americas). Note that some phones may now require a 3rd-party app to record true 240 fps video. My iPhone stutters the recording, obscuring flicker in the built-in Camera app and the info for the video says ~180 fps instead of 240 fps even if the app is set to 240 fps. Instead, I use the Moment app to record 240 fps video, but play it back in the built-in Photos app. Extreme brightness flicker and extreme color-to-color flicker can be detected.

• My smart phone films slow-motion videos at 240 frames per second (fps). It captures a single frame of video by scanning from one long side of the field of view to the other long side. 

• When lights have 120 Hz flicker caused by the mains alternating current (AC) in the United States, the bright and dim portions of each frame of the video flip-flop in consecutive frames. It takes 2 frames to create a complete cycle and then the pattern starts over again. Each frame shows one half of the flicker cycle:

(1/2 cycle per frame) x (240 frames per second) = 120 cycles per second = 120 Hz

The image below shows 6 consecutive frames from a slow-motion smart phone video filmed at 240 fps of the LED fixture that was in the common hallway of my apartment building from 2016 until we replaced the fixtures with nearly flicker-free LEDs in the summer of 2022 to eliminate the harmful health impact of the flickering LEDs; the entire frame is shown for each image. This pattern is typical of 120 Hz flicker, with a full cycle taking 2 frames, although the contrast between bright and dim varies for different lights with 120 Hz flicker. More subtle flicker may be difficult to notice when consecutive frames are placed side-by-side. The flicker for the light below is obviously visible if the slow motion video is played at the typical playback speed of 30 fps. I find the flicker when such videos are played to be quite painful and triggering of symptoms, so I have chosen to show consecutive frames, rather than the actual video, since the difference in brightness in consecutive frames is visible enough in this format.

Similar images and videos from an NYC Public school, the NYC Subway system, and a grocery store are shown on the Public Health Risks page.

The image below shows 4 consecutive frames from a slow-motion smart phone video filmed at 240 fps of the LED lights at the Blick Art Materials store at 6th Ave and 20th in NYC, filmed on April 23, 2022. These lights appear to be white when observed normally, but actually flicker between slightly red and slightly green whitish light at 120 Hz. Less than a minute in this light while wearing a hat and shade 5 welding glasses triggered a headache behind my right eye (mostly pain without much pressure) and nausea with loss of appetite that lasted at least 6 hours, until I fell asleep. Note that data in the Powerpoints at the top of this page suggests that color-to-color flicker may be especially harmful for me, perhaps explaining the toxicity of these lights.

The flicker of the color-changing LED device below is even more complex. The flicker seems slower than 120 Hz. although it's hard for me to figure out how many frames constitute a cycle, partly because the device is continually changing colors, so the time that individual colors are displayed varies. This device was very triggering of my symptoms. The image shows 21 consecutive frames from a slow-motion smart phone video filmed at 240 fps; frames are slightly cropped around the device. Notice how the frames with 2 colors show how the smart phone records each slow-motion frame by scanning from one long side of the image to the other long side, with the color changing while the scan is in progress in some cases.

• When lights have rapid flicker, each image frame of the video has bright and dim stripes or bands. This happens because the smart phone pans from one side of the field of view to the other while capturing the image, and the multiple bright and dim phases that occur over that time will produce banding. I count the number of times the banding pattern in an image repeats, estimating if the number of times the pattern repeats is between two whole number. For example, if there are between 6 and 7 bright stripes per image:

(6.5 cycles per frame) x (240 frames per second) = 1560 cycles per second = 1560 Hz

The images below show the very subtle flicker from the LED strip lighting that triggered my months-long symptoms in 2018. This was the first time, at age 42, that lighting had caused severe headaches and other neurological symptoms for me. Each of the 2 panels shows 6 consecutive frames from a slow-motion smart phone video filmed at 240 fps. The entire image for the first frame is shown, then only the rightmost portion of each of the next 5 frames is shown to the right side of the preceding frame. The vertical lines between the 2 panels indicate the divisions between the frames. The top panel shows the original images. The left portion of the frame shows the LED strip through a gap in the covering diffuser panels. The right portion of the frame shows a diffuser panel that is covering additional LEDs in the strip. The bottom panel shows the same images as the top panel, except that the images have been enhanced together in Photoshop to maximize the visibility of the horizontal bands that are parallel to the long edges of the frames. In the original video, very subtle bands of increased brightness seem to move across the screen as the slow motion video is played back. This is an example of the flicker percent being about at the limit of what can be detected using a smart phone. I see between 6 and 7 bands across the frame, which would indicate that the frequency of the flicker is about 1500 Hz, as calculated above. However, the data I provide for these lights in the graph above and in the linked spreadsheet lists 1000 Hz because that was the highest frequency setting for these lights for which a lighting analyst provided statistics. There are other examples of LED strip lighting that cause my symptoms, but for which I haven't been able to detect the flicker with a smart phone, although measurements from lighting analysts show flicker.

Sometimes the banding may be much more obvious if the dim phase of the flicker is dimmer, such as in this image of a color-changing LED device. This following image shows one complete frame from a 240 fps smartphone video. This is the original image except that a black rectangle covers a portion of the image; the colors and contrast have not been enhanced. There are 5 cycles of banding visible over the device and probably a similar number of bands over the other parts of the frame. So if there are about 10 cycles of banding, (10 cycles/frame) x (240 frames/second) = about 2400 cycles/second = ~2400 Hz for the flicker frequency.

Note that if you are attempting to detect very fast flicker with an iPhone, the banding will be parallel to the longer side of the phone. I didn't realize this when I first started testing lights in 2018, but one should ideally orient the phone so that an LED strip light spans the short distance across the phone (from left to right, rather than from top to bottom) so that you have a chance of observing the banding pattern as a variation in brightness, either across the strip light or across a part of the ceiling that is approximately evenly illuminated. Orienting the phone in the other direction, which is the more intuitive way that maximizes the length of the light in the video, makes it nearly impossible to observe the banding because there isn't a region of approximately uniform brightness in which to look for brightness variation. Focusing the camera on the brightest part of the light can help to maximize the contrast in the images. The banding pattern is generally more visible for a dimmed strip light than for a strip light at full brightness; see Background: LED Lights for a discussion of dimming strategies for LED strip lights, which often introduce or enhance flicker.

Lessons from mistakes in flicker testing:

• Whether testing lights with a flicker meter or testing a person's sensitivity, ensure that the light being tested is the only artificial light source.

From my perspective as a scientist, this seems obvious. However, when working with a lighting consultant, I realized that this was not their standard practice. They tended to have all of the lights on and then they'd hold a handhold meter up near individual light sources. This probably is fine under most circumstances, but when trying to obtain data about lights that are flicker-free or have very low flicker, other lights with higher flicker in the vicinity can contaminate the tests of the lights with low flicker. 

• When using a flicker meter, place it on a stable surface and control it remotely - don't hold it in your hand. Slight variations in light intensity due to an unsteady hand changing the distance between the meter and the light definitely create variations in the recorded wave. With my meter, my pressing the "measure" button on the meter always jostles the meter enough to impact the measurement.  It's necessary to eliminate this kind of variation so any variation from the actual light source can be analyzed. Especially when measuring flicker from lights with no or low flicker, it's important that the meter is kept completely motionless when taking a reading by placing the meter on a tripod and using an external computer to trigger the measurement.. 

• Take flicker measurements for each kind of light individually, rather than general measurements for whole rooms. Only taking whole-room measurements might obscure the degree of flicker of particular lights. The experience above, attempting to test the hanging globes when some other peripheral lights in the vicinity were on, shows how even a small amount of light from a flickering source can induce symptoms for sensitive people. The example above also shows how a meter would not detect the degree of that flicker if other lights observed by the meter have very low flicker. Also, the measured flicker would be significantly reduced in any whole-room measurements taken in rooms receiving sunlight.

• Take flicker measurements in the absence of sunlight. Sunlight reduces flicker measurements.

• Take flicker measurements on site with the actual installed fixtures. While trying to find flicker-free commercial light solutions, we have learned that it's theoretically possible that additional lighting control systems being used now in commercial facilities might introduce flicker. We did not find this to be the case for the systems we're testing in spring 2022. However, when the lights giving me symptoms in the fall of 2018 were tested, they were only tested using a mock-up at the manufacturer and were not tested on site at my workplace. This led to questions in 2021-2022 of whether the fall 2018 lights had actually had the characteristics on site that they had during manufacturer mock-up testing - leading to the need for me to try lights with similar flicker characteristics again in order to assess whether I am in fact sensitive to lights with as little as 0.7%, ~1000 Hz flicker. Luckily, we had access to such lights and didn't need to purchase additional equipment, but this could have been a situation incurring unnecessary expense. Yes, when tested in March 2022, lights with these characteristics were immediately painful. The initially minor pain behind my right eye intensified over the course of about 5 minutes, at which time the test was aborted for safety reasons. This occurred even though there was a reasonable amount of sunlight in the room. The lights felt just like the lights from the fall of 2018, including how the pain behind my right eye would spike when I walked directly under the lights. Lighting consultants measuring flicker should measure the actual lights on site, not only to learn more about the characteristics of lights that do or do not cause symptoms in sensitive individuals, but also to protect sensitive individuals from needing to unnecessarily repeat exposures to harmful lighting. Lights should also be measured on site because it's possible that fluctuations in the power supply or electrical interference from other equipment might introduce additional flicker.

• Record and share the graphs of flicker meter readings as well as all software-calculated flicker statistics. The shape of the flicker waveform probably matters in determining the symptoms experienced by sensitive individuals. Data needs to be collected to determine what kinds of flicker profiles affect sensitive individuals. Graphs reveal whether there are patterns in the flicker at different frequencies than the one frequency calculated by the software. The graphs are also important in assessing whether the meter might have been unstable during the readings. Also, sharing all available flicker statistics is necessary to be able to make comparisons between lights.