Sunlight
Select very low flicker LED bulbs - there is something protective in their flicker waveforms as other lights with lower % flicker cause greater health effects.
Philips 2013 A19 soft white dimmable AmbientLED bulb (11 W; 60 W equivalent, model 9290002268). Note the graph of light output vs. time by LEDbenchmark in this link showing the completely flicker-free light output (flat-line); LEDbenchmark indicates the flicker is <1% at 17,360 Hz with a flicker index of 0.002. Purchased in 2013. As of January 2024, these are still the best LED bulbs that I have ever used. They have never caused the any symptoms for me and 10 years after installation still have negligible flicker. When I measured these lights, I detected 1.1% flicker and ≥50,000 peaks/sec in a complex 120 Hz pattern (see data in powepoint above), When in my dimmable ceiling fixture, the pattern is semi-random and there is additional randomness on a slow scale, perhaps due to a very slightly unstable connection. There may be something specific about the electrical supply in this fixture that makes these bulbs tolerable for me.
Waveform Centric Home 2700K A19 LED bulb (10 W, 60 W equivalent, model 4005C.27) when relatively new.. ~0.8% flicker in a complex pattern repeating 120 times a second. Purchased in 2021.I haven't used this version long enough to assess the failure mechanism, but see notes for the 3000K version..
Waveform Centric Home 3000K A19 LED bulb (10 W, 60 W equivalent, model 4005C.30) when relatively new. Purchased in 2021. ~0.9% flicker in a complex pattern repeating 120 times a second. These bulbs increasingly flicker as they age, developing up to 100% flicker. See data in the Powerpoints linked above.
LEDs powered by constant direct current. Even battery power does not necessarily ensure that the current to the LEDs is constant. Be very careful that the LEDs are powered on a very simple battery circuit with no pulsing. For example some battery-powered LED lamps I've tried have pulsed the current and triggered what felt like electrical shocks, perhaps akin to seizure, in my brain. The flicker had a complex, rapid pattern when measured with a meter that couldn't be detected with a phone video - the lamps were engineered to flicker even though they were battery-powered. Do not trust manufacturer marketing that says "flicker-free." Testing with a professional flicker meter is recommended if the circuitry isn't obvious. I use simple battery-powered LED string lights and battery-powered DC LEDs in Mason jar lanterns that use AA or AAA bateries and have no dimming or flicker function. See flicker data in the powerpoint linked above.
A very bright LED blue light transilluminator used for imaging DNA in agarose gels from 2011-2019, and similar blue-light transilluminators from 2019-the present. There are 3 different brands of this kind of device and all have AC/DC power adapters and do not have significant flicker. They have never triggered or exacerbated my symptoms and I've even tried looking at them for a long period of time without effect. See flicker data in the powerpoint linked above.
Prior to the late spring of 2021, incandescent lights had not bothered me. Now many do, causing a little pain or nausea, and after long exposures, migraine headaches. As I slowly recovered from a severe April 2021 flicker exposure, I gradually became more tolerant of incandescent lights again. In November 2021, during a full day spent at a home with incandescent lighting, I could tolerate the incandescent light for a few hours without pain, but after that time felt pain behind my right eye when in the light, developed a minor headache and become unusually tired by the early evening. In contrast, in June, these same incandescent lights were painful after a few minutes and caused or exacerbated headaches. In 2023, Incandescent lights with flicker less than about 5%, using as few bulbs as possible, are the best AC lights for me. Typical household incandescents have higher flicker than this (~8-12% flicker) and can cause migraines for me now that I've become extremely flicker-sensitive, especially when there are many bulbs in use. I tested a variety of incandescent bulbs with a flicker meter to find ones with lower flicker. Note that flicker can vary widely from one bulb type to another for a brand. I am currently using these incandescent Edison bulbs at home:
Globe Electric Edison Incandescent 60 W A19 E26 2200K 245 lumen (model 01325): 3.66% sinusoidal flicker
Hudson Bulb Co. Edison Incandescent 60 W ST58 E26 Spiral filament 2100K 230 lumen (X002K5ZZGN): 4.32% sinusoidal flicker
SCOMX lingruiyi Edison incandescent CA flame tip 40 W E12 280 lumen (lingruiyi15 X00314456D): 4.6% sinusoidal flicker in a nondimmable lamp and in a dimmable fixture from 5.6% flicker at full brightness to 9% flicker when fully dimmed
A few brands of incandescent halogen MR16 50W GU5.3 12 V spotlights : ~3.5% sinusoidal flicker
Flicker frequency of 120 Hz to at least ~1500 Hz (very rough estimate from video) has been a trigger. The flicker percent of LED lights that trigger symptoms has ranged from 100% down at least 0.7%. I have experienced LED symptoms from every LED light from which I can detect any flicker on a slow-motion smart phone video. I have also experienced LED symptoms from LED lights that manufacturers have confirmed have some flicker, but for which I can't definitively detect flicker using a slow-motion smart phone video. The following are LED lights that have caused symptoms for me for which I have statistics:
2013 Cree A19 dimmable LED, (model BA19-08027OMF-12DE26-1U100), 120 Hz flicker, probably between about 20% and 40% flicker based on similar models reviewed on LED benchmark. I had this bulb in a lamp behind my couch and tended to have this light on only when watching TV on a CCFL screen. Prior to the fall of 2018, I felt very vague disorientation in this light, but tried to ignore the feeling, thinking that I just must not like the difference in color temperature compared to incandescent bulbs. Once my LED symptoms started in the fall of 2018, this bulb was noticeably painful and exacerbated my LED symptoms. I replaced it with an incandescent bulb. in 2021, I replaced the incandescent with a completely flicker-free LED.
Waveform "flicker-free" SUPERWARM 1700K A19 LED bulb (10 W, 50 W equivalent, model 4005.17. Flicker stats provided by manufacturer. Purchased in 2021. I initially thought that these bulbs were safe, but I only used them for a few minutes at a time. Later, when using them in a reading light, I developed "LED" symptoms. Later meter measurements revealed that they flicker much more than the 2700K and 3000K varieties of Waveform Cenric Home LED bulbs. (Meter measurements will be posted in upcoming months - now April 2023).
Apartment #1 common hallway LED fixtures installed in 2016. These have 100% flicker at 120 Hz. I'm exposed to these lights for less than a minute each time I enter or leave my building. Prior to 2018, I had a vague sense of not particularly liking the lights. Once my LED symptoms started in the fall of 2018, these lights became increasingly painful and seemed to exacerbate my symptoms. By the summer of 2019, these lights weren't painful anymore for the seconds I was in the hallway. They became quite painful again and exacerbated my LED symptoms beginning with my new severe round of LED symptoms beginning in April 2021. They are still painful now in November 2021.
Apartment #2 common hallway LED fixtures installed in 2021. These have 100% flicker at 120 Hz and usually have additional PWM dimming but switch to full brightness (also with 100% flicker) on a motion sensor. These hallways are particularly painful to traverse - much more painful and exacerbating of LED symptoms than the lights in apartment #1.
An LED projector with 120 Hz flicker illuminating a screen about 20 meters away that was the main light source in an auditorium during a conference in June 2021 while I was still experiencing sleep problems and rapid LED symptom restart in response to flicker following the serious exposure to 3 hours of LED flicker on April 1, 2021. I felt vague spatial disorientation immediately upon entering the auditorium and developed sharp pain behind my right eye after less than 5 minutes. At that point, I checked the auditorium lights with a slow-motion smartphone video and saw that the projected light on the screen alternated between bright and quite dark at 120 Hz. I put on a hat and shade 5 welding glasses that sat closely on my face, but had gaps in a foam liner and didn't make a complete seal. I tried keeping my eyes shut, just listening to the talks, but this was not sufficient to stop the sharp pain behind my eye. Headache pain and pressure started to develop. I then covering the inside of one of the lenses with foil from a food wrapper and with paper that I had on hand. This blocked, most of the light on one eye, but some light still leaked around the edges of the lens. I still felt the sharp pain and the headache pain and pressure worsened. As usual, the sharp pain behind my right eye would stop immediately when I left the auditorium and entered an area illuminated only by sunlight. I skipped half of the morning talks and half of the afternoon talks to sit in the sunlight and give my head a rest. This incident caused headache, head pressure, nausea, and disorientation lasting several days and exacerbated ongoing sleep problems. This incident indicates that I will develop symptoms from flicker even when I'm not focusing my eyes on anything.
"White" LED bulbs that have 120 Hz flicker where the flicker is between slightly red and slightly green light (see photos below from a Blick Art Materials store). 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.
Various LED bulbs tested by me in 2021 in an attempt to find LED bulbs that would not cause symptoms for me - all of these caused symptoms for me that started in seconds to minutes and all have 120 Hz flicker that was easily detected on a slow-motion smart phone video:
Philips dimmable PAR 20 advertised on Amazon as “flicker-free” (model 9290013169); fine print on packaging indicates that they are only free of "visible" flicker.
Sunco dimmable PAR20 advertised on Amazon as “flicker-free” (model L9-PAR20DWP-7W); I called Sunco and a sales rep acknowledged that they are only free of "visible" flicker.
Philips dimmable A19 LED advertised on Amazon as “flicker-free” (model 9290019409B); fine print on packaging indicates that they are only free of "visible" flicker.
GE soft white 60W-equivalent dimmable A19 LED (product code 67615)
Target soft white 60W-equivalet non-dimmable A19 LED (model A800830).
After reporting Philips PAR20 9290013169 bulbs to the Consumer Product Safety Commission for their causing of my LED symptoms in 2021 (they had been recently added to someone else's apartment), Philips kindly sent us replacement bulbs to try, but they also caused symptoms for me - symptom onset was just slightly slower than for the original bulbs.
The replacement "eye comfort" bulbs that still cause symptoms (disorientation initially, interestingly without sharp pain behind the eye initially, followed by dull headache near eye and temple that lasted 72 hours after 30 min of exposure) are model 9290019867 and Philips shared that they have the following flicker stats: 13.813% flicker, flicker index = 0.040, Pst = 0.294, SVM = 0.453. I could also tell that they flicker at 120 Hz using a smart phone slow motion video. Instead of using these bulbs, we reconfigured all of the ceiling fixtures to have open cans that wouldn't overheat with incandescents and we re-installed incandescents - even though their flicker is slightly irritating, it's vastly better than the flicker of LEDs. We would prefer completely flicker-free LEDs both for health and energy efficiency reasons, but cannot find a current manufacturer of this PAR20 style bulb, although Philips used to make them in 2013 - see LEDbenchmark.
This is what I know about the flicker stats of industrial-style LED strip lights that have caused my longest-lasting LED (and see the following graph and document with spreadsheet):
LED strip lights: ~1000 Hz flicker based on my slow-motion smart phone videos and manufacturer data. Flicker percent from 0.7% at full brightness (typical usage) to 30% when dimmed (tried once) based on manufacturer data.
LED strip lights: ~800 Hz based on my slow-motion smart phone video. Flicker percent from 0.7% to 8.13% at full brightness based on lighting consultant measurements.
LED strip lights: ~1500 Hz flicker with a probable square waveform based on sharp delineation between dim areas and brighter lines in my slow-motion smart phone video. Flicker percent might have been roughly around 20% based on video. Caused immediate pain when entering a room with these lights dimmed and caused other longer-lasting symptoms.
LED strip lights: Waveform 4000K Centric Daylight strip lights being controlled by a Waveform CENTRIC SERIES™ Flicker-Free Non-Dimmable Power Supply for LED Strip. These seem to have 1.27% flicker at 120Hz (see measurements below). Since I've been regularly using flicker-free light bulbs from this same company since May 2021 without symptoms (Waveform Flicker-Free A19 bulbs), it was immensely disappointing to learn that this power supply advertised as "flicker-free" causes symptoms for me and is not actually flicker-free. In the presence of the strip light controlled by the Waveform Centric Series Non-Dimmable Power Supply, I immediately felt a vague disorientation, but I didn't have the sharp pain behind my eye that I often get when in the presence of flickering LED light. On the first day of testing on 3/23/22, I was already feeling strong headache pain and pressure from exposure to more severely-flickering LEDs in testing of other lights just before (see "Lessons from mistakes in flicker testing" section below). After about 45 minutes in this room with only light from the strip attached to the Centric Series Non-Dimmable Power Supply, the pressure in my head began to go down and I stayed in this light for about another 45 minutes without noticing the existing headache worsening. So even though the light made me feel slightly disoriented, I decided to try it again the next day. On the second day of testing for 1.5 hours, facing away from the bright strip, I immediately felt vaguely disoriented in the light, had occasional twinges of sharp pain behind my right eye, and occasionally felt more noticeably disoriented - at one point I had to grab the chair to maintain my balance when standing up. I also felt too nauseous to finish the second half of a sandwich for lunch. However, since all of those symptoms didn't seem definitively conclusive, I came back the next day. On the third day, I stayed in this room, facing away from the strip, for 5 hours and 50 minutes (11am-4:50pm). The light again immediately made me feel vaguely disoriented. I occasionally felt a slight twinge of pain behind my right eye. After 3 hours, I was feeling very tired and was not able to concentrate on the planning work I was doing in a paper notebook. I walked around the room to try to become more alert. I decided to try to do a Sudoku puzzle (in a paper book), both as a break from work and because I use these puzzles to gauge whether my short-term memory is compromised. I worked on a new puzzle (so this meant inserting easy starting numbers) for about 3 minutes, during which time I felt it was a little hard to focus. My speed felt reasonable, but I made a careless mistake of repeating a number and not noticing the error immediately (unusual when I’m more normal). After those few minutes I started to notice a pain in my right temple, so I put the book away and didn’t look at anything else requiring reading or thinking - just in case one of those activities had been a trigger. Over the next hour and 20 minutes, the pain in my temple intensified and I also started to feel noticeably nauseous and have increasingly intense spatial disorientation. Interestingly, I didn’t have any pressure in my temple, which is one of my common symptoms from screen use or from very intense flicker exposure. I tried shutting off the lights for a few seconds and closing my eyes. My balance with my eyes closed was slightly abnormal the first time I checked just after 3:30, but became noticeably compromised after another hour. Having difficulty maintaining my balance with my eyes closed is usual for me if I am spatially disoriented. By the time I left, the head pain was moderately intense and the spatial disorientation and nausea were moderately severe. I’ve experienced worse of both in the past, but this was fairly bad. I felt slightly better immediately upon getting into the sunlight at 4:50. That evening, my headache lasted until ~7pm and I was unusually tired in the evening and fell asleep hours early. Nausea, spatial disorientation, and lack of appetite continued the next day (Saturday) and until mid-afternoon on Sunday. Although these lights seemed to be fairly clearly symptom-inducing by this point, I decided to try them one more time to confirm that the effects were reproducible. On Monday, the 4th day of testing, I stayed in the room for 3 hours (9:45-12:45), facing away from the strip. The lights felt uncomfortable (vaguely disorienting) right away and I started feeling definitely motion-sick within 20-30 minutes. By 11:30 I had a headache. I hadn’t been reading -just talking on the phone and also starting to sort materials and mapping how to cut them - basically an arts and crafts project to create a new kit for teaching. By about 11:45, I was having trouble concentrating enough to even keep working on this - a task not requiring hardly any thinking. I turned off the lights briefly and had a very hard time maintaining my balance - I swayed backwards into the wall. To test myself further, I tried Sudoku again and only managed to place 2 numbers in about a half hour - super slow. Usually an entire puzzle in this book takes me about 20 minutes. I just couldn’t concentrate. I noticed that my headache worsened when working on the Sudoky puzzle. The headache would lessen slightly if I stopped looking at the puzzle. Upon leaving, I had a headache, nausea, spatial disorientation, and a lack of appetite that lasted 2 more days, through Wednesday. Tuesday evening I felt like a left-side migraine might be starting (slight pain in left forehead and pain behind both eyes from light brightness) so took ibuprofen and those migraine symptoms stopped, but the previously-existing flicker-induced symptoms did not stop. I had increased sensitivity to flicker and had other symptoms from minor flicker triggers the rest of the week. Saturday morning, I woke up early with a partially developed migraine headache centered in my right temple - the first time for this location for a migraine in over 30 years of occasionally having migraines on the left side (This 4/2/22 migraine was a throbbing headache worsened by movement or light brightness with intense nausea - the overall intensity wasn't yet at full force for a migraine and I took ibuprofen which lessened the headache pain, stopped the other migraine symptoms, and probably prevented symptoms increasing to the point of vomiting). Although it was in a new location - at the site of my flicker symptoms - this migraine fits a pattern I've noticed recently of migraines happening within ~5 days of a significant flicker exposure. Residual headache and fatigue following the migraine lasted into Sunday morning.
Conclusion: This lighting setup causes symptoms that are too severe to be compatible with working in the light. The loss of short-term memory and severely compromised ability to concentrate make me unable to do my job.
Data for Waveform 4000K Centric Daylight strip lights being controlled by a Waveform CENTRIC SERIES "Flicker-Free" Nondimmable power supply
Flicker frequency: 120 Hz
% flicker (for magenta region highlighted in graph): 1.27%
% flicker (overall, determined by meter software): 2.325%; Note: I was told verbally that in a repetition of this test, the lighting consultant obtained a reading of 1.3% flicker with the meter software.
Calculation of % flicker:
Distance from min to max point measured in Photoshop: 689.60 px
Height of indicated portion of wave (magenta line) measured in Photoshop: 375 px
If the minimum of the magenta line was set at 0.2260, the maximum of the magenta line would be at 0.2318. [(0.2367-0.2260) *(375 px/689.60 px)+0.2260 = 0.2318]
% flicker = 100% * (max-min)/(max+min) = 100% * (0.2318-0.2260)/(0.2318+0.2260) = 1.27%
It is unclear to me what creates the broad peaks and valleys in the graph of luminosity vs. time. I asked this question of the lighting consultant over the phone the day the testing was being done. They didn't have a definitive answer, other than to say that the readings were made with a handheld meter that wasn't necessarily kept completely steady. Theoretically, the amount of light observed by the meter would depend on the exact distance of the meter's sensor from the light source. I was told verbally that after our conversation, they repeated the test, obtaining a meter reading of 1.3% for the flicker of these lights, but I was also told that the same broad fluctuations happened even when the meter was held steady against the wall (I'm still confused as to how both statements can be true). I wonder to what extent there really is sub-120Hz variation in light intensity and I wonder to what extent it might contribute to my symptoms.
This strip light was installed on a wall and was the only light source in the room during testing with the meter and during testing on me. The flicker was not visible on a smart phone slow motion video.
5. LED T8 tube lights: Waveform 4000K CENTRIC DAYLIGHT™ Full Spectrum Flicker-Free T8 LED Tube Light. Two of these T8 tube lights were installed in a new fixture from PLT Lighting in which the tubes were directly connected to the line power, as directed by Waveform to achieve flicker-free light. There was no ballast attached. These have 2.679% flicker. There are 120 peaks and valleys in the wave per second, suggesting the flicker frequency is 120Hz as the computer software determined. However, there are also 60 sharp valleys per second, indicating that this pattern in the flicker has a frequency of 60 Hz (see measurements below). Since I've been regularly using flicker-free light bulbs from this same company since May 2021 without symptoms (Waveform Flicker-Free A19 bulbs), it was immensely disappointing to learn that these LED tube lights advertised as "flicker-free" causes symptoms for me and are not actually flicker-free. These were the last lights I tested on 3/23/22, after I already had a headache with pressure in my right temple that followed testing of a first kind of light and after I had already tested the second kind of light, the strip lights controlled by the Waveform Centric Series Non-dimmable Power Supply, for about 1.5 hours. When the Waveform T8 LED Tube Lights were turned on, I immediately felt a new slight twinge of pain behind my right eye. This pain intensified over about the next 10 minutes. At this point the pain was moderately intense and I decided to abort the test and to turn off the Waveform T8 LED Tube Lights for safety reasons. The sharp pain behind my right eye stopped as soon as the Waveform T8 lights were turned off. The sharp pain behind my eye did not start again when the lights with the Waveform Centric Series Non-dimmable Power Supply were turned back on. The sharp pain behind my right eye is a symptom I experience only in the presence of flickering LED lights. The intensity of this pain tends to be correlated with the severity of the flicker, the length of time I have been in that flickering light, and it tends to start more quickly if I have been recently sensitized by other flicker. Given how obviously painful the Waveform T8 Tube Lights were after only a few minutes, I decided it was not worth the risk to my health to keep them on longer to try to learn what kinds of long-term symptoms they might cause for me.
Data for Waveform 4000K CENTRIC DAYLIGHT™ Full Spectrum Flicker-Free T8 LED Tube Lights
Flicker frequency: 60 Hz
% flicker: 2.679%
The overall luminosity oscillation is at 120 Hz, as indicated by the flicker meter software, but there are extra-low points occurring with 60 Hz regularity. I count 30 of these extra-low valleys in the 0.5 seconds of time displayed in the graph of luminosity vs. time.
[No additional calculations were done for a subset of this graph since the original readings seemed relatively steady].
Two T8 tube lights were installed in a new overhead fixture, connected directly to the mains power supply. They were the only light source in the room during testing with the meter and during testing on me. Their flicker was visible to a small degree on a smart phone slow motion video.
6. LED T8 tube light replacements in old fluorescent T8 fixtures; 120 Hz and an estimated 20-100% (probably closer to 100% flicker) based on a smart phone slow-motion video. A few minutes of exposure caused symptoms lasting weeks. Details of my symptoms and images are provided below in the section "How I test for flicker." These lights were installed in the hallways of a New York City Public School building on 2/16/22, making me quite concerned for the health and ability to learn for the potentially sensitive fraction of the million NYC Public School children if these lights are being installed throughout the system.
7. Recessed "2x2" LED lights with a striped pattern; 120 Hz flicker and additional ~2000 Hz flicker on top of that based on my slow-motion smart phone videos. Flicker percent from 4.3% at full brightness to 9.8% when dimmed based on lighting consultant measurements. I suspect that part of the reason that I found these lights to be extremely painful might have been because, in addition to the temporal flicker, they had a spatially repetitive pattern of lines of light (see photos below) that was not obscured by any diffuser. The pattern was very obvious when looking at the light - the brightness and contrast of in the bottom photo (vertical recessed fixture section of the top photo) were adjusted to approximate how the light looked to the naked eye.
2. Clusters or 2-dimensional arrays of flickering LEDs seem to be worse than single point sources when considering normal light bulbs.
3. Industrial-style LED strip lighting or 2-dimensional LED arrays seem to much more quickly trigger initial pain and subsequent symptoms than single household bulbs, even when the flicker of the industrial LED strips is much faster than 120 Hz. I don't know why these lights are so triggering for me. Perhaps it has something to do with the percent of the retina exposed to the flicker or it’s something about the spatial arrangement of the LEDs. Perhaps it’s related to the brightness of the LEDs or some aspect of the exact frequency or shape of the waveform. Perhaps it has something to do with there being multiple LEDs in a line - even though they are covered by diffuser panels, the sharp pain behind my right eye tends to spike intensely if I walk directly under one of these fixtures, following the path of the fixture. Perhaps the lights have been white-tunable and thus have included color flicker. I only know that this kind of light has been linked to triggering all of my worst episodes of symptoms, including my initial months-long 2018 symptoms (only LED strip lights in the workplace), the episode at the neuro-ophthalmologist's office in January 2019 that triggered peripheral blindness and symptoms lasting 8 days (45 minutes of only LED strip lights), and my months-long April 2021 symptoms (3 hours of mostly LED strip lights along with some LED bulbs and 2x2 LED lights).
4. Dimmed LED lights that use PWM are very likely to quickly trigger symptoms, either beginning as pain behind the right eye or beginning as intense nausea and spatial disorientation. When nausea and disorientation are the first symptoms, I suspect that the LED flicker may be interfering with my normal perception of the environment in a way that makes me feel motion-sick.
Click here for a pdf containing the above graph and a spreadsheet containing the data and sources of statistics for flickering LEDs that cause my neurological symptoms (plotted above) as well as data and sources of statistics for completely flicker-free LED lights that do not cause any neurological symptoms for me. See Background: LED Lights for a discussion of the IEEE recommended limits on lighting flicker. See Background: Health Effects of ≥ Flicker 100 Hz for a discussion of the referenced flicker visibility studies.
After having experienced LED flicker sensitivity since 2018, I have now (3 years later) also become sensitive to the flicker of many incandescent lights. Symptoms begin as either mild stinging pain behind my eye or as very mild nausea/disorientation, both of which are less intense forms of my initial LED symptoms. 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. I haven't been in a situation yet where the symptoms I experience from incandescent lights become more severe than having nausea and/or a headache, although I have also largely avoided incandescent lights since this started and have since installed completely flicker-free LED bulbs in my home and workplace.
I am also more sensitive to fluorescent light flicker than I used to be. Although rarely near them, I hadn't noticed any sensitivity to household CFL bulbs prior to April 2021, although they now trigger symptoms for me with an intensity that is a little greater than that of incandescent bulbs.
In retrospect, frequent mild headaches at the end of work days prior to 2018 might have been at least partly due to overhead fluorescent light flicker. After replacing new workplace flickering LED light with incandescent lamps in 2018, limiting screen time and having an incandescent desk lamp on when using the screen, and after having recovered from the prior LED flicker exposure by the spring of 2019, I realized that I wasn't ever getting mild headaches at the end of the day anymore, despite everything besides the lighting being largely the same as before 2018. My lack of awareness at the time that fluorescent lighting might have been a cause of headaches for me is consistent with the observation that none of the study participants thought that lighting caused their headaches in the Wilkins et al. (1989) study linking fluorescent light flicker to causing headaches and eyestrain.
My workplace (from July 2019 until February 2022) had overhead fluorescent tube lights that I could tolerate for a few hours if I was looking toward the east-facing windows that allow bright morning sunlight into the classroom while teaching in the morning. These lights would start to give me a headache after the sun shifted in the afternoon or on cloudy days. I tended to keep them off when I wasn't teaching and used completely flicker-free LED bulbs in desk lamps. This experience is consistent with Wilkins et al. (1989) who found that headaches were less common in office workers using fluorescent lights if they worked in a part of the building receiving more sunlight, although in this study, there wasn't enough information to know whether this was because of the sunlight in combination with the fluorescent lights or because fluorescent lights might have been kept off more often in brighter offices.
Some screens are worse than others.
My work laptop is a MacBook Air 13”, early 2015, running Mac OS 10.14.6 Mohave. When I’m LED symptom-free, I can use it for at least a few hours without pain if I avoid software with flicker. When I’m hypersensitized by having LED symptoms, it’s fairly immediately painful and aggravates the LED symptoms.
My work desktop computer from July 2021 to February 2022 was a PC running Windows 7 and had a CCFL monitor. I could tolerate this combination of operating system and screen better than any other, although I still needed to shine a flicker-free light in my eyes and limit my usage time to prevent symptoms. I had more tolerance when using the old desktop versions of software on this computer than when using web-based software or when using websites. I suspect that websites and web-based software may be introducing more graphic/color-based flicker than the old Windows 7 compatible software.
My phone is a 2016 iPhone SE. When I’m LED symptom-free, I can use it for at least a few hours without pain. When I’m hypersensitized by LED symptoms, I can sometimes use it for a few minutes without issue, but other times it’s fairly immediately painful and aggravates the LED symptoms. Once the April 1, 2021 exposure to LED flicker started severe symptoms, even seconds using the phone screen would cause pain and exacerbate or restart other LED symptoms. I'm very slowly becoming less sensitized, and now, in December 2021, I can use the phone screen for a few minutes to several minutes, depending on the application and how much scrolling is involved. Before getting the first LED symptoms in 2018, I would often read books on my phone without any issue. I now only read on paper or on an eInk Kobo tablet.
An LED television causes symptoms regardless of the program. A CCFL television usually does not cause symptoms, however some videos have inherent flicker that does trigger symptoms for me.
Some software is worse than others. For example, Apple preview and Zoom quickly trigger pain and symptom onset or me. I can use other software for fairly long periods of time. Certain software immediately triggers nausea/spatial disorientation, including following a late April 2021 update to the web version of Microsoft Outlook used for my work email. Something “invisible” about the updated graphics immediately made me intensely nauseous with enhanced spatial disorientation each time I tried to look at it. I switched to only using the old desktop application which doesn't have this quality.
Scrolling is almost always triggering of symptoms, especially when graphics surrounding the scroll window flash as the scrolling happens, such as in the Google Sites editor used to build this website.
Eyes tracking across characters when reading creates flicker on the retina. I avoid reading on LED screens when possible because it’s a little triggering. Reading a paper source or on a static eInk screen (Kobo eReader) is not triggering. Reading on my Dasung eInk Not eReader that I use as an external computer monitor is better than on the LED computer screen, but also can be triggering due to its rendering of the computer graphics feed (that can have inherent flicker) and because scrolling is usually involved. There may also be a problem with the 45 Hz refresh rate of the Not eReader creating flicker.
Electronic interference creating obviously visible flicker on my work desktop computer monitor, if also connecting to a TV using a split VGA cable or if printing, causes symptoms.
As a child beginning at about age 5, I noticed that I would get a mild headache on the few occasions that I watched multiple hours of television on a CRT set in one sitting. I never told an adult because I had received the messaging that "television is bad for you" and I didn't want to confirm this for the adults. I noticed that watching one movie was OK, but two in a row would give me a headache. In retrospect, I think this may have been an early manifestation of my flicker sensitivity, since CRT screens flicker as the image is drawn repeatedly line-by-line across the screen (reviewed in Wilkins, 1995. Warning: This book contains multiple patterns that may bother sensitive individuals. The book warns those with epilepsy and migraine not to look at the frontispiece. Patterns in the book triggered my "LED" symptoms when I ignored the warning).
Visible flicker in the Zoom video feed
Even a few seconds of Zoom may trigger LED symptoms, although if my exposure has been minimal, the symptoms generally last only about 6-24 hours. If I keep having repeated Zoom exposures in rapid succession, the symptoms may extend for multiple days. The longest was about 10 days. I have since figured out some strategies for minimizing the effects of Zoom, including keeping the monitor off during staff meetings. Also, when teaching over Zoom during the pandemic, I kept the student videos as small as possible, covered students with the worst flicker in their videos with another window, keep all of the room lights on, and shined a bright non-flickering LED light directly into my eyes from about 18 inches away to minimize my sensing of the screen flicker. These strategies helped me to either avoid LED symptoms if I only used Zoom for perhaps a 45-minute class where I was mostly screen-sharing, so not looking much at the students. If I had to interact a lot with the student videos and/or if I have to teach multiple consecutive classes, I’ll still get LED symptoms, but it was mild compared to those that I had before adopting the above strategies.
Low-quality web camera video feeds that have a very grainy look where the grains are visibly flashing. This happened in some videos of interviewees on news programs, particularly early in the pandemic when people were scrambling to figure out how to make things work with the equipment they had on hand. In multiple instances, I felt immediate pain and had to look away from the screen.
Movies with rapidly flashing patterned graphics (such as 2018 Spiderman: Into the Spiderverse), even if viewed on a non-LED (CCFL LCD) TV.
Movies with repetitively flashing lights (such as the flashing hyperdrive lights outside of the ship window in Star Wars Episode VIII or the flashing lights of the prison break episode in season 1 of The Mandalorian).
Animated cartoons or live-action videos that have relatively few unique images in the video, but that insert intermediate frames that are a blend of the previous and next images, probably to reduce visible choppiness to the video. I have used a slow motion phone camera to analyze such videos after they immediately caused the pain behind my eye and sense of disorientation that I associate with the onset of LED symptoms. The flicker is present whether these videos are streamed via a streaming device to a television or watched on a computer using a website. Other animated cartoons and videos delivered to the same television using the same streaming device do not repeatedly flicker and do not trigger symptoms for me.
Something else in some newer videos streamed to my television, but not most videos. I'm still trying to figure out what the issue for me is, but suspect that there might be flickering of colors or some other less obvious form of flicker.
This screensaver for a BenQ projector shows a cityscape with constantly changing colors. This was immediately intensely painful to view. The image below shows 120 consecutive frames from a slow-motion smart phone video filmed at 240 frames per second (these 120 frames comprise half a second). I suspect that the flickering between disparate colors to gradually fade between colors is what makes this video so intensely painful.
I keep the optional front-light on my Dasung Not eReader off and view the screen in sunlight. The screen has a 45 Hz refresh rate and much lower density of pixels than a normal monitor so there is a choppier appearance to the graphics and the grayscale rendering. The eInk monitor does not appear to flicker and does not trigger LED symptoms when used with most software. The list below is the subset of instances where there is flicker.
Zoom video feed flicker
Software that has flicker on the normal LED computer monitor that triggers LED symptoms from the normal monitor. Some of this flicker becomes quite visible when rendered on the eInk monitor, such as for the Apple Preview application and the Google Sites software being used to build this website. Nightshift mode on the Mac adds flicker throughout the screen. The flicker in desktop Adobe CS6 applications or in desktop Microsoft Office for Mac programs is very low, except when part of the screen is highlighted. The highlighted portion flickers significantly. I suspect that both highlighting and nightshift mode might use flicker (temporal dithering) to create the altered colors. These induce symptoms using the normal LED monitor, but the symptoms seem to be worse from these on the Dasung eInk monitor, perhaps because the flicker is quite visible on the Dasung.
Scrolling.
I’d estimate that flicker sources affect me to a comparable degree on both the normal laptop LED screen and on the eInk monitor. If anything, I’d guess that it may be slightly worse on the eInk monitor for Zoom and other software. There isn’t the underlying issue of backlight flicker on the eInk monitor, but the software or video flicker tends to be more pronounced on the eInk monitor.
For example, I am sensitive to the image of escalator treads in Figure 15 of the IEEE Std. 1789-2015 report on flicker that is included as an example of the kinds of patterns to which people with photosensitive epilepsy may be sensitive. When viewed on an LED monitor, I feel strong aversion and moderate nausea and spatial disorientation. Also, on the LED monitor, scrolling the page made the image appear to flicker, which restarted a new round of LED symptoms lasting 3 hours
LED billboards in Times Square – immediately painful; first noticed in June 2017.
Signs backlit by 2-dimensional arrays of LEDs with flicker. These are common on the streets of NYC (at bus stops and as additional advertisements) and on Subway platforms. It’s hard to avoid looking at them when walking.
LED-illuminated display and refrigeration cases at stores
Colored LED lights with color flicker. A Sparoom scent diffuser with changing LED colors was extremely painful. The colors flicker at 120 Hz. For example, red and blue LEDs alternately flicker to create the illusion of purple.
Visibly flickering bicycle lights, including flashing head and tail lights and LEDs on wheel spokes. Ones that alternate between red and blue might be more painful for me than flickering white lights, but all of them are quite painful.
Walking rapidly past an iron fence with early morning sun shining brightly through the bars is slightly painful
LED car headlights and tail lights are painful. I can feel a sharp pain from LED headlights even through shade 5 welding glasses and with my eyes closed. While walking at night, when waiting to cross streets, I close my eyes and look away from traffic, listening for when the traffic pattern changes. Many times, I've felt a rush of pressure in my temple while wearing the welding glasses and with my eyes closed as car or bicycle headlights have passed over my eyes.
LED lights in lamp posts in parks. Many of these still have orange lights in the lanterns that I assume are sodium vapor lamps and the flicker of these lights causes a little pain for me now, when I'm fairly sensitized to flicker. However, these are increasingly being replaced by white LED lights that are very painful for me. I sometimes feel a a brief rush of pressure against the back of my right eye along with sharp pain as I walk rapidly past one of these LED lamp posts, even while wearing a hat and welding glasses and looking away from the lights - I'm not consciously aware that the LED lamp post is coming.
Thousands of different LED light bulbs illuminating the exteriors of buildings, shop windows, or temporary sidewalk restaurants. Individually, they are painful and cumulatively they can restart or exacerbate longer term LED symptoms after less than a minute of exposure. While I have not checked every one of these bulbs for flicker, every LED bulb that has caused symptoms for me that I have checked has flickered. I also checked all of the sidewalk illumination on a 12 block route I need to use frequently after dark to figure out which side of the street had less flicker for each block. Of the dozens of restaurants with temporary outdoor seating, all used flickering LEDs to illuminate the temporary seating area except for one restaurant that used incandescent Edison-style string lights. The lights from all of these places cause me pain, even through shade 5 welding glasses, except for the one restaurant with incandescent lights. Additionally, on the lower-flicker side of the street, one business has LED lights that aren't abnormally bright, but that have particularly intense (high percentage) flicker. Even though while wearing welding glasses at night, I tend to focus almost exclusively on paying attention to traffic and avoiding other pedestrians, rather than paying any particular attention to businesses, for months after my serious flicker exposure in April 2021, I would unexpectedly feel a a brief rush of pressure in my right temple along with sharp pain as I walked past this particular business while looking mostly at the sidewalk and away from the businesses. I had never anticipated that this business was coming up, but the pressure and pain sensations always correctly alerted me that I was passing it.
Intense reaction to dim ceiling-illuminating LED strip lighting programmed to create a blue-purple color in an apartment lobby for the holidays. I did not have LED symptoms at the time. It is likely that these lights created the precise color by flickering red and blue LEDs. These lights immediately triggered spatial disorientation and nausea so intense that I struggled to remain upright and struggled considerably to make it through the lobby.
Moderate reaction to walking across a sidewalk with pale yellow rectangular bricks and dark grout in a zigzag pattern (in front of New York School of Interior Design on E. 70th St). The pattern makes me feel very spatially disoriented and a little nauseous even when I do not have LED symptoms. I look at the sky if I cross this sidewalk.
The painting Elysium by Bridget Riley, when viewed in-person at the Metropolitan Museum of Art. The painting is several feet wide and high. It’s disorienting and I feel the need to look away when I walk by. I was unaware until now in November 2021 that there are other reports of health effects associated with repetitive patterns in work by this artist (reviewed in Wilkins,1995, along with extensive discussion of visual stress induced by repetitive patterns, Warning: This book contains multiple patterns that may bother sensitive individuals. The book warns those with epilepsy and migraine not to look at the frontispiece. Patterns in the book triggered my "LED" symptoms when I ignored the warning.).
Some other periodic patterns, including some with radial periodicity, have a similar effect.
LED lights with flicker – if I am not already hypersensitized, depending on the lights, it may take about 5-20 minutes of exposure to trigger LED symptoms. If I am hypersensitized by a recent flicker exposure, sharp pain behind my eye tends to be immediate and seconds of LED flicker can be sufficient to trigger a new round of symptoms. Occasionally, such as when testing LED strip lights with 1.3%, 120 Hz flicker, in the first minutes I've only felt a vague disorientation that has taken up a few hours to intensify into a more severe and obvious feeling of motion sickness (nausea, spatial disorientation, lack of appetite) and obvious inability to concentrate with compromised short-term memory.
LED screen use – the time needed to trigger a LED symptoms is highly variable, not only because of variation in my sensitivity level, but also depending on the software, whether there is a flickering video feed, and the nature of the ambient room light (bright, non-flickering light is best). Under the best-case scenario, I may be able to work on the laptop using the normal LED screen for a few hours without triggering LED symptoms. In the worst-case scenario, seconds of certain kinds of flicker may be triggering.
Videos with flicker - seconds to minutes of exposure can trigger LED symptoms.
Flicker on eInk monitor - seconds to minutes of exposure can trigger LED symptoms.
Making the dark phase of the flicker brighter by maximizing continuous light from:
Sunlight
Other bright ambient light from completely flicker-free LED bulbs
Maximizing my exposure to continuous light sources is my preferred method for preventing LED symptoms and the method that I implement most of the time. In addition to working best, it has the added advantage of not involving the use of tinted lenses that could potentially increase my photosensitivity over time.
If the above is not possible, making the bright phase of the flicker darker by wearing a hat and wearing shade 5 welding glasses is slightly helpful. This helps a little when needing to very briefly traverse an environment with flicker, but isn’t a solution because flicker can still be felt through the glasses (it's enough to worsen or cause new symptoms) and it’s too hard to see to be able to do any useful work. I only do this when I need to walk in NYC at night or briefly navigate a store or other space with flickering LED lights, and then the protection is often insufficient.
Covering both eyes completely with a 100% blackout mask to block headlights and city lights while a passenger in a car at night.
If flicker is from only one direction (such as from a screen), patching one eye and using sunglasses for the other eye (while keeping the screen at full brightness) may somewhat delay, but not prevent, symptom onset for me. When I'm in a more sensitized state following a recent flicker exposure, symptoms will start immediately when I look at a screen, even when patching one eye and wearing sunglasses. A patch attached to glasses that isn't fully blacking out the light isn't sufficient to help much at all if the ambient light flickers since the light gets around the patch. I am curious about why the patch might delay symptom onset with screen use - is it just because I'm sensing half the amount of flicker, or is there some role for the binocular perception of flicker in making symptoms worse? The fact that patching one eye delays symptom onset for me when using a screen is similar to how patching one eye could reduce, but not eliminate, the frequency of seizures in a patient for whom the observation of striped patterns induced seizures (described in Wilkins, 1995. Warning: This book contains multiple patterns that may bother sensitive individuals. The book warns those with epilepsy and migraine not to look at the frontispiece. Patterns in the book triggered my "LED" symptoms when I ignored the warning).
Covering both eyes with my hands if I don't have a blackout mask and am forced to wait in an environment with flicker.
Keeping any LEDs that have more flicker when dimmed set at full brightness
When possible, schedule travel from place to place in NYC to happen in the daytime and choose routes that maximize sunlight and minimize LED light. For example, walking along the river or through parks during daylight hours are best. Night is the worst because of the increased number of LED light sources and the greater contrast between the light and dark. I now avoid walking under scaffolding/sidewalk sheds (common in NYC) because the majority of scaffolding is lit by flickering LED lights, many with quite intense 100% flicker. A few seconds spent under such scaffolding at night while wearing my welding glasses and hat has triggered symptoms lasting hours. I am more likely to develop LED symptoms by walking in NYC on a cloudy day than on a sunny day, so when I have the option, I may delay trips until sunny days. I adjusted my work hours once daylight savings time ended in the fall of 2021 so that I could continue walking home before sunset.
Using blackout curtains at night to block flickering LED light from apartments across the street.
Minimize screen time in general.
Completely eliminate non-essential screen use, such as social media, games, or non-essential internet surfing.
Minimize use of software that has inherent flicker.
Try to close my eyes when scrolling.
Minimize types of screen use that involve a lot of scrolling, such as online shopping and editing documents.
Minimize reading on any screen with a backlight or with frequent refreshing. Only read books and journal articles on paper or on a KOBO eInk tablet that has a completely static screen that doesn't refresh. (Reading on paper or the KOBO, I'm completely symptom-free).
Maximize bright continuous sunlight or ambient completely flicker-free LED light when working on a screen.
Closing my eyes - ambient light through the eyelids is sufficient to cause symptoms.
Eyeglasses with Blokz Blue Blocker lenses (Zenni)
Wearing a hat – it helps a tiny amount, but not nearly enough.
Wearing a hat and thick lab-grade SCT-orange glasses that block all UV and blue light – it helps a tiny bit more than the hat alone, but the effect of both together is still minimal. These glasses are designed to block the light of the blue-light transilluminator for excising DNA bands from gels when doing genetic engineering. They block more than the same range of light blocked by “migraine” glasses.
Wearing a hat and amber sunglasses – it helps the same minimal amount as the hat with SCT-orange glasses.
FL-41 glasses (Wilkins, 1991) and in a recent paper, Wilkins explains why these glasses should not help for LED lights (Wilkins, 2021 and also see discussion in Background: Health effects of blue light).
Red, blue, and green tinted lenses referenced as providing some relief for concussion patients (Clark et al., 2017). I initially liked wearing them, especially the blue lenses, but they had little to no effect in terms of preventing LED symptoms when I tried using a screen.
I tried doing exercises with a Brock string because some individuals on the LEDstrain.org forum said it helped them with eye convergence issues, and since it didn't cost much and couldn't hurt me, I thought it was worth a try. I didn't notice this helping me at all. Multiple ophthalmologists and optometrists have never identified a convergence issue for me, although I know that routine eye exams do not test for this. I would have scored 0 points on the Convergence Insufficiency Symptom Survey prior to 2018, out of 56 possible points, with a score of 21 or higher indicating possible convergence insufficiency. After I started having LED symptoms in 2018, I would have scored 1 point - that I infrequently had trouble remembering what I had read (occurs only when LED symptoms are particularly bad and generally inhibit my short-term memory). I also have headaches from LED symptoms, but these almost always happen when I'm not doing close work or reading. However, this survey has been shown to have no predictive value of convergence insufficiency, so my score may not indicate very much. I found the Brock string exercises to be extremely simple for me to do and I can easily keep both eyes focused on the tip of my nose, never have double vision, have no issues when reading on paper or on the KOBO - either at a typical reading distance or with my glasses off and the book close to my nose. I never have any "LED" symptoms in situations that don't involve light flicker or repetitive patterns. I would have scored 1 point on the Binocular Vision Dysfunction Questionnaire prior to 2018 for occasionally having headaches. I now score 8 points for having frequent headaches, occasionally experiencing nausea/disorientation with near or far screen use, depending on the screen flicker, and always experience glare from bright flickering LED lights, but not other bright lights. A score of 15 or more indicates possible BVD and I scored 0 points on all of the "red flag" questions. I don't think I personally have an eye convergence deficit or another kind of binocular vision disorder, but it sounds like this may be a real issue for others with sensitivity to LED lights or screens. Update April 2022: When testing the Waveform Centric Series Nondimmable Power Supply with strip lights on 3/28/22, I was able to test further. This lighting setup essentially made me feel motion sick as the most prominent set of symptoms (spatial disorientation, nausea, loss of appetite), have a moderate headache without the usual pressure, and have trouble concentrating with loss of short-term memory. I also noticed that looking at a Sudoku puzzle in this light would make the headache pain worse than when I was not looking at the puzzle. I tried testing binocular vs monocular vision while looking at the puzzle by holding my hand vertically between my eyes - so both eyes could see, but only one eye could see the puzzle at a time. I figured out that looking at the puzzle with one eye was worse than not looking at the puzzle. Looking at the puzzle with both eyes was somewhat worse than looking at the puzzle with one eye. The result didn’t seem to be much different if I covered one eye and looked at the puzzle than if both eyes were open and only one eye could see the puzzle. I still don’t think there’s any good evidence for my symptoms being due to a misalignment of binocular vision, however, it's interesting that using one or both eyes to read could intensify the headache in this particular lighting situation. I wonder whether this might be similar to how reading in the car makes some people (not me) carsick. I have not noticed a similar effect of reading on paper intensifying symptoms when I am not in flickering light. I don't yet have other experiences reading on paper in flickering light. Update Sept. 2022: I saw an optometrist who is an expert in binocular vision disorders and she did not detect any binocular vision disorder for me. The tests included using a Maddox rod.
Reducing stress – while the teaching job I had left after the 2018/2019 school year was fairly high-stress, my current job is very low stress. I’m generally happy, have strong coping skills, and am nearly always free of anxiety. Even dealing with months-long concussion-like symptoms from LED flicker doesn't make me feel stressed. Part of me is fascinated by trying to figure out the underlying science of the issue. Despite not being stressed, I’m still regularly getting symptoms from LED flicker.
Fidget spinners, smart phone slow-motion videos, a high speed camera, an oscilloscope, or another kind of flicker meter can all be used to detect flicker. Prof. Arnold Wilkins suggests that consumers can use fidget spinners to detect flicker in lights. If the light flickers, the pattern produced as the fidget spinner spins will appear to move both forwards and backwards due to the "wagon wheel" effect created by intermittently illuminating the spinner. In a continuous light source, there is only the smooth blur of motion occurring in the direction of the spin. In my experience, a fidget spinner can be used to detect only the most obvious kind of LED flicker that has a low flicker frequency and fairly high flicker percent. A smart phone slow-motion video can detect much more subtle flicker than a fidget spinner. However, a smart phone cannot detect all flicker, including that from some lights that have caused my symptoms. For example, with a smart phone slow-motion video I could just barely detect the 2.679% flicker of the Waveform T8 tubes, but I could not detect the 1.3% flicker of from the Waveform Centric Series Non-Dimmable Power Supply with strip light at all. An oscilloscope or flicker meter is necessary to detect the most subtle flicker. Unfortunately, I do not own such a meter.
Note: the numbers in the directions below are the values used in North America that accompany the 60 Hz AC mains power and would need to be modified for other parts of the world that have 50 Hz AC mains.
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. I'm curious about whether these might be an example of color-tunable LEDs or whether they were just designed to always have the same degree of flicker between reddish and greenish white light. 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.
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.
See more examples of flickering lights being installed in NYC Public Schools and throughout the NYC public transit system on the Public Health Risks page.
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. This was the case when a lighting technician tested Waveform 3000K A19 Centric Home Flicker-Free 10W bulbs. I had been using 2700K and 3000K versions of these light bulbs without symptoms since May 2021 at home and at work. The 3000K bulbs that were tested 3/23/22 were installed in hanging globes in a room with refractive glass walls that curved around the the two long sides of the room. Hallway lights on either side with 7.88% flicker and the lights from rooms across the hall on either side with 9.19% flicker also shone into the room where the Waveform bulbs were tested. A lighting consultant sent me the images below of the flicker meter readings. It didn't occur to me to ask at the time whether they had turned off the nearby flickering lights hallway lights or the flickering lights in other rooms when taking the readings. Later that day, I went to the facility to test various lights on myself and a lighting consultant led me in with a blindfold. We started in the room with the Waveform 3000K A19 Centric Home bulbs, since those had the lowest flicker readings of the 3 kinds of lights we would be testing. I was told that all of the other nearby lights were off and it was safe to remove the blindfold. I could see that the hallway lights were off through the open doorways. During the testing I thought that the only lights were the globe lights with the Waveform bulbs, so it was unexpected that almost immediately after arriving I was feeling tingles of pain behind my right eye (a typical first symptom in the presence of flickering light), but if I’d look directly at the Waveform A19 globe lights, that pain would go away. It was strange that I was feeling pain in the presence of a style of light bulb that I’ve used extensively in the past without issue. It was also initially confusing that looking directly at the lights would make the symptoms better instead of worse if those were the only light sources. Those feelings of pain behind my right eye gradually became more pronounced. Eventually after about 35 minutes, I noticed that there were lights on in a room across the hall that were barely in my peripheral vision and when I turned to put those lights further behind me, I didn’t feel the tingles of pain behind my eye. In retrospect, I also think that there may have been other light sources compromising this test too since the surrounding refractive glass walls seemed fairly bright this first day of testing - especially compared to the next day. On this first day, 3/23/22, after about 40 minutes in this room I felt strong headache pain and pressure behind my right eye in the right temple area. This headache pain and pressure lessened slightly after about 45 minutes in the next testing area, but lingered the rest of the evening.
The next day, other people helped me to repeat tests and they carefully turned off all of the other lights on the floor that it was possible for them to turn off. They found that one light in a nearby room across the hall wouldn't turn off, so they positioned me looking away from it with a pillar behind me, blocking light from that room. This second day, the glass walls of the room with the A19 bulbs looked very dark. There was much less illumination in the room overall - the light from the Waveform bulbs wasn't particularly bright, and the light generally had a warmer feel than it had the day before. I suspect that the previous day, a significant amount of the room's illumination came from other lights on the floor with cooler color temperatures. On the second day of testing, the light from the Waveform A19 bulbs felt good, like how sunlight feels for me, and I didn't develop any symptoms at all after an hour and a half in the light. My conclusion was that these bulbs are still safe for me when installed in this facility.
It only occurred to me a few days later that other lights might have been on when the flicker meter reading was made for the Waveform A19 bulbs - especially since there had been enough flickering lights left on in the vicinity by the lighting consultants to start my symptoms on the first day of testing and one nearby light couldn't be turned off at all. I asked the lighting consultant, who couldn't recall having turned off either hallway lights or the lights in rooms across the hall when the flicker meter readings were made for the Waveform A19 bulbs. They just said that the technician had held up the meter by hand near the hanging globes. Given how difficult it was for others to get all of the floor lights off the next day - since a special lighting control system was involved and the lights could be triggered back on by motion sensors and one light wouldn't even go off - I believe it would have been remembered if this had been done when the flicker meter readings were taken for the Waveform A19 bulbs. I especially think this was likely not done since not all of the other lights were turned off when testing the effect of the Waveform bulbs on me during the first day of testing.
So given the above, there's no way to know how much, if any, of the flicker in the below graph is from the Waveform A19 bulbs and how much is from the known flicker of the surrounding lights. For example, if hypothetically the Waveform A19 bulbs didn't flicker and if 5% of the light reaching the meter came from other light sources and that other light had 8% flicker, the amount of flicker recorded by the meter would be 0.4%, which is about what was recorded. Therefore, nothing can be concluded from this test about whether any of the recorded flicker actually came from the Waveform A19 bulbs. It's very disappointing not to have data from only the Waveform A19 bulbs. In order to collect data on what kind of flicker profiles can be tolerated by sensitive people, it's very important to obtain flicker readings that are definitively only from the light source being tested by turning off all other lights.
Combination of nearby lights with 7.88% and 9.19% flicker and Waveform A19 Centric Home 3000K 10W bulbs in hanging globes:
Flicker frequency: ? - I count 60 wave peaks in the graph's 0.5 seconds, meaning that the frequency would be 120 Hz. However the meter software says the frequency is 2 Hz and the left histogram with flicker frequency on the x axis has a scattering of frequencies between 2 Hz and 16000 Hz.
% flicker (for magenta region highlighted in graph): 0.44%
% flicker (overall, determined by meter software): 0.8943%; Note: this was a hand-held reading.
Calculation of % flicker:
Distance from min to max point in Photoshop: 617.85 px
Height of indicated portion of wave (magenta line) in Photoshop: 293 px
If the minimum of the magenta line was set at 0.2283, the maximum of the magenta line would be at 0.2303. [(0.2324-0.2283) *(293 px/617.85 px)+0.2283 = 0.2303]
% flicker = 100% * (max-min)/(max+min) = 100% * (0.2303-0.2283)/(0.2303+0.2283) = 0.44%
Is the irregularity in the luminosity vs. time graph not all due to the technician taking a handheld measurement, but somewhat due to the actual light output of the bulbs being slightly irregular or from irregularity in how light from nearby lights is reaching the meter? The Waveform statistics for the A19 bulbs say "0 Hz" for their flicker frequency, suggesting that there is perhaps not enough flicker from the bulbs themselves to measure?
When using a flicker meter, place it on a stable surface - 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 may create variations in the recorded wave. It's necessary to eliminate this kind of variation so any variation from the actual light source can be analyzed. For example, it isn't possible to know whether the gross variation in the above graph is from the meter being hand-held and moving slightly different distances from the light or due to variation in actual light output. 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.
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.