Unanswered questions in biomedical flicker research

What isn't yet known about the health effects of flicker in the scientific literature?

To my knowledge, there is not yet evidence in the research literature to answer these questions:

1. How frequently do people have a biological response to LED light flicker?

2. What are those biological responses and how common are they?

   1. • What is the scope of symptoms that sensitive people experience in response to LED lights? How common are headaches, difficulties in concentration, short-term memory problems, dry eye, or forms of eye strain in the general population in response to LED flicker? How common are spatial disorientation, nausea, or sleep abnormalities?

      • How do other photophobia syndromes like migraine, dry eye, and concussion influence flicker sensitivity? For example, do patients with post-concussion syndrome have greater sensitivity to LED flicker compared to what they experienced prior to their concussion?

      • Is a mild form of spatial disorientation what some people experience when they say that they feel uneasy or “don’t like” LED lights? 

      • How common are sub-clinical biological effects of flicker, such as possible mild headaches, mild disorientation, and effects on learning?

      • Which part of the nervous system limits the neurological response to ≥100 Hz flicker in people whose health seems not affected by flicker? Does it happen at the level of the eyes, trigeminal pathway, thalamus, cerebral cortex, or some other part of the peripheral or central nervous system?

      • Which parts of the nervous system detect and respond to light flicker in sensitive individuals? Are there differences compared to unaffected individuals?

      • What role does the inflammatory process around the eyes or in the brain play in responding to ≥100 Hz flicker in sensitive individuals? 

      • Does the neuropeptide CGRP, which plays roles in creating neuroinflammation and in sensitizing the central nervous system to light in the photophobia conditions dry eye disease, migraine, and concussion, also play a similar role in the biological response to light flicker in sensitive individuals?

3. Although there has been research on "critical flicker frequency" (the frequency at which flicker seems to blend together, creating an optical illusion that fools people into thinking the light is continuous), how fast does flicker need to be in order for the nervous system to be completely unable to sense it? Is there any difference in such a hypothetical frequency limit for patients with migraine, dry eye, concussion, or other potentially sensitive groups compared to the general population?

4. What parameters are needed for LED flicker to eliminate negative biological effects? What are the parameters of the frequency and other wave characteristics (shape, modulation depth, duty cycle, light intensity, etc.)?

5. How does the spatial arrangement of multiple LED lights affect the biologic impact of LED flicker? Are different standards needed if lights are in arrays or clusters?

6. What are the safe parameters for LED lamps that have flickering colors (becoming common in “color-tunable” LEDs, "white-tunable" LEDs and in novelty lighting)?

7. Does LED flicker sensitivity increase with more exposure to flicker? 

8. Are people with more years of experiencing other photophobia syndromes more sensitive to LED flicker?

9. Are there any appropriate neurological diagnostic tests for detecting sensitivity to flicker ≥100 Hz?

10. To what degree is research on the biological effects of slow <100 Hz flicker also relevant for the ≥100 Hz flicker of LED lights?

11. What kinds of labeling or flicker statistics need to be provided so that consumers and lighting design specifiers can evaluate whether lights have ≥100 Hz flicker or are completely flicker-free before purchase? Note: the lighting industry's 2016 decision that flicker that isn't obviously visible isn't going to be called "flicker" by the lighting manufacturers anymore (CIE TN 006: 2016) makes current communication about flicker between lighting manufacturers and consumers at the least confusing, if not dangerously misleading.

Lighting experts have been asking for help from the neurological community. In 2015 while developing the first LED flicker guidelines, the IEEE international committee of experts on flicker recognized the absence of relevant direct research on the biological effects of LED flicker, and urged

“In fact, an additional purpose of this document is to urge industry and research laboratories to continue to critically evaluate data from research and from field experience and make additional recommendations when supported by data. The risk analysis discussion in Clause 7 should allow research entities to identify areas of valuable research topics that could further the understanding of the human biological effects of light flicker.”(IEEE std 1789, 2015, Section 1.3).

As recently as July 2019, LED flicker expert Naomi Miller from Pacific Northwest National Laboratory (PNNL) urged for research on the biological effects of LED flicker to begin and she offered the collaboration of PNNL with the neurological community during a talk for the Illuminating Engineering Society (Naomi Miller. Metrics in Motion: Flicker and Glare. July 11, 2019 webinar, minute 59 and as her final message in minute 111). She also said in reference to the problem with a recently-developed, more relaxed flicker metric than the IEEE recommendations, SVM, being based only on the average observer rather than sensitive individuals, “Recent work has shown that 25% of people are more sensitive than other people and these tend to be the people that suffer some of the worst effects such as migraines and headaches” (minute 57).

Is LED sensitivity a larger problem than is currently apparent to the medical community?

LED sensitivity is a relatively new problem since LEDs are new and symptoms of sensitivity may not overlap completely with previously-described conditions.

There is a hint in a recent large-scale public health study that there may be a new, common class of headache with light sensitivity that doesn't clearly fit prior headache diagnostic criteria (Li et al., 2020, discussed in Background: Health Effects of ≥100 Hz flicker).

There are many reasons why LED flicker might currently be under-reported to neurologists or other medical professionals as a trigger of adverse health outcomes:

1. Novel syndrome: The symptoms caused by flicker may differ from those of previously characterized headaches and both affected individuals and neurologists might not know how to classify the symptoms. Some affected individuals may not even report a headache among their symptoms. Depending on the scope of their symptoms, individuals might also first consult optometrists or ophthalmologists, rather than neurologists. The multiple categories of specialists that may reasonably be consulted makes it more difficult for any one specialty to recognize the scope of the problem. Depending on the severity, affected individuals might not seek medical care and might dismiss their symptoms as just headaches, just nausea, or just feeling uneasy around flickering LEDs. It may be that any headache pain can be “pushed though” to keep working, unlike for common migraine. 

2. Access to care: There’s the barrier of often needing to experience symptoms for several months before being able to schedule an appointment with a headache-specialist neurologist in at least some parts of the United States, so many individuals may not have jumped through the necessary hoops, especially if their symptoms are mild and they can avoid most flicker by still using incandescent lights. Primary care physicians may not refer patients to neurologists if their symptoms don’t clearly fit existing headache diagnostic criteria. Once patients have waited months to see a headache-specialist neurologist, they may have already found a way to limit their LED flicker exposure and their symptoms might have abated, so that any clinical neurological signs may no longer be present. Since LED flicker is not currently widely-recognized as a potential acute cause of injury to the nervous system, patients do not currently have access to specialized acute care.

3. Able to avoid LED flicker: Through luck and/or privilege, some individuals can avoid using flickering LEDs in both the home and workplace. A substantial percentage of the LED bulbs in the initial mass-market roll-out were completely flicker-free, so some households are already using “safe” completely flicker-free LEDs. Older buildings may still be using older types of lighting.

4. Difficulty identifying an "invisible" trigger: It can be difficult to recognize that hidden flicker from screens or ambient lights is the trigger of symptoms because it requires conscious perception of LED light as the trigger. And then it may require self-experimentation with special equipment to deduce that it is the flicker aspect of the light, rather than color or brightness, that is the trigger of symptoms. When LED screens are triggering, it’s even more difficult to figure out the nature of the trigger given the complex interplay of hardware and software. There are also multiple potential sources of flicker on screens and they may vary on different devices and with different software. Tech savvy individuals on LEDstrain.org have been trying to figure out the parameters of screen triggers, but the complexity of computers makes it extremely difficult.

Since a patient's first significant symptom onset might begin when exposed to LED lights at a new or recently-remodeled building, the possibility that some other aspect of the construction process might be triggering symptoms might further delay identification of the cause.

5. Severe symptoms might arise only after more flicker exposure: Tangential research on flicker below 100 Hz (Karanovic et al., 2011)  suggests that sensitivity to flicker in migraine patients may increase with more years of having had migraines, so there might be an age/experience-related component to sensitivity to LED flicker as well.

Even if patients report LED sensitivity to neurologists, ophthalmologists, optometrists or other medical professionals, there is a historical bias in the medical community to downplay the potential seriousness of even traumatic injury to the brain, such as in concussion, so it would not be unreasonable to expect it to be even more likely for medical professionals not to recognize the potential seriousness of injury to the nervous system via light, which of course does not involve any physical contact. The absence of objective biomarkers of nervous system or neuroinflammatory changes (other than upon autopsy) in post-concussion syndrome or for the photophobia experienced in migraine or dry eye disease raises the likely possibility that there also may not be readily measurable, objective biomarkers to diagnose sensitivity to LED flicker or to detect potential nervous system or inflammatory signalling.

Further complicating matters for patients is another long-standing bias in the medical community to believe that photophobia, sensitivity to light, has a purely psychological, rather than physiological basis. Even through research in recent years has demonstrated the physiological basis of photophobia syndromes, there may still be difficulties for patients. Medical professionals might still think that the "sunglasses sign," wearing tinted glasses in the clinic, is a sign of a purely psychiatric disorder or a sign that the patient has made up the symptoms to seek attention, rather than evidence that the light in the clinic is having a genuine impact on the patient's physical health. Howard & Valori (1989), describe clinician's use of the sunglasses sign, as an indicator that a patient does not have a physiological condition, as a long-standing, widespread assumption in clinical practice that had not been validated by evidence. The preponderance of currently-available evidence suggests that there are physiological causes of photophobia and that the vast majority of patients exhibiting the sunglasses sign have photophobia with a genuine physiological basis (reviewed in Digre and Brennan, 2012 and Katz and Digre, 2016). 

Thus, even though the biological safety of LED light flicker was questioned during the development of this technology, no relevant tests of the safety of LED light flicker were done prior to the mass public rollout of the technology. The public has been unknowingly exposed to light flicker without any clear path for recognizing or reporting symptoms, and in the context of a medical system that has not been primed to recognize potential symptoms or to understand that despite historic clinical biases, patients reporting symptoms should be taken seriously. Despite having the ability to manufacture safe, flicker-free LED lights, manufacturers have largely chosen not to do so in recent years and have at the same time adopted language and marketing practices that have the effect of concealing the presence of flicker in their lights.

The goal of this website is to begin to focus research attention, regulatory attention, and public attention on the potential health impacts of LED light flicker and the complex forms of flicker on device screens. The hope is also that lighting manufacturers, computer hardware and software designers, and video creators will voluntarily take action to remove both obviously visible flicker and hidden flicker from their products.