Flicker

Flicker is a rapid, unintended change in light intensity caused by imperfections in power supply or light source design - both visible and invisible forms of this fluctuation place strain on the nervous system, trigger headaches and eye fatigue, and fundamentally differ from biologically beneficial therapeutic pulsing.

Flicker (also referred to as light flickering, light fluctuation, or light modulation) is a technical term for periodic, uncontrolled changes in the light output intensity of a source over time. Unlike intentional therapeutic pulsing, flicker is an undesirable technical phenomenon - a byproduct of imperfect power supply or lighting device design.

Flicker can be visible (a person directly perceives the flickering) or invisible (the flicker frequency is above the threshold of conscious perception, yet the nervous system still registers it). Invisible flicker is more insidious - the user does not consciously notice it, but its biological effects on the brain, eyes, and nervous system continue.

Where and why flicker occurs

Light from electrical sources is in most cases powered by alternating current (AC) with a frequency of 50 Hz (Europe) or 60 Hz (USA). This alternating current causes the intensity of the light output to change cyclically - at a 50 Hz grid frequency, fluorescent lamps and LED bulbs typically flicker 100 times per second (double the grid frequency).

The quality of flicker depends on an electrical component called a driver. A high-quality driver effectively smooths the alternating current and minimizes flicker. A cheap or poorly designed driver allows current fluctuations to pass through, which directly translate into light output - resulting in flicker with high modulation.

The main sources of flicker in everyday life include:

• LED bulbs and luminaires with cheap drivers - a common issue in the low-cost product segment
• Fluorescent lamps with magnetic ballasts - a classic source of 100 Hz flicker in office and industrial environments
• Screens and monitors - especially at reduced brightness controlled via PWM (pulse width modulation)
• Dimmable LEDs - incompatibility between the LED device and dimmer can create strong flicker
• Outdoor LED advertising boards and displays

How flicker is measured

Flicker is not a single parameter, but a combination of several measurements. According to international standards (IEEE 1789-2015, IEC TR 63158, EU regulation 2019/2020), flicker is mainly characterized by:

Percent flicker / Modulation Index (MI %)
Expresses the depth of flicker modulation - the ratio of the difference between maximum and minimum light output to their sum. A value of 0 % means no flicker (constant intensity), while 100 % means the light turns completely off in each cycle. The IEEE 1789 standard for 100 Hz flicker recommends a maximum of 8 percent flicker (for a safe level without observable effects).

Flicker Index (FI)
Takes into account the waveform shape - not only amplitude, but also the duration of different light levels within a cycle. It ranges from 0 (no flicker) to 1 (maximum flicker).

Flicker frequency (Hz)
The number of flicker cycles per second. With a 50 Hz power supply, the typical flicker frequency is 100 Hz. Frequencies below 80 Hz are generally considered more dangerous - visual stimuli in the range of 15 to 20 Hz are the most likely to trigger epileptic seizures in photosensitive individuals.

Stroboscopic effect (SVM)
When objects move under flickering light, an optical illusion may occur - rotating or moving objects appear slowed down or stationary. This effect is dangerous in industrial environments with fast-moving machinery.

Health effects of flicker

Flicker is not just an aesthetic issue - it has measurable biological effects on the nervous system, eyes, and brain. The IEEE 1789 standard categorizes these effects based on frequency and modulation intensity:

Low frequencies (3 to 70 Hz) - high risk
In this range, flicker can directly trigger visual stimuli in the brain. Frequencies of 15 to 20 Hz are the most sensitive range for photosensitive epilepsy. It is estimated that around 2 to 14 percent of epilepsy patients suffer from photosensitive epilepsy, and about 1 in 4,000 people in the general population. In people with migraines, a 2024 study found that 41 percent of individuals developed headaches after 60 to 120 minutes of exposure to flicker patterns commonly found in architectural lighting, compared to only 8 percent in the control group. [R]

Medium frequencies (70 to 160 Hz) - low to moderate risk
Typical frequency of common LED lighting powered at 50 Hz. Flicker in this range is not directly visible to most people, but the nervous system still registers it. Long-term exposure is associated with headaches after exposure, eye strain, reduced concentration, and disruption of the sleep rhythm.

Higher frequencies (160 to 400 Hz) - low risk, but not zero
According to the IEEE 1789 standard, frequencies in the range of 100 to 400 Hz may still negatively affect the human body, especially at high modulation. The stroboscopic effect (SVM) remains relevant up to frequencies of 2,000 Hz.

Digital Eye Strain
Meta-analyses of digital eye strain studies from 2023 and 2024 report a prevalence of this syndrome in the range of 66 to 74 percent of screen users. Headaches are among the most commonly reported symptoms - in a large UK survey (2021 to 2022), 62.3 percent of respondents with digital eye strain reported them. Flicker from screens controlled by PWM dimming is one of the identified contributors.

Flicker vs. pulsing: a fundamental difference

The most important distinction that Mitochondriak® repeatedly emphasizes: flicker and pulsing are not the same.

Mitochondriak® devices are designed to eliminate flicker (high-quality driver without grid ripple) while enabling therapeutic pulsing with precisely defined frequencies. These are technologically opposite concepts within a single device.

How to detect flicker in your environment

There are several practical methods to detect flicker without specialized measuring equipment:

Smartphone test (for low frequencies):
Point your smartphone camera at a suspected light source and observe the image on the display. If you see dark bands moving up or down, the light source is flickering at a frequency captured by the camera (typically below 100 Hz). This test does not detect flicker above 120 Hz (depending on the camera frame rate).

Pencil test (for 100 Hz flicker):
Quickly move a pencil or pen in front of the flickering light. If instead of a smooth line you see a series of repeating images of the pencil, the light is visibly flickering.

Specialized measuring devices:
Professional lux meters with flicker meter functionality measure percent flicker (MI %) and flicker index (FI) in accordance with IEEE 1789 and IEC standards. Consumer devices start at a few dozen euros.

Related terms

• Pulsing - intentional, precisely controlled flickering of a therapeutic device; the biological opposite of flicker
• PWM (Pulse Width Modulation) - a method of dimming LEDs by switching them on and off; at low frequencies, the main source of flicker in screens
• Driver (ballast) - an electronic component regulating LED power supply; its quality directly determines the level of flicker
• Percent flicker (MI %) - a measure of light output modulation depth; from 0 % (no flicker) to 100 %
• Flicker Index (FI) - a metric that accounts for waveform shape; from 0 to 1
• Stroboscopic effect - an optical illusion of moving objects under flickering light
• Flicker fusion threshold - the frequency above which the human eye no longer directly perceives flicker (typically 60 to 90 Hz); flicker above this threshold still has subconscious effects
• Blue light - another biological issue of LED lighting; flicker and blue light are distinct but independently harmful aspects of poor-quality lighting
• Circadian rhythm - the body’s biological clock; flicker, similarly to blue light, can disrupt its proper functioning
• Photosensitive epilepsy - a condition in which flicker and flashing light (15 to 20 Hz) can trigger a seizure; affects 2 to 14 percent of epilepsy patients
• IEEE 1789-2015 - an international standard of recommended practices for current modulation in high-brightness LEDs to minimize flicker-related health risks

Frequently asked questions about flicker

What is flicker and why is it harmful?

Flicker is rapid, periodic fluctuation of a light source caused by variations in electrical current or design flaws in the device. It can be visible (people directly perceive flickering) or invisible (occurring above the threshold of conscious perception, yet still registered by the nervous system). Both visible and invisible flicker can cause headaches, eye strain, impaired concentration, and in sensitive individuals migraines or epileptic seizures.

What is the difference between flicker and pulsing?

Flicker is an undesirable technical defect - flickering caused by poor power supply or a low-quality driver. Pulsing is an intentional therapeutic parameter in red light therapy devices, where frequency, duty cycle, and pulse shape are precisely configured according to a biological goal. Flicker places strain on the nervous system; therapeutic pulsing at proper frequencies (e.g., 10 Hz) improves light penetration into tissues and has regenerative effects.

Can I see flicker with the naked eye?

It depends on frequency and modulation depth. Flicker below 60 to 80 Hz is directly visible to most people as flickering. Flicker above 80 Hz is usually not directly visible, but the stroboscopic effect (moving objects under flickering light) can still be perceived by many. The most reliable quick test is recording the light source with a smartphone camera - flicker appears as dark moving bands.

What flicker frequency is dangerous?

The most critical range is 15 to 20 Hz - this is where the likelihood of triggering an epileptic seizure in photosensitive individuals is highest. Frequencies of 3 to 70 Hz are categorized by IEEE 1789 as high risk. Frequencies of 70 to 160 Hz (typical for LED lighting powered at 50 Hz) are low to moderate risk, but still problematic at high modulation. Even higher frequencies (100 to 400 Hz) can have negative biological effects.

How can I tell if my LED bulbs flicker?

The simplest method is the smartphone test: point your phone camera at the bulb and observe the image. If you see dark horizontal bands moving up or down, the bulb is flickering. Another method is the pencil test: quickly move a pencil in front of the light - if instead of a smooth line you see multiple “ghost” images of the pencil, it is visible flicker. Accurate measurement requires a lux meter with a flicker meter function.

Are flicker-free bulbs more expensive?

A high-quality flicker-free driver is more expensive to manufacture, which is reflected in the product price. Cheap LED bulbs on the market very often flicker - sometimes with modulation exceeding 90 percent. Investing in flicker-free products is justified in the long term through energy savings, longer lifespan, and most importantly the elimination of chronic nervous system strain caused by invisible flicker. Mitochondriak® devices are designed with flicker elimination as a fundamental requirement.

Is screen flicker the same problem as lighting flicker?

Yes, and in screens it is often even more problematic - the light source is much closer to the eyes and the user looks directly into it. Many LCD screens at lower brightness use PWM dimming, which generates flicker at frequencies of 100 to 400 Hz. OLED screens can be more sensitive to flicker at very low brightness levels. For screen use, it is therefore important to use monitors with flicker-free certification (hardware dimming) or set screen brightness to maximum and adjust ambient lighting instead.

Summary

Flicker is unwanted, periodic fluctuation of a light source caused by imperfections in power supply or driver design. It can be visible (below 80 Hz) or invisible (above 80 Hz), and both types have proven biological effects - from headaches and eye strain to triggering migraines and epileptic seizures in sensitive individuals. Flicker is fundamentally different from intentional therapeutic pulsing, which is biologically beneficial. Mitochondriak® devices eliminate flicker through high-quality drivers while enabling therapeutic pulsing - making them the technological opposite of what flicker represents.

Scientific sources and standards

• IEEE Std 1789-2015: Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers. standards.ieee.org
• EU Regulation 2019/2020 on ecodesign - requirements for SVM and PstLM for LED luminaires.
• Miller N et al. Visibility and annoyance of phantom array effect varies with age and history of migraine - 41 % of people with migraine developed headache after flicker exposure. Lighting Research & Technology. 2024. PMC12371629
• FAQ on Visually-Provoked (Photosensitive) Epilepsy - Epilepsy Foundation, 2025. PMC11995705