Definitions

Note: When synonyms exist, this website preferentially uses the terms with the most colloquial usage for the purpose of communicating with the public even though other terms may be used more frequently by specific interest groups.

Light-Emitting DIode (LED): A semiconductor-based light source that emits light when electrons fall from a higher energy state to a lower energy state. LEDs are more energy-efficient and have much faster (nanosecond) response times to changes in the driving current compared to traditional light sources. (Learn more at LEDbenchmark.com).

Synonyms for LED: Solid state lighting (SSL)

Flicker: Variation in light intensity with time.

Synonyms for flicker: Temporal Light Modulation (TLM), temporal modulation, light modulation, light oscillation, visual beats, visual ripple, discontinuous illumination, pulsed wave

Note that the lighting industry now only applies the term "flicker" if the flicker can be directly, visibly observed as fluctuations in light intensity. The lighting industry uses "temporal modulation" to refer more generally to a variation in light intensity with time, regardless of whether or not it can be directly observed (CIE TN-0060). This website will use the term "flicker" to refer to any variation of light intensity with time, partly because it is a term that is familiar to a general audience, partly because doing so excludes subjective observations, and partly because there is no biological basis for assuming that health effects of flicker can only be present if that flicker is consciously visible.

Visible flicker: Variation in light intensity that occurs at a frequency that is sensed by the brain and consciously perceived - flicker that a person is conscious of seeing. Historically, flicker was generally thought to be visible when slower than about 60 Hz (60 flashes per second), but could also be obviously visible as flashes of light up to about 100 Hz, depending on the person and on environmental conditions [reviewed in IEEE std 1789, 2015]. However, there is now evidence that much higher frequency flicker can also be visible (see notes and references below for CFF).

Critical flicker fusion (CFF) frequency: A hypothesis that there is a flicker frequency above which light appears to be continuous, meaning that the flicker is invisible. The idea behind this hypothesis is the assumption that the nervous system fuses the discrete images created by the flicker. Thus, flicker fusion makes the flicker "invisible." However, there is now significant evidence that "There's no such thing as flicker fusion" (Arnold Wilkins, 2014). The historic estimates of CFF depend upon the following assumptions  that are rarely present in real-world scenarios:

• The light is not moving

• The person's eyes are not moving (no gaze shift)

• Objects in the environment are not moving 

If the eyes move relative to the light, flicker up to at least 11,000 Hz can be visible as a phantom array (Brown et al., 2019). The stroboscopic effect can be visible if objects are rapidly moving under flickering light up to at least 10,000 Hz (Bullough et al., 2012). Davis et al. (2015) also suggest that at least 800 Hz flicker may be visible as flashes of light if the field of view isn't uniform and the eyes are allowed to move naturally. Together, these data refute the assumption that people can't see flicker above 100 Hz. They also suggest that if there is a point at which flicker becomes "invisible" to humans, it is at some point in excess of 11,000 Hz.

Synonyms for critical flicker fusion frequency (historic): critical period of duration, time differentiation threshold, duration of retinal impressions

Synonyms for flicker fusion (historic): Persistence of vision 

"Invisible" flicker: The variation in light intensity is fast enough that the light appears to human observers to have a constant intensity.  Historically, flicker with a frequency higher than the CFF has been called "invisible"and this kind of flicker is not usually consciously perceived as separate flashes of light. Flicker faster than 100 Hz has historically been considered to be "invisible" (reviewed in IEEE std 1789, 2015). However, there is now evidence that much of the flicker in the historic "invisible" range is actually visible too. At least 800 Hz flicker can be visible as separate flashes and at least 10,000 Hz flicker can be seen as a phantom array or indirectly via the stroboscopic effect (see notes and references for CFF). Together, these data refute the assumption that people can't see flicker above 100 Hz. They also suggest that if there is a point at which flicker becomes "invisible" to humans, it is at some point in excess of 11,000 Hz. Additionally, even though flicker faster than the historic CFF is often difficult or impossible to consciously see, sensitive individuals may experience pain, spatial disorientation, or nausea with immediate or rapid onset during exposure to flicker ≥100 Hz that makes them aware of the flicker even though they don't necessarily notice seeing discrete flashes of light. 

Synonyms for invisible flicker: Subliminal flicker

Phantom array effect: If a person's eyes and a flickering light move relative to each other, the light may be consciously seen as a series of discrete images that create a pattern across the person's field of view. For example,during a rapid gaze shift, a car's LED tail light may appear as multiple red lights in a pattern arranged from one side of a person's field of view to the other, rather than appearing as a single red blur.

Synonyms for phantom array effect: ghosting

Stroboscopic effect: An object moving rapidly across the field of view appears to move in choppy jumps due to its repeated, transient illumination by flickering ambient light. This may create the optical illusion that rapidly rotating objects are either stationary or rotating backwards, depending on the frequency of rotation relative to the flicker frequency. The stroboscopic effect can be dangerous when rotating industrial machinery appears to be stationary.

Temporal Light Artefacts (TLAs): Collective term for the various visible manifestations of flicker: visible flashes of light ("direct effect"), phantom arrays, and the stroboscopic effect.

Hertz (Hz): Unit of frequency that indicates how many times an event repeats each second (cycles per second). For flicker frequency, 120 Hz indicates that the light flashes 120 times a second.

Waveform: A graph of light output versus time. While incandescent lights have waveforms with sinusoidal shapes, LED lights can have one of an extremely large variety of waveform shapes. The waveforms often tested in flicker visibility studies include constant DC, sinusoidal, cut sinusoidal, and square. Actual LED lights sold to consumers have a seemingly near-infinite variety of different waveforms, many with complex patterns that have multiple pattern frequencies. Many such waveforms are shown on the Testing page of this site. 

Sinusoidal waveforms have rounded peaks and valleys and, when perfectly sinusoidal, have the general shape of the graph of the function y=sine(x).

Square waveforms have abrupt changes between off and on, so the luminosity vs. time waveform graph is line segments connected by 90° (square) angles. It looks like a row of separate tall buildings.

Constant Direct Current (DC) waveform is a flat line without any temporal modulation at all. 

Flicker metrics: Methods for numerically describing the light flicker waveform or for mathematically calculating a number describing some aspect(s) of the flicker waveform that could be used as estimates of the visibility of the flicker. 

The two flicker lighting standards documents described in the next section, IEEE 1789 (2015) and TM-39-25 (2025) both define several flicker metrics. IEEE 1789 describes the metrics flicker frequency, flicker percent, and flicker index and discusses the rationale for initial recommendations for limiting flicker in LED lights. TM-39-25 defines the flicker perception metric (Mp), flicker visibility measure (FVM), short-term flicker severity measure (PstLM), stroboscopic visibility measure (SVM), physiological percentage flicker (PPF), and phantom array visibility measure (PAVM). The flicker portion of Pacific Northwest National Laboratory Naomi Miller’s webinar on flicker and glare (after minute 30) describes several flicker metrics and has a technical review of LED flicker (Miller, 2019). Also see CIE TN-006, and a US Department of Energy graph of recommended limits on flicker. 

• Flicker frequency: how many times the flicker repeats per second, with units of Hertz (Hz) 

• Flicker percent (%): 100% * (max-min)/(max + min), where max and min refer to the maximum and minimum light output, respectively. 

Synonyms for flicker percent: modulation depth, % modulation, PF

• Flicker index: A value between 0 and 1; flicker index = (area above the mean light output)/(total area under the curve), referring to areas in a light output versus time graph of the flicker waveform. Unlike flicker frequency and % flicker, the flicker index depends on the shape of the flicker waveform. Occasionally in a minority of references, flicker index may be multiplied by 100% and then expressed as a percent.

• Duty cycle: the fraction of time that the waveform is above 10% of the maximum light output. The duty cycle is a value between 0 and 1 (sometimes expressed as a %) and is most often used to describe square waveforms. For example, a light with a 25% duty cycle is on a quarter of the time and off three quarters of the time.

• Flicker perception metric, Mp: A metric for estimating the visibility of direct flicker that can only be used for flicker from 5 Hz to 65 Hz. When Mp=1, an average observer can notice seeing visible flicker 50% of the time. A lower value for Mp indicates that the flicker is less visually noticeable. TM-39-25 gives the example that for a sinusoidal waveform with 30% flicker, Mp=1 at 54 Hz. This metric is an estimate of the visibility of the flicker for a theoretical "average" normal observer and does not predict the visibility of the flicker for any specific person. It does not predict the visibility of flicker for potentially sensitive subgroups (such as people susceptible to headaches or seizures).

• Short-term flicker severity measure, PstLM: A metric for estimating the visibility of direct flicker that can only be used for visible flicker <80 Hz. When PstLM=1, an average observer can notice seeing visible flicker 50% of the time. A lower value for PstLM indicates that the flicker is less visually noticeable. NEMA 77 recommends a limit of PstLM=1 for LED lights. This metric is an estimate of the visibility of the flicker for a theoretical "average" normal observer and does not predict the visibility of the flicker for any specific person. It does not predict the visibility of flicker for potentially sensitive subgroups (such as people susceptible to headaches or seizures).

• Stroboscopic Visibility Measure (SVM): A metric for estimating the visibility of the stroboscopic effect of flicker >80 Hz up to 2000 Hz. When SVM=1, an average observer can notice seeing flicker through the stroboscopic effect 50% of the time.  A lower value for SVM indicates that the flicker is less visually noticeable. NEMA 77 recommends a limit of SVM=1.6 for indoor LED lights and no limit for outdoor lighting. This SVM limit is much less stringent than the IEEE std 1789 (2015) NOEL recommendations. SVM was developed based on normal observers' observations of a spot on a rotating disc (Perz, M. et al., 2015) and, thus does not necessarily reflect people's ability to detect flicker through other tests of the stroboscopic effect or the phantom array effect. This metric is an estimate of the visibility of the flicker for a theoretical "average" normal observer and does not predict the visibility of the flicker for any specific person. It does not predict the visibility of flicker for potentially sensitive subgroups (such as people susceptible to headaches or seizures).

• Phantom Array Visibility Measure (PAVM): A metric for estimating the visibility of the phantom array effect of flicker (Tan et al, 2024). It was developed from visibility tests of 90 Hz to 2000 Hz flicker and then checked at higher frequencies using study data reporting the visibility of the phantom array effect up to 15,100 Hz. When PAVM=1, an average observer can notice seeing flicker through the phantom array effect 50% of the time.  A lower value for PAVM indicates that the flicker is less visually noticeable. This metric is an estimate of the visibility of the flicker for a theoretical "average" normal observer and does not predict the visibility of the flicker for any specific person. It does not predict the visibility of flicker for potentially sensitive subgroups (such as people susceptible to headaches or seizures).

Flicker (Temporal Light Modulation) Lighting Standards

Lighting standards are advice to the lighting industry for how to best design lights. Lighting standards for flicker have been developed both to reduce the visibility of flicker from lights and with the goal of limiting the harm to human health from light flicker. Flicker lighting standards are suggestions to manufacturers - there are not yet any regulations requiring adherence to such standards. Some health data exists for flicker below 90 Hz in regards to limiting seizures and has been considered when creating flicker standards. However, no formal health data for flicker above 90 Hz was collected to inform flicker standards. Individuals potentially sensitive to health effects of flicker were excluded from the flicker visibility studies that were the basis of creating flicker visibility standards.

Until 2025, a commonly-cited flicker standard was IEEE std 1789, 2015). It includes a graph of % flicker vs. flicker frequency and shades in different regions of this graph based on risk to people. This graph has been shown on flicker meters. While only flicker visibility studies were the basis for the IEEE 1789 recommendations above 90 Hz, this document very strongly implies that IEEE 1789 also predicts the risk to human health of flicker at various frequencies and modulation depths, and thus strongly implies that it predicts safety. However, There is no evidence to support such claims and increasing evidence from people anecdotally reporting injuries from LEDs provide evidence that flicker visibility does not predict health outcomes.

No Observable Effect Level (NOEL): Proposed limit on LED flicker in IEEE std 1789 (2015) that combines flicker percent and frequency. Although purported to predict health outcomes, it is actually only based on flicker visibility data, not on any health data above 90 Hz. Below 90 Hz, the NOEL recommendation is that flicker percent should be less than 0.01 x frequency. Above 90 Hz, the NOEL recommendation is that flicker percent should be less than 0.0333 x frequency. This standard is now superseded by TM-39-25. See Background: Heath Effects of ≥100 Hz flicker for an analysis of the basis for the NOEL recommendation.

In April 2025, the Illuminating Engineering Society and American National Standards Institute released new guidance on minimizing the visibility of flicker in architectural lighting (ANSI/IES TM-39-25) in which they emphasize that minimizing flicker visibility may be insufficient to prevent adverse biological health effects and that significant research in several areas is needed to begin to develop safety recommendations for LED lights. TM-39-25 comments upon the language in IEEE1789 that implies those flicker criteria were sufficient to limit biological harm. TM-39-25 states that IEEE 1789 was based only on flicker visibility data, not other health data for flicker above 90 Hz and is unlikely to prevent harm to human health. (See Background: Heath Effects of ≥100 Hz flicker). TM-39-25 essentially updates and replaces the older IEEE 1789 (2015) as a standard for minimizing flicker visibility. TM-39-25 also acknowledges that the language in IEEE 1789 suggesting that invisible flicker was safe for human health was "aspirational" thinking not based on evidence (for flicker over 90 Hz) and says  "there is growing recognition that while IEEE 1789 limits were necessary as a first step, they are unlikely to be sufficient to prevent ham to human health in some populations." 

The April 2025 Illuminating Engineering Society and American National Standards Institute guidance on minimizing the visibility of flicker in architectural lighting (ANSI/IES TM-39-25) describes lights that are "better" in terms of having low flicker visibility as having either  

("Metric Method") the combination all of these three: 

Mp ≤ 0.4

SVM ≤ 0.4

PAVM ≤ 0.4 

or

 ("Alternate Frequency Method")

flicker frequency ≥ 10,000Hz for sinusoidal waveforms 

or 

flicker frequency ≥ 20,0000 Hz for all other waveforms 

or 

Direct Current (DC) waveform without any temporal modulation at all. 

TM-39-25 emphasizes that minimizing flicker visibility using the above "better" standard may be insufficient to prevent harm to human health and that health-related research is needed. TM-39-25 recommends that to protect human health, lights should either not flicker at all (constant DC waveform), or if that is not possible, proposes an untested idea that flicker at frequencies above 25,000 Hz without lower-frequency subpatterns, might be sufficient. (See Background: Heath Effects of ≥100 Hz flicker).  

Examples of categories of flicker: 

• Pulse width modulation (PWM):  A strategy for dimming lights by decreasing the duty cycle of the light. In the lighting industry, "PWM" refers only to this specific strategy for dimming lights. In the simplest example, PWM dimming repeatedly and rapidly turns the light off for a fraction of the time, creating the illusion that the light is dimmer since people only visually perceive the average brightness if the flicker is fast enough.

• Forms of full-screen flicker:

  • "PWM" - sometimes this may be actual PWM of the backlight or of all of the OLEDs in a display. Other times, screen websites may use "PWM" to refer more general to backlight flicker or other full-display flicker, whether or not it fits the technical definition of PWM (screen flicker measurements are difficult to obtain (see.Nick Sutrich's Android Central article on PWM dimming of displays)..

  • "DC-like dimming" for some OLED displays avoids harsh power cuts of PWM dimming, but has a more moderate dip in display brightness in time with the refresh rate (see.Nick Sutrich's Android Central article on PWM dimming of displays).

• Screen pixel flicker:

  • Pixel flicker to create a basic color palette (I haven't found a specific term for this):  flickering each red, green, or blue subpixel between brighter and dimmer values to create the specified color.

  • LCD pixel inversion: Reversals of the direction of the current supplied to the subpixels shifts the position of light transmission through the subpixel and may also alter the brightness.

  • Temporal dithering: Extra pixel flicker added by the computer's operating system or graphics processor to expand the color palette and smooth gradients by flickering pixels between different colors. 

  • Frame rate control (FRC): A form of temporal dithering, typically referring to when a display uses temporal dithering to expand the color palette and smooth gradients. For example a display that can natively produce 8 bit color may use FRC to produce 10 bit color (see full explanation in LED screens).

  • Other forms of pixel flicker, including font smoothing and anti-aliasing which use flicker to smooth the edges of otherwise jagged text or shapes, respectively.

References

Brown, E., Foulsham, T., Lee, C-s., Wilkins A.. Visibility of temporal light artefact from flicker at 11kHz. Lighting Research and Technology, 52, 371-376 (2019).  https://www1.essex.ac.uk/psychology/overlays/2019-249.pdf

Bullough et al. Detection and acceptability of stroboscopic effects from flicker. Lighting Research & Technology 44 (2012) 477-483; first published online 2011. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.876.126&rep=rep1&type=pdf

Davis, J., Hsieh, YH. & Lee, HC. Humans perceive flicker artifacts at 500 Hz. Sci Rep 5, 7861 (2015). https://doi.org/10.1038/srep07861

Illuminating Engineering Society. ANSI/IES TM-39-25, Technical Memorandum: Quantification and Specification of Visual Responses to Temporal Light Modulation (a.k.a. Flicker). New York: IES; 2025. https://store.ies.org/product/technical-memorandum-quantification-and-specification-of-visual-responses-to-temporal-light-modulation-a-k-a-flicker/?v=0b3b97fa6688 

The Institute of Electrical and Electronics Engineers, Inc. IEEE Std 1789™-2015: IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers. 2015. http://www.bio-licht.org/02_resources/info_ieee_2015_standards-1789.pdf

Miller, Naomi. Metrics in Motion: Flicker and Glare. July 11, 2019 webinar. https://www.ies.org/lighting-education-facility-showcase/metrics-in-motion-flicker-and-glare/

Perz, M. et al., Modeling the Visibility of the Stroboscopic Effect Occurring in Temporally Modulated Light Systems. Lighting Research and Technology, 47, 281-300 (2014). https://doi.org/10.1177/1477153514534945

Tan J, Miller N, Royer M, Irvin L. Temporal light modulation: A phantom array visibility measure. Lighting Research & Technology. 2024;56(7):772-789. doi:10.1177/14771535241239611

Wilkins, A. CFF is a seductive but misleading concept. Lighting Research and Technology, 46, 368 (2014). http://www1.essex.ac.uk/psychology/overlays/2014-222.pdf