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

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

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, 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 a variety of waveform shapes, including constant, sinusoidal, cut sinusoidal, and square.

Flicker metrics: Methods for numerically describing the light flicker waveform. The IEEE report (IEEE std 1789, 2015) describes flicker frequency, flicker percent, and flicker index and discusses the rationale for initial recommendations for limiting flicker in LED lights. The flicker portion of Pacific Northwest National Laboratory Naomi Miller’s webinar on flicker and glare (after minute 30) provides an update on more recently-developed flicker metrics and 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 and is most often used to describe square waveforms.

  • Short-term flicker indicator, Pst: A metric for visible flicker <80 Hz. When Pst=1, an average observer can detect visible flicker 50% of the time. A lower value for Pst indicates that the flicker is less noticeable. NEMA 77 recommends a limit of Pst=1 for LED lights.

  • Stroboscopic Visibility Measure (SVM): A metric for flicker >80 Hz up to 2000 Hz. When SVM=1, an average observer can detect flicker through the stroboscopic effect 50% of the time. A lower value for SVM indicates that the flicker is less 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.

No Observable Effect Level (NOEL): Proposed limit on LED flicker in IEEE std 1789 (2015) that combines flicker percent and frequency. 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. See Background: Heath Effects of ≥100 Hz flicker for an analysis of the basis for the NOEL recommendation.


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).

Bullough et al. Detection and acceptability of stroboscopic effects from flicker. Lighting Research & Technology 44 (2012) 477-483; first published online 2011.

Davis, J., Hsieh, YH. & Lee, HC. Humans perceive flicker artifacts at 500 Hz. Sci Rep 5, 7861 (2015).

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.

Miller, Naomi. Metrics in Motion: Flicker and Glare. July 11, 2019 webinar.

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).

Wilkins, A. CFF is a seductive but misleading concept. Lighting Research and Technology, 46, 368 (2014).