The alternation of light and dark which can be visually perceived.

A phenomenon where a lamp blinks repeatedly, often caused by power supply issues, or a faulty ballast or driver.

Flicker is a directly visible change in brightness of a light source which can be due to fluctuations of the light source itself, or due to external causes such as due to rapid fluctuations in the voltage of the power supply (power-line flicker) or incompatibility with an external dimmer.

Various scientific committees have assessed the potential health, performance and safety-related aspects resulting from temporal light modulations (TLMs) including light flicker. Adverse effects of flicker include annoyance, reduced task performance, visual fatigue, headache and epileptic attack by photosensitive persons. The visibility aspects of flicker are given in a technical note of CIE; see CIE TN 006:2016. In general, undesired effects in the visual perception of a human observer induced by light intensity fluctuations are called Temporal Light Artefacts] (TLAs).

Root causes

The light emitted from lighting equipment such as luminaires and lamps may vary in strength as a function of time, either intentionally or unintentionally. Intentional light variations are applied for the warning, signaling (e.g. traffic-light signalling, flashing aviation light signals), entertainment (like stage lighting) with the purpose that flicker is perceived by people. Generally, the light output of lighting equipment may also have unintentional light level modulations due to the lighting equipment itself. The magnitude, shape, periodicity, and frequency of the TLMs will depend on many factors such as the type of light source, the electrical mains-supply frequency, the driver or ballast technology and type of light regulation technology applied (e.g. pulse-width modulation). If the modulation frequency is below the flicker fusion threshold and if the magnitude of the TLM exceeds a certain level, then such TLMs are perceived as flicker. These TLM-properties may vary over time due to aging effects. Component failure within lighting equipment or end-of-life behavior of lighting equipment can also give rise to flicker. Furthermore, external factors such as incompatibility with dimmers or the presence of mains-supply voltage fluctuations (power-line flicker) are root causes of flicker.

Flicker can also be perceived from naturally modulated light sources like candlelight or a sunlit water surface or it may be experienced while driving along a row of trees lit by the sun. TLMs and resulting flicker can be seen also while driving with a certain speed along a street or through a tunnel lit by lighting equipment positioned with regular spacing.


Temporal light modulations become visible if the modulation frequency is below the flicker fusion threshold and if the magnitude of the TLM exceeds a certain level.

There are much more factors that determine the visibility of TLMs as flicker:

The shape properties of the light waveform (e.g. sinusoidal, rectangular pulse and its duty cycle); see Figure 1;
The average light level of the light source and its contrast with the background light level within the environment;
The viewing angle and changes in direction of view by the observer;
Physiological factors such as age and fatigue.

All observer-related influence quantities are stochastic parameters because not all humans perceive the same light ripple in the same way. That is why the perception of flicker is always expressed with a certain probability. Detailed explanations on the visibility of flicker and other temporal light artifacts are given in CIE TN 006:2016 and in a recorded webinar “Is it all just flicker?”.

Adapted from content published on wikipedia.org
Last modified on December 5, 2019, 11:46 pm
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