In visual perception, flicker is a human-visible change in luminous flux of an illuminated surface or 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.
Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium.
Flicker exists for other organisms having different perceptual thresholds.
Light meters and image sensors can potentially detect flicker at much higher frequency bands than human vision. Shutter speeds used in motion photography can interact with high frequency flicker to produce visual artifacts in the captured imagery that betray flicker that would not otherwise be noted.
The spectral sensitivity of the human eye to flicker depends upon the mode of visual perception. Due to the flicker fusion threshold of foveal vision, steady vision can rarely detect flicker above 90 Hz, whereas flicker can be perceived during visual saccades up to or beyond 1 kHz.
Flicker due to mechanical factors such as AC line frequency (typically 50 or 60 Hz) will have a stable frequency structure, whereas the flicker of a damp or failing light bulb will often have a chaotic or erratic frequency structure.
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 of 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).
Light emitted from lighting equipment such as luminaires and lamps may vary in strength as function of time, either intentionally or unintentionally. Intentional light variations are applied for warning, signalling (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 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 candle light 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 a 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 levels.
There are much more factors that determine the visibility of TLMs as flicker:
All observer-related influence quantities are stochastic parameters, because not all humans perceive the same light ripple in the same way. That is why perception of flicker is always expressed with a certain probability. Detailed explanations on the visibility of flicker and other temporal light artefacts are given in CIE TN 006:2016 and in a recorded webinar “Is it all just flicker?”.
For objective assessment of flicker, a widely applied and IEC-standardized metric, the short-term flicker indicator (PstLM) is used. This metric is derived from the short term flicker severity metric PstV that is applied in the power quality domain for testing electrical apparatus for their potential to cause flicker via voltage fluctuations on the electrical mains network (see the IEC publications IEC 61000-3-3 and IEC 61000-4-15). The short-term flicker indicator PstLM is implemented in a light flickermeter which processes the light measured by means of a light sensor. The light flickermeter consists of four processing blocks which include weighting filters to account for frequency dependency of the visibility of TLMs as well as statistical processing to enable assessing a-periodic TLMs. The specification of the light flickermeter and the test method for objective flicker assessment of lighting equipment is published in IEC technical report IEC TR 61547-1.
It is recommended to calculate the value of PstLM using a light waveform recorded for at least three minutes. This enables proper assessment of flicker occurring at low repetition frequencies.
NOTE - Several alternative metrics such as Modulation Depth, Flicker Percentage or Flicker Index are being applied. None of these metrics are suitable to predict actual human perception because human perception is impacted by modulation depth, modulation frequency, wave shape and if applicable the duty cycle of the TLM.
The perceptibility scale of PstLM (and PstV) is chosen such that a value of 1.0 corresponds to a level at which 50% of human test subjects judge the flicker to be both noticeable and irritating (Figure 1).
The objective light flickermeter can be applied for different purposes (see Figure 2 and IEC TR 61547-1):
Other examples of light flicker can sometimes be associated with ceiling fans or wind turbines This occurs when the rotation of the blade continuously blocks the light source (i.e. indoor ceiling light or Sun), causing visual flicker.