Flicker (light)

Last updated

In visual perception, flicker is a human-visible change in luminance 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.

Contents

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. [1]

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.

Temporal light modulation

Temporal light modulation (TLM) is a slightly broader concept defined by the International Commission on Illumination (CIE) as fluctuation in luminous quantity or spectral distribution of light with respect to time. [2] The effect is typically connected to lighting products such as lamps and luminaires where the light is modulated in order to provide some functionality, such as dimming or color change. TLM can cause temporal light artifacts (TLA) such as the stroboscopic effect or phantom array effect. TLM has been linked to headache and migraine, and in rare cases epileptic seizures. [3]

Effects

Various scientific committees have assessed the potential health, performance and safety-related aspects resulting from TLMs, including light flicker. [4] [5] [6] 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. [7] 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

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. [8] Furthermore, external factors such as incompatibility with dimmers or presence of mains-supply voltage fluctuations (power-line flicker) are root causes of flicker. [9]

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. [10]

Visibility

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 [7] and in a recorded webinar "Is it all just flicker?". [11]

Objective assessment of flicker

Light flickermeter

Figure 1: Flicker visibility threshold curves for three different types of light modulations; values of modulation depth as a function of modulation frequency at which flicker is perceivable on average (Pst=1 curves) Figure4Wiki flicker sensitivity curves.jpg
Figure 1: Flicker visibility threshold curves for three different types of light modulations; values of modulation depth as a function of modulation frequency at which flicker is perceivable on average (Pst=1 curves)

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 [12] and IEC 61000-4-15 [13] ). 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 [14] [15] 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. [16]

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.

Matlab toolbox

A Matlab light flicker assessment toolbox including a function for calculating PstLM and some application examples are available [17] on the Matlab Central via the Mathworks Community.

Acceptance criterion

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).

Figure 2: Generic setup to test lighting equipment for its flicker performance. Figure4Wiki flicker influence quantities.jpg
Figure 2: Generic setup to test lighting equipment for its flicker performance.

Test and measurement applications

The objective light flickermeter can be applied for different purposes (see Figure 2 and IEC TR 61547-1 [16] ):

Publications of standards development organisations

  1. IEC TR 61457-1:2017: [16] light flickermeter specification and verification method, and test procedure for voltage fluctuation immunity and dimmer compatibility.
  2. NEMA 77-2017: [18] among others, flicker test Methods and guidance for acceptance criteria.

Ceiling fans and wind turbines

Other examples of light flicker can sometimes be associated with ceiling fans [19] or wind turbines [20] This occurs when the rotation of the blade continuously blocks the light source (i.e. indoor ceiling light or Sun), causing visual flicker.

See also

Related Research Articles

<span class="mw-page-title-main">Pulse-width modulation</span> Representation of a signal as a rectangular wave with varying duty cycle

Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle.

<span class="mw-page-title-main">Fluorescent lamp</span> Lamp using fluorescence to produce light

A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than an incandescent lamp, but is less efficient than most LED lamps. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. For comparison, the luminous efficiency of an incandescent bulb may only be 16 lumens per watt.

The flicker fusion threshold, also known as critical flicker frequency or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is a concept studied in vision science, more specifically in the psychophysics of visual perception. A traditional term for "flicker fusion" is "persistence of vision", but this has also been used to describe positive afterimages or motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.

<span class="mw-page-title-main">Stroboscopic effect</span> Visual phenomenon

The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples at a sampling rate close to the period of the motion. It accounts for the "wagon-wheel effect", so-called because in video, spoked wheels sometimes appear to be turning backwards.

Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform. In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.

<span class="mw-page-title-main">Hydrargyrum medium-arc iodide lamp</span>

Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp, made specifically for film and entertainment applications. Hydrargyrum comes from the Greek name for the element mercury.

<span class="mw-page-title-main">Wagon-wheel effect</span> Optical illusion

The wagon-wheel effect is an optical illusion in which a spoked wheel appears to rotate differently from its true rotation. The wheel can appear to rotate more slowly than the true rotation, it can appear stationary, or it can appear to rotate in the opposite direction from the true rotation.

<span class="mw-page-title-main">Visible light communication</span> Use of light in the visible spectrum as a telecommunication medium

In telecommunications, visible light communication (VLC) is the use of visible light as a transmission medium. VLC is a subset of optical wireless communications technologies.

<span class="mw-page-title-main">Electrical ballast</span> Device to limit the current in lamps

An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.

The Comité International Spécial des Perturbations Radioélectriques was founded in 1934 to set standards for controlling electromagnetic interference in electrical and electronic devices and is a part of the International Electrotechnical Commission (IEC).

0–10 V is one of the first and simplest electronic lighting control signaling systems, used as an early fluorescent dimming system. Simply put, the control signal is a DC voltage that varies between zero and ten volts. Two standards are recognized: current sourcing and current sinking.

<span class="mw-page-title-main">Multifaceted reflector</span> Light bulb

A multifaceted reflector light bulb is a reflector housing format for halogen as well as some LED and fluorescent lamps. MR lamps were originally designed for use in slide projectors, but see use in residential lighting and retail lighting as well. They are suited to applications that require directional lighting such as track lighting, recessed ceiling lights, desk lamps, pendant fixtures, landscape lighting, retail display lighting, and bicycle headlights. MR lamps are designated by symbols such as MR16 where the diameter is represented by numerals indicating units of eighths of an inch. Common sizes for general lighting are MR16 and MR11, with MR20 and MR8 used in specialty applications. Many run on low voltage rather than mains voltage alternating current so require a power supply.

<span class="mw-page-title-main">LED lamp</span> Electric light that produces light using LEDs

An LED lamp or LED light is an electric light that produces light using light-emitting diodes (LEDs). LED lamps are significantly more energy-efficient than equivalent incandescent lamps and fluorescent lamps. The most efficient commercially available LED lamps have efficiencies exceeding 200 lumens per watt (lm/W) and convert more than half the input power into light. Commercial LED lamps have a lifespan several times longer than both incandescent and fluorescent lamps.

Power-line flicker is a visible change in brightness of a lamp due to rapid fluctuations in the voltage of the power supply. The voltage drop is generated over the source impedance of the grid by the changing load current of an equipment or facility. These fluctuations in time generate flicker. The effects can range from disturbance to epileptic attacks of photosensitive persons. Flicker may also affect sensitive electronic equipment such as television receivers or industrial processes relying on constant electrical power.

IEC 61000-3-2Electromagnetic compatibility (EMC) – Part 3-2: Limits – Limits for harmonic current emissions is an international standard that limits mains voltage distortion by prescribing the maximum value for harmonic currents from the second harmonic up to and including the 40th harmonic current. IEC 61000-3-2 applies to equipment with a rated current up to 16 A – for equipment above 16 A see IEC 61000-3-12.

Temporal light effects (TLEs) is the general term for all possible effects resulting from temporal light modulations (TLMs).

<span class="mw-page-title-main">Temporal light interference</span>

Temporal light interference (TLI) is an unacceptable degradation of the performance of an equipment or system that has an optical input for its intended functioning and is caused by a temporal light modulation disturbance. A temporal light modulation (TLM) disturbance may be either an intentional or unintentional temporal light modulation (TLM) of lighting equipment such as luminaires or lamps. Examples of equipment that can be interfered are barcode scanners, cameras and test equipment.

Temporal light artefacts (TLAs) are undesired effects in the visual perception of a human observer induced by temporal light modulations. Two well-known examples of such unwanted effects are flicker and stroboscopic effect. Flicker is a directly visible light modulation at relatively low frequencies and small intensity modulation levels. Stroboscopic effect may become visible for a person when a moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz) and larger intensity variations.

References

  1. Wilkins, Arnold J (27 July 2017). "The scientific reason you don't like LED bulbs — and the simple way to fix them". theconversation.com. The Conversation . Retrieved 3 August 2021.
  2. "CIE TN 012:2021 Guidance on the Measurement of Temporal Light Modulation of Light Sources and Lighting Systems". CIE - International Commission on Illumination. 2021. doi:10.25039/TN.012.2021 . Retrieved 3 October 2023.
  3. CIE 249:2022 Visual Aspects of Time-Modulated Lighting Systems. CIE - International Commission on Illumination. 2022. doi:10.25039/TR.249.2022. ISBN   978-3-902842-68-8.
  4. IEEE Std 1789:2015, IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers (link).
  5. SCENIHR (Scientific Committee on Emerging and Newly Identified Health Risks), Health effects of artificial light , 19 March 2012 ( ISBN   978-92-79-26314-9).
  6. SCHEER (EC Scientific Committee on Health, Environmental and Emerging Risks), Final Opinion on potential risks to human health of Light Emitting Diodes (LEDs), June 2018.
  7. 1 2 CIE TN 006:2016, Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models (pdf).
  8. Fluorescent lamp end-of-life flicker problems.Fluorescent lamp#Flicker problems
  9. R. Cai, Flicker interaction studies and flickermeter improvement, PhD thesis , Eindhoven : Technische Universiteit Eindhoven, 2009.
  10. See flicker problems in the Queensway Tunnel.
  11. Sekulovski, Dragan; Signify (Apr 4, 2016). "Is it all Flicker?". YouTube. Archived from the original on 3 September 2020. Retrieved 1 March 2018.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  12. IEC 61000-3-3 (ed. 3.1), Electromagnetic compatibility (EMC) – Part 3-3: Limits – Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤ 16 A per phase and not subject to conditional connection.
  13. IEC 61000-4-15 (ed. 2), Electromagnetic compatibility (EMC) –Part 4-15: Testing and measurement techniques – Flickermeter –Functional and design specifications.
  14. J. Drápela, J. Šlezingr, A light-flickermeter – Part I: Design, Proceedings 11th International Scientific Conference Electric Power Engineering 2010, pp. 453.
  15. Lance Frater, Light Flicker and Harmonic Modelling of Electrical Lighting, PhD thesis, University of Canterbury, Christchurch, New Zealand, 2015.
  16. 1 2 3 IEC TR 61547-1 (ed. 3), Equipment for general lighting purposes – EMC immunity requirements – Part 1: An objective light flickermeter and voltage fluctuation immunity test method.
  17. Light flicker assessment toolbox Matlab Central
  18. NEMA 77-2017: Temporal Light Artifacts: Test Methods and Guidance for Acceptance Criteria.
  19. Kent, Michael; Cheung, Toby; Li, Jiayu; Schiavon, Stefano (2020). "Experimental evaluation of visual flicker caused by ceiling fans" (PDF). Building and Environment. 182: 107060. Bibcode:2020BuEnv.18207060K. doi:10.1016/j.buildenv.2020.107060. S2CID   225305290.
  20. Harding, Graham; Harding, Parmela; Li, Arnold; Wilkins, Stefano (2008). "Wind turbines, flicker, and photosensitive epilepsy: Characterizing the flashing that may precipitate seizures and optimizing guidelines to prevent them". Epilepsia. 46 (6): 1095–1098. doi: 10.1111/j.1528-1167.2008.01563.x . PMID   18397297. S2CID   16197834.