Flickering, stroboscopic or flickering illumination is a phenomenon in which the intensity of light (or its spectral composition or spatial distribution) varies over time and the user perceives this variation directly as a disturbing perception or indirectly as an optical illusion.

 

Biological effects

Some diseases are characterized by increased acquisition of the rhythm of passing light by brain waves. Periodic changes in light can trigger a seizure in people with rare photosensitive epilepsy. Sensitivity to contrasting periodic patterns in the temporal and spatial domains has also been described in migraine. Erratic light also causes dilation of ocular blood vessels and increases blood flow through the retina [1], suggesting that the eye uses more energy during such exposure. Blinking lighting is also associated with visual fatigue, a decrease in work performance or poorer subjective assessment of lighting.

 

Stroboscopic phenomenon

When rotating objects are illuminated with a soft light, a stroboscopic effect can occur, in which the observer has the impression that the observed object is at rest or moving at a lower speed or in the opposite direction. Based on this misinformation, he may, for example, touch a rotating part of a machine tool or the blade of a running fan and cause injury. The rectilinear motion of periodic structures may appear jerky in a dim light, which distracts attention from the activity being performed.

 

A series of apparent images

(ghosting, phantom array)

If a source of passing light or an object illuminated in this way is in the field of vision, the individual sections of the retina are exposed with different intensities as the eye moves. For example, the flashing LED brake lights of a car appear as an intermittent series of apparent images during rapid (saccadic) eye movement, which may surprise the driver, briefly fix his attention, and thus increase reaction time.

 

Application

However, the flickering or flashing light has its uses. The strobe effect is used, for example, for touchless speed measurement, for adjusting motors, for examining vocal cords, for creating the effect of staggered movement during dance creations or at discos. Thanks to synchronised flashes, phases of rapid action can be photographed or filmed. Flickering light is used to activate brain waves in EEG. In the elderly, flashing light is known to slightly increase scores on cognitive tests. Audio-visual stimulation devices (AVS, psychowalkmans) work on the principle of flashing light combined with sound. Recent research points to the possibility of using flashing light in patients with Alzheimer's disease [2].

 

Temporal light artifacts (TLA)

This collective English name is used for phenomena related to the perception of temporal changes in illumination. It can be variations in intensity, chromaticity or spectral composition.

 

Standards

The standard [3, p. 17] requires that lighting systems be designed to avoid flicker and strobe effects. With electronic ballasts for fluorescent lamps, this requirement could be considered fulfilled, but with the advent of light-emitting diodes (steep volt-ampere characteristic; luminous flux follows changes in current very quickly) and the great variety of their power supply circuits, it has again come into focus.

Flicker (also flikr)

According to [4, p. 28], flicker is a sensation of unstable visual perception caused by a light stimulus whose brightness or spectral distribution varies over time. These terms are used in the fields of electromagnetic compatibility (EMC) and power quality (PQ).

Scrambling

According to [5, p. 45], flicker is a subjective impression of instability of visual perception caused by a stimulus whose brightness or spectral composition varies. It is used in lighting technology.

In English, both are called flicker and both definitions are the same in English. The definition according to the CIE recommendation [6, p. 7] adds the condition of a static observer in a static environment.

The standard [7, p. 57] defines mixing as the change in luminous flux of a light source or luminaire due to voltage fluctuations of its power supply.

Passing visibility

Passing can be observed if its frequency is lower than the merging frequency. This is approximately 40 to 50 Hz under normal conditions (hundreds of lux). A limiting value (for higher light conditions) is given as 60 to 90 Hz. Above the Talbot's law spill frequency, only the mean value of the fluctuating stimulus is registered. People are most sensitive to flicker frequencies of 7 to 13 Hz, see Figure 1.

Fig. 1. Frequency dependence of the modulation depth at which the flicker is at the limit of observability - lower value represents higher sensitivity [7, p. 49]

In the field of sharp vision, one perceives red light flickering most strongly, in the peripheral field blue light flickering dominates. The different temporal responses of the visual apparatus to individual colours are illustrated by an experiment called Fechner colours, where a colour perception is created on the retina when a rotating black and white pattern is observed.

The observer is more likely to see the flicker in reflected light or as a disturbance in the peripheral visual field. However, when looking directly at a passing light source or luminaire, the passing may not be apparent. Above the flicker frequency, the light fluctuations are no longer directly visible, but can be observed as a stroboscopic phenomenon (when an object is moving) or a series of apparent images (when the observer's eye is moving). The cutoff frequency is usually given as 3 kHz.

Electrical causes

The electrical causes of light scattering generated by light sources and luminaires can be divided into:

- in a trouble-free state:

• caused by fluctuations or changes in the supply (mains) voltage,
• caused by the actual design or internal wiring of the light source or luminaire, its components or interference between its components;

- switching failure under normal power supply conditions (e.g. end of life of light source, degradation of power supply component).

 

Power quality

From the point of view of power quality, it is desirable that the supply voltage does not contain waveforms that cause flickering of the lighting. These are in particular phenomena affecting the peak voltage value, such as normal fluctuations in voltage magnitude, rapid voltage drops and increases, or interharmonic components. For details, the reader is referred to EN 50160, the Rules for the Operation of Distribution Systems and their annexes, or to the company standards PNE 33 3430 or the international standard IEEE Std 1453™.

 

EMC of luminaires

From the point of view of design, it is desirable that luminaires and their individual electrical components should be adequately resistant to disturbances from the mains, while at the same time not contributing more than is reasonable. The electromagnetic immunity of luminaires is specified in EN 61547, which includes, inter alia

requirements for the behaviour of the equipment during short-term voltage drops. The limitation of voltage variations, voltage fluctuations and flicker generated by equipment connected to the public supply network is addressed in EN 61000-3-3 and -11, but luminaires and light sources are not normally considered as sources of these types of disturbance.

 

"Self-shuffling"

Even if the power quality and electromagnetic compatibility requirements are met, the user may not be satisfied with the lighting in terms of mixing. This may be due to the design of the power supply used - in combination with the fast response of the light emitting diodes. The power supply may be a component of the luminaire, a separate unit (adapter) or integrated in the light source (LED replacement bulbs). With the current pressure for miniaturisation and price competitiveness, it is easy to come across power components (including those integrated in light sources) where the manufacturer has not thought too much about mixing. It is therefore necessary to be able to compare individual light sources and their power components in terms of the time course of illumination they provide under normal conditions - preferably by converting the time course of the light quantity into a number. There are several such procedures, ranging from simple to complex.

 

Light Waviness Factor

The light waviness factor is probably the oldest measure of scattering and is defined by [9, p. 193]

@rovnice1@

where Фmin and Фmax are the global minimum and maximum of the luminous flux Ф(t) per period, see Fig. 2. Synonyms are modulation depth, Michelson contrast or flicker percentage. Other notations are FP, Fpercent or mod%. With a dark background, the instantaneous luminous flux can be replaced by the instantaneous representative illuminance. This has the advantage of being easy to understand and, in the past, easy to implement using analogue minimum and maximum detectors. The disadvantage is that this procedure does not take into account frequency or waveform, which are essential for blending. This allows comparisons to be made between similar light sources passing at the same frequency, e.g. different fluorescent lamps operating at mains frequency, or fluorescent lamps with different degrees of electrode wear. The red waveform in Figure 2 illustrates the Ф(t) of a fluorescent lamp with one electrode partially deactivated. A typical value of kf under mains power is 7% for a 60W bulb and about 30% to 60% for a fluorescent lamp [9, p. 194].

@obr2@
Fig. 2. The light waviness factor kf is the same for all three waveforms

 

Passing index

This quantity was designed as an improved replacement of kf and is defined by the relation [9, p. 193]

@rovnice2@

Where A1 is the area bounded by Ф(t) above the mean value line, A2 is the area bounded by Ф(t) below the mean value line, see Figure 3. The English name is the flicker index and the designation FI or Findex. It is given as a dimensionless number from 0 to 1 so as not to be confused with the flicker factor (in percent). Compared to kf, the integral approach partially accounts for the shape of the waveform. This approach, like kf, does not take frequency into account. A typical value at mains supply is 0.03 for a 60W bulb and approximately 0.1 for a fluorescent lamp, which is also the recommended maximum value [9, p. 194].

@obr3@
Fig. 3. The mixing index f is the same for all three waveforms

 

Spectral methods

The desire to create a more waveform-aware measure has led to the development of various spectral methods such as FVM, SVM or Assist Mp. These methods are based on a discrete Fourier transform that converts the measured time waveform into its frequency spectrum. The amplitudes of the individual spectral components are summed after multiplication by the weighting factors for each frequency. If the phase component of the spectrum is not taken into account in this process, such a scale gives a value for the least favourable of all time histories corresponding to the amplitude component of the spectrum. See Figure 4 for a comparison of two such waveforms.

@obr4@
Fig. 4. Two different waveforms with the same amplitude component of the spectrum

 

Objective measurement

The design and characteristics of the flicker meter (flicker meter) are addressed in standard EN 61000-4-15, which contains a methodology for determining the short-term and long-term flicker perception rate from the voltage waveform. At the core of the instrument is a cascade of filters and quadratures that model a 60W incandescent lamp (which was the reference light source at the time of the meter's development) and the response of the average observer's eye/brain under reference observing conditions to changes in light, and whose output is processed by statistical methods. In October 2017, the revised IEC TR 61547-1 was published, which describes an objective flicker/flicker meter suitable also for new light sources and provides a methodology for determining flicker perception measures from both voltage and illuminance waveforms. The literature contains, among others, references to the works of Assoc. Drápela and Ing. Šlezingr from Brno University of Technology. Publicly available software implementations of these algorithms for MatLab® or the freely available GNU Octave allow, for example, to process waveforms recorded with a digital oscilloscope or to perform simulations.

 

Measuring instruments

Professional flicker/flicker meters are usually part of EMC test instruments or power quality analyzers that evaluate the voltage time history in accordance with EN 61 0004-15. The design of an objective flicker meter evaluating the time history of the light flux is described in detail in [8]. New instruments are likely to be designed in accordance with IEC TR 61547-1. Orientation measurements For quick orientation in luminaire development or component selection, it is often sufficient to measure the current flowing through the LEDs, to which the luminous flux is proportional over a wide range. If this is not possible, the current waveform can be obtained from the voltage waveform of the LEDs using their (steep) volt-ampere characteristics. To measure the instantaneous (representative) illuminance, a simple probe with an integrated light-to-voltage converter, e.g. TLS257, described in [10], can be connected to the oscilloscope. There are also handheld flicker meters on the market that display the frequency, wavelet factor and flicker index, the visibility of the SVM stroboscopic phenomenon or the waveform.

 

Conclusion

Flicker has almost been forgotten, but the advent of light-emitting diodes has brought it back as an important phenomenon in lighting technology that affects both safety and user satisfaction with lighting. The next section will be devoted, among other things, to flicker in recording technology.

 

Used and recommended literature:

[1] GARHÖFER, G. et al. Diffuse luminance flicker increases blood flow in major retinal arteries and veins. Vision Research. 2004, 44(8), 833-838. ISSN 00426989. Also available from: https://goo.gl/jdDMdn

[2] IACCARINO, Hannah F. et al. Gamma frequency entrainment attenuates amyloid load and modifies microglia. Nature. 2016, 540(7632), 230-235. DOI: 10.1038/nature20587. ISSN 0028-0836. Also available from: http://www.nature.com/doifinder/10.1038/nature20587

[3] EN 12464-1:2012. Light and lighting - Lighting of work areas - Part 1: Indoor work areas.

[4] CSN IEC 50(161):1993+A1:1999+A2:2000. International Electrotechnical Dictionary. Chapter 161: Electromagnetic compatibility.

[5] ČSN IEC 50(845):1996+Z1:2000. International electrotechnical dictionary. Chapter 845: Lighting.

[6] CIE TN 006:2016. Visual Aspects of Time-Modulated Lighting Systems - Definitions and Measurement Models.

[7] IEEE Std 1789™-2015. IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers.

[8] DRÁPELA, Jiří. Objective flickrmeter. Institute of Electrical Power Engineering: Brno University of Technology [online]. [cit. 2018-01-25]. Available from: https://goo.gl/1m7GZS

[9] HABEL, Jiří et al. Lighting technology and illumination. Prague: FCC Public, 1995. ISBN 80-901-9850-3.

[10] How to measure light flicker in LED lamps [online]. Richtek Technology Corporation, 2006 [cited 2018-01-04]. Available from: https://goo.gl/WvAmjE

Review: doc. Ing. Jiří Drápela, Ph.D., Department of Electrical Power Engineering, FEKT BUT in Brno.

Author. Antonín Fuksa, NASLI & Blue step
Published in Světlo 1/2018