Perception of artificial light - flickering

Perception of artificial light - flickering

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I noticed something strange a couple of years ago. I was walking down the street at night, when it struck me that the street lights were flickering. But, when I turned to take a direct look at the light source it did not flicker any more. In other words, when the light is not the main focus of your eye, it can flicker.

From then on, I started to focus on these flickerings and they are everywhere. Street lights, lights in a house, but also computer monitors or even projections on a screen. Again, the flickering only happens when you do not look directly at the source.

Does anyone have an explanation for that? Is it the brain's fault, or the eye's?

It's difficult to give an exact answer without actually observing the light and performing measurements. I have a theory, though.

Your peripheral vision is hyper-sensitive to changes in light - an evolutionary trait that provided quicker reactions to predators sneaking up on you. As such, even the tiniest fluctuations in light can be registered with your peripheral vision; much more so than your primary field of vision. In this case, it may well have been that the light was flickering very subtly, or at a very high frequency, which your primary field of vision could not pick up. When your peripheral vision was in play, it immediately spotted the flicker.

This is reliant on a principle called the flicker fusion threshold, which essentially states that there is a threshold of various light properties (including flicker frequency, colour, intensity, etc.) at which an eye begins to distinguish a steady light from a flickering one. Research has demonstrated that species who have high rod density are good at picking up flickering, whereas species with low rod densities have a much poorer threshold. Cone density doesn't seem to factor in very much, since flickering is primarily a change of intensity rather than hue.

This transfers into the human eye, since we have a high rod density (and low cone density) at the edges of our retinas, i.e. in our peripheral vision. If you had the eyes of another species (as creepy as that would be) with a high rod density, you may well have been able to detect the flicker quite easily with your primary field of vision.

To directly answer your question - yes, I believe this is primarily the "fault" of the eye, rather than the brain. The brain has a role in identifying the flicker at a subconscious level, in terms of fight or flight response, but the eye is responsible for providing the elevated sensitivity at the peripheral.

The Mythical “Flicker-Free” Luminaire

by Daniel Murby, Senior Optical Engineer

What is Flicker?

Everyone has experienced light flicker. Whether noticed or not, light flicker refers to the change of visible light intensity with respect to time. However, all flicker is not created equal and different observers respond to flicker in different ways. The lighting industry uses a broad term, Temporal Light Artefacts (TLA), to describe this light behaviour. Within TLA there are two subcategories: 1) Flicker and 2) Stroboscopic Effects. Flicker categorises lower frequency, higher-impact light modulations that are readily perceptible by most occupants. Flicker leads to negative health consequences including headaches and dizziness. In applications that require high levels of visual acuity, such as manufacturing, flicker can be dangerous. Stroboscopic Effects, on the other hand, are more nuanced and exist at higher frequencies. These effects tend to manifest when objects or light sources are moving at high speeds - for example in sports or while operating a motor vehicle. Although there are legitimate applications where flicker is preferred or advantageous, such as emergency vehicle lighting or entertainment strobes, the vast majority of illumination applications aim to minimise flicker and its effects. In order to achieve this goal, it is necessary to understand the origins of flicker, how to measure it, and how to implement mitigation solutions

Where Does Flicker Come From?

A light source, whether LED or not, does not inherently flicker on its own. Instead, flicker is introduced by the power delivery, conversion, and control system to which the LED is coupled. Although flicker manifests itself visually in light output, flicker actually starts as a function of voltage. Typical powerlines provide alternating current (AC) voltage (a sinusoidal waveform with a typical frequency between 50 and 60 cycles per second, or hertz (Hz)). LEDs cannot be powered by AC voltage on their own. Instead, the power input must be altered to direct current (DC) voltage. This conversion process is handled by the power supply. In most cases, the power supply will return a flicker frequency at twice the line frequency due to how the sinusoidal power is corrected, or rectified. For example, it is very common to see flicker frequencies of 100Hz in European markets and 120Hz in North American markets, since those markets operate on 50Hz and 60Hz mains frequencies respectively.

Beyond power conversion, the other key source of voltage-induced flicker occurs when an LED lighting product is dimmed. Depending on the dimming system and its compatibility with the luminaire, additional flicker characteristics may appear - especially when operating at dimmed light levels. One dimming method common in many digitally-controlled lighting solutions is the use of pulse-width modulation (PWM). In a PWM system, to reduce perceived brightness, the same LED is rapidly switched on and off. The perceived light level corresponds to the percentage of time spent in the "on" state. Since the low ("off") and high ("on") states are already defined, the expected flicker mitigation technique is to increase the switching frequency however, this approach often requires more expensive electrical hardware, increased heat generation (or phrased alternatively: lower electrical efficiency), and in some cases, the risk of audible noise generation.

How is Flicker Measured?

There are several different flicker measurement techniques used in the industry. These techniques have various positive and negative attributes. Fundamentally, a successful flicker measurement needs to account for three critical characteristics: the magnitude, frequency, and waveform of the light modulation.

Flicker Percentage

Likely the easiest to calculate, flicker percentage is used to measure the magnitude of the perceived flicker. Flicker percentage is calculated by taking the difference between the peak light output value and the trough light ouput value and dividing it by the sum of those two values. This difference is then converted to a percentage. While straightforward to calculate and representative of magnitude, flicker percentage does not take frequency nor waveform into consideration and cannot be used to characterise temporal light artefacts alone. At best, a flicker percentage may provide a rough estimate for how flicker-prone a luminaire may be - at worst, it can disqualify suitable fixtures, such as those using PWM-dimming systems.

Flicker Index

Flicker index approaches measurement by first calculating the average light output for a given flicker cycle. Flicker index then calculates the total area under the waveform curve above, and the total area below that average light output. Finally, the total area above the average light output is divided by the sum of both areas. The advantage to this method is that it relies on relatively simple calculations but considers the shape of the waveform rather than solely relying on amplitude values like flicker percentage. Flicker index is typically represented on a 0-1 scale, with a lower score representing a lower flicker perceptibility.

Frequency-Domain Analysis

The latest flicker measurement techniques, including stroboscopic effect visibility measure (SVM) and flicker perception, have a critical difference from Flicker Index and Flicker Percentage. Specifically, these "perception metrics" rely on a Fourier transform of the light output to convert it from time domain to frequency domain. This conversion successfully accounts for both frequency and amplitude, creating a more comprehensive representation of the perception of light flicker. A Fourier transform will dissect complicated light output signals into the constant frequency "building blocks", allowing a light source to be evaluated for presence of flicker at frequencies known to cause issues to occupants. To create a meaningful score, flicker perceptibility metrics compare the fundamental frequencies with empirical data surrounding human impact. These frequencies can be weighted against empirical human perception test results to determine the risk of negative impact on human occupants. While empirical tests can introduce subjectivity, it allows a direct comparison of light sources against each other and often makes selecting light sources easier. Some of these metrics, such as Flicker Perception, limit the frequency range to what would classify as flicker, typically <120Hz. Other metrics, such as IEEE's PAR1789-2015 standard, expand the frequency range to 1000Hz to account for stroboscopic effects as well. Frequency-domain flicker analysis involves more testing and data analysis than earlier flicker scores such as Flicker Index and Flicker Percentage, but by combining change amplitude and change frequency, these metrics provide the greatest amount of information on luminaire performance.

The "Flicker-Free" Luminaire

It is not uncommon to see luminaires marketed as "flicker-free", especially if they are LED. While this labelling might ease some anxiety about LED technology adoption, these statements can be misleading. Almost all light sources, whether a candle, an incandescent bulb, or an LED luminaire, exhibit changes in light output with respect to time and therefore - flicker. This is especially true of any luminaire that is powered over AC mains - the filament will produce a measurable flicker as it reacts to alternating current supplied to it. The language used should be changed and "flicker-free" statements, whether from luminaire manufacturers or power supply and driver manufacturers, should be challenged. Instead, the goal should be to eliminate the harmful impacts of flicker and stroboscopic effects in the environment in which the luminaires are installed. The two critical paths for eliminating the impact of flicker, rather than intrinsically flicker itself, are lowering the modulation percentage and increasing the frequency. Different applications and project budgets will demand different levels of performance, but any regularly occupied space should be actively designed to eliminate the risks of exposure to flickering artificial light. In all cases, flicker treatment involves design tradeoffs like cost and fixture size - generally greater flicker reduction requires more electrical components and higher fixture costs. When evaluating the merit of a luminaire's flicker claims, look for both a modulation percentage and a frequency, as those two pieces together will provide the clearest indication of flicker performance.

Special Cases

Sometimes considering human physiological impact is not enough and lighting design requires additional flicker investigations and precautions. One critical application area is television and video production. Flicker is important to monitor in these situations because the recording equipment may "see" flicker that the eye does not naturally detect based on the frame rate of the recording. For example, slow motion replays frequently used in sports broadcasting require higher frame rates that can exacerbate flickering field illumination. For this reason, professional sports arenas have strict guidelines on what light sources can be used during broadcasted events. Depending on the light source used, cameras may need to be ‘synced' to the light source to ensure consistent light levels frame-to-frame. Lumenpulse designs many of its fixtures to utilise high enough frequencies to maintain compatibility with video production.

Lumenpulse relies on flicker studies to understand the characteristics of the light output and to ensure the highest quality light delivery in all applications. Light flicker, whether visually detectable or not, can create problems for occupied spaces. By understanding the nature of light flicker, how to measure it, and marrying the lighting design to the desired application, proper illumination can be realised. Chasing the "flicker-free" luminaire is not a worthwhile endeavour and could end up costing more than the lighting application requires. Instead, Lumenpulse recommends understanding your application and the requisite levels of flicker to minimise any negative impact. This includes fully understanding any compliance requirements, for the space - WELL Standard, for example, has flicker requirements listed. Flicker will continue to grow in importance as more human impacts of artificial light are studied and specified - investing in a quality flicker metre that can output the right flicker measurements and using the metre as a tool to make design decisions, mockup evaluations, and product qualifications is a key step for any specifier.

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Electric Light Factsheet

Electric lighting became available less than 150 years ago but it provides a convenience most people would find difficult to live without today. The use of artificial light has undoubtedly increased our productivity by effectively extending the working day and improved our safety. However, many of our modern lighting sources also come with an electromagnetic radiation and hazardous material burden. In addition, artificial lighting is one of the major consumers of electricity. The health and environmental impacts of modern electric lighting will be covered in this course. For information on how light, natural and artificial, impacts the health of our visual, circadian, and skin systems see The Human Response to Light course. As Building Biologists, we have the opportunity to design and/or modify the lighting in spaces in a way that optimizes human health while minimizing the environmental footprint.

Today, most modern cultures rely on artificial light in the form of electrically powered light bulbs, also known as lamps. Lamps can be divided into three main categories: 1) incandescent, 2) discharge, which includes linear and compact fluorescent, and 3) solid-state, which are LEDs. Each lamp technology has different characteristics that are important to consider for making healthy choices for humans and the environment. In addition, the associated components of lighting, such as dimmers and electrical wiring should also be evaluated from a Building Biology perspective.

In Building Biology, we use nature as the gold standard. Thus, we look to the properties of natural light to guide us when choosing lamps. The emission spectrum of light refers to the distribution of the different wavelengths and thus colors. Sunlight follows a smooth and continuous emission spectrum with no wavelengths missing. The distribution of colors in sunlight changes throughout the day. Dawn and dusk have high levels of red and yellow and are low in blue, while midday is very blue-rich. Thus, we look for lighting with similar spectral emissions knowing that our needs change throughout the day. Caution should be used when choosing lamps labeled as full-spectrum as this term is not regulated and often does not represent the qualities of sunlight.

In addition to mimicking natural light we want electrical lighting with low levels of electromagnetic radiation and hazardous chemicals, while ideally low in energy use, and flicker free.

Incandescent bulbs work by conducting a current through a thin tungsten filament, resistively heating it to the point of incandescence. They are considered energy inefficient as most of the energy they consume (

90%) is released as heat rather than light. However, they are also free of flicker, low in all electromagnetic radiation, and low in hazardous materials. A regular incandescent bulb has a smooth spectral distribution close to that of sunlight during dawn or dusk.

Discharge lamps use electric current to excite a gas to illumination. These can be pressurized to achieve different spectral qualities. There are many types of discharge lamps. The most commonly used for general illumination in buildings are fluorescent lamps. Fluorescent bulbs are low-pressure discharge lamps where the gas is mercury. When mercury gas is excited by an electric current it emits mostly UV rays. The inner surface of the bulb thus must be coated with phosphors to convert the UV into visible light. There are two types of fluorescent lamps – linear, long tubular bulbs with a separate ballast, and compact, small lamps with the ballast built in. Fluorescent bulbs are 4 to 6 times more energy efficient than incandescent bulbs but can have issues with flicker and dirty electricity. Magnetic fields are also high for the magnetic-ballasted linear lamps. Of great concern is their mercury content, which poses a human and environmental health hazard at all life-cycle stages –production, use, and disposal. In addition, their spectral distribution is uneven with discrete narrow peaks at particular wavelengths and negligible emissions between peaks. Fluorescent lamps are, therefore, not recommended.

LEDs use solid-state, that is, semi-conductor technology. The technology of LEDs has advanced rapidly since they were first developed in the 1960s, and continues to do so. It is now possible to produce white LED lighting that comes close to emulating sunlight with a wide range of temperatures from 2700 K to 6500 K. LEDs are also very energy efficient. There is a large variation in the type and quality of components used in LED lamps today, resulting in end products that can differ tremendously in quality and thus, attributes related to health. Some LEDs generate high levels of dirty electricity and flicker. They also contain hazardous levels of lead and copper. Most LEDs on the market have a spectral distribution with two main peaks, one in blue and other in the warm color range. However, advances have seen the development of candlelight style LEDs, which have a spectral distribution very similar to sunlight. High-quality LEDs can be a good choice as long as dirty electricity levels are tested before using.

Dimmers are another important aspect to consider when making healthy lighting choices. Dimmers generally generate high levels of dirty electricity and can even be a fire hazard when combined with non-dimmable lamps. The lowest dirty electricity option is not to use dimmers at all. Instead, use multiple light sources with different illumination levels. Some dimmers, called constant current reduction dimmers, that are designed to be used, and matched with, compatible LED light bulbs are low in dirty electricity.

Lamp technology has advanced rapidly in the last few decades, not only with solid-state technology but also with the development of smart lighting and visible light communication. Smart lighting refers to wirelessly controlled lighting. As with any wireless technology, smart lighting is not recommended in Building Biology. Visible light communication, also called light fidelity (LiFi), is the transmission of data on light waves. The light waves themselves may be safe, however, the data transmitted on the light waves have to be modulated onto a carrier signal. At this nascent stage, the health implications of LiFi are unknown.

Humans are not the only organisms affected by electric lighting. The presence of light at night is a major stressor on wildlife, especially nocturnal animals. Light at night is a form of pollution and is called ecological light pollution in reference to plants and animals. Ecological light pollution can attract or repel different organisms leading to potential changes in their foraging, reproduction, migration, and communication. A change in the behavior and health of one species ultimately affects others, leading to disturbances at the ecosystem level. Ecological light pollution, for example, can set migrating birds on the wrong path, discourage female turtles from nesting, and even cause algal blooms in lakes by discouraging zooplankton from migrating to the surface to eat algae at night. For the health of all organisms, it is important to minimize this type of pollution.

(In plants, light is a crucial factor in their photosynthetic activities. Their photosynthetic organs (e.g. leaves and shoots) are drawn to light whereas their non-photosynthetic organs (e.g. roots) are repelled by it. This is important to ensure that the leaves will collect as many photons as possible while the roots are oriented towards the soil to absorb water and nutrients. While blue or white light makes the roots respond by growing away from it, red light, according to studies, makes the roots grow towards it. Reference 1)

Electric lighting is considered an integral part of life for most people today. Building Biology takes a holistic approach that looks at all aspects of lighting concerning the health of humans and the environment. This includes the type of lamps we choose as well as associated technology and reducing light at night pollution.

Want to learn more on this issue? Click the comprehensive online course, here below.

Reference 1/Further reading:
Photobiology Definition and Examples – Biology Online Dictionary. Biology Articles, Tutorials & Dictionary Online.


Through positive phototaxis, many flying insects are attracted to ALAN (Verheijen, 1960 ). Some exhibit characteristic spiraling flight patterns, while others approach the light directly. Some orbit the light source, frequently changing their angular velocity and direction to remain within its vicinity (Muirhead-Thompson, 1991 ), while others perch on or under the light, apparently stunned. Physiological and behavioral explanations of this phenomenon abound (see Nowinszky, 2004 ), and their explanatory power varies with species. The light compass theory (Baker & Sadovy, 1978 Sotthibandhu & Baker, 1979 ) suggests insects that orient themselves by maintaining a constant angle to light rays, historically emitted only by the moon or stars, will spiral into artificial light sources. Other theories involve the illusion of open sky (Goldsmith, 1990 ) or dark “Mach bands” at light–dark borders (Hsiao, 1973 ), or disorientation due to “dazzling” (Robinson, 1952 , Verheijen, 1960 , Hamdorf & Höglund, 1981 see Desensitization below).

Historically, light traps have been used by scientists to survey community composition, monitor beneficial insects (Nabli, Bailey, & Necibi, 1999 ), and control insect pest populations (e.g., Goretti, Coletti, Veroli, Giulio, & Gaino, 2011 Pawson, Watt, & Brockerhoff, 2009 Wallner & Baranchikov, 1992 ). The most common insect orders attracted to and captured in light traps are Diptera, Coleoptera, and Lepidoptera (Mikkola, 1972 van Grunsven et al., 2014 Wakefield et al., 2016 ). Light-trapping equipment can differ from ALAN in important ways: Experimental light traps usually emit more short wavelengths, are often without glass shields (which filter UV), and are placed near the ground (Degen et al., 2016 ). However, experiments that vary the intensity and spectral composition of light traps can still offer insight into the potential effects of ALAN on positively phototactic insects. The impact of ALAN on negatively phototactic insects such as cockroaches and earwigs has not yet been well-explored (Bruce-White & Shardlow, 2011 , but see Farnworth, Innes, Kelly, Littler, & Waas, 2018 ), despite a clear potential for adverse effects (see Siderhurst, James, & Bjostad, 2006 ).

Among the common positively phototactic insects, moths (Frank, 1988, 2006 MacGregor, Pocock, Fox, & Evans, 2015 ) and aquatic insects (Perkin, Hölker, & Tockner, 2014 Yoon, Kim, Kim, Jo, & Bae, 2010 ) are best studied. Comparative surveys have shown that, relative to their calculated visibility, short wavelengths are disproportionately attractive to many insects (Barghini & de Medeiros, 2012 Mikkola, 1972 see also Wakefield et al., 2016 for a discussion of infrared wavelengths). Although most insects can perceive short wavelengths (Briscoe & Chittka, 2001 Kelber & Roth, 2006 ), certain families of moths are more attracted to them than others (van Langevelde, Ettema, Donners, WallisDeVries, & Groenendijk, 2011 Somers-Yeates et al., 2013 , see also Wölfling, Becker, Uhl, Traub, & Fiedler, 2016 ). LPS lamps rarely attract moths (Plummer et al., 2016 Robinson, 1952 Rydell, 1992 ), even though most species can detect the yellow wavelengths they emit (Briscoe & Chittka, 2001 Mikkola, 1972 ). Some nocturnal insects are disproportionately attracted to polarized light sources as well (Danthanarayana & Dashper, 1986 see Recognition below).

About 30%–40% of insects that approach street lamps die soon thereafter (Eisenbeis, 2006 ), as a result of collision, overheating, dehydration, or predation (Minnaar, Boyles, Minnaar, Sole, & McKechnie, 2015 Yoon et al., 2010 ). The presence of foraging bats does not repel moths from ALAN sources (Acharya & Fenton, 1999 ), and under mercury vapor light, Operophtera brumata and O. fagata moths lacked their normal evasive responses to simulated ultrasonic bat signals (Svensson & Rydell, 1998 ). Depending on its placement, ALAN may also impede the movement of insects among habitat patches, lure them into bodies of water, or divert them into traffic (Frank, 2006 ). Insects not killed immediately may become trapped in a “light sink,” unable to forage (Langevelde, Grunsven, et al., 2017 ), search for mates, or reproduce—especially when different sexes are disproportionately attracted to ALAN, as is the case for many moth species (Altermatt, Baumeyer, & Ebert, 2009 Altermatt & Ebert, 2016 Degen et al., 2016 Frank, 1988 Garris & Snyder, 2010 see also Farnworth et al., 2018 ). Ecological traps that result in mortality or reproductive failure are predicted to lead to rapid population decline and ultimately extinction (Kokko & Sutherland, 2001 Robertson, Rehage, & Sih, 2013 ). Long-term records confirm that positively phototactic macro-moths (Langevelde, Braamburg-Annegarn, et al., 2017 ) in lit habitats (Wilson et al., 2018 ) have undergone disproportionate declines in abundance over the past 50 years.

Perhaps due to selection, when newly eclosed moths from urban populations are tested under standardized conditions, they are less attracted to ALAN (Altermatt & Ebert, 2016 ). ALAN in urban settings may also be generally less attractive due to a reduction in background contrast (Frank, 2006 ), although one study comparing declines in macro-moth abundance at light-trap sites with and without artificial night sky brightness did not support this suggestion (Conrad, Warren, Fox, Parsons, & Woiwod, 2006 , see also White, 2018 ).

Light flicker and your health – most artificial sources of light have flicker!

Flicker is common characteristic of most artificial (man-made) light sources, and another way that artificial light is different than natural light (which has no flicker). Flicker is due to the way light sources are powered and depends upon the type of ‘lamp’ as well as the electronics used to drive the light-source. The electricity which comes out of a regular wall socket alternates at 50 or 60 Hz (AC = alternating current) which means the power feeding most lights is cycling on/off 100 or 120 times a second. As a result, many light sources flicker at 100 or 120 Hz, since they have power twice in every cycle. Almost all artificial light sources are associated with flicker, including TV screens and computer monitors. Flicker is a known cause of headaches and eye strain and has also been tied to reduced concentration and visual performance. Studies show that some individuals are more sensitive to flicker than others. In addition to headaches and eye-strain, flicker can increase heart rate, induce dizziness or nausea and even trigger seizures. Unfortunately, the commonly used flicker metrics don’t provide enough information to judge the quality of a light source. Here we describe the health impacts of flicker, provide measurements of common light sources (incandescent, LED, fluorescent), and discuss what makes some artificial light sources more disruptive than others.

Documented effects of Light Flicker from various sources of artificial light (source).

Flicker refers to the change in the intensity of the light source as a function of time. In simple terms, the two things which matter are (1) how much the light intensity varies (flicker amplitude) and (2) how many times per second the light is flashing or cycling (flicker frequency). The two most common flicker metrics (flicker percent and flicker index) are a measure of (1) how much the intensity of the light varies. As you would expect the higher the number, the more the intensity or amplitude of the light varies (the worse the flicker). Old fashioned incandescent bulbs flicker at 100-120 Hz but with less than a

10% variation in intensity/amplitude which is generally not enough to cause discomfort. Equally important is (2) the rate at which the light varies (how many flashes per second). As it turns out the slower the light cycles or flashes the more disruptive the flicker! First generation fluorescents (with magnetic ballasts) flickered at 100-120 Hz, leading to complaints of headaches and eyestrain. Newer fluorescents (with electronic ballasts) cycle at 20-60 kHz (>100x faster) which is thought to be outside the range of human perception – but most still have some 120 Hz oscillation. Poorly designed LED lights flicker at 100-120 Hz and can be quite disruptive, while higher quality LED lights flicker at lower amplitude and much higher frequencies. In an extreme case of disruptive flicker, a 1997 Pokémon cartoon showed flashes at 10 Hz (10 cycles/second) causing seizures in children with no prior history of epilepsy. Humans can see lights flicker up to 60-100 Hz (60-100 cycles per second) which is called the critical fusion frequency, but negative health effects have been documented at frequencies up to 200 Hz and there is speculation of impacts at even higher frequencies. Net-net the lower the flicker percent (or flicker index) and the higher the flicker frequency, the less disruptive the light!

Not all artificial light sources are created equal. Most cheap LED bulbs have moderate 120 Hz flicker. With proper electronics it is possible to design and build a light source with no flicker!

Guide to measuring Light Flicker:

(1) amplitude of the flicker (how much the intensity varies per cycle):
– Flicker Index = (Area 1)/(Area 1 + Area 2)
– Flicker Percent = 100% × (Maximum Value – Minimum Value)/(Maximum Value + Minimum Value)
*Flicker Index of less than 0.05, and/or a Flicker Percent of less than 10% are ‘good’ (the lower the number the better)

Flicker Percent and Flicker Index are two common measurements of the magnitude or amplitude of Light Flicker. For more on measuring flicker click here.

(2) frequency of the flicker (how many times per second does the light vary):
– Flicker Frequency = # of cycles / second
*Flicker Frequency of >500 Hz is ‘good’ (the higher the frequency the better)

Comparing low frequency (120 Hz) and high frequency (850 Hz) light flicker. low frequency flicker (120 Hz) is much more disruptive than high frequency (>500 Hz) light flicker. Note that the amplitude of the flicker is the same, but due to the lower (120 Hz) frequency the red flicker is much more disruptive than the blue!

(3) flicker amplitude as a function of dimming (how flicker changes as you dim the lamp)
– Flicker Index and Flicker Percent at 50% dim and 95% dim levels
*Flicker Index and Flicker Percent should not increase dramatically as a light source is dimmed

Flicker amplitude increases with dimming for incandescent and LED bulbs, while the Flicker Frequency does not change (stays fixed at 120 Hz).

Unfortunately there is no universally accepted standard for ‘safe flicker’ (maybe ‘safe flicker’ is just an oxymoron?), but practically speaking a Flicker Percent of less than 10%, and a Flicker Index of less than 0.05 is considered ‘good’ (the lower the number the better). While most artificial light sources flicker at 120 Hz, the higher the flicker frequency the better. Frequencies above 1000 Hz thought to be imperceptible for humans (no measurable biological/physiological impact). California has proposed setting a crude threshold of no more than 30% flicker for Title 24 regulations. The IEEE (Institute of Electrical and Electronics Engineers) has published a more nuanced set of ‘recommended practices’ under IEEE 1789. The IEEE guidelines take into account both the amplitude and the frequency of the flicker, and suggest a limit of no more than 8% Flicker in Europe (at 100 Hz) and 9.6% in the US (at 120 Hz). While these thresholds are not perfect, and some people are more sensitive to flicker than others, they do provide a useful framework.

Summary of IEEE 1789 ‘Recommended Practices’ for Light Flicker. Light sources with a lower Flicker Percent and higher Flicker Frequency are less disruptive. (source of this figure).

Comparing the Light Flicker of various artificial light sources:

Additional reading and references about Light Flicker:

– Understand the lighting flicker frustration – high-level lighting industry overview on the topic of Light Flicker

Quantity vs. quality of LED lighting. Evaluation measures with consideration of physiology of vision, efficiency, and photobiological safety.

L ED technology and its application in luminaires pose a new challenge for quantitative and qualitative evaluation of lighting, taking consideration of its influence on the comfort of performing visual tasks, on photobiological effects, and on photobiological safety. The field of LED lighting is in a particular way interpenetrated by issues related to the design, construction, light efficiency, psychology, and physiology. This leads to the need of introducing new useful measures and standards, enabling us to take a specific measure or make a specific evaluation, and determine equipment used to address such needs.

It seems obvious, that illumination which has direct influence on people should be evaluated for its impact on visual and cognitive processes, on memory, state of readiness and performance, circardian rhythm, sleep quality, and general health status. Regretfully, our knowledge on how to determine and measure positive or negative impact of illumination on these processes is still very limited and is the subject matter of
many studies and R&ampD projects. In addition, the evaluation must also take into account the illuminance levels, exposure time, and spectral power distribution.

It also has to be recognized that the design and application of lighting systems should take into account other matters, seemingly unrelated to heath and biology, like spectral power distribution, proper color rendering, and practical aspects related to the manufacturing processes, cost price, electrical efficiency, etc.

What should be considered then, when selecting lighting components and products in order to meet market demands and user needs? To better understand available ways of verification and the factors influence humans, as well as quality and quantity measures, we can distinguish three, somewhat overlapping, categories:

  1. Vision related quality measures,
  2. Photobiological safety related measures,
  3. Photobiological efficiency related measures.

The highest number of available studies and measures which help better evaluate LED illumination products are related to vision.
The most obvious ones are the well known photometric and colorimetric values, specifically:
– illuminance and luminance [ lx i cd/m^2 ],
– colorimetric values, including color temperature closest to CCT,
– Ra color rendering index (CRI) and the new color fidelity index Rf.

New color rendition index
In the last few years, due to LED light specific spectral distribution, some traditional measures were replaced with others, and some were expanded. At the end of April 2017, the CIE (International Commission on Illumination) published a new standard, i.e.: CIE 224:2017 Colour Fidelity Index for accurate scientific use about a new color fidelity index called Rf. This index takes into account the specificity of LEDs as a source of white light. The commonly recognized and earlier used color rendering index CRI has limitations when evaluating LEDs, which in recent years led to controversies on this matter. The lighting market demanded an update or expansion of the method. The rendering of colors depends on the light spectrum distribution, which is the amount of radiation within a specific wavelength which reaches out vision after being reflected from illuminated objects. This allows us to see colors and shapes. If the light consists of only specific wavelengths, corresponding to specific colors, the perception of colors from other ranges is limited or impossible. It should be noted that the CRI was developed over 40 years ago, when LEDs were not used for general lighting purposes. The studies and methodology of calculating the color index at that time, did not take into account the specific radiation from LED sources. CIE’s new publication presents the Rf index as a
supplementary index for scientific purposes. This addition is largely in keeping with publication TM-30 by IES (Illumination Engineering Society of North America) published in 2015. For the purposes of developing the new methodology thousands of different LED sources and lamps were analyzed, and the number of color samples was increased from 8, used to calculate the Ra, to 99, used to calculate the Rf. This improved the universal nature of the index when applied to the evaluation of LED illumination with a different spectral distribution and different color temperature. The new index has been introduced as an addition to Ra ( Rendering index averaged ) for scientific purposes,
rather than its replacement, due to the fact that all standards about lamp functional requirements, e.g. the ECO regulation, refer to Ra (CRI). Thus, it would be difficult to introduce an immediate change. In addition, TM-30 does not take into account all color rendering aspects and it would be too early to announce the end of the Ra (CRI), and switch only to the new index. Nevertheless, it is a very good step towards adapting standards to available technology and market expectations. Presently, all professionals from the lighting industry and demanding customers can compare and select LED products for color rendition quality in an additional and more objective way.

Optical radiation safety measures
With respect to photobiological safety we have the provisions of the existing IEC 62471 standard to evaluate photobiological safety of light sources and luminaires (including LED ones).

Photobiological effects have been studied for many years, to better understand their spectral efficiency function. Of special importance for human safety and health is an in-depth understanding of the photobiological impact of optical radiation on human eyes and skin.

The EN 62471 standard lists values of hazards defined with the use of three functions: spectral efficiency and hazards to eyes from near ultraviolet (UV-A), hazards to eyes from infrared radiation in the 780-3000 nm spectral range, and thermal hazards to skin caused by radiation in the 380-3000 nm spectral range. The values of hazards should be given either as values of irradiance or effective irradiance, or as values of effective radiance.

Given the very broad spectral range specified in the standard and the fact that such an evaluation is a very complex measurement process requiring specialized equipment and high qualifications, the practical application of the standard was very limited. Many manufacturers marketed their products only declaring compliance with the standard, or ordering only partial measurements in specialized laboratories.

LED lamps and luminaires used for general lighting and in industrial applications emit mainly optical radiation in the visible wavelength range. Consequently, differently from other types, LED lamps and luminaires create photobiological hazards only from blue light.

The methods of evaluating photobiological safety of light sources and luminaires, emitting blue light, were presented in technical report IEC TR 62778. The document gives a lot of practical information which may help understand measurement principles, simplification of measurement, and consequently – common application of the standard’s provisions. This may help improve the safety of products introduced to the market. Details of the methods and instruments were described in this post

The principles of lamp classification into risk groups with respect to hazards were specified in the standard, and lamps and luminaires were divided into four risk groups:

  • risk-free group (RG0),
  • risk group 1 (low risk) (RG1),
  • risk group 2 (moderate risk) (RG2),
  • risk group 3 (high risk) (RG3).

To learn more about the available instruments please visit the GL Optic application page here

Table 1 Blue light emission limits for individual risk groups

The evaluation of photobiological risk from blue light usually requires quite labor intensive measurements, however, existing correlations between photometric and colorimetric values vs. blue light hazard effective values in some cases permit a significant simplification of the measurement. The measurement result determines the risk group, RG0 or RG1, or – if the source of light or luminaire is classified as RG2 risk group – it determines the threshold illuminance Ethr.

The method described herein uses dependencies existing between values describing blue light hazards, and photometric and colorimetric values of a light source. Naturally, this method applies only to sources of white light. Having determined the color temperature of a light source closest to T cp , the result should be cross-referenced in Table 2 to find the corresponding threshold illuminance E thr .

Table 2 Conservative estimation of E thr as a function of color temperature closest to T cp

Human Centric Lighting
The impact of illumination on human biological functions, including the circardian rhythm, is a new topic, presently under in-depth analysis. The very fast growing in popularity concept of Human Centric Lighting or Circadian Lighting, in one of its assumptions, uses the influence of changing color temperature and the illuminance value on the human alertness and performance. With the discovery of a new receptor, i.e. the ipRGC (Intrinsically Photosensitive Retinal Ganglion Cells) the era of evaluating the influence of non-image forming mechanisms on the human organism began. The process of melatonin secretion and suppression in the human organism is responsible for the change of human activity, and is stimulated by exposure to light of a specific wavelength (not a color temperature). On the basis of studies, several different effectiveness curves were published in the recent CIE S026 standard.

In the US, the WELL organization studying issues of designing buildings, including lighting that takes into account human needs, published a standard showing a graph of spectral effectiveness of suppressing the secretion of melatonin EML (Equivalent Melanopic Lux). Also technical report CIE TN 003-2015 on neurophysiological photometry forms a basis to standardize the methodology of measuring biologically effective illumination.

By using the available methods we can install conversion tables in measurement equipment and calculate the EML intensity, verifying in this way what kind of light will have the most effective influence on the human organism.

However, it must be remembered, that color temperature is not enough. According to the rule of additive color mixing, there are many ways of obtaining the same color resulting from mixing different constituents. Therefore, even if we are able to obtain a specific color temperature, it may not necessarily be the most efficient one to influence the melatonin level. For example, if by mixing RGB colors we obtain white light with a cold temperature, it may turn out that specific wavelengths are missing in the distribution, and this light will only seemingly “look” like biologically efficient. Read more about the available spectral light meter offered by GL Optic here

Therefore, what is decisive in suppressing the secretion of melatonin, consequently influencing work productivity and concentration, is the value of optical radiation of a specific wavelength. One should also bear in mind the possible negative impact of artificial illumination on the human organism which during the day also requires the right amount of rest and sleep. An imbalance in that respect may also lead to very serious health problems.

Perception of artificial light - flickering - Biology

Selected as an Editor's Choice by the Natural History Book Service (UK)

"Ecological Consequences of Artificial Night Lighting is an excellent reference that will undoubtedly raise awareness of the need to conserve energy, do proper impact assessments, and turn the lights down."

"It should be a primary source for anyone dealing with work related to lights and their impacts on living organisms. It's on my primary source reading list."

- Albert M. Manville II in The Condor

"Anyone interested in how human civilization affects natural environments will want to see this book the literature reviews will be a treasure-trove for biologists beginning to learn about the problems."

- Robert L. Crawford in The Auk

"This is a book with a mission and a soul. At the front of each part is an extract of prose from one of a variety of authors that make us remember that nights are meant to be dark, beautiful and exciting. It is an academic book, but one that is written and presented in such a way that it will appeal to anyone with an interest in ecology."

-Paul Elliott in Biological Conservation

"The book is seminal in its field. It comes at a most opportune time, when our entire social system revolves in myriad ways around brighter lights. The articles in this volume are meant to unsettle the common perception that more light is indicative of a higher level of development. It brings forth ecological ramifications, drawing upon a variety of fields such as geography, physics, and biology. Well researched and well written, the articles open up doors to further research in the field, giving ample opportunities for multidisciplinary approaches towards environmental issues. It has also served to bring the concept of light pollution into mainstream thinking regarding pollution, an agenda hijacked by the more pressing problems of air and water pollution. The book is a must read for students, academicians, and laypersons alike."

-Sucharita Sengupta in TerraGreen

"This book is very readable and should be of interest to a wide audience, ranging from professional biologists and managers to students. Conservation practitioners will find strong support for the need to incorporate artificial night lighting into impact assessments and conservation planning."

- Lyn C. Branch in The Quarterly Review of Biology

"A powerful compendium. Surely eye opening for many ecologists. . Recommended for wildlife ecologists and anyone fighting light pollution."

- Sarah O'Malley in Northeastern Naturalist

"The adverse effects of night lighting on the environment have only come to the attention of scientists during the last fifty years. This intriguing book, edited by Catherine Rich and Travis Longcore, brings together historic accounts and recent scientific findings in a broad spectrum of writings on the significant influence of night light on plants and animals. Divided into six parts according to taxonomic groups, the text documents seminal studies showing that overabundant artificial lighting has played a role in the lives of plants and animals by disrupting regular rhythms and creating serious hazards for fauna. In conclusion, the editors offer a thought-provoking reminder, '. as we light the world to suit our needs and whims. doing so may come at the expense of other living beings . .'"

- Marilyn K. Alaimo in Chicago Botanic Garden Book Reviews

A reader might anticipate from its title that Ecological Consequences of Artificial Night Lighting holds a in-depth technical focus on night lighting's impact on nature - but it goes beyond chronicling science to consider how human activities from lighting affects animals and plants in a variety of ways. 'Photopollution' exists nearly everywhere thanks to mankind's activities: here are examples not only of effects on plants, insects and animals but how to mitigate them. Sections seek a readable approach by pairing vignettes of events and experiences of nighttime creatures with plenty of science and analysis of the physiological and behavioral effects of light pollution. It's these vignettes which make this book accessible not only to college-level students of science, nature and ecology but the general non-scientist public library browser, as well.

- Diane C. Donovan in California Bookwatch

While certain ecological problems associated with artificial night lighting are widely known -- for instance, the disorientation of sea turtle hatchlings by beachfront lighting -- the vast range of influences on all types of animals and plants is only beginning to be recognized. From nest choice and breeding success of birds to behavioral and physiological changes in salamanders, many organisms are seriously affected by human alterations in natural patterns of light and dark.

Ecological Consequences of Artificial Night Lighting is the first book to consider the environmental effects of the intentional illumination of the night. It brings together leading scientists from around the world to review the state of knowledge on the subject and to describe specific effects that have been observed across a full range of taxonomic groups, including mammals, birds, reptiles and amphibians, fishes, invertebrates, and plants.

Ecological Consequences of Artificial Night Lighting provides a scientific basis to begin addressing the challenge of conserving the nighttime environment. It cogently demonstrates the vital importance of this until-now neglected topic and is an essential new work for conservation planners, researchers, and anyone concerned with human impacts on the natural world.


Night, Venezuela • Alexander von Humboldt

2. Effects of Artificial Night Lighting on Terrestrial Mammals (Paul Beier)

3. Bats and Their Insect Prey at Streetlights (Jens Rydell)


4. Effects of Artificial Night Lighting on Migrating Birds (Sidney A. Gauthreaux Jr. and Carroll G. Belser)

5. Influences of Artificial Light on Marine Birds (William A. Montevecchi)

6. Road Lighting and Grassland Birds: Local Influence of Road Lighting on a Black-Tailed Godwit Population (Johannes G. de Molenaar, Maria E. Sanders, and Dick A. Jonkers)


7. Protecting Sea Turtles from Artificial Night Lighting at Florida's Oceanic Beaches (Michael Salmon)

8. Night Lights and Reptiles: Observed and Potential Effects (Gad Perry and Robert N. Fisher)

9. Observed and Potential Effects of Artificial Night Lighting on Anuran Amphibians (Bryant W. Buchanan)

10. Influence of Artificial Illumination on the Nocturnal Behavior and Physiology of Salamanders (Sharon E. Wise and Bryant W. Buchanan)



12. Artificial Night Lighting and Insects: Attraction of Insects to Streetlamps in a Rural Setting in Germany (Gerhard Eisenbeis)

13. Effects of Artificial Night Lighting on Moths (Kenneth D. Frank)

14. Stray Light, Fireflies, and Fireflyers (James E. Lloyd)

15. Artificial Light at Night in Freshwater Habitats and Its Potential Ecological Effects (Marianne V. Moore, Susan J. Kohler, and Melani S. Cheers)


Night, Massachusetts • Henry David Thoreau

16. Physiology of Plant Responses to Artificial Lighting (Winslow R. Briggs)

Editors: Catherine Rich, Travis Longcore
Subject: Ecosystem Studies: Biodiversity and Wildlife
Binding: Paperback 6x9, 458 Pages, Maps, Photos
Publisher: Island Press
Pub Date: 2006
Price: $29.95 (in 2006)
ISBN: 1-55963-129-5

Why do some artificial light sources have low color rendering accuracy?

When artificial light sources emit light, they may be missing significant portions of the visible spectrum, even if the light appears white.

Instead of emitting light as a result of heating, LEDs emit light as a result of electrons being converted to photons - a significantly different process compared to daylight and incandescent lights.

Therefore, the emitted light tends to be of a very specific color, such as blue, red or green, rather than a combination of colors.

Chemicals called phosphors are applied to these devices in order to tweak the spectrum in a way that the resulting light appears white. Unfortunately, this resulting spectrum is oftentimes far different from the natural light sources we are used to.

The light emitted from the light source might appear bright and white, but once you shine it onto an object, the reflected color may appear inaccurate.

This is where CRI will help explain the likelihood that a light source shining on an object will allow it object to appear accurate.

Watch the video: Artificial lights to make plants grow. SLICE (February 2023).