# Impact of Lighting on Humans and the Biotope: Physics, Measurement Systems, and Influences

Pascal Dupuis, Marie-Pia Marchant, Leoš Kukačka, Georges Zissis

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Pascal Dupuis, Marie-Pia Marchant, Leoš Kukačka, Georges Zissis. Impact of Lighting on Humans and the Biotope: Physics, Measurement Systems, and Influences. IEEE Industry Applications Magazine, 2024, pp.2-13. 10.1109/MIAS.2024.3488369. [hal-04861263](https://hal.science/hal-04861263v1)

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<figure><img src="image_1.png" /></figure>
# Impact of Lighting on Humans and Biotope

Pascal Dupuis*, Senior Member, IEEE, Marie-Pia Marchant*, Leoš Kukačka†, Senior Member, IEEE, Georges Zissis*, Fellow, IEEE

*Université Toulouse III Paul Sabatier, LAPLACE, UMR 5213 (CNRS, INPT, UPS), 118 rte de Narbonne, 31062 Toulouse, France

†Technical University of Liberec, Institute of Mechatronics and Computer Engineering, Studentská 1402/2, 461 17 Liberec, Czechia

**Abstract**—Light just seems "natural" to us, as well as lighting when the ambient level is too low for our needs. This paper is about the Physic and Technologies behind light and lighting, together with a brief description of the Human perception mechanisms. It goes over norms and measurement systems, and summarizes a few of the influences of lighting on Humans, but also on Biotope.

Index Terms—Lighting; CIE; standards; norms; visual perception; ALAN.

## I. Introduction

Archaeologists exploring a cave are thrilled when discovering a seemingly unremarkable substance: charcoal. There is only a single instance in which those remnants travel into caves: though controlled combustion. Inflamed sticks, torches, fireplaces in the ground to name a few. And a single species behind this technology: humans. Our distant ancestors were able to extent their territories through the use of devices to produce light and heat in the darkness and coldness of caves.

As diurnal species, we rely on light to analyze and use our environment. To find potential resources. To avoid becoming a resource to some predator. To detect friends and foes. The lack of light means very simple tasks become quite complicated. That a single noise can be some leaves rustling under the wind, or the approach of a predator.

Light is thus a necessity towards Humans activities, and generates other effects on our organisms as well. Those aspects are reviewed in the upcoming parts, which are organized as follows. Sec. II explains how what we consider as "natural light" is produced by the Sun and reaches our immediate environment. Then it summarizes the mainstream technologies used to generate lighting.

Sec. III briefly explains the main characteristics of the Human visual perception. It is followed by Sec. IV which is about static or slowly varying disturbances to the Human perception. Sec. V is about non-visual effects on Human in daily and nightly conditions, as well as disturbances on the biotope caused by artificial lighting. When the perturbing frequency increases, the phenomenon become known as Temporal Light Modulation (TLM) and is described apart in Sec. VI.

As this paper is mostly a review of lighting, there are no conclusions. Lighting technologies and knowledge's are in a constant state of flux. This is an instantaneous picture at the time of the writing.

## II. Human vision and Natural light

### A. The Physic behind light: the Sun

A crude approximation of the Sun is to describe it as a plasma of spherical shape, whose surface emission properties can be approximated as a Black Body with a temperature of 5778 K. The photons exiting this surface transit through space and hit the Earth's upper atmosphere 7 min later. The level of radiations, at the average Sun - Earth distance, is $1\,300\,W\,m^{-2}$ between 250 nm and 3000 nm.

This extra-terrestrial light is then filtered through the atmosphere according to a number of phenomenon:

* Ozone ($O_3$) molecules absorb radiations below 300 nm. The DNA strains are made of 4 kind of basic building blocks; each of them present an absorption peak at 260 nm; one of them also possesses a peak at 280 nm. Damages to the DNA are the causes of mutations, cancer, cellular death, etc.

* Molecules such as $O_2$ and $H_2O$ presents a few narrow absorption peaks;

* Molecules much smaller than the photon wavelengths cause Rayleigh scattering with a dependence to wavelength roughly $\approx \lambda^{-4}$, meaning blue photons whose wavelength is the half of the wavelength of red photons, are scattered 16 times more. That explains how the sky is blue.

* Particles bigger than the photon wavelengths produce Mie scattering. This phenomenon is mostly independent of the wavelength. Clouds, which are volumes of micro water droplets, appear in neutral color: white when their emissivity is similar to the ambient luminance, and gray if their emissivity is lower.

* Other variables also play a role: the air humidity, the level of dusts and aerosol generated by pollution, the ground reflectivity, etc.

Sunlight depends on a number of geographical and weather related factors. CIE and ISO have defined normalized outdoor daylight conditions (ISO 15469:2004). Those standards are based upon work from the National Renewable Energy Laboratory (NREL), which mandated Dr. Christian Gueymard, Solar Consulting Services, to write FORTRAN code to simulate the effect of the aforementioned factors on the solar spectrum distribution.
<figure><img src="image_2.png" /><figcaption>Fig. 1: Solar Radiations at sea level, on a sandy soil</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2: Lunar Extraterrestrial Irradiance</figcaption></figure>

The code, described in [1], can be obtained at [2]. It is used with the author's permission to reproduce its output.

The graph at Fig. 1 is generated using the datasets contained in the Smarts299 code. The general layout is created with the R packages "photobiology" and "ggspectra" [3]. The upper curve is the irradiance at an height of 12 km, which is the cruise altitude of commercial airplanes. The intermediate curve is the emission of a Black Body; the amplitude has been fitted to approximate the previous curve for wavelength at 1000 nm and above. The lower curve is the sunlight at sea level on a sandy soil. On the average, the amount of radiation at the ground level is 1000 W m⁻², modulated by latitude, amount of clouds, dust, aerosols, etc.

ESA, the European Space agency, has made available [4] a toolbox named LIME offering similar computations for the Moon spectrum. The surface of the Moon contains "maria" (seas), basaltic plains of volcanic origin; impact craters, mountains, etc. Dust is omnipresent. The Moon is the brightest object in the night sky, with an apparent size close to that of the Sun, making it possible to completely cover the Sun during total eclipses.

Fig. 2 was generated by the LIME toolbox for the city of Toulouse, France, on Augustus, 19th 2024 at 06:25 PM UTC corresponding to the full Moon. The toolbox generates data over the range 350 nm to 2500 nm. The data above 2500 nm are obtained by extrapolation from the Planck Body curve at 5 778 K. As it can be observed by comparing the two figures, the Moon spectrum is quite similar to the Sun spectrum; except its level is order of magnitudes lower. The Moon reflectance as a function of wavelength present smooth changes; its effect is similar to asphalt which, under Sunlight, appears as dark gray, i.e. a neutral color.

## B. Artificial Indoor Light

As said in the introduction, artificial light sources have permitted Humans to compensate for their inability to perceive their environment in very low light levels. Contrast this with animals like the house cats whose eye special structure permits them to hunt at night.

Humans used for eons indoor light sources based upon fire, like candles and oil lamps. One of the brightest of its time appeared in France in 1783, wearing the name of their inventor: Antoine Quinquet. Vegetable and animal oil lamps were supplanted by kerosene lamp after 1850.

As of today, such lamps are still used frequently in countries where electricity is too expensive. Those lamps are considered harmful by the World Health Organization (WHO) due to their production of aerosols affecting the lungs, the eyes, ... The availability of LED's is expected to permit to reach substantial energy savings in Asiatic countries as explained [5].

## C. Artificial Public Light

The first forms of public lighting using gas appeared as early as the 18th century.

For enthusiastic observers, it became a fashion to ask for more lights in big cities. Artificial lighting became quickly a synonym of "progress", "vibrant life", "public safety", "good management", to name a few. Nevertheless, the New York Herald, European edition, published an article on Jan 31st, 1890, referring to a pandemic of "flu", stated that "The disease has raged chiefly in towns where the electric light is in common use, ..." It can be visualized at the archive of the French National library [6].

A less extravagant claim [7] mentions the possibility that the so-called "Russian Pandemic" from 1890 to 1894 was actually linked to a corona virus; its transmission was amplified by the availability of long-distance trains between major cities.

Reminder: "Correlation is not Causation".

The cost vs utility of Public lighting mandated a lot of evolution's. From incandescent light bulbs to Mercury Vapor to low pressure Sodium which became available once glass with enough strength to withstand the temperature and chemical properties involved in the plasma.

In Belgium, the government decided around 1967 and the start of its first nuclear power plant to absorb its nightly production by lighting the highways. Using Low Pressure Sodium lamps, night travels became a path among fifty shades of orange.
Over the years, low-pressure gas discharge lamps were themselves supplanted by high-pressure discharge lamps whose luminous output spectrum is made of bands instead of narrow lines, leading to better color rendering properties.

Both kind of devices require moderate high voltage pulses (2–5 kV) to ignite when the interior gas is at low temperature; that pulse being generated close to the lamp is compatible with usual low-voltage insulating materials. But re-igniting high-pressure devices directly after extinction requires High Voltage discharges, sometimes in the range of 50 kV. This was a major drawback to the development and market introduction of high-pressure discharge lamps.

White LED's appeared around 2010 and quickly flooded the market. Leading to maintenance issues: the typical lifetime of public lighting installations in tunnels is expected to be around 40 years. Such places are sheltered from usual atmospheric factors and their maintenance requires careful planning at the level of Regional authorities to redirect the traffic in case of closures. That lifetime is the norm at the CETU, the French Center which is in charge of overseeing all tunnels in France.

### III. Visual perception

#### A. Definition

The eyes of living creatures can be sensitive to the light level only (non-focusing) or are complex optical systems able to generate images of the surrounding (focusing). Human eyes are of the later kind. They contain a number of mechanisms to target some direction and adapt to the average light level. The human vision can analyze images where the ratio of the highest to the lowest luminance is around 1000. In daily situation, when able to weight the red / green / blue components, the vision of average observers is said to be tri-chromatic. That vision is primarily performed by cells called "cones" whose central activation wavelength was previously described in terms of RGB. Today the appellation "Short, Medium, Long" is preferred.

The scale at the top of Fig. 1 illustrates the correspondence between the photons physical wavelength and the associated monochromatic color. There exist alterations of that perceptual mechanism when some receptors are weak or non-functional, leading to persons with various levels of color-blindness.

Fig. 3 emulates three such conditions: deuteronopia; characterized by the lack of perception in the green area; protanopia with a lack of perception in the red area. The last line corresponds to a vision without any color perception. The first condition occurs within around 6% of males; the second to around around 2% of males. Their occurrence in females is much lower.

In the range of 0.01–3.0 cd m⁻², the vision is called mesopic. It implies the aforementioned cones and another kind of cells called rods; color perception is diminished.

Below that range is the scotopic or "night" vision, using rods only. In that mode, spatial resolution is lower, and

<figure><img src="image_4.png" /><figcaption>Fig. 3: Simulation of color deficiency perceptions</figcaption></figure>

there is no color perception. But the maximum sensitivity is roughly three times higher than in photopic mode.

Describing the Human Vision mechanism is quite complex. The receptors densities are inhomogeneous; the raw signals are pre-processed (combined) before leaving the eyes; different brain zones manage different tasks. As a rule of thumb, it takes 0.4 s to analyze a shape; 0.6 s to recognize a color, and around 0.8 s to analyze and process a whole scene.

The "standard" sensitivity for the average observer was proposed by the CIE in 1924 and definitively accepted in 1931. The scotopic value was proposed in 1951. The mesopic sensitivity is a weighted mean of those two and was formally accepted in 2023 as ISO/CIE 23539:2023.

Sources observed from some distance are best described using spherical geometry. Therefore, the associated SI units are the luminous flux and luminance, defined in terms of solid angle. The corresponding physical units are lm and cd m⁻².

The luminous flux is obtained from the radiometric flux by applying a weighting corresponding to the CIE 1931 photopic sensitivity. Luminance measurement have to be performed using an imaging system.

Non-imaging receptors far from their illuminating sources can be approximated by planar surfaces. The associated SI is the lx, the fraction of luminous flux per unit surface. Luxmeters usually contains a flat sensor associated to a curved diffusive cover applying a weighting to emulate the CIE1931 sensitivity function and to conform to a cosine law.

The later sensors are much less expensive than the former. Historically, lighting rules were expressed in lx. But in recent years it was understood that illuminance is only part of the picture to assess the adequacy of the light level with respect to the considered tasks. Norms, particularly in road lighting, may use either illuminance either luminance.

#### B. Adaptation

The eye's iris and pupil act as a diaphragm, modulating the flux reaching the retina, the zone at the back of the eye
containing the photo-receptors. The diaphragm aperture is controlled by the scene average luminance as explained previously. This permits to cover a domain spanning nine orders of magnitude, including scotopic vision but at some given level the usable range is one to one thousand.

As a whole, the adaptation process to light level changes involves chemical and neural mechanisms. The overall speed of change is not symmetric. Adapting from dark to bright light takes around five minutes. Adapting from bright conditions to darkness takes around 10 minutes for the cones, and around 40 minutes for the rods.

Those figures are for young peoples. Elderly peoples take a much longer time to adapt from bright to dark.

## C. Norms Evolution

For centuries, colors were described by painters, printers, ... The color was obtained by selectively absorbing some parts of the visual spectrum. What was reflected in the case of paintings or transmitted in the case of glass was able to achieve a colored perception. An early attempt to quantify colorimetric perception was established by Albert Munsell (1858 – 1918) which was a painter and an art teacher. His research leaded to the Munsell Color System, which underwent many revisions from the original Atlas of 1905. This system is still in use today. Munsell introduced the idea that a pure color like yellow, observed at luminances above the adaptation luminance, tends to fade to white. At very low luminance, yellow is perceived as light or dark brown.

Colors generated by a light source have been known from a long time. The book of Genesis mentions the rainbow. Isaac Newton, during a lock-down caused by a plague outbreak, invented calculus and experimented with prisms; revealing that what our eyes analyze as "white light" is a mix of many components. For the physicist, a pure color is associated to a narrow-bandwidth distribution of photons.

The CIE publication of 1931 mentioned earlier leaded at first to a system of three absolute values called "X", "Y", and "Z" corresponding roughly to pass-band filters in the red, green and blue regions. It became quickly apparent that two kinds of information were mixed together. As said previously, due to the eye adaptation mechanism, changing some light source by a few percents produced a perceptible change in its brightness, but not in the color appearance. This is why from the original set of indicators, denoted as X, Y, Z, emerged a new one x, y, Y where the small capital letters are the corresponding absolute values divided by their sum. Y became the luminance and (x, y) the colorimetric coordinates. The so-called "CIE 1931 color triangle" encodes a number of distinct colors and tones at normalized luminance. This explains why that diagram does not contain nuances of pink or brown.

By experimentation on this CIE 1931 color definition it was found that the locus of non-distinction between quite similar colors was an ellipse in the color triangle; but its size and orientation is dependent of their position. The

system was revised in 1960 as "Uniform Color Space", and later revised in 1976 in order to reduce the inhomogeneity of those ellipses. A companion model, the CIE 1976 LUV encodes brightness and chromaticity with respect to a reference white point. That way, it is possible to correct the rendering of a scene when the viewing references are significantly different from the original scene. Think f.i. in terms of viewing a picture on a computer screen.

All the models so far are elaborated with respect to normalized luminance. In 1976, the CIE also defined the "Lab" model, which covers the whole range of object luminance with respect to the adaptation luminance. That model is thus able to render pink and brown tones, pure as well as pastel colors. Its intent is to take into account the Human color perception mechanisms. Further research lead to more plausible models, i.e. validated by neuroscience: CIECAM02 (2002) and CIECAM16 (2016). Those models takes their idea from the various iterations of the CIE color models, but also from the Munsell theory.

## IV. Visual disturbances

### A. Glare

When a sufficient amount of light enters the eyes through an oblique path, this increases, by reflection, the average amount of perceived light. By reducing the contrast levels, the image analysis is more difficult. This effect is typical in situations where the person has to focus in some direction close to a bright source. Glare may also result when some region of interest has to be analyzed in a very bright background.

A typical case for such issue occurs in guided vehicles. French designer Paul Arzens created a style for Electrical locomotives for the French National Railways known as "nez cassé" ("broken nose"), which was applied between 1965 and 1988. An example of such design is depicted in Fig. 4, reprinted with its author's permission.

The idea is to have a reduced windshield with a backward orientation, overlapping a narrow inclined surface. That way, the optical path for troublesome lighting is notably diminished.

The train driver focuses on the tracks and the adjacent signals. This design was used before signaling could be repeated electronically into the cabin. Observe in particular that the space behind the windshield is much darker than the front panel under the rising sun.

### B. Visual adaptation

Another potential issue occurs for vehicles entering or exiting a tunnel. The driver eyes are accustomed to some level, then have to adapt to another level. The more problematic case is while entering tunnels. The apparent loss of perception may create a sensation of fear, leading drivers to brake. To avoid this, tunnel regulations usually require to shield the entrance from bright sources; like f.i. the open sky. Furthermore, the tunnel lighting is increased on the entry lane, over a distance which is a function of the authorized speed.
<figure><img src="image_5.png" /><figcaption>Fig. 4: Glare effect on a guided vehicle</figcaption></figure>

Those two mechanisms favor a gradual adaptation of the vision, bringing it gently to a lower level requiring less artificial sources of light.

The next two pictures are taken with a fish-eye camera during the night and will serve to illustrate some major misuse of public night lighting. An usual metric to define a lighting system is its homogeneity; described f.i. as the ratio of maximal to minimal values. System designers like to emphasize attention points.

The first picture (5a) is taken on the Toulouse University campus. There is a bus lane alongside a wooden area. On the right, a bus stop is flooded with light. Meaning passengers exiting a vehicle are in photopic condition; and have to walk into a darker area. Their ability to perceive their environment will take a few minutes to be restored.

The second picture (5b) is taken in Eastern Europe. That town is a popular winter sport resort. The town council required a marked crossing to be "well lit".

Two LED's luminaries were installed over it. On the picture, we first notice a supermarket on the left which is closed but lamps are still at full regime. The road surface was slightly wet, which increases the issues.

At the center of the picture is a white spot: the marked crossing at a distance of 50 m, which is the safety braking distance for car drivers. During the experiment, people wearing safety clothes with high contrast and high reflexivity were present on that crossing.

The test results showed that no observers were able to discriminate those pedestrians at 50 m. There was no contrast at all. That study was mandated by insurance companies after two occurrences of pedestrians being killed by cars while crossing that zone.

# V. Non-visual effects

## A. Daily conditions

Since at least the start of the 20th century, the link between Sun exposure and the synthesis of the "D" vitamin was established by Physicians. The lower layers of the epidermis are able to synthesize this vitamin when

<figure><img src="image_6.png" /><figcaption>(a) Bus Stop at Toulouse Campus</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b) Marked crossing observed from 50 m</figcaption></figure>

<caption>Fig. 5: Fish-eye picture of two nocturnal scenes</caption>

exposed to UV-B (280-315 nm). Only a limited sources of food contain naturally interesting levels of this vitamin.

Up to the '80s, Sun tan was regarded as a sign of good health. Yet it appears that UV-A (320-400 nm) produces free radicals which themselves are responsible of DNA damages and cancers. They are also responsible of skin aging. UV-B have been identified as the main source of Sunburns.

The French Agency for Food, Environmental and Occupational Health & Safety (ANSES) reported in 2019 about various risks linked to exposure to high intensity blue sources. One of the long term risk is age-related macular degeneration.

That report gave rise to various claims in the News about "the dangers of Blue Light". While it is true that in a bright space lit by LED's it is possible to get to the blue light hazard threshold, consumer-grade lamps, smartphones and screens in usual situations are not concerned. Advertizing for blue-light filtered glasses is just advertizing.

The most important non-visual effect of the Sunlight is the modulation of the Circadian Rhythm. Blue photons simulate the eyes melanopsin receptors, which suppress melatonin, the hormone linked to sleep. In the evening,
that effect disappears and there is rebound effect, easing the sleep transition. Older people living the whole day in artificial lighting with the same level of blue content are known to suffer from insomnia. The use of LED lighting with an increase of the blue level in the morning and a reduction in the afternoon has been shown to significantly improve the sleep quality.

The epidermis of premature newborn is transparent to infra-red, meaning they lose their thermal energy by radiating it to their surrounding. In neonatal care, it is customary to provide them with supplemental heat through infrared heating in order to help them regulate their internal temperature.

Infrared heating is used in physical therapy in order to relax sore muscles, to increase the skin capillary blood flow, etc.

Recent laboratory studies have revealed that red and infra-red light plays an important role in improving vision and providing general support to various metabolic pathways. The work suggests that red light can increase the activity of mitochondria in the body's cells and this can lead to a higher metabolic activity.

When retinal cells are exposed to red light for a period there is evidence of improved color perception. See f.i. [8] on animal models.

There are also suggestions that infra-red radiation may be used to cure various troubles linked to diabetes mellitus: improvement of glucose metabolism on mice [9], restoration of peripheral sensations in Humans [10], reduction of depression symptoms [11], etc.

## B. Nightly conditions

When Low Pressure Sodium Lamps were the norms, astronomers used dichroic filters to remove the Sodium "line" around 589 nm. With the introduction of LED's, the light levels at nights became stronger for many reasons. On the one hand, the perturbing spectrum was much wider and its blue component gave rise to re-emission in the pollution dome covering cities. On the other hand, as producing night lighting became less expensive, there was a tendency to increase the number of light poles.

Astronomers started openly voicing their concerns around 2012. Their growing inability to study stars during the night received the name "Artificial Light At Night" (ALAN). A body of literature quickly emerged to criticize the overuse of public lighting for no good reason. Some of those studies showed a diminution of the sleep quality by humans. It is also hypothesized that some cancers are hormone dependent; so changing the hormones production pattern may increase the risk of such cancers.

As of today, the utility of some high level scientific observatories is questioned. One such example is the "Pic du Midi", in the French Pyrénées. That observatory is plagued both by lights of remote cities in Spain and France, but also by local skying stations. It was recently decided to keep it working as a didactic equipment.

The ALAN community has also documented numerous effects on the fauna and flora. For the fauna, there are

disturbances in the sleep / wake pattern, in the perception of seasons leading to birds nesting too early, in the ability for nocturnal fauna to use the Moon as orientation reference as many street lights have an apparent size similar to the Moon leading to disorientation or "capture" phenomena: insects circle indefinitely around lamps.

For the flora, documented disturbances include the seasonal fall of leaves occurring later than usual, leading to increased sensitivity to the first frosts. Some cross-interactions with nocturnal pollinators have also been observed, leading to a reduction of the amount of seeds.

A potential mitigation came with the introduction of smart lighting systems. The "smartness" results from the combination of a perception mechanism and a retro-action to a LED's ballast, which modulates the light level according to the street actual occupation level.

## VI. Temporal Light Modulation

With electricity powered lighting technology, users expectations on the quality of lighting have risen. Temporal light modulation (TLM) refers to the variation in the intensity or color of light emitted by artificial sources over time. The history of TLM began with the advent of electric lighting in the late 19th and early 20th centuries, particularly with the invention of the incandescent bulb.

### A. TLM Characteristics and Cause from Technologies

When supplied with AC voltage, incandescent lamp output will be modulated at twice the fundamental frequency of the power network. With 50 and 60 Hz systems this means 100 or 120 Hz. These frequencies are invisible to the eye in static conditions. Furthermore, thanks to the thermal inertia of the lamp filament, this modulation is attenuated and consequently does not cause any technical issues. The flicker level is lower in high-wattage and low-voltage lamps.

However, owing to the Stefan-Boltzmann law, any low frequency supply voltage disturbances will be transferred to the light output and amplified by a factor of approx. 3.6 (with standard 230 V 60 W incandescent lamp; this number may vary depending on the lamp type). Depending on the severity, these light changes can be seen as irritating by humans. Early after introducing the incandescent lamps, users have begun complaining about flicker, i.e., the perception of instabilities of the electric light level. The incandescent light source technology was unable to cope with flicker by itself and the only solution was to stabilize the supply voltage and keep the voltage disturbances to a reasonable level.

The situation is very similar with fluorescent tubes with the difference that the light modulation caused by AC supply is filtered out by the phosphor layer only to a very limited level and thus the modulation can achieve relatively high levels (tens of per cent). With the frequency being high enough to be invisible to an observer in static conditions, such high level of modulation can easily cause stroboscopic effect if an illuminated object is moving.
With LED's light sources the situation is relatively different, as there are many more potential causes of TLM. With the LED's being able to produce light only with DC supply, each light source must be equipped with an electronic circuit (driver) whose job is to stabilize the supply for the LED's to allow their usage in AC networks. The character and intensity of TLM produced by the LED light source is inextricably bound with the driver and the way it is designed.

In general, one can identify several potential causes of TLM with LED lamps:

* The AC supply itself can cause TLM at twice the AC frequency unless the driver is designed in a proper way. This is similar to incandescent lamps and fluorescent tubes, with the difference that the TLM can be much more serious with LED's.

* Similar to fluorescent and incandescent lamps, supply voltage distortion (voltage dips, swells, transients, inter-harmonics, sub-harmonics etc.) can propagate to the LED output and cause flicker.

* PWM switching of the LED output which is normally used to implement dimming feature in some drivers causes the light to be modulated at hundreds of Hz, which is invisible in static conditions, but can cause issues when motion is involved.

* Driver switching frequency can propagate to the LED output if the circuit architecture is built around switched topology, which is the case of all modern drivers. This is barely an issue as the switching is usually done at very high frequencies irrelevant for humans. The same is true for Li-Fi communication.

* Compatibility issues of the LED drivers with phase cut dimmers. Low-end drivers may either filter out the command level, either interact in strange ways; high-end drivers will "recognize" the waveform and modulate consequently their output level.

It is therefore apparent that the effects of TLM on human observers depend on whether physical motion is involved or not. In each situation the character and cause of the light modulation are different and needs different treatments to be addressed.

## B. Flicker

As already mentioned, flicker is experienced by humans in static conditions as a subjective perception of instability of the light [12]. As such, it is one of artifacts (Temporal Light Artifacts, TLA) that can be caused by TLM. With lighting systems it is regarded as a nuisance. It can be caused either by changes of luminance (common with LED light sources), changes of the wavelength spectrum, or a combination of both (as is the case with both incandescent lamps – thanks to Planck's law – and fluorescent tubes – thanks to re-emission lifetime of the phosphor layer).

The severity of flicker depends heavily on the frequency. Slower sinusoidal changes of luminance (below approx. 5 Hz) can be compensated by the contraction and dilation

<figure><img src="image_8.png" /><figcaption>Fig. 6: Flicker visibility threshold for sinusoidal modulated light in two conditions: central vision with 3.4° target [15], and panoramic vision [16]. Ordinate values are in log10 of light modulation level expressed in per cent. Intensities above the threshold will be visible; intensities below will be invisible. Averaged from over 30 human individuals.</figcaption></figure>

of pupil. Higher frequencies (above approx. 20 Hz) are filtered by the retina and neural system. The peak sensitivity occurs around 10 Hz, where modulation as small as 0.1% can sometimes become visible.

The highest frequency at which flicker can be experienced under specific conditions is called the Critical Flicker Frequency (CFF). The actual value of CFF will vary depending on the conditions, but usually it is between 30 and 60 Hz.

Research on the visibility of flicker dates back to the end of 19th and beginning of 20th century when the works of Ferry [13] and Porter [14] formulated what we now call the Ferry-Porter law. That law states that CFF is a linear function of the logarithm of the mean light intensity. Later research during the 20th century showed that the CFF is also a function of many other factors, both subjective (observer's age, weariness, eye strain, overall psychic state, effects of medicament's and drugs, adaptation to flicker exposure etc.) and objective (modulation depth, size, shape and position of the flickering source with respect to the observer, background lighting intensity etc.). Thanks to that, changes in CFF can be sometimes used as an indicator of a person's overall physical state.

One of key aspects affecting the CFF is the position of the light source image on the retina. In central view it is advantageous to have the image stabilized, and flicker sensitivity is smaller. In peripheral vision the individual must be able to spot danger easily, and flicker sensitivity is higher, allowing the observer to be alerted with less intensive stimulus. Fig. 6 shows flicker visibility threshold measurements performed at Technical University of Liberec in two distinct conditions. The difference in sensitivity is sometimes tenfold.
<figure><img src="image_9.png" /><figcaption>Fig. 7: Weighting functions which are used for evaluating the severity of discussed artifacts – stroboscopic effect [12] and flicker [17]. The weighting functions reflect the intensity of light modulation needed to induce the given effect at given frequency. The risk of visible flicker drops above approx. 50 Hz while the risk of stroboscopic effect increases above approx. 30 Hz.</figcaption></figure>

The severity of flicker is usually evaluated in terms of visibility and irritability. Flicker visibility is an instantaneous measure ("now you see me, now you don't"). Flicker irritability is a concept related to the flickermeter where statistical evaluation of long term voltage measurements needs to be performed. The flicker irritability threshold is defined as "the highest level of flicker which is tolerated without discomfort by a specified sample of population" [18]. It results from this definition that the concept of flicker irritability allows evaluation of stochastic signals containing aperiodic phenomena and permits the aggregation of results over extended periods of time into a single indicator, the flicker severity measure, $P_{st}^{LM}$.

Apart from simply being annoying, long term exposure to flickering light can easily induce headaches, eye strain, or, with sensitive individuals, even migraines [19]. This is why European legislation requires the $P_{st}^{LM}$ produced by light sources sold on EU market to be within given limits.

These effects were reported in literature even when the flicker frequency was above CFF [19]; however, these findings are not considered conclusive [20] and it is judged that more research is needed in this area.

In general, the severity of effects of flicker depends on the flicker level and exposure time. Aggravation of autistic behavior or epileptic seizures are often mentioned in literature among effects of flicker on humans [19], however, with normally operating lighting systems (even with poor quality LED drivers) the light modulation does usually not have such character so as to cause these drastic effects on humans. The modulation is either too fast or too mild.

With older lighting technologies, the only way to reduce flicker was to minimize the supply voltage disturbances: this is the responsibility of the energy distributor.

To help evaluate the amount of flicker relevant voltage distortion, the IEC flickermeter algorithm was developed in the 1980s [21]. This algorithm simulates a reference

60 W incandescent lamp and models the visibility of flicker by a human observer.

With modern LED's light sources the situation is somewhat different. Due to the need of a driver whose properties dominate the light source's light modulation characteristics, the responsibility for flicker has partially moved from the energy distributor towards the lamp manufacturer, and must be addressed as a matter of electromagnetic compatibility [17].

### C. Light Modulation in Dynamic Conditions

Flicker is an artifact caused by light modulation in static conditions. However, ideal static conditions are only achievable in laboratory conditions.

When motion is involved, the situation is different. Depending on whether it is the light source that is moving, the illuminated object or the detector (eye), different disturbing artifacts may occur [12]. The motion effectively translates the, otherwise invisible, temporal light modulation to potentially visible spatial patterns.

The occurrence of these artifacts will be bound to the motion speed of objects and limited by the angular resolution of human retina. In usual conditions of people everyday life, the upper frequency limit for the occurrence of stroboscopic effect is in the order of kHz [19].

Among such artifacts, the most common linked to lighting systems occurs when the illuminated object is in motion, and in these situations we talk about stroboscopic effect. With pulsing light, a fast moving object will be illuminated for a short period of time at different places along its trajectory of motion. In case of rotational motion, stroboscopic effect can occur if the pulsing light is in sync (or near sync) with the angular frequency of the rotating object. In this case the object can appear stationary or rotating much slower (even in opposite direction) than in reality. This artifact is known to movie fans as the stage
coach wheel effect. This poses a safety hazard in various environments, such as factories.

The risk of stroboscopic effect occurrence can be assessed via the Stroboscopic Effect Visibility Measure (SVM) [12]. This SVM is also limited by European directives. With LED light sources, the SVM can be decreased via careful design of the LED driver. With fluorescent tubes, the only solution was to split the luminaries into three electrical branches and supply each branch from different AC phase, which effectively de-synchronizes the tubes modulation.

When the light source is in motion or the observer is moving with respect to the light source, we talk about the phantom array effect. It manifests as a momentary perception of multiple light sources where there is actually only a single light source. Although not too relevant to lighting systems, an example of a phantom array effect is a situation of a late night driver watching the taillights of the car before him with those being modulated. At times the tail lights may appear as if there were more of them.

Also during rapid eye movements (saccades) the eye can notice TLM even when the frequency is too high to be seen in static conditions. Intrasaccadic TLM perception has been demonstrated at frequencies up to 500 Hz [22].

## D. Effect of TLM on biotope

Homo sapiens is not the only species out there exposed to artificial lighting. It can be expected that other species will be affected by TLM of artificial lighting systems. However, not all species will be affected the same way.

The key indicator for the way how individuals are affected by TLM is the CFF. Below the CFF, visible flicker is the main irritating artifact. Higher CFF means the individual will experience flicker in a wider scale of situations. According to many studies carried out in recent decades and summed up in [23], [24], the CFF across different species can range from below 10 Hz to up to 200 Hz or even 400 Hz.

In general, fast moving, i.e. flying species, such as birds and insects, tend to have higher CFF. Such higher CFF not only means a broader band of visible frequencies, but also an increased sensitivity to frequencies within that band. This means that f.i. a 20 Hz light modulation could be perceived by these species at modulation intensities which are well below the visibility threshold of humans.

However, the most vulnerable species will be those for whom 100 Hz modulation is visible. For technical reasons explained above, 100 Hz and 120 Hz are very common modulating frequencies in artificial light sources. At the same time, they are invisible for humans and thus they are not regulated as strictly. The recommended modulation limits for avoiding stroboscopic effects are much more relaxed than what would be required to avoid visible flicker, had those frequencies been visible to humans in static conditions.

Fast flying birds, when flying under artificial lighting, could also potentially experience phantom array effects

regardless of their CFF, as they are in a situation similar to drivers driving at night in dynamic conditions. However, the authors are not aware of any literature documenting this phenomenon and it only hypothetically arises from likening the bird's visual system to that of human. Some limited research suggests that this is a reasonable thing to do [25].

## E. Measurement of TLM and Risk Assessment for TLA

The TLM is usually quantified using Michelson contrast (also called Percent Flicker although it does not evaluate flicker) and so called Flicker Index (ditto) [19]. Especially the Michelson contrast is used very often. Those two metrics do not assess any severity, risk or probability of occurrence of any TLA as they only describe the physical TLM waveform. Nonetheless, certain frequency based limits exist to minimize the risks of stroboscopic effect [19]. However, in order to assess the severity of potential effects of the TLM on humans, the use of different metrics is required.

The aforementioned SVM metric evaluates the waveform in such a way to estimate the probability of occurrence of stroboscopic effect. A similar metric exists for phantom array effect [12].

As for flicker, the traditional way to estimate flicker severity was based on voltage, as controlling the mains voltage distortion was the only way to mitigate it. There have been several voltage based flicker severity indices developed during the 2nd half of the 20th century, including the $\Delta V_{10}$ metric and the IEC flickermeter [21], an algorithm producing the $P_{st}$ (short term flicker severity) and $P_{lt}$ (long term flicker severity) metrics.

They both utilize a model of an incandescent lamp and thus are able to estimate the flicker severity if there were a person exposed to the light produced by an incandescent lamp supplied by the analyzed voltage waveform.

With the advent of LED light sources in the 21st century, the need arose to assess flicker severity directly from the luminous flux, as the link between voltage distortion and flicker became much less predictable for the reasons explained earlier. The easiest way to obtain such metric was to remove the lamp model from the IEC flickermeter and adapt it so that the luminous flux is an input for the algorithm instead of the voltage.

The light flickermeter was developed [17] this way. It is essentially a human observer model and the metric produced is denoted $P_{st}^{LM}$.

All hereby mentioned metrics are anthropocentric. No standardized TLA assessment procedures exist for fauna. Yet.

## References

[1] C. A. Gueymard, “The SMARTS spectral irradiance model after 25 years: New developments and validation of reference spectra,” Solar Energy, vol. 187, pp. 233–253, 2019. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0038092X19305110

[2] https://www.nrel.gov/grid/solar-resource/renewable-resource-data.html.
[3] P. J. Aphalo, “The r4photobiology suite,” UV4Plants Bulletin, vol. 2015, no. 1, pp. 21–29, 2015.

[4] C. Toledano, S. Taylor, A. Barreto, S. Adriaensen, A. Berjón, A. Bialek, R. González, E. Woollams, and M. Bouvet, “LIME: Lunar Irradiance Model of ESA, a new tool for absolute radiometric calibration using the Moon,” Atmospheric Chemistry and Physics, vol. 24, no. 6, pp. 3649–3671, 2024. [Online]. Available: https://acp.copernicus.org/articles/24/3649/2024/

[5] N. U. Islam, M. Usman, and T. Jamil, “Energy-savings using Solid-State Lighting: A case study of India, Pakistan, and Bangladesh,” Energy Policy, vol. 160, p. 112676, 2022. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0301421521005413

[6] https://gallica.bnf.fr/ark:/12148/bpt6k47751647/f2/item.

[7] P. Berche, “The enigma of the 1889 Russian flu pandemic: A coronavirus?” La Presse Médicale, vol. 51, no. 3, p. 104111, 2022, history of modern pandemics. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0755498222000045

[8] I. Kokkinopoulos, A. Colman, C. Hogg, J. Heckenlively, and G. Jeffery, “Age-related retinal inflammation is reduced by 670 nm light via increased mitochondrial membrane potential,” Neurobiology of Aging, vol. 34, pp. 602–609, 2 2013.

[9] G. Silva, C. Ferraresi, R. T. de Almeida, M. L. Motta, T. Paixão, V. O. Ottone, I. A. Fonseca, M. X. Oliveira, E. Rocha-Vieira, M. F. Dias-Peixoto, E. A. Esteves, C. C. Coimbra, F. T. Amorim, and F. de Castro Magalhães, “Infrared photobiomodulation (PBM) therapy improves glucose metabolism and intracellular insulin pathway in adipose tissue of high-fat fed mice,” Lasers in Medical Science, vol. 33, pp. 559–571, 4 2018.

[10] D. A. Arnall, A. G. Nelson, L. López, N. Sanz, L. Iversen, I. Sanz, L. Stambaugh, and S. B. Arnall, “The restorative effects of pulsed infrared light therapy on significant loss of peripheral protective sensation in patients with long-term type 1 and type 2 diabetes mellitus,” Acta Diabetologica, vol. 43, pp. 26–33, 5 2006.

[11] A. Brouwer, D. H. van Raalte, M. Diamant, F. Rutters, E. J. W. van Someren, F. J. Snoek, A. T. F. Beekman, and M. A. Bremmer, “Light therapy for better mood and insulin sensitivity in patients with major depression and type 2 diabetes: a randomised, double-blind, parallel-arm trial.” BMC psychiatry, vol. 15, p. 169, 7 2015.

[12] Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models, Technical Note CIE 006, 2016.

[13] E. S. Ferry, “Persistence of vision,” American Journal of Science, vol. s3-44, no. 261, pp. 192–207, Sep. 1892. [Online]. Available: http://www.ajsonline.org/cgi/doi/10.2475/ajs.s3-44.261.192

[14] T. Porter, “Contributions to the study of flicker. Paper II,” Proceedings of the Royal Society of London, vol. 70, no. 459-466, pp. 313–329, Sep. 1902. [Online]. Available: https://royalsocietypublishing.org/doi/10.1098/rspl.1902.0032

[15] L. Kukačka, J. Hergesel, J. Nečásek, P. Bílek, M. Vik, J. Meyer, R. Stiegler, M. Pourarab, and J. Drápela, “Flicker Visibility with Different Spectra of White Light,” in 2023 IEEE Industry Applications Society Annual Meeting (IAS), Oct. 2023, pp. 1–6, iSSN: 2576-702X. [Online]. Available: https://ieeexplore.ieee.org/document/10406563

[16] J. Svobodová, “Flicker Visibility Threshold Measurements with Various Light Intensity Levels,” Master’s thesis, Technical University in Liberec, Liberec, Czechia, 2024.

[17] Equipment for general lighting purposes – EMC immunity requirements – Part 1: An objective light flickermeter and voltage fluctuation immunity test method, Technical Report IEC 61547-1 ed.3, Jul. 2020.

[18] International Electrotechnical Vocabulary (IEV) – Part 845: Lighting, Std. IEC 60 050-845, Dec. 2020.

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

[20] D. Sekulovski, S. Poort, M. Perz, and L. Waumans, “Effects of long-term exposure to stroboscopic effect from moderate-level modulated light,” Lighting Research & Technology, vol. 52, no. 6, pp. 775–789, Oct. 2020, publisher: SAGE Publications Ltd STM. [Online]. Available: https://doi.org/10.1177/1477153519881473

[21] Electromagnetic compatibility (EMC) – Part 4-15 Testing and measurement techniques – Flickermeter – Functional and design specifications, Std. IEC 61 000-4-15 ed.2, 2010.

[22] J. Davis, Y.-H. Hsieh, and H.-C. Lee, “Humans perceive flicker artifacts at 500 Hz,” Scientific Reports, vol. 5, Feb. 2015. [Online]. Available: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4314649/

[23] R. Inger, J. Bennie, T. W. Davies, and K. J. Gaston, “Potential Biological and Ecological Effects of Flickering Artificial Light,” PLoS ONE, vol. 9, no. 5, p. e98631, May 2014. [Online]. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4038456/

[24] K. Healy, L. McNally, G. D. Ruxton, N. Cooper, and A. L. Jackson, “Metabolic rate and body size are linked with perception of temporal information,” Animal Behaviour, vol. 86, no. 4, pp. 685–696, Oct. 2013. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0003347213003060

[25] J. R. Jarvis, N. R. Taylor, N. B. Prescott, I. Meeks, and C. M. Wathes, “Measuring and modelling the photopic flicker sensitivity of the chicken (Gallus g. domesticus),” Vision Research, vol. 42, no. 1, pp. 99–106, Jan. 2002. [Online]. Available: https://linkinghub.elsevier.com/retrieve/pii/S0042698901002681