Flicker & Stroboscopic Effects - GET LIGHT RIGHT · Flicker & Stroboscopic Effects Gerard Harbers People can get sick and can even get seizures from flickering lights. Even if

Flicker &Stroboscopic

EffectsGerard Harbers

People can get sick andcan even get seizures fromflickering lights. Even if youdon’t get sick, you might stillget a headache, get tired, orfind it hard to concentrate ifexposed to flickering lights.And did you know that youcan be affected, even if youdon’t see the light flicker?

If flickering lights can have these effects, why dolights flicker? Sometimes lights are made to flash onpurpose, to create extra attention.This is the case withflashing lights on a fire truck, or a flashing neon light infront of a store. But more often these variations in lightoutput of lights are a caused by physics principles andtechnology which make lamps work.

Although electriclights are around for a longtime, there is still a lot ofconfusion about this topic.There are only limitedstandards and regulations,and flicker and stroboscopiceffects are still activelyresearched. And with newlamp technologies such as

LEDs, standards need to be updated. Which takes time,and technology is moving ahead quickly. In addition,tools to measure and characterize flicker s are not readilyavailable, or expensive, not easy to use, or not accurate.

This white paper describes key research on flickerand stroboscopic effects, how they can be measured, andspecified.

GETLIGHTRIGHT

©2019 Gerard Harbers

Photo by Carolina Heza on Unsplash

PRE-RELEASEWORK INPROGRESS

Introduction

You probably know that flickering lights – lights which have a rapidlyvarying output – can make people sick, and can even induce seizures:known in the medical profession as photosensitive epilepsy. However, thisextreme case is not very common: about three out of ten thousandpeople have this condition. Seizures are typically triggered with lightsflashing in the range from 1 to 60 Hz, and most common in the rangefrom 3 to 30 Hz.

Photosensitive epilepsy occurs more often under the age of twenty. Afamous example of a mass induced seizures is the broadcast of a:Pokemon cartoon broadcast TV in Japan on December 16, 1997. About685 children got seizures while watching this cartoon and were lateradmitted to a hospital. Over 75% of these children never had a seizurebefore. Two of the patients remained in the hospital for more than twoweeks! It is maybe needless to say that this episode got broadcast onlyonce, and is referred to as the "Pokemon Shokku" or "Pokemon Shock" inJapanese press .The scene which caused most harm was a a scene withred/blue flashes alternating with a frequency of about 12 Hz.

But even if you do not have photosensitive epilepsy, you might still beaffected by fast modulating lights. Even if you don't notice anythingwrong with them, and even if they modulate at much higher frequenciesthan 60 Hz. Even at higher frequencies, with seemingly nothing wrongwith the lights, so called stroboscopic effects might cause a headache, ormake you find it hard to concentrate.

At these higher frequencies it are the movements of your eye incombination with the modulating lights which disturb the visualprocesses in your brain. Although you don’t notice them, your eyesmakes small movements –called saccades– to process the imagesprojected on your retina. With a constant illumination these processeswork fine, but they can fail if the images your eyes and brain receive getbroken up by flashing lights.

By the way, it is not only modulation from light might which can makeyou sick: also patterns –either moving or static– can make you sick.Probably the first ever recorded case of a stroboscopic induced seizuredates back to 125 AD: Apulius got a seizure while looking at a rotatingpotter's wheel!

Here –in this white paper– the main topic are the health risks ofunintended light output and/or color variations of lamps and LEDs.Besides an overview of the key results of research in this field, I will alsodiscuss how flicker and stroboscopic effects of lights can be measuredand analyzed, and show examples of light modulations and associatedquality metrics.

But let start with the basics first: what is flicker, what are stroboscopiceffects, how do these differ, and why do they make your sick?

GETLIGHTRIGHT FLICKER & STROBOSOPIC EFFECTS 2


A Japanese cartoon containingscenes with flashing blue and redframes induced seizures in manychildren watching it; they were sosevere that 685 children wereadmitted to the hospital to recover.

Flicker

Flicker is a directly noticeable variation in intensity of a light.The wordis used for noticeable variations, that is that we see and are aware of thefact that a light flickers. Typically we are able to ‘see’ flicker if thevariations are below 90Hz. If light varies at higher frequencies it is hardto directly see a light variation. It are stroboscopic effects related to thesefast modulating lights which can get you tired or give you a headache.Stroboscopic effects and will be discussed in the next section.

Light output variations can be created intentionally, as with trafficsignals, or unintentionally, for example as a result of power linevariations.

This term flicker was introduced by the lighting and electrical powerindustry, to describe and specify the variations in the light output oflamps.These variations can be caused by varying input power – such 50or 60Hz mains power frequencies – or by their operating conditions.

Lamps can have these intensity variations due to their physical operatingprinciple, or due to the variation in their input power, in particular dueto the 50Hz and 60Hz main power frequencies.

In the early stages of electric lighting flicker was not a big problem.Thefirst mass produced lamps were incandescent lamps, and these have verylittle flickers, even if they are operated on a direct 50 or 60Hz AC mainssupply source.The reason for that is that the filament is run at a veryhigh temperature, and has thermal mass. You can notice this if youswitch off a light bulb: it has a slight but noticeable decay time before itstops emitting light.

The first mass produced lamps which had flicker problems werefluorescent lamps.These lamps use an electronic discharge, and the lightoutput is directly related to the current flowing through the tube.Theselamps can not directly be operated from the mains power line, but needa ballast. In modern lighting installations this ballast is an electronicballast, running at very high frequencies resulting in very little flicker,but early on magnetic ballasts were used.These provide a regulatedcurrent and inverted voltage to the tube at a frequency twice thefrequency of the supply voltage. In the USA this is typically 120 Hz,while in many other countries this is 100 Hz. Although these frequenciesare too high to be easily directly noticed, they created many issues forpeople sensitive to light variations.

Stroboscopic Effects

A stroboscopic effect is observed when a flashing light – called a strobelight – creates visual illusions with motion objects.They are sometimesused for creative effects at parties, or used to study or measure themotion of fast moving objects.They can also occur without a strobe lightin case video recording with a camera: you might have seen the wagon



A police car using flashing lights tocreate attention. This is an exampleof intentional light modulations.But even if lights are intentionallymodulated, care has to be taken toselect the right modulation patternsas to not induce seizures.

Photo by Matt Popovich on Unsplash

wheel effect in old movies, were at a certain speed the wheel appear tobe not or slowly rotating.

The difference between flicker and stroboscopic light artifacts is thatflicker is referred to as the effects of temporal light artifacts related tostatic objects, and stroboscopic light artifacts to the effects on movingobjects.

Stroboscopic light artifacts have a much higher frequency range thanflicker light artifacts due to their aliasing effects.

A light modulated at a relatively high frequency of 500Hz incombination with a moving object create image patterns at your retinawith spatial frequencies in harmful flicker frequency range.

A variant of the stroboscopic effect is the phantom image or ghostingphenomenon.This is caused by your eye moving, and instead of seeingan object as a single image, it appears multiple times in a row. It happensthat when you look at an object, your eye is making constantly smallmovements: in fact, if this would not happen the objects you are lookingat would slowly fade.

Normally your brains – or more formally: the visual processing centra inyour brain – corrects an object's position with the eye movements, andyou will see a stable image.This is very similar what happens nowadaysin digital cameras called image stabilization.

If you have a flickering light however, your brain can get confused, andhas to work extra hard to make sense of the data it is receiving, and myfail to do so. With the result you start feeling tired, getting a headache,or –even worse– induces a seizure.

As concerned to the medical terms: these are called Photic or PatternInduced Medical Effects, and Photic or Pattern Induced Seizures.

Who suffers from flicker?

Fisher (2005) reports the following on this:

▪ About ~0.3 to 3% of us get an abnormal EEG response ifexposed to modulated lights and patterns.

▪ The chances that you actually get a seizure are much lower:about 1 per 10,000 on average, or 1 per 4000 in the age group of5 to 24.

▪ If you happen to have some form of epilepsy, the chance thatyou get seizures from flicker is 2 to 14%.

Also an abnormal EEG response is – well – not normal, and should beavoided. Which means that between 3 to 30 people out of a group of



1000 should be seriously concerned about lights and lighting with regardto flicker and stroboscopic effects.

And if you are directly or indirectly responsible for the lighting in aspace, do you want induce these negative effects to the visitors,customers, or workers?

There are people who take their own lamps with them if they travel, andreplace the lamps in their hotel rooms during their stay!

What is it in flickering lights which make

you sick?

In the same article written by Fisher (2005) et al, the followingparameters are mentioned to have an effect on the risk of getting aseizure:

▪ The flicker frequency is an important parameter. A very lowfrequency less than 1Hz gets barely noticed, and also very highfrequencies, well above 4000Hz, don't pass the first phase ofvision.

▪ Its intensity. If the light is very dim, variation might getsnoticed, but is not known to have any negative effects onpeople.

▪ Its modulation depth, or percent flicker, as this is sometimescalled as well. For example, does the light vary from 0 to fullintensity, or does it vary between 90% and 100%? Depending onfrequency and intensity of course, the first has a high risk ofhaving negative effects on people, while the latter case has not.

▪ Do you see the flicker with one or two eyes? If the flickeringlight is on one side, and is mainly visible from one eye, theeffect of flicker is less than if observed with two eyes.

▪ The aerial extent of flicker within the field of view. A largerarea of flicker in your view is worse, than only a small area.

▪ If you are directly looking at a flickering light, or if it is at alarger angle in your field of view.

Measuring Flicker

How is flicker, or –more formerly– how are temporal light variationsmeasured?That is done with –you guessed it– a flicker meter. Andalthough the term flicker is used for light modulations in the range from1 to 90 Hz, I will use here this term describe an instrument whichmeasures temporal light modulations over the frequency range relevant



for human vision and photic medical effects.

As opposed to other photometric or colorimetric instruments, flickermeters are not very common, and are mainly used in professionals in thelighting industry.

References

▪ Fisher (2005) Photic – and Pattern – induced Seizures: AReview for the Epilepsy Foundation of America WorkingGroup, Robert S. Fisher, Graham Harding, Giuseppe Erba,Gregory L. Barkley, Arnold Wilkens, Epilepsia 2005 Sep,46(9):1426-41.



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Flicker & Stroboscopic Effects - GET LIGHT RIGHT · Flicker & Stroboscopic Effects Gerard Harbers People can get sick and can even get seizures from flickering lights. Even if