Strobes and Epilepsy

thiswayup · Nov 2, 2007

Condensed from a lighting SFX manufacturer's site:

http://www.birket.com/strobes/Library/Strobes%20v%20Epilepsy,%20rev070704.htm

About one in 4000 individuals has photosensitive epilepsy. Repetitive flashing lights may induce seizures in these individuals. The flash frequency of concern is from 5 Hz to 70 Hz, with most individuals only susceptible in the range of 15 Hz to 20 Hz.

·A flashing strobe (or a close combination of multiple strobes sequenced together) must not be programmed to flash in the 5 Hz to 70 Hz frequency range.

·Slower flash rates, and randomly flashing lights are not known to be a cause of photosensitive epilepsy.

·Point sources of light are much less likely to induce seizures than a diffuse source of light which covers a large part of a person's field of vision.

·To induce a seizure the light must be present in the center of the field of vision as opposed to the periphery.

·Reducing brightness or increasing distance between a photosensitive viewer and the light source is effective for preventing photosensitive epileptic seizures.

·Lights flashing in the distance, even in the frequency range of concern, are not known to cause seizures when in the presence of other lights of a more natural or chaotic nature.

·The probability of inducing a seizure is greatly increased (by up to a factor of ten) if the light source is arranged in a regular pattern, such as a raster scan image... Stated another way, avoid adding spatial contrast (pattern) to temporal contrast (flickering).

Each of these points is derived from reading Graham Harding's "Photosensitivity: a vestigial echo? The first Grey Walter lecture." in the International Journal of Psychophysiology, 1994, volume 16, pages 273-279.

Something to think about. If you are tempted to use a "tactical" strobe to e.g. direct traffic at night, I'd suggest that you might want to re-think - these tend to be at exactly the worst frequency. Otoh, an SOS strobe is not going to cause problems. And a manufacturer could possibly equip their lights with a safe tactical strobe by breaking their strobe pattern, so that gaps between flashes vary in length.

Marduke · Nov 2, 2007

thiswayup said:
Condensed from a lighting SFX manufacturer's site:

Something to think about. If you are tempted to use a "tactical" strobe to e.g. direct traffic at night, I'd suggest that you might want to re-think - these tend to be at exactly the worst frequency. Otoh, an SOS strobe is not going to cause problems. And a manufacturer could possibly equip their lights with a safe tactical strobe by breaking their strobe pattern, so that gaps between flashes vary in length.

In general, people with epilepsy severe enough that a strobing light causes a seizure will not be allowed to drive, so no real problem. Rule that most states usually have is that if you have a recurring grand mal seizure condition (it wasn't a one-time thing), you are not allowed to drive for 6 months, or until a doctor can show that you are otherwise safe to drive again.

thiswayup · Nov 2, 2007

Marduke said:
In general, people with epilepsy severe enough that a strobing light causes a seizure will not be allowed to drive, so no real problem.

That's an excellent point.

Otoh, if 1 in 4000 people are vulnerable, you might still want to think before waving a round a 15Hz strobe.

stevet47 · Nov 2, 2007

Marduke said:
In general, people with epilepsy severe enough that a strobing light causes a seizure will not be allowed to drive, so no real problem. Rule that most states usually have is that if you have a recurring grand mal seizure condition (it wasn't a one-time thing), you are not allowed to drive for 6 months, or until a doctor can show that you are otherwise safe to drive again.

That is how it is in MD and PA. I know two different people (1 from PA, and 1 from MD) who couldn't drive for 6mo. because they had a seizure. Luckily, they are both alright now.

elgarak · Nov 2, 2007

Night-Ops had a small section about this when people had doubts about the strobe on the Gladius when it came out. Cannot find the link right now, but it boiled down to that:

- The probability to encounter an epileptic is pretty low to begin with, since the condition is rare in itself.

- The probability to encounter a photosensitive epileptic is even lower -- only about 1 in 400 of all epileptics are photosensitive.

- The majority of photosensitive epileptics are females, which are less likely to be encountered in violent tactical situations, for which the strobe is intended.

So it's very unlikely to strike a seizure, when the Gladius is used as intended.

Sure there's a problem when you use the strobe to direct traffic. That's not what the Gladius strobe is designed for. Frankly, people who do that should be whacked on the head with their lights. To direct traffic, you get a constant on light with a cone. For warning, you'll get a low frequency blink. Cause a tactical strobe is very irritating. That's what it's designed for. Which is pretty bad for using it in a high tension situation like traffic congestions.

mchlwise · Nov 2, 2007

From the information posted, it seems rather unlikely that a flashlight will cause a seizure.

Flashlights are point sources of light, likely to be in the periphery rather than in the center of field of vision, are likely to be used in conjunction with other lights (such as streetlights, lights in the distance, etc.), and aren't used in a pattern (you usually only have one).

It sounds like it's certainly possible to induce a seizure with the strobe mode, if you get right in someone's face with the light and you're in an area without other lights around.

I think though that something like a bicycle using a strobe mode is unlikely to affect a photosensitive driver, who as stated should probably not be driving anyway.

:shrug:

Sgt. LED · Nov 2, 2007

Well although it's very unlikley, maybe someone will have a huge seizure and sue some flashlight maker for millions! Company's will have to consider the cost of lawsuits and stop the annoying mode! Then I have a better chance of getting a design I like without that crap built in! My dream for the day......:laughing:

thiswayup · Nov 2, 2007

elgarak said:
Night-Ops had a small section about this when people had doubts about the strobe on the Gladius when it came out. Cannot find the link right now, but it boiled down to that:

- The probability to encounter an epileptic is pretty low to begin with, since the condition is rare in itself.

It's happened to me - an "encounter" not epilepsy - not with a torch, and not my fault, but it was one of the most horrible things I have ever seen.

- The probability to encounter a photosensitive epileptic is even lower -- only about 1 in 400 of all epileptics are photosensitive.

I can't see why a manufacturer of SFX lighting would over-hype the number, and they seemed to specify, quoting specified peer reviewed research, 1 person in 4000 of the total population - that's a much higher figure.

If you're standing by the side a busy road signalling traffic for an hour, you might well have a thousand people go past. Hopefully you'll be lucky, and even if one is photosensitive, conditions won't be "right".

- The majority of photosensitive epileptics are females, which are less likely to be encountered in violent tactical situations, for which the strobe is intended.

If someone is attacking you, and he has an epileptic fit, I think you should be excused from caring, thank you. My concern is with other uses of strobe. Reading this, it seems fine to use a tactical frequency strobe for "peaceful pusposes" at a distance, but using it close-up may cause something unfortunate.

So it's very unlikely to strike a seizure, when the Gladius is used as intended.

Sure there's a problem when you use the strobe to direct traffic.

That's not what the Gladius strobe is designed for.

Frankly, people who do that should be whacked on the head with their lights.

Umm, I don't think that's quite fair. If these things aren't discussed, people won't know. I don't think there's a major problem here, just something useful for people to know. It is a bit alarming to think of all the lights now being sold with 15Hz strobes, without any attempt to warn users these might be dangerous to some people.

My bet is that tac strobe will be removed from most lights shortly for legal reasons, and that you'll need to show a LEO ID to buy it. Of course, most people here will think that's a UI improvement.

GreySave · Nov 2, 2007

I have used a strobe at a traffic accident (during the daytime and not pointed directly at the traffic closing from behind). At that time there were no real alternatives since I was on foot, just happened to be there when it happened, and had only what gear I had on me to use. The wrecked vehicle's lights were inoperative due to heavy front end damage and the resulting damaged battery, and the vehicles were stopped directly in the lane of traffic. It was either use it or have no warning at all until professional help arrived, which was not an especially enticing thought given the way that folks speed on that stretch of roadway. The light was stationary on the deck lid of the vehicle pointed slightly off to the side so the hotspot was not shining right at traffic. The light was not used for active traffic control purposes.

I think the concerns about seizures are valid. Even if the affected person is not driving, they could be a passenger in an approaching vehicle. Given that thought, I would probably revert to the SOS mode if I was to be placed in that situation again. The strobe was extremely effective and I have no doubt that using it in SOS mode would produce the same results.

thiswayup · Nov 2, 2007

Sgt. LED said:
Well although it's very unlikley, maybe someone will have a huge seizure and sue some flashlight maker for millions!

How many lights with strobe are now being sold each year in the US? If it's tens of thousands, then the odds of someone having something horrible happen to them are fairly high. Seriously, if I was Fenix or one of their retailers, I'd be very worried. Putting a bright strobe on their lights, at exactly the most dangerous frequency - ouch.

Company's will have to consider the cost of lawsuits and stop the annoying mode! Then I have a better chance of getting a design I like without that crap built in! My dream for the day......:laughing:

I *knew* you'd say that...

thiswayup · Nov 2, 2007

mchlwise said:
From the information posted, it seems rather unlikely that a flashlight will cause a seizure.

Flashlights are point sources of light, likely to be in the periphery rather than in the center of field of vision, are likely to be used in conjunction with other lights (such as streetlights, lights in the distance, etc.), and aren't used in a pattern (you usually only have one).

If a light is close enough to light up the field of vision - or even just the fovea - then in terms of retina response rather than physics, it is no longer a point source. With a modern cree pocket torch this happens at what, 10m at night? As for pattern, etc - those aren't necessary conditions, they're risk increasing factors. If you look at the page as a whole, what is actually being said is "avoid ~15Hz unconditionally, and even then these other things are bad".

I think though that something like a bicycle using a strobe mode is unlikely to affect a photosensitive driver, who as stated should probably not be driving anyway.

Given that a fit can be both dangerous and distressing, I don't think you should consider triggering one to be copacetic just because someone is a pedestrian. Which I am sure isn't what you meant.

mchlwise · Nov 2, 2007

thiswayup said:
Given that a fit can be both dangerous and distressing, I don't think you should consider triggering one to be copacetic just because someone is a pedestrian. Which I am sure isn't what you meant.

No, that's NOT what I meant.

What I was trying to say was: based on the information posted it seems that there is a sensitivity to it in a small number of people, but that under most normal flashlight usage it's not something to worry that much about.

Most people don't use strobe without reason, and I can't see a situation where someone would unintentionally get in someone's face with a strobe light, and the likelihood of that person being sensitive were that unlikely event to happen seems pretty small.

Maybe someone on a bike... on a bike/jogging trail at night... using strobe... adjusted poorly so it's pointing at eye level instead of the ground in front of the bike... coming suddenly upon a jogger coming from the other direction... who happened to be sensitive. :shrug:

I'm not trying to belittle or dismiss the very real sensitivity that some people very really have. It just seems like it's easily avoided in most situations. Don't get in someone's face with strobe mode. Of course... you probably don't want to do that anyways. :crazy:

elgarak · Nov 2, 2007

thiswayup said:
It's happened to me - an "encounter" not epilepsy - not with a torch, and not my fault, but it was one of the most horrible things I have ever seen.

I can't see why a manufacturer of SFX lighting would over-hype the number, and they seemed to specify, quoting specified peer reviewed research, 1 person in 4000 of the total population - that's a much higher figure.

If you're standing by the side a busy road signalling traffic for an hour, you might well have a thousand people go past. Hopefully you'll be lucky, and even if one is photosensitive, conditions won't be "right".

If someone is attacking you, and he has an epileptic fit, I think you should be excused from caring, thank you. My concern is with other uses of strobe. Reading this, it seems fine to use a tactical frequency strobe for "peaceful pusposes" at a distance, but using it close-up may cause something unfortunate.

Umm, I don't think that's quite fair. If these things aren't discussed, people won't know. I don't think there's a major problem here, just something useful for people to know. It is a bit alarming to think of all the lights now being sold with 15Hz strobes, without any attempt to warn users these might be dangerous to some people.

My bet is that tac strobe will be removed from most lights shortly for legal reasons, and that you'll need to show a LEO ID to buy it. Of course, most people here will think that's a UI improvement.

The discussion/information from Night-Ops was directed at LEOs. A LEO has to deal with someone (which I mean with 'encounter'), he strobes. What is the probability that the someone is photosensitive and has a seizure, and sues the LEO? As Night-Ops pointed out, the probability that the someone is photosensitive is quite low (1 in 4000, wiki says 2 in 10,0000, about 0.02 per cent), and probably female. In short, there's quite little danger. I don't think the numbers are high enough to justify 'outlawing' of strobes, though high enough that people need to be informed of the danger. [Caveat: The 1 in 400 number was mine, and is probably wrong and I misremembered. Wiki says 3 to 5 per cent of all epileptics, and 2 in 10,000 in general population. About 0.02 per cent].

About the "not quite fair", I think it is. Fenix for instance has no idea, IMO, what the strobe is for. CPFer wants it, CPFer gets it. And frankly, a lot of people have no idea either what the strobe is for, and what it can do.

Of course we have to inform people. That's what this thread does. And I stand by my opinion that a fast, 10 to 20 Hz strobe is not a general tool, and that it should not be a general feature on flashlights. Most people have absolutely no use for it.

That's not to say that it does not have any uses. But the people that have uses for it are of a very limited group.

All the uses that most people have for strobing and blinking can be achieved with less dangerous, less irritating, slower strobes (1 Hz or slower).

Strobes and Epilepsy

thiswayup

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Marduke

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thiswayup

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stevet47

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elgarak

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mchlwise

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Sgt. LED

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thiswayup

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GreySave

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thiswayup

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elgarak

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