Strobe LIghting

[probuttonpusher]

Hi,

 

I'm working with a small based entertainment company running various shows throughout the year and I'm wondering if I could get some advice.

The lighting setup uses 8 - 10 Par 56 LED cans. All daisy chained to the same desk They are useful for us to multi-colour however we have started using the strobe feature more frequently. Although these are only small lighting units I cannot find any safety regulations regarding using the strobe feature. I'd like to know if possible what my limitations are with this feature as we have 4 of the cans facing the audience while sat on a goal post lighting stand. How long am I allowed to use the strobe feature before turning it off for a period of time?

 

Any advice would be greatly appreciated.

 

Thanks.

[ImagineerTom]

There's no specific rules prescribing a precise time since there are so many variables.

 

What I would say though is that "normal people" (ie not lighting/disco/band people who have become desensitised to it) find a strobe effect VERY disorientating and unpleasant if it's for more than a few seconds. THAT should be your number one concern.

[rossmck]

Also keep in mind it's unlikely they'll strobe at exactly the same rate, which can make the effect worse for some people and can increase the risk of a seizure in those that suffer from photosensitive epilepsy (PSE)

[J Pearce]

The old purple book has the following guidance (the new guide hasn't written this chapter yet):

• Warning at entrance/on ticket/in programme
• Keep rates below 4Hz - below this less than 5% of the flicker sensitive portion of the population will be at risk of an attack
• Where more than one unit is in strobe they should be synchronised
• Mount lights above head height
• Bounce off surfaces to reduce glare effect
• Should not be used in corridors or on stairs
• Long periods of strobing should be avoided

This is not law, but as ever if you go against guidance have a risk assessment sorted and be ready to justify it in front of a judge and expensive lawyer.

 

There was a HSE guidance document available, but it seems to have been taken down from the site, and searching only produces comments saying there is a guidance note on the HSE website.

[bigclive]

Definitely warnings and definitely limit the use. It's a very tacky effect that should only be used in small bursts for effect.

[kerry davies]

There was a HSE guidance document available, but it seems to have been taken down from the site, and searching only produces comments saying there is a guidance note on the HSE website.

 

Lots of stuff that is not covered by legislation is being taken down from the HSE site and the removal of the Purple Guide to an industry website is an example of this. They are dealing with law and they allow industry experts to provide Codes Of Practice and guidance. The only link on their website takes you to an NHS website which basically says PSE is rare, 5%, 5Hz etc. Nothing more.

 

Clive is right, it's just naff. Another case where less is more and judicial use far more effective than overloads.

[top-cat]

Well, let's be honest. Strobe isn't 'tacky' or 'naff'. It may be in some environments, but not others. In nightclubs you will struggle to convince people that strobes are naff, because they, the punters, love it.

 

Frankly if the strobes are on the stage and your lighting position is in the audience, YOU should be a reasonable judge to how intense your effects are, because you are observing them from the audience and as a result are therefore subject to those same effects.

[musht]

Thing with an LED Par on `strobe` is it can flash quickly, but isn`t really a strobescope.

 

Pulse length is too long to freeze action and intensity is a lower than a xenon or big LED strobe.

 

Does make me wonder that if you are using the effect more frequently, mebbe want an actual purpose built strobe added to your rig to suit the events your doing...

[Wingwalker]

There's been lots of talk over the years with many technicians on this very subject. Most professional strobe units will have a flash rate of between 1 and 15 flashes per second (FPS) but as of yet no-one has deemed what is a safe limit from one set of timings to another as a FPS of 1 second will actually be brighter thant that of 15FPS - although the difference to the naked eye is barely visible.

 

The normal level of safety precaution is to provide clear notice beforehand that stroboscopic lighting is in use in your performance. The best way to do this is to have a notice in the entrance way that everyone walks through into your auditorium as not everyone will purchase a program or listen to an announcement before the show. In addition to this, it is "reccommended" that you have your effect on for no longer than 30 seconds at any one time AND that you do not use the strobe effect as your sole light source. i.e. it is advisable to have a general lit state of at least 40% so that an audience member can divert their eyes if required.

 

As with most things with this type of effect - less is more, so think about how effective you can make your performance if you are going to use it rather than how long can I get away with it.

 

Interestingly enough, my wife has epelipsy and tells me that the best way she normally gets around this is to shut just one eye rather than both. The reason behind it is that in her case for a seizure to take place her brain has to receive two sets of signals simultaniously. Obviously this will be different from person to person but in her case she is able to look at say the piercing bright blue strobe of an Ambulance or Police car as their sets of lights flash from one side of the vehicle and then the other, rather than both sides at the same time. That split second delay coupled with closing just one eye has saved her from many seizures over the years. And when you live in London, that's a lot of flashing blue lights... :D

[bigclive]

When I designed and manufactured fairground strobes many moons ago I did a lot of research which was limited greatly by the lack of Internet at that time. I found various articles suggesting that frequencies above about 15Hz were considered a risk and capped my own units at 12.5Hz. I also added a limiter that automatically turned the strobe off after 256 flashes (about 20 seconds at full speed).

 

During experimentation I ran a strobe at 25Hz and immediately saw lines radiating from all surfaces and felt very nauseous.

 

As for strobe use in clubs... The audience doesn't like it as much as you might think. Small bursts with purpose - yes. Long or continuous operation - no.

[Brian]

The definitive book on PSE...

 

[Grum]

One thing I never understand on this is why people get so worried about Strobe lights but completely ignore any other kind of flashing light.

 

OK, I know in this instance the OP is asking about LED cans, and rightly so however I have been to plenty of venues that do not consider the flash rate of any other type of light. One very well known venue in London even has a policy that the strobe rate has to be set in advance with management and must not be changed during the show, they cannot be used for more than so many seconds at a time and not more than a certain number of seconds every so many minutes etc.

 

That same venue has no problem with moving lights on a high flash rate (strobe mode) even if the units are out of sync effectively creating a faster perceived flash rate.

 

I have personally pointed over 50 strobes directly at audiences of over 10,000 people on many occasions had them flashing at a high rate for extended periods (over 50% of a song in total) and whilst it may seem excessive it did suit the song. At no point have I ever received any complaints nor have there been any health problems reported.

 

Whilst it is true that in court a lot of emphasis from a prosecution would be put on established/published guidance notes but any decent defence would be able to counter that with recognised industry practice and lack of recorded problems. If this was as big a problem as some people seem to think it is then there'd be lots of incidents being reported each week. Hell, Academy venues alone would each be putting at least one person a week in hospital.

 

Yes it is true that a very small number of people suffer from PSE but you are talking about a VERY small percentage of people who generally know they have a problem and also know how to deal with it. I am an amputee and as a result I often fall over when walking on uneven surfaces yet I don't see anybody rushing to move all music festivals to car parks to reduce trip hazards and I can promise you there are a lot more amputees than there are PSE sufferers. The attitude to strobe lighting is a massive overreaction to a very real but small problem that is often badly executed by people who don't fully understand what they are trying to achieve.

[dbuckley]

Thing with an LED Par on `strobe` is it can flash quickly, but isn`t really a strobescope.

 

Indeedy, but it is a useful alternative to "true" strobe discharges.

 

And even modern "proper" strobe lights aren't really stroboscopes in the conventional sense of the word. An old-school capacitor discharge strobe had a very short "on" period, measured in microseconds, and would freeze even quite fast motion. Modern strobes, like Atomics, have an on period of milliseconds, and the higher the intensity, the longer the duration.

 

But typically, none of this matters, we use strobes in a theatrical context for a look, rather than for the science. And on small stages with small rigs, which generally correlates to shows with limited budgets, LEDs are often adequate to give a "good enough" strobe "effect". I doubt I'll ever be Grum and get to push go on 50 Atomics :)

[bigclive]

The use of traditional intelligent lights for strobing has a much softer attack and decay than a traditional xenon strobe, since the shutter has to move in and out. LED fixtures do offer very high rise times on the flash, but at the moment they have to compensate for lack of actual intensity by increasing the width of the flash pulse. I'm sure this will change as they get progressively brighter, and you will get people using them as very high speed strobes.

[musht]

Did some stuff a while back with someone using LED based strobes for high speed fluorescence under a microscope, getting into microseconds, rise time certainly ain`t a problem, not exactly dance floor sized though.

 

Man who started it all Professor Harold `Doc` Edgerton:

 

http://web.mit.edu/6.933/www/Fall2000/edgerton/www/intro.html

 

To get an LED to light bright for short period requires dropping a much larger current through it than normal continuous operation, just pulsing it at normal current will dim it, thats how PWM works, so somethng to be a strobed requires the hardware be built around it, not something that can be solely done by software.

 

SGM X5 is an LED strobe with blinder mode rather than an LED wash with strobe mode:

 

http://www.sgmlight.com/shop/x-5/c-23/p-105

 

A cheap high power xenon DMX strobe for under 100 quid will give the impact of a strobe effect rather than just a fast flashing light. Admittedly it`s a big fragile box to lug around for a nights use probably measured in seconds total.