Questions about Laser Safety, specifically Pangolin/William Benner doc 'Making Shows

[buffo]

I think I see the source of your confusion. Let's back up a moment:

Back to the rationale for step 1), maybe it is measuring/establishing a baseline irradience against which single pulse, multiple pulse MPE & average power can be compared?

Essentially, yes. Step 1 measures the irradiance of the beam at full power at the closest point of audience approach. Remember: Irradiance is power per unit area, nothing more. It's a fundamental property of your projector. The only thing that will change it is if you are operating the laser at reduced power levels, or if you move further away from the projector and the beam diverges (spreading the power over a larger area). Apart from these two variables, irradiance will not change. That's why you measure it first.

Note, however, that this step tells you nothing about the *safety* of that beam. It's merely meant to measure the maximum irradiance that the projector is capable of producing. (More on this below.)

Assuming that is the case, step 4) computes the single pulse MPE, from actual measurements of pulse width and pulse repetition rates.

Right. Step 4 basically tells you how much irradiance is too much, given the time figures you provide for the effect you are measuring. You can then compare that limit with the actual irradiance of your projector, which you measured in step 1.

The concept here is that a moving effect with a SHORT pulse duration can have a higher irradiance than one with a long pulse duration.

My confusion comes from 'if the irradiance measured in step 1 is greater than the single pulse MPE, stop right there - the effect is not safe for even 1 pulse of laser light'

Exactly. This means that the irradiance (remember - power per unit area) is too high even for such a short duration exposure. In order for such high irradiance to be eye-safe, you have to reduce the duration of the exposure even more.

Surely it's the other way round? I.e. if the MPE measured in step 4) is greater than the irradiance measured in step 1), that's unsafe?

No. In that case, the calculation is saying that some large irradiance level is OK, and your actual irradiance from your projector is lower than that, so you are safe for a single pulse.

Remember: Irradiance is a fundamental property of your laser. It is a function of the power of the laser and the size of the beam. Period. So yes, you can reduce the power (analog modulation) to reduce the irradiance, but at maximum power, it will only deliver so many watts, and the beam diameter is more or less constant for a given distance away from the aperture. Thus, irradiance at max power (and at a given distance) should be a constant. It won't ever go higher. (Ignoring slight variations in the performance of the laser over time, of course. This is why you have to check it before each show.) Divide that maximum power over the area of the beam, and you get irradiance. It's that simple.

The way it's written it says if the single pulse MPE is less than the irradiance of a static beam, that's unsafe. I thought you want your single pulse MPE less than the irradience of a static beam. Seems illogical to me, or I have got it wrong?

You have it wrong, sorry to say. But that's OK, this is an important point, and once you understand it this should all make more sense.

The theory behind all these calculations is based on one simple fact. If you limit the time of your exposure, you can be exposed to more. This works for lots of things - sound, light, heat, chemical agents, ionizing radiation, annoying relatives , and even politicians!

Think about a sunburn: if you lay out in the sun all day you get a burn. But a 5 minute exposure to the sun doesn't give you a burn. Why? The duration of the exposure isn't long enough to cause damage. Same idea here...

A safe irradiance level for a static beam is often quoted as 2.5 mw/cm squared. What is important to note is that this limit is based on the assumption that after about .25 seconds, you will either blink or avert your gaze. So it's really the limit for a single-pulse exposure lasting .25 seconds.

But if you reduce the exposure time to around one millisecond, the allowable irradiance increases to 10 mw. Plug the numbers into the formula and see for yourself. (1.8 x t ^.75)/t (Answer will be in mw/cm^2)

Likewise, if you are determined to stare into a laser beam and willfully override your blink reflex (not recommended, obviously), you can use the same calculation to see what a safe irradiance level is for - oh, say a 10 second exposure. (1.8 x 10^.75)/10. And that works out to around 1 mw/cm^2.

So you see, at shorter exposure durations, higher irradiance levels are allowed. That's why we do the calculation. We want to see how much is OK for a really short exposure. But if the irradiance of our projector still exceeds that, then we're screwed. We have to either reduce the power, increase the beam diameter, or make the exposure even shorter, and then try the calculation again.

I would have thought that it is desirable for the MPE of effects projected into the audience to be LESS than the Irradience of a static beam.

I think you are confusing irradiance (which is a fundamental property of the beam coming from your projector) with a maximum permissible exposure level, which combines an irradiance level with a specific duration of exposure. Does that help?

Adam

EDIT: Looks like James and I were composing our messages at the same time.

[Pangolin]

Hi guys,

This is a long thread with several participants. I have not read all of the feedback given by James and Adam, but both of these guys are really smart, so I expect their answers should be right on.

I'm glad that Simon is trying to understand this stuff. Too many people simply put lasers into the audience assuming it will be safe because, after all, it's only light. (Light which has caused burn marks on my light-colored wallpaper in my office, so imagine what it can do to humans watching a show under the wrong circumstances...)

In a private email to Simon, I suggested that he attend a class put on by either Greg Makhov from LSDI or by James Stewart. I'm glad that James has responded. In addition to suggesting classes, I also provided a few quick points:

1. Having done laser safety related calculations and measurements over and over during a 15-year period of time, it can be demonstrated that it is impossible to use the raw, undiverged laser beam for audience scanning unless the beam power is around 15 milliwatts or less. (This is not 15 milliwatts per square centimeter, but really 15 milliwatts. A raw laser beam with 15 milliwatts power will be over 15 milliwatts per square centimeter unless the audience is located sufficiently far away that the beam diameter is over 1 centimeter, and you certainly wouldn't want to watch such a laser show...)

2. When I see shows being done, I walk into the audience and don't look toward the projector. Instead I look at the size of the beam as it is crossing my body. If the beam is smaller than a ping-pong ball, I don't want to watch the show.

The article you refer to was originally written in 1997. It was revisited from time to time, but not much changes have been made over the years. The 1997 timeframe coupled with the many emails I must respond to each day has washed this article completely from my memory, so I am not sure if I made the assumption that the reader already understood these two points or not...

Best regards,

William Benner

[simonb]

Thanks everyone.

I am beginning to see the light!

Seriously, I can appreciate that in step 1) a static beam is used just to allow its Irradience to be measured to act as a baseline for comparison. That makes sense now!

I did initially wonder about your meaning of the ‘accessible emission’ of the laser effect.

That made me remember the parameter 'Accessible Exposure Limit' (or AEL) that used to be mentioned in the old UK PM19 Laser Safety Guidelines doc from the Health & Safety Exec.

This older document (circa 1980) introduced AEL alongside MPE. It was established from guidance docs that:-

AEL is the Measure of intensity when the eye can avert/blink reflex, such as in a concert or disco lasershow, with value 2.5mW/cm^2.

MPE is the measure of intensity when the eye is not allowed to avert/blink reflex, such as when using surveying theodolite etc. It had a value of 1mW/cm^2.

I was never happy with this, as your eye always has the ability to avert from the beam, except for maybe someone experiencing side-effects of drugs etc.

Accordingly I always took MPE as the way to go, & used the specified lower limit of 1mW/cm^2.

Recently re-reading some of the sample calculations documents surrounding PM19, I now see it has a lot of errors.

One in particular is entitled "Display Laser Information to comply with HSE Guidance Note PM19", from the UK firm 'Laser Systems' of Cwmbran. I can paste in some of the calculations if anyone is interested.

Sorry I digress. But it is strange looking at a calculation that attempts to prove that directing a 1W argon laser 20m onto a mirror ball is eye-safe for a person sat 1 metre from the ball. It shows the Irradience at said person is at 21mW/cm^2, but then goes on to say that only 8.1mW of that laser light enters the persons eye, & that it is safe because it takes only 10.4 micro-seconds to scan across the 7mm iris, against a limit of 25us gained from PM19.

How things have changed.

Thanks,

Simon.


P.s. Laser Sustems were the firm that made the Magnum & Maestro laser controllers btw. I have lots of pictures/docs from when I worked for them in 1988.

[buffo]

I can appreciate that in step 1) a static beam is used just to allow its Irradience to be measured to act as a baseline for comparison. That makes sense now!

The idea of it being a baseline measurement is correct, but there is a hidden issue here that I failed to address. Irradiance is a measure of power per unit area. But the maximum permissible exposure limit is fundamentally an ENERGY limit, not a power limit. We choose to express it in terms of irradiance to make it easier to work with, but the actual limit is based on the total energy delivered to the eye.

The reason is that the MPE calculation takes time into consideration. (Duration of exposure, remember?) Power is energy per unit time. So power multiplied by time (duration) equals energy delivered. And so it follows that irradiance multiplied by time equals energy delivered per unit area, which is MPE.

We actually work the equation backwards to obtain an irradiance level (for that given effect) because that way it's very easy to compare it to the maximum irradiance of our projector (which we already determined in step 1). As long as the projector's maximum irradiance doesn't exceed our calculated MPE, we are OK. Does that make sense?

I did initially wonder about your meaning of the ‘accessible emission’ of the laser effect.
<snip>
AEL is the Measure of intensity when the eye can avert/blink reflex, such as in a concert or disco lasershow, with value 2.5mW/cm^2.

MPE is the measure of intensity when the eye is not allowed to avert/blink reflex, such as when using surveying theodolite etc. It had a value of 1mW/cm^2.

I was never happy with this, as your eye always has the ability to avert from the beam, except for maybe someone experiencing side-effects of drugs etc.

You are misinterpreting what they mean. When looking through a surveying instrument, you need to view the laser beam more or less continuously to align the transit. If you blink, you can't see the beam. So in that case, you must keep your eye open, and if the irradiance level is too high during that time, you could deliver too much energy to your eye and cause damage. Thus, for such a long exposure, 1mw/cm^2 is the limit. (And in fact, you are eye-safe for up to 10 seconds of continuous exposure at that irradiance level.)

On the other hand, if you are not required to hold your eye open, then you can use .25 seconds for the exposure time, since most people will blink or otherwise avert their gaze when the light becomes bright enough to be uncomfortable. One quarter of a second is the assumed reaction time for an average person, though admittedly alcohol or drugs could delay this. Using the shorter time period allows you be exposed to a higher irradiance level.

So if the application includes some reason that might make you not blink your eyes (like the surveying example above), then you can't use .25 seconds as your exposure time.

Also, there is a stipulation in the rules that say if your audience might be using binoculars, telescopes, or other devices to magnify their view (such as people at a sports stadium watching a game), then you must use the much larger aperture of a pair of binoculars, rather than the 7 mm diameter of the human iris. And obviously, this will drastically lower the MPE, since a telescope or a pair of binoculars will both gather a LOT more light.

it is strange looking at a calculation that attempts to prove that directing a 1W argon laser 20m onto a mirror ball is eye-safe for a person sat 1 metre from the ball. It shows the Irradience at said person is at 21mW/cm^2, but then goes on to say that only 8.1mW of that laser light enters the persons eye, & that it is safe because it takes only 10.4 micro-seconds to scan across the 7mm iris, against a limit of 25us gained from PM19.

Hmmm... There are probably a lot more assumptions in that example that are not listed. For one, remember that the beam from the laser will have spread out quite a bit as it's traveled the 20 meters to the ball. So that reduces the irradiance by a certain amount before it even hits the ball.

Then you need to know how large the mirrors on the ball are. If they are exceptionally small (say, 3 mm square), they will only be able to reflect a very small fraction of the total beam area. This significantly reduces the irradiance of each reflected beam.

Then there's the rotation speed of the ball to consider. I must admit that I was surprised to see a duration of 10.4 microseconds though. I would have expected something at least an order of magnitude or two longer. But I suppose if the ball was large enough and spinning fast enough, it's possible.

Still, if you plug that duration into the formula, you get a single-pulse MPE limit of 31 mw/cm^2. So if their measurement of 21mw/cm^2 at a point 1 meter from the ball is correct, then yes, the effect is safe - at least for a single pulse.

Adam

[JStewart]

No problem, and glad we were of help.

Just as a quick bit of further clarification, AEL’s are used primarily to define product classifications. 1mW being the Class 2 limit for a small source visible beam, and 5mW for 3R etc, etc. They are related to MPE, but not the same. – MPE is the maximum amount of light considered not to harm to humans. ALE is the maximum amount of accessible light emitted from a product, for a particular class.

‘Accessible Emission’ in the context I used was simple that; the amount of light that is accessible by a person. i.e. the amount of light your laser projector is producing at the closest point of contact.

1mW/cm^2 for a visible beam would be good for most exposures, as it also works out to be the limit for a 10s exposure. Most laser operators however, will argue that the 10s MPE limit is too limiting artistically.

Believe me, there are still many dodgy calculations of varying vintage doing the rounds, and turning up in paperwork produced by certain individuals…

James