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

[simonb]

The William Benner doc mentioned is one of the most informative ones I have found, but a few item confuse me:-

Step 1)

It uses the method of (carefully) projecting a static beam into the venue, but the preceding paragraph says its effects that need to be evaluated. Unless steps 4), 5) & 6) measure effects which are then compared against this static beam? Or more correctly, it's Irradience?

It then mentions 'laser powers below 15mW'. I cannot easily see where this figure is extrapolated from. Unless it is somehow related to the area of a 7mm diameter pupil. Any ideas?

And that if the beam diameter is less than 1cm, it's already an unsafe exposure unless <15mW is being used. Not sure why it's 15mW, & does it mean beam area of 1 square cm rather than beam diameter? The para about a calibrated laser power meter says active area of 1cm square. But a beam of diameter 1cm has an area of pi x diameter/4? Or pi x radius squared? [incidently, Whats the Maths linking these equations? Does radius squared somehow equate to diameter/4 do to some law of indices/powers?]

Conversly, am I also correct in saying just because the beam dia/area is more than the 1cm detector area, it doesn't mean things are safe - you could be using some huge 2000mW Class 4 laser?

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?

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

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'

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? 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? Could it be Irradience at the NOHD where even a static beam is eye safe?

Using the example equations, step 1) establishes a static beam irradiance of 7.5mW/cm square, whilst step 4) states a single-pulse of 18mW/cm square.

As written it suggests to me that if the irradiance of 7.5 is greater than MPE of 18 it's unsafe - or the other way round if step 4) MPE is less than step 1) Irradience that's unsafe. Is the worked example equation in the 4th line from the bottom of step 4) correct, as it says 0.0018 not 0.018 which is 18mW - an extra zero crept in?

Similar argument exists if step 5) Multiple-pulse MPE is less than step 1) Irradience being unsafe.

Step 6) says if average power delivered by this effect is greater than Average MPE, the effect is not safe. I can see this one is correct! But is the example equation right? Similar to step 4) It uses 0.0018 for 18mW again, another extra zero?

Do you see what I mean, or have I been working on this too long?

Sorry that I could not find a better forum to ask about these points. I have emailed Pangolin & await their results.

Fundamentally, are MPEs Irradience based, I.e. if I had a line generation optic that spread a laser beam into a straight line, & that line had a power density (is that same as irradience?) of 1mW/cm squared, would that beam/line be safe if it hit you in the eye?

What happens with time? Is such a viewing safe indefinitely, or is there a duration after which it is unsafe, even just a minute pulse. Maybe this is where those equations describing pulse width come from.

I am trying to understand safety & MPE, & also the scientific and mathematical basis behind them. I have Asperger Syndrome.

I look forward to your reply.

Many thanks,

Simon B.

[JStewart]

Hello Simon,

Good questions.

I haven’t gone through Bill’s document in detail as I’m pushed for time at the moment, but I’ll see if I can help with some of the queries.

15mW - Bill is suggesting a 1cm^2 detector is to be used for measurements. So this means that any “mW” figures that are quoted, for beams that overfill the detector are actually “mW per square cm”. i.e. irradiance values. So this means that 15mW figure is “15mW per square cm (cm^2)”.

Now the irradiance limit for a 0.25s accidental exposure works out to be 2.5mW per cm^2, in the US (ANSI notation). Other parts of the world use IEC notation which is expressed as per square meter – but 2.5mW / cm^2 is the same as 25W / m^2.

Using irradiance figures that are quoted per square cm is easier to get a handle on as they seem nearer to the power contained in the beam. But if you are working in the UK, it is more usual to see watts / m^2.

MPE is based on duration, so the less time light spends in the eye, the higher level it can safely accommodate. Therefore shorter durations lead to an increase in permissible MPE. But for lightshow use, this changes normally only by a factor of 5-10 times. So taking a figure of 6 x 2.5mW, gives you a figure of 15mW. Which is probably not a bad estimation for a permissible irradiance of a fast scanning effect.

In terms of the different MPEs that must be assessed, it is right to say that if the exposure exceeds the MPE for a single pulse exposure, then don’t go any further.

In the current standard, the most restrictive of the three different types of MPE has to be the limiting case. Single Pulse MPEs can accommodate higher amounts of permissible exposure. The multi-pulse MPE, will be more restrictive. Therefore, if there is a failure on the single pulse MPE, there’s little point in proceeding further.

The extra zero you are seeing in the calculation is the probably the reference to the MPE formula, which for the timebases for scanning lightshow beams will be 1.8*t^0.75 *10^-3 J per cm^2. This is not referring to 18mW.

Where did the J per cm^2 come from? Well really the MPE for pulses should be expressed as “Radiant Exposure”, however, it is useful to convert Radiant Exposure to Irradiance, as a normal power meter can be used to evaluate. Irradiance is power density, so think of Radiant Exposure as being energy density.

As for long term exposure to laser light? It starts to depend on the wavelength (e.g. blue vs red), but for red after 10s, the MPE levels out at 1mW / cm^2, (which this side of the pond is 10W / m^2).

Hope this helps. (and if you have an iPhone, there is little app that can sort many of these issues )

James

[simonb]

Thanks for all the replies. I look fwd to reading them, once the football is over!

I have emailed the author too. I just cannot see the reasoning that if the measured MPE is LESS than a static beam, it is unsafe.

I used to work for a professional UK laser display company back in the 80s, using 4 to 20W argon lasers. Back then the safety document was PM19. The company had to provide Calcs to show the laser Accessible Emission Limit (AEL) was 2.5mW/cm^2 on less if the audience was exposed.

England have just taken the lead against Sweden in the Euros. I don't normally watch football, but this is quite good!

Thanks,

Si.

[simonb]

Thanks James,

You mentioned an iPhone app. Please can you tell me what it is?

Thanks again,

Simon

[taggalucci]

It's called "Laser Show Safety" and it's easily found in the App Store.

[JStewart]

Simon, not sure what you mean by "the measured MPE is LESS than a static beam, it is unsafe.", and I don't have time to trawl through that doc at the moment. But as a general rule -> the less time you spend exposed to the light, the more light you can view without exceeding the limits. And the limits change (i.e. allow more light) the shorter the exposure duration gets. Hope that helps.

James

[White-Light]

Doc I linked Simon to is here:

http://www.laserist.org/files/Making...0enjoyable.pdf

The quote is:

If the irradiance measured in Step 1 is greater than the multiple-pulse MPE, stop right there -- the effect is not safe for that exposure time and would have to be reduced and re-measured before performing before an audience.

Step 1. Measure the laser beam irradiance at the closest point of audience access. To do this, project a
non-moving beam into the venue.

Sent from my laptop.

[simonb]

Dear James,

Thanks White-Light. I have pasted in step 4 in its entirety below.

In essence its says if the static beam Irradience is GREATER than the single pulse MPE, it's NOT safe.

Logically translates to 'if the single pulse MPE is LESS than the static beam Irradience it is unsafe'. Same thing for the multiple pulse MPE also in step 5.

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.

Am I missing something here, and have I got the meaning backwards. I am trying to find out how to evaluate the safety of audience scanning a blue 445nm 1W laser module.

"Step 4. Compute the single-pulse Maximum Permissible Exposure [MPE] for this effect. This is the safety guideline or government regulation prescribing the maximum amount of irradiance (laser power density) that is considered safe for a given pulse-width. To compute the single-pulse MPE [see note 1] (in Watts per square centimeter), raise the pulse-width (in seconds) to the 3/4 power, multiply the result by 0.0018 and divide the entire result by the pulsewidth (in seconds). For example, if the pulse-width is 100 microseconds (0.000100 seconds) the calculation would be (0.000100) 3⁄4 X .0018 / 0.000100 = 0.018 W/cm2 or 18 milliwatts per square centimeter. (To do this with the scientific calculator provided with Microsoft Windows, enter 0.0001, press the X^Y key, enter 0.75 (equivalent of 3/4), press *, enter 0.0018, press /, enter 0.0001, and press =.) If the irradiance measured in Step 1 is greater than the single-pulse MPE, stop right there -- the effect is not even safe for one pulse of laser light (one scan across your eye) and must not be performed before an audience".

Maybe step 1 establishes a static beam & measures it at ts NOHD, against which the various MPEs are compared?

sorry to be a burden, but I cannot precede in establishing & understanding safety with things like this not straight in my mind!

Thanks again for all the help.

Si.

Doc I linked Simon to is here:

http://www.laserist.org/files/Making...0enjoyable.pdf

The quote is:





Sent from my laptop.

[simonb]

Dear James,

Thanks White-Light. I have pasted in step 4 in its entirety below.

In essence its says if the static beam Irradience is GREATER than the single pulse MPE, it's NOT safe.

Logically translates to 'if the single pulse MPE is LESS than the static beam Irradience it is unsafe'. Same thing for the multiple pulse MPE also in step 5.

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.

Am I missing something here, and have I got the meaning backwards. I am trying to find out how to evaluate the safety of audience scanning a blue 445nm 1W laser module.

"Step 4. Compute the single-pulse Maximum Permissible Exposure [MPE] for this effect. This is the safety guideline or government regulation prescribing the maximum amount of irradiance (laser power density) that is considered safe for a given pulse-width. To compute the single-pulse MPE [see note 1] (in Watts per square centimeter), raise the pulse-width (in seconds) to the 3/4 power, multiply the result by 0.0018 and divide the entire result by the pulsewidth (in seconds). For example, if the pulse-width is 100 microseconds (0.000100 seconds) the calculation would be (0.000100) 3⁄4 X .0018 / 0.000100 = 0.018 W/cm2 or 18 milliwatts per square centimeter. (To do this with the scientific calculator provided with Microsoft Windows, enter 0.0001, press the X^Y key, enter 0.75 (equivalent of 3/4), press *, enter 0.0018, press /, enter 0.0001, and press =.) If the irradiance measured in Step 1 is greater than the single-pulse MPE, stop right there -- the effect is not even safe for one pulse of laser light (one scan across your eye) and must not be performed before an audience".

Maybe step 1 establishes a static beam & measures it at ts NOHD, against which the various MPEs are compared?

sorry to be a burden, but I cannot precede in establishing & understanding safety with things like this not straight in my mind!

Thanks again for all the help.

Si.

Doc I linked Simon to is here:

http://www.laserist.org/files/Making...0enjoyable.pdf

The quote is:





Sent from my laptop.

[JStewart]

I think I can see where the confusion is creeping in…

Steps 1 to 3 are used to determine the ‘accessible emission’ of the laser effect. Now a standard laser power meter can’t determine this on the fly so there are a number of steps necessary to do so. And it is why we have to think of the emissions in terms of irradiance, rather than the radiant exposure, which the MPEs are actually written in for the time domains a typical laser effect would create.

First you must determine the irradiance of a static beam. – NOT to learn the MPE of that static beam, but instead, so that you can apply the irradiance to the pulse duration in step 2, to figure out the accessible emission.

The only reason the beam has to be static, is so that you can measure it with a standard power meter.

When you get to step 4, if the irradiance that you measured in step 1 is greater than irradiance calculated for the single pulse MPE, which is determined in step 4 itself, then it makes complete sense that the effect will be unsafe. i.e. it will be above the applicable MPE. …Remember, you are not comparing against a static beam itself, you are instead comparing against an irradiance, that because of instrument limitations, had to be taken with a static beam. And the whole reason you are having to convert from J.m^2 to W.m^2 (i.e radiant exposure to irradiance), is so that you can check the accessible emission with a standard laser power meter.

The multi pulse MPE becomes more restrictive still, so there is little point in proceeding any further, if the MPE for a single pulse has already been exceeded.

Hope that helps.

Best regards,

James