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MPE

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Toshimichi Otsubo:

Johann and Jens

Thank you very much.  Amazed to hear that you could track Etalon solidly with such a weak-energy laser.

I am still learning and a bit confused.

I applied the same procedure as Herstmonceux and Stuttgart to the WLRS-IR based on what you wrote.  It does not seem eye-safe in my quick computation.

(W) WLRS:  1064 nm, 0.4 mJ, 400 Hz, 10 ps FWHM, beam diameter 75 cm

The MPE per pulse in the first wikipedia graph is:
  MPE for (W) = 3e-8 J/cm^2 (where real (W) = 9e-8 J/cm^2)
which suggests WLRS-IR is NOT EYE-SAFE.  Am I right?

The MPE per 0.1 s and per 1.0 s in the second wikipedia graph is:
  MPE for (W) = 2e-3 J/cm^2 per 0.1 sec (where real (W) = 4e-6 J/cm^2)
  MPE for (W) = 1e-2 J/cm^2 per 1.0 sec (where real (W) = 4e-5 J/cm^2)
which looks ok.

The N^(-0.25) rule ("Rule 3" in the document from Jens message) will make the MPE simply lower, if I understand correctly.

The PDF document Jens suggested looks useful for visible (400-700 nm) wavelengths - thanks.  I would be glad to see the same thing for 1064 nm.

Toshi

Johann:
Toshi

here are the numbers for the MPE (or class 1M) that i found for 1064 nm:

ANSI Z136.1-2007:
27*t^0.75 J/cm2 ~ 1.5e-7 J/cm2
and N^-0.25

ANSI Z136.1-2014:
2e-6 J/cm2
no reduction from repetition rate

IEC 60825-1:2014:
2e-2 J/m2
N^-0.25 for N>600 Hz

for WLRS this leads to:

2007:
~0,6 mJ

2014 (ANSI & 60825):
~8 mJ !!!

it seems that the situation for 1064 nm has relaxed in the latest documents. For all of this numbers the WLRS should be "eyesafe" in the mentioned configuration.

In Wikipedia i found from figure 1: 2e-7 J/cm2
and in figure 3: ~9e-8 J/cm2


However, there is another statement that i found in IEC 60825-1:2014, concerning a Protective Housing:
"Each laser product shall have a protective housing which, when in place, prevents human
access to laser radiation (including errant laser radiation) in excess of the AEL for Class 1,
except when human access is necessary for the performance of the function(s) of the product."

So in principle MPE (class 1M) is not eyesafe. The AEL for class 1 from IEC 60825-1:2014 is:

7.7e-7 J in a 50 mm aperture => ~ 1.2e-7 J/cm2
or ~0.17 mJ for WLRS

There is no clear definition of what needs to be done, when going with lasers through the atmosphere. So it is depending on the corresponding authority, if class 1M is sufficient...

Best regards,
Johann

Toshimichi Otsubo:
Thanks Johann.  Toshi

jsteinborn:
It looks like we need some kind of math support for formulas :-)

Matt?

danielhampf:
Hello everyone,
since our system is mentioned here I thought I should comment. I have just spoken with the laser safety expert in our institute about this. Since I personally know very little about this topic, I will only summarise his comments.
He has checked the above posts and finds many statements which he thinks might be wrong or at least misleading, and therefore dangerous. His main point was: Calculating MPE is a quite difficult business, and almost impossible to do for a general case. Pulse duration, pulse repetition rate, wavelength, beam diameter and divergence are only some of the factors to take into consideration.
We have a detailed analysis of our current system, that defines what precautions are necessary at what point. Indeed, we are currently eye-safe at a distance of a few hundred meters from the transmitter. If we decrease our divergence in the future, this might change. Our procedures will always reflect the specific analysis.
Toshi, since you helped us so much with the data analysis, I suppose we could try to help you with a safety analysis of a proposed system. However, our laser safety officer is reluctant to do public statements in this forum which people might misunderstand, but rely on and possibly do dangerous things with. But if you could send me your specs via PM, I will try to get an analysis from him.
Best regards,
Daniel

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