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Eye safety when using a green laser pointer for optics inspection?

eyepieces optics
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#1 grahamtv

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Posted 29 June 2020 - 09:19 PM

What eye safety do you practice when using a green laser pointer to inspect the lens sets of your eyepieces and other optics?
 

Recently, CN member, “j.gardavsky” gave me this tip: “Scrutinizing the eyepieces: When I get a new-to-me eyepiece, I try with simple means to "look inside". The first thing is to pass a green laser beam,...and then I am looking how the light is scattered on the micro inclusions in glass (the brightened laser beam trace), the internal reflections, diffusion spots on the glass interfaces which indicate a low polish quality, ...With the pinhole imagery, the smallest bubbles in glass and the dust particles become visible in and on the outer lenses.”

 

It does seem to be a very effective practice, but the reflected intensity from even dust particles is so great, that it gave me pause to question the safety of the practice.
 

I’ve not been able to find many other references here on CN, about the practice of using a GLP for this purpose. I thought surely the practice was more common, but even a Google search makes it appear that this may be a fairly recent idea. [tg2 -edit to remove linked reference to Cambridge article.]

 


Edited by grahamtv, 30 June 2020 - 10:05 AM.


#2 BGRE

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Posted 29 June 2020 - 10:07 PM

The very strong IR pump beam component emitted by some green 532nm DPSS laser pointers is a safety issue.
Direct green lasers don't have this issue.

#3 ButterFly

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Posted 29 June 2020 - 11:17 PM

The only safe way to use a gun as a can opener is to not do so.

 

I don't know what one would do armed with the knowledge that there are air bubbles in the glass.  Surely you would stick the eyepiece in a scope before you throw it away.  Ignorance is bliss sometimes.

 

If you insist on using a laser around reflective surfaces, wear safety glasses appropriate for the wavelength being used and utilize traps to avoid blinding others.  Watch Styropyro on youtube to see ways that mad scientists handle ultra hazardous hand held laser death rays safely - for them anyway.


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#4 MKV

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Posted 30 June 2020 - 06:09 AM

The only safe way to use a gun as a can opener is to not do so.

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#5 grahamtv

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Posted 30 June 2020 - 06:35 AM

Please note that my question is NOT intended to be provocative but serious. 

I did realize that any question concerning lasers MIGHT be contentious, and I do well understand and appreciate the point being made about using tools only for their designed and intended purposes, however it’s not yet clear to me that this cannot be a safe and effective practice. 
 

(Note that I referenced an article from Cambridge and not a teenager on YouTube. And I’ve intentionally posted this question in the Optics forum here on CN.)


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#6 Jon Isaacs

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Posted 30 June 2020 - 07:08 AM

The basic idea with laser pointers is that the eye reflex is fast enough to close before damage is done.  The 5mw limit on laser pointers is based on this.

 

I worked in a research laboratory at a major university for 30 years. To use a laser of any sort in the laboratory, one (including me) had to be trained and received a yearly refresher course.  Before an experiment could be approved, a laser safety officer from the university would have to inspect the experimental setup, all safety requirements had to be in place.  The officer would then present the findings to a laser safety committee for approval.  Only then could the experiment be performed. 

 

From what I saw looking through that Cambridge University link, that would never have been approved.  The device used would not qualify as a laser pointer in the United States nor in most of the world.  

 

Jon


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#7 BGRE

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Posted 30 June 2020 - 07:14 AM

keep the laser power at a safe level and use a camera as the detector. Using appropriate laser googles that block the laser wavelength adequately will also be prudent especially if the laser used has high power.

532nm DPSS lasers are somewhat problematic as the leakage of the ~ 808nm pump beam as well as the 1064nm output from the crystal to be considered. Both of these are invisible and potentially are both far more powerful than the desired green output.
A ~ 520nm direct green laser is far safer in that it only emits a visible beam.
Suitable direct green laser diodes and modules are readily available.

Since nonlinear processes aren't involved in this test a longer camera exposure can easily be used to compensate for low laser power.

The Cambridge article indicates that a filter with a neutral density of 4 was used with a camera.
This indicates that a 2 microwatt laser is more than adequate or say a 1mW laser with a lower attenuation ND filter (ND = 3?)).

N.B. reflective ND filters can produce hazardous reflections with a high power laser.

Edited by BGRE, 30 June 2020 - 07:24 AM.

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#8 sixela

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Posted 30 June 2020 - 07:22 AM

Guys, we're talking about using a sub-millimeter round pinhole that lets through only a fraction of the beam of a 1mW direct diode green laser pointer, not something to vaporise satellites in orbit. Even if it were a DPSS with really bad IR filtering (note: I do not condone such devices, but they do exist), any IR leaking is usually uncollimated and most of the energy won't make it through the pinhole...

 

The question remains: even though these devices are of class 2 (which means that we CAN actually talk about them here), is there a safety risk because you have the potential for optics to focus the beam energy?

 

My gut feeling is "no", since you're looking at what doesn't make it through the eyepiece, i.e. usually scattering in the micro-inclusions in the glass (and if you're unlucky, a dust particle), or the reflections on multicoated surfaces designed to suppress these.

 

But it would be good to have actual information that validates or invalidates it, or measures to reduce even a tiny risk, rather than general handwaving about class 3 lasers and their dangers.


Edited by sixela, 30 June 2020 - 07:31 AM.

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#9 BGRE

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Posted 30 June 2020 - 07:29 AM

There's no pinhole mentioned in the Cambridge university article.

#10 sixela

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Posted 30 June 2020 - 07:33 AM

Follow the first link in the original post to see what we're talking about. We're not trying to duplicate the Cambridge setup.

 

This is what the original poster is talking about:

 

[Note this is only the upper portion of a Delos]

gallery_316833_11917_21065.jpg

 

And yes, a class 2 laser will do fine for this. And you don't want to use all the energy from that laser, but the beam from a small pinhole (which has high divergence, but you don't care since you're not going to be meters away from the eyepiece).


Edited by sixela, 30 June 2020 - 08:02 AM.

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#11 sixela

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Posted 30 June 2020 - 07:54 AM

My personal method: get your eye enough off-axis that even reflections on shiny parts of the eyepiece wil never make it to your eye, use a phone on a stand to take a picture.



#12 Jon Isaacs

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Posted 30 June 2020 - 08:05 AM

Guys, we're talking about using a sub-millimeter round pinhole that lets through only a fraction of the beam of a 1mW direct diode green laser pointer, not something to vaporise satellites in orbit.

 

The Cambridge paper did mention a laser.  The TOS prevents me from mentioning the output.  

 

If one is using a 1mw laser, it's unlikely there would be a problem.  There have been cases where people were flashed by what was supposed to be a 5mw green laser and suffered long terms after effects.

 

Jon


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#13 grahamtv

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Posted 30 June 2020 - 10:22 AM

I regret that I overlooked the specific mW reference in the Cambridge article. I had attempted to be quite cautious about the TOS. Thank you for pointing that out, Jon. I've removed the link from the original post. 

 

Let's all assume that we 're only talking about quality green lasers that are truly < 5mW.  I believe that we should be able to speak openly about quantifiable risks and how to effectively manage them.

 

I am simply asking whether there are appropriate (and if so, what) safety precautions that a non-professional can take and and safely and effectively use a GLP for optics inspection. If the best answer is "leave this to the professionals," so be it, but please provide a little more substance for that answer.

 

p.s., I am only 6 months into the hobby with no background in optics or lasers (nor much "hard" science of any sort for that matter), but naturally curious and believe strongly in understanding and managing risks.



#14 ButterFly

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Posted 30 June 2020 - 12:33 PM


I am simply asking whether there are appropriate (and if so, what) safety precautions that a non-professional can take and and safely and effectively use a GLP for optics inspection.

There is no real gain to a non-professional doing this.  Everything one could fix can be determined by others, more safe, means.  But sure, one can be curious and I do encourage that if and only if one can do it safely.  That "teenager" on youtube is rather apt and curious, but proceeds safely.
 

 

If the best answer is "leave this to the professionals," so be it, but please provide a little more substance for that answer.

 

 As I pointed out:

 

 

If you insist on using a laser around reflective surfaces, wear safety glasses appropriate for the wavelength being used and utilize traps to avoid blinding others. 

It would be nice if we could rely on manufacturer specs to say that their output at focus is what they claim.  Unfortunately, many commercially available options can be far off spec.
 

 



#15 Oregon-raybender

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Posted 30 June 2020 - 03:28 PM

OK, I really want to pour water on this. Please, please do not use any laser for inspecting optics. There is not one professional optical company I know of who uses a laser for inspecting glass, coatings performance, etc.. (on less it is a requirement by the customer who needs it)

 

I have been a laser safety officer, there is zero chance this method would be used for any inspection. Yes, some times a bright light is used, a 40 watt or 12 watt florescent. If you want to go down this path, fine but use a bright LED, (not the sun) What are you looking for? There is no Mil- spec, ISO or any standard that is used by optical manufacturers that calls out this requirement. So please stop doing this. I think it is wrong to pass this type of nonsense to people who have little or no knowledge of optical specs. A green laser is the worse type to use due to the high risks of eye damage, the most used in a professional instrument is 1 mw, in the red. Even then

one never looks into or at the beam. So there is no "non-professional" way to use any laser for this. Sorry, but as far as I am concern, this and the reference in the #1 should be removed for safety reasons. IMHPO

 

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#16 Steve Dodds

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Posted 30 June 2020 - 04:16 PM

For inspecting for polish and scratches I use a 4000 lumen LED flashlight with a focusable beam, that focuses it down to .5: It's brighter than a laser and covers a larger area.


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#17 Oregon-raybender

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Posted 30 June 2020 - 05:26 PM

LED is fine. However there is no industrial standard used by optical manufacturers for LED. I seen only telescope makers using such extreme lighting for inspection. I am very aware of marketing issue verses the performance issue ( a slight scratch or a few pits, not visible under polish) Unless it is required by the customer, the price of such of perfect optic, is well astronomical. That is why most

manufacturers don't even quote mirrors for ATM market. Which is great, because it does open the door for others, but one must deal with the expected perfection in polish, coating, PV, RMS, CC, 99% Strehl, etc... I just imagine in the coming years that white interferometers for 1A surface roughness and 100X microscopes will be needed for inspection of mirrors for the ATM market. Just like performance test using interferometers and fringe software or KE or Bath is called for now, but the weakness is seeing and poor eye sight, like having 200 mph car but can only go 50. If wonder you why Nikon or Cannon lenses are so pricy, it's because the only spec, is perfect. They regrind and re-polish as needed. That is why the glass center thickness starts at the high end, just in case it has to be re-processed.

 

The wide beam is a better choice since you can seen the pits better if under polished and check for scratches.

 

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#18 Mark Harry

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Posted 05 July 2020 - 06:28 AM

99+% of the time, I use red. On very unusual, long radius curves on mirror blanks of smallish diameter, I have to resort to a green to locate the return beam when the tester is half way across the cellar, or more. That's the only time I have used green, and that's maybe been 2-3 times over the last 2 decades or so, and for very brief instances just enough to get the rough idea where the return beam is. This is mostly common sense. Stand well to the side, and never attempt to get in line with the beam. Red should be sufficient to inspect microfinish on most optical surfaces.
On mirror blanks, just getting the thing clean enough to get rid of all compound residue is the main thing so as to see a truely 'black' optical polish.


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