Filtering out laser light

[Jeffrey Warren]

Hi all, had a thought today; we've been trying to minimize the bright peak of laser light at 405 nanometers when using the Oil Testing Kit. The peaks interfere with our height and area equalization when comparing oil samples. 

Are there filters, and particularly Rosco gels (thinking of cost) that will filter out anything below, say, 410 nm? 

Jeff

[Chris Fastie]

I think the IR cut filters that we remove from cameras also block UV (because it also compromises photos). Maybe one of those would block enough of the 405nm to make a difference.

[Hank Roberts]

Ask the support people at Rosco. Unfortunately, near as I can tell, there's no easy search tool or automated way to find an answer like that.
http://www.rosco.com/us/corporate.cfm
I wish they had a page where you could adjust a hypothetical spectrum (like using sliders on audio output) to draw the spectrum you want, and then tell you what filter they have (or combination of filters, sometimes stacking them is needed)

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[Chris Fastie]

Oh wait, I have a 405 nm laser pointer and IR cut filter sitting right on my desk.

[Chris Fastie]

Yup, it seems to block almost all of the laser pointer beam.

[Jeffrey Warren]

Haha, nice, chris! But do you know what the cutoff is? Where does it begin to dim the spectrum? 

[Hank Roberts]

PS, y'all probably all know this but just in case, it bears repeating

-- 405nm may look very dim violet because it's right out at the edge of what's normally visible (the older you get, the yellower the natural lenses in your eye). But those seemingly dim $7 405nm laser pointers are _very_ bright -- highly energetic photons. What an adult may barely see as visible can still do eye damage quickly -- and instantly to a youngster.

Read: http://www.photonlexicon.com/forums/showthread.php/6166-405-nm-LASER-SAFETY

Some of us older folks have cataract replacement lenses that don't block any wavelength -- for me, with one operated eye, the difference is astonishing.
The normal lens sees a faint violet. The plastic lens passes all the photons and that eye sees a brilliant white dot surrounded by bright violet.

http://www.komar.org/faq/colorado-cataract-surgery-crystalens/ultra-violet-color-glow/

I verified this with one, instantaneous, eyeblink looking at the laser dot on a wall 40 feet away -- all it took to scare me into looking this stuff up.

Kids, don't try this at home, especially not with the remaining eye.
You won't notice the blind spot -- but your eye doctor will see it.

On Jul 23, 2015, at 10:34 AM, Chris Fastie <cfa...@gmail.com> wrote:

> Oh wait, I have a 405 nm laser pointer and IR cut filter sitting right on my desk.
>

[Jeffrey Warren]

I mean, there is already an IR cut filter in the spec 3 webcam, but we still get laser peaks. But maybe the cut filter you have is different? 

[Jeffrey Warren]

Yes - I have a pair of yellow glasses which I use. 

[Chris Fastie]

Hank, thanks for the reminder. I had the laser pointer on for just a few seconds but noticed that I was really bothered by it. I hope everybody is wearing eye protection when you play with these things. I hope eye protection is included whenever one of these 405 laser pointers is distributed.

The IR cut filter I used was removed from a PowerShot S100. It filtered out almost all of the laser. I just tried it on the filter removed from a PowerShot A2300 and another PowerShot. Neither of these filtered out quite as much of the laser light. So the cut filters are not created equal.

[Hank Roberts]

People are just astonishingly clueless about this.
The normal eye -- which varies from person to person -- barely sees 405nm, although the photons are very high-energy.

People seem to think there's a discontinuity between visible and UV -- that if they can see it it's safe -- and of course that's wrong.
That end of the human range of vision varies a bit, some see it better than others, but nobody sees that wavelength as very bright compared to the wallop it packs energetically.

Take a look at this thread, mostly people who really have no idea what they're doing:

http://laserpointerforums.com/f38/how-do-you-see-405nm-53344-3.html

That's just amazing.

The caution is -- at the edge of visible wavelengths -- you can't trust what your eyes see, with or without some kind of "filter" -- treat this like invisible dangerous radiation. Same on both ends of the visual range.

Most of the energy in those photons isn't visible.

If I didn't have the comparison right in my own head -- one aging yellowish natural lens, and one crystal clear UV-transmitting cataract replacement lens -- I'd have had no clue about this issue.

[Jeffrey Warren]

I definitely am cautious, and thanks for the additional warning. Because I knew i couldn't see the light itself, mostly, I actually shined it *through* the yellow glasses onto paper, which fluoresces. It's interesting -- you can see the fluorescence of paper when wearing the yellow glasses, but if you shine it through the yellow glasses onto paper, the paper doesn't fluoresce. Of course only did this very momentarily and didn't look directly at it or anything, but it's a nice way to see what you're talking about. 

[Hank Roberts]

I'll stop now (grin). I'm just twitchy about this because I can see the intensity with my cataract-operated eye and compare it to the faint violet glow that the aging naturally-yellowed lens transmits -- and I know that the youngsters are far more sensitive to what gets through their un-yellowed natural lenses.

And the key info -- the laser pointer forum hosts generally _really_ don't like to see this posted -- is that damage takes days, weeks, and months before the consequences of a brief exposure finish happening.

"... evidence of photochemical retinal injury may be seen as early as 3 h after exposure. The first alterations were seen in the outer segments of the photoreceptor cells, which appear swollen and tortuous. Additionally, the lamellar structure of the outer segment discs becomes disrupted. Pyknosis (condensation of chromatin in the cell nuclei) and swelling of the mitochondria then occur in the inner segments. Subsequently, there is an increase in the number of phagosomes and myeloid bodies in the RPE, the damaged photoreceptors disappear, and the RPE ends up adhering to Mueller cells. Tso et al studied photochemical retinal injuries in the rhesus monkeys. They described the histologic response to photochemical injury as occurring in three stages: the acute stage occurs within 24 h of the photic insult and is characterized by retinal oedema, RPE pigment disorganization, irregularity of the photoreceptors, and the presence of abnormal pigmentary cells in the subretinal space; the second stage, or reparative stage, occurs approximately 1 week after the initial insult and is characterized by a macrophage response; the third stage, or chronic degenerative stage, can occur weeks to months after the photic injury..."

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3144654/

Eye (Lond). 2011 Jan; 25(1): 1–14.
Published online 2010 Oct 29. doi: 10.1038/eye.2010.149
PMCID: PMC3144654
Retinal light toxicity

So -- seeing as these lasers can be bought for under $10 from China postage free, and are way, way more powerful than officially claimed -- I do try to hammer on this when I see it come up.

No, really, I'm done now (grin)
At least unless I see people playing with them and publishing spectrograms covering that range. That would be scary.

[Jeffrey Warren]

Hi, Hank - while the Oil Testing Kit's laser is enclosed, would you like to add a section on the hazards of 405 nm lasers and a caution to always use yellow glasses here: http://publiclab.org/wiki/oil-testing-kit#Hazards

It'd be really helpful. 

Best,

Jeff

[Hank Roberts]

I'd already said "enough" from me.

But more public discussion is needed -- maybe someone reading can help.

I'm _really_ not qualified to tell people what is safe use for this kind of laser. I'm pointing out concerns raised and often ignored.

That's really important. I can't give anyone assurance that "do this" is enough to avoid eye damage, especially as there's a general concern that blue light damage can be cumulative over time.

I would not trust "yellow glasses" that are not rated specifically safe for this wavelength

NOTE: All the rated protection I've found for wavelengths in this range appears to be dark green!

What we see is not a good measure of the energy being delivered to the eye because our eyes are so weak at detecting this.

The measure the protection lenses use is optical density.

If you want to collect the discussion somewhere, and include a pointer to that with a warning that there are concerns and no assurance that any exposure to this band is safe -- it might be useful to do that.

[Dave Stoft]

Yes, there are two issues here: 1) eye protection, which is a must and shipped with the kit and 2) Jeff's point about the 405 peak.

As to the 405 peak, the only means I know of to block such a sharp, energetic spike is either a) a very narrow notch filter tuned to 405 or a very sharp LowPass filter set to about 430 -- both very expensive. Anything else will likely have a significant and un-characterized impact on the spectral curves.

However, considering that it would be extremely difficult to use any spectral data very near the peak, the real consideration is whether the 405 spike's magnitude is altering the spectral data at longer wavelengths or if the 405 spike is somehow over-loading the camera pixels (flairing) and indirectly adding noise or false signals. If not, then the spectral data can simply clip off the 405 line (and some nearby data because the Spec bandwidth is typically 5+ nm so with the high energy of the laser you'd need to ignore maybe 405+/- 15nm ..... and since the data below 400 is questionable because the camera's sensitivity is falling so fast, it would be easier to just chip off 430 and higher(frequencies). Raman spectroscopy uses a notch filter because they are interested in spectral data that is super close to the carrier wavelength.

[Dave Stoft]

There are two (2) basic mechanisms which PLab should implement to avoid this hazard and the associated liability (and even bad press); 1) include a pair of UV protective glasses with instructions to use them and 2) encapsulate the laser and laser light such that under normal use virtually no laser light escapes the measurement instrument.

PLab works to engage the public in science projects so PLab should demonstrate just as much concern over safety when using PLab products. Anything else is bad PR to say the least.

[Jeffrey Warren]

Yeah, on second thought, I suppose setting a wavelength range in software works well enough for what we're doing -- agreed, Dave!

--

[Jeffrey Warren]

Of course we are concerned with peoples' safety. Let's try to keep this in a single topic thread, though, folks - Can we move over to the well-titled one here: "UV LASER danger?" https://groups.google.com/forum/#!topic/plots-spectrometry/Td3palFws_s