Would it be safe to look at 1.6W dot?

[Pi R Squared]

Grass will be the most reflective for green, but blue isn't far from green so it is probably very reflective for blue, of course if the grass is wet then it will also be more reflective.

Alan

 

[paul1598419]

Actually, if you take green as being 532nm then the difference between 635nm red and 445nm blue is 16nm closer to the blue side than red. I think they are close enough that the difference would be negligible. I still think white would be more reflective and since it isn't polished, the light reflected is not coherent. I don't know why the grass would seem more of a problem except for the darkness.

 

[leukoplast]

I've said it before and I'll say it again, I look at the dot from my 445nm 3 watt laser at a distance of 4 feet at an off white wall and it causes me no harm at all. I just finished a build for vortish that is a 445nm at 1500mW and I have been looking at it from about 4 feet and no problem at all. The dot is reflected off a non-polished surface and though the higher powers may get you to see the remnants of the light from the dot, it is by no means detrimental to your vision. No more than the flash from getting yourself photographed with the flash on. It just isn't focused any longer and therefore is perfectly okay to view.

That is certainly interesting, and you have had your eyes checked and confirmed there was no damage done?

The thing that gets me is what the class 4/IV rating indicates:

Class 4

LASER RADIATION
AVOID EYE OR SKIN EXPOSURE TO
DIRECT OR SCATTERED RADIATION
CLASS 4 LASER PRODUCT

Class 4 is the highest and most dangerous class of laser, including all lasers that exceed the Class 3B AEL. By definition, a class 4 laser can burn the skin, or cause devastating and permanent eye damage as a result of direct, diffuse or indirect beam viewing. These lasers may ignite combustible materials, and thus may represent a fire risk. These hazards may also apply to indirect or non-specular reflections of the beam, even from apparently matte surfaces—meaning that great care must be taken to control the beam path. In most U.S states it is illegal to sell preassembled class 4 lasers, however a citizen can construct a class 4 laser for personal use. Class 4 lasers must be equipped with a key switch and a safety interlock. Most industrial, scientific, military, and medical lasers are in this category.

And the older system:

Class IV

Lasers in this class have output powers of more than 500 mW in the beam and may cause severe, permanent damage to eye or skin without being magnified by optics of eye or instrumentation. Diffuse reflections of the laser beam can be hazardous to skin or eye within the Nominal Hazard Zone. Many industrial, scientific, military and medical lasers are in this category. Many handheld lasers ("laser pointers") at this output level are now available in this category.

Both make it clear that even diffuse reflections can cause eye damage. The thing is, class 4 could mean 500mW, or 500,000 Watts. And from what I understand, the NOHD is only for direct viewing of the beam from the laser, not from diffuse reflections.

With 445nm you also have the blue-light hazard which from what I read isn't immediately obvious but is something that builds over time.

 

[paul1598419]

My eyes are checked yearly by an ophthalmologist, and it shows no damage at all. Class IV lasers are any laser with a power rating above 500mW. There are many powers that are much higher than 3 watts that could be possibly damaging to the eye if reflected from a matte surface. We don't normally have access to these lasers. If I owned a 10 to 50 watt laser it would be a different story entirely.

 

[leukoplast]

Actually, if you take green as being 532nm then the difference between 635nm red and 445nm blue is 16nm closer to the blue side than red. I think they are close enough that the difference would be negligible. I still think white would be more reflective and since it isn't polished, the light reflected is not coherent. I don't know why the grass would seem more of a problem except for the darkness.

I'm pretty sure laser light is always coherent, regardless if it is bouncing off something or not. Like when you shine a laser in a room and the reflected light on the walls looks grainy, that is the effect of coherent light. Which is why reflections are so bad. Think about it, say you shine a laser through a mirror, will the reflected light no longer be coherent?

From what I understand, the light waves are still parallel to each other, but there will be more or less of them depending on what they get reflected off of.

So if he is pointing the laser at wet grass, which is reflective and glossy, then that is why it looks really bright because the concentration of the light being reflected back is higher than if viewing on a matte surface that reflects the light evenly over a larger area.

 

[paul1598419]

I assure you that the reflected light from an unpolished and grainy surface loses it's coherence in the respect that it is all coming at different angles and different phases. Coherent light is not in itself dangerous to look at. For instance, the light from a beam that is spread out by a biconcave lens is perfectly okay to view dead on if the beam is spread out over a wide enough area. The light is coherent, but the power is lower by the inverse square of the distance from the lens. Also, you are assuming the grass is wet. It was stated that the grass was dark and nothing about dew on the grass was mentioned.

 

[leukoplast]

Well I certainly am no expert on coherence, so I cannot say with absolute certainty that diffuse laser light is coherent or not. I personally believe it is, because at what point do you draw the line of grainy or matte? Does the light all of a sudden become coherent again once it has just the right amount of reflection off a particular surface?

Let me ask you this, if the diffuse light is not coherent then how am I able to re-focus diffuse laser light scattered off a matte piece of white paper back into a focused dot using a magnifying glass? I just tried this a moment ago, and it worked like a charm. Shined laser at white paper, got magnifying glass and moved it and my hand (away from the paper) to the right spot and wholah, focused green dot on my hand from the diffuse reflections alone (I did this with a 532nm 3mW greenie).

If the light was incoherent, I don't think this would be possible as the waves should be completely randomized and all over the place, impossible to refocus again back into a neat round dot. However the magnifying glass was able to re-align all the waves (just as our eyes do) and created a focused green dot, just as if it was coming out of the laser (but less bright of course). With the right optics and setup, I bet somebody could make a laser beam of collimated light just from diffuse reflections off a matte surface.

----

As for the grass, most grass is somewhat glossy and is pointed in every which direction, so him shining the laser into it yields tiny reflections that appear bright because the laser is essentially reflecting off a thousands of glossy surfaces and reflecting back to his eyes with more intensity than if shining on a plain matte surface. Wet or not it seems a simple explanation to me.


EDIT: Just for fun I decided to measure the power output of the dot from my magnifying glass. Lasercheck was about 3 ft from the paper, and the magnifying glass is some cheap acrylic 3.25" one. I measured nearly 14 microwatts, lol. So definitely not much from a 3.20mW green pointer but still pretty neat.

 

[paul1598419]

You're right. You aren't an expert in coherent light. You can take the light from any source and with a positive focal length lens focus it into a dot. Try it with sunlight sometime. It is how we used to burn ants when I was 7 years old. Just because you can get a dot at a certain distance away from reflected light does not mean it is coherent. Now if you can take a lens and recreate a beam, then you have produced coherent light.

 

[leukoplast]

You're right. You aren't an expert in coherent light. You can take the light from any source and with a positive focal length lens focus it into a dot. Try it with sunlight sometime. It is how we used to burn ants when I was 7 years old. Just because you can get a dot at a certain distance away from reflected light does not mean it is coherent. Now if you can take a lens and recreate a beam, then you have produced coherent light.

Sorry to break this to you, but the suns rays are completely parallel once they reach earth. Same goes for star light (due to how far away they are).

And no, you cannot focus any light source to a neat dot with a magnifying glass. Go ahead, try it with a light bulb or LED flashlight, all you will get is a projector image of the LED surface/reflector or the bulb.

Just tried this with my fluorescent desk lamp for kicks, focused it as far down as I possibly could... and guess what? It was the shape of the fluorescent bulb!

Kinda neat looking actually, I could even read the warning labels inside the lamp shade thing that sits next to the bulb.

 

[Atomicrox]

For crying out loud. Every time a thread like this pops up there's just so many people spreading misinformation (on both sides - excess of caution and lack thereof) it's not even funny.

Won't bother replying to any particular comments, but if you use the search you'll find a post from one or two years ago where I and some others did the math with the formulas/MPE from a proper laser safety paper and came to the conclusion that no high power laser pointer available at the time (up to 3W if I remember) could do damage if you looked at the dot on a perfectly white matte surface from 1m away.

Of course real life surfaces aren't perfectly matte, so you should add some extra margin and point it at an angle that won't reflect back on you. 3m is what I usually consider safe for a white wall, but even then I don't "stare" at the dot because it bothers my eyes.

Note that this does NOT apply to burning - melting surfaces get highly reflective and could easily cause an accident. Also don't go pointing around without looking at what you're hitting - there might be a nail, a water droplet or even some glass on the way.


Actually, I'll bother to comment about one specific post - I have tried that experiment with LPMing the reflected light. I find it hard to perform properly, at least with a thermopile sensor - even the radiated heat from your body affects the readings by a few mW and the laser light being absorbed on the non-sensor surface probably affects the reading as well. That said the most I ever got from a reflection was about 10mW, and that was from a 3W laser being pointed at a piece of white paper a few cm away.

 

[paul1598419]

Yes, I did leave that part out. If you shine your light source at a pin hole so that the "waves" coming from your source are parallel, you can focus any source of light down to a point. They still aren't coherent. That was the whole point of this exercise. You stated that the light from a matte surface was collimated and the sun proves that it isn't. Just because you can use a lens to make a source of light focus down to a point does not mean that the light is laser coherent light. That was my one and only point. That and the fact that the beam from a matte surface is not damaging to your eyes.

 

[leukoplast]

Yes, I did leave that part out. If you shine your light source at a pin hole so that the "waves" coming from your source are parallel, you can focus any source of light down to a point. They still aren't coherent. That was the whole point of this exercise. You stated that the light from a matte surface was collimated and the sun proves that it isn't. Just because you can use a lens to make a source of light focus down to a point does not mean that the light is laser coherent light. That was my one and only point. That and the fact that the beam from a matte surface is not damaging to your eyes.

You are missing the point here, sure there may be some way to make a nice "dot" from an incoherent light source by using different methods and experiments. However with MY experiment and the same piece of equipment, a magnifying glass, an incoherent light source doesn't focus down to a dot, while the diffuse reflections from the coherent light source did. If the diffuse light was indeed incoherent, I shouldn't have been able to focus it back down to a nice round dot.

The light bouncing off the paper may no longer be in parallel with the laser beam, but it is still coherent in nature, just that there is smaller clusters of coherent waves traveling at different angles that grow wider with distance (which when using a magnifying glass, can re-align all those waves back down to a point).

Here is a crude drawing of this.

Obviously realistically there would be a million more lines covering a full 180 degrees from the paper, however for this example just imagine those lines are small pockets of coherent light waves traveling outward from the paper.

Laser light doesn't magically become incoherent just because it is diffuse, it just means the concentration of individual coherent light waves are more numerous and spread out. Kind of like when you de-focus your laser and get a big dot on the wall, the light is still coherent.


Also you really should be more careful when saying things like "That and the fact that the beam from a matte surface is not damaging to your eyes." If it's a fact then prove it, otherwise you shouldn't be making such unproven bold claims, as people not educated on lasers may believe that and could possibly get themselves hurt. These multiwatt lasers are nothing to screw around with, and people should certainly not think that looking at the 3 watt laser dot on a wall from a couple of feet away is "perfectly safe". And it's even worse since you are incredibly unspecific (what constitutes for a matte surface?). Most walls in homes are not completely matte, but kind of glossy. But many people would think "Yeah, that is a matte surface therefore according to paul1598419 it is ok for me to stare at the dot".

 

[paul1598419]

It's not worth trying to get you to understand lenses and light. I give up. If you want you can google lenses and light waves and maybe you'll get it.

 

[The Lightning Stalker]

Yes, I did leave that part out. If you shine your light source at a pin hole so that the "waves" coming from your source are parallel, you can focus any source of light down to a point.

No, you can't. A pinhole acts as a lens and will
only make an image of the light source. That is all
any lens does. The dot from a laser is an image of
the area where the light is coming from.

 

[paul1598419]

Lighting Stalker, agreed. I was thinking of a slit arrangement for using a prism spectroscope where a light source is shined at a slit and allowed to be deflected by a prism and viewed though another slit and then a series of lenses to see the spectrum. It has nothing to do with this. But, surely you don't believe that a laser dot reflected from a diffused surface can be collimated again into to a beam of collimated light? And do you believe that the light from a matte surface is a danger to the eyes at a distance of a couple of feet? I am talking about a 3 watt 445nm laser. My point was that the nearly parallel light waves from the sun which is not collimated or of a single frequency and can be focused to a dot is proof that the dot is meaningless when it comes to collimated light.

 

[leukoplast]

It's not worth trying to get you to understand lenses and light. I give up. If you want you can google lenses and light waves and maybe you'll get it.

Since I started this conversation with you, I have done tons of searching and research on the subject to try to make my information as accurate as possible to prove, or at least reinforce, my case.

However once you posted about how you can use the suns light to create a focused dot as your defense (you not realizing that the suns light is parallel and is why you can focus that dot), I knew it was a lost effort.

I ask once again, please prove to me that looking at the 3W+ laser dot on a matte surface a few feet away absolutely doesn't hurt a persons eyes. Give me, and everybody else, FACTUAL PROOF. Otherwise, you should keep your opinions to yourself about how looking at the dot of a 3 WATT laser within a few feet isn't harmful.