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5mW limit too conservative?

EpicHam

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A meter away it drops by a million well that's good to know
This is NOT true!
https://docs.google.com/viewer?url=...ac.uk/aboutus/healthandsafety/LaserSafety.pdf

Even at 5mW , if you don't blink and look away within a few seconds of being hit by the laser , you WILL be causing permanent damage

If you point it at a wall , there are less documentation regarding the reflected diffuse radiation .

The main reason for this is that the wall spreads out the light significantly. If the laser beam were to bounce off of a mirror, it would remain a narrow beam, and could still potentially harm your eye if you stared into it and managed to suppress your blink reflex. However, when the beam bounces off a rough surface like a wall, the reflected light is no longer a beam. Instead, it begins to spread out in the shape of a hemisphere as it propagates away from the wall.

You can tell that the reflected light is spreading out in this way because the spot looks the same on the wall no matter where you are in the room. That means it is reflecting equal amounts of light in all directions. By contrast, if you look directly at the source of the laser (which I do not recommend) you will find it is significantly brighter the closer you get to looking straight into the beam.

We can estimate how much less light hits your eye when looking at the laser spot on a wall (versus looking into the laser itself) by comparing the area over which the light is spread out. When the light first hits the wall, it is all concentrated in a narrow beam, with a diameter around a few millimeters. That gives it an area of roughly 7 square millimeters.

After bouncing off the wall, the beam spreads out to fill a hemispherical surface. At a distance of 1 meter, this hemisphere would have an area of over 6 million square millimeters. If you are even farther away, the area will continue to increase as the square of the distance, and correspondingly the intensity hitting your eye will continue to decrease.

In conclusion, the intensity of light hitting your eye when looking at the spot on the wall from 1 meter away is easily on the order of 1 million times smaller than the intensity if you looked directly into the beam.

But you don't have the complete specification of the object in question. How do you ensure its completely matte ?
How do you ensure you aren't within the "killzone" of the laser cone?

Don't take any chance when using lasers above 5mW indoors.



We kinda already answered that question back at his other thread , but I guess he wanted more opinions from different people I guess
 
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Eudaimonium

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Doesn't the laser decrease power when being redirected off a mirror? That, adding to the distance, even if something small as 2 meters, won't make the impact on the eye considerably smaller than a short range direct 15mW, probably resulting in no permanent damage?
About 99% gets reflected back on average, and atmospheric loss through 2 meters is too small a percentage to be taken into account for.

So if you started with 15mW, you are going to get 15mW.

NO, it would not make it safe. YES, you are going to get permanent damage from 15mW laser reflected off the mirror into your eye.
 

Spooky

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A meter away it drops by a million well that's good to know
Blaster my friend, that's "cherry picking"

assuming the laser hits a matte surface , its intensity decreases by a factor of a million once it reaches 1 m away.
Condition C only exists if both Condition A AND Condition B are met.

Little Jonny flew to 40,000 feet in his fathers plane, little Jonny put on his parachute, little Jonny jumped from the plane, Jonny landed safely.

3 Conditions

A: Jonny is in a plane at 40,000 feet
B: Jonny has put on a parachute
C: Jonny jumped from the plane
Result: Jonny landed safely

If we just take A and C = Result then from that we can confide that jumping from aircraft at 40,000ft without a chute is SAFE when clearly it isn't.

Guests will either read "lasers given certain conditions at 1m are safe" or they will skip the conditional information and simply see "Lasers at 1m are safe".

Eventually "belief" becomes fact, Snopes is full of erroneous information that people believe that has no basis in fact yet much of the information there is repeated with the best of intentions by a huge number of people.
When individuals safety and / or the future of an entire hobby is under threat it is imperative that only facts are believed and not modified versions of them.

cheers

Dave
 
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EpicHam

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Blaster my friend, that's "cherry picking"



Condition C only exists if both Condition A AND Condition B are met.

Little Jonny flew to 40,000 feet in his fathers plane, little Jonny put on his parachute, little Jonny jumped from the plane, Jonny landed safely.

3 Conditions

A: Jonny is in a plane at 40,000 feet
B: Jonny has put on a parachute
C: Jonny jumped from the plane
Result: Jonny landed safely

If we just take A and C = Result then from that we can confide that jumping from aircraft at 40,000ft without a chute is SAFE when clearly it isn't.

Guests will either read "lasers given certain conditions at 1m are safe" or they will skip the conditional information and simply see "Lasers at 1m are safe".

Eventually "belief" becomes fact, Snopes is full of erroneous information that people believe that has no basis in fact yet much of the information there is repeated with the best of intentions by a huge number of people.
When individuals safety and / or the future of an entire hobby is under attack it is imperative that only facts are believed and not modified versions of them.

cheers

Dave
People often chastise me for over-exaggerating the importance of SAFETY
But thats EXACTLY the reason why I do it.

Partial information intake kills .

But the deadliest logic fallacy of all is
"My friend got hit in the eye with a laser once, he's fine. So it MUST BE OK to shoot people in the eye with lasers. And I don't need goggles for using high powered lasers indoors"

THAT!
KILLS!
 

steve001

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People often chastise me for over-exaggerating the importance of SAFETY
But thats EXACTLY the reason why I do it.

Partial information intake kills .

But the deadliest logic fallacy of all is
"My friend got hit in the eye with a laser once, he's fine. So it MUST BE OK to shoot people in the eye with lasers. And I don't need goggles for using high powered lasers indoors"

THAT!
KILLS!
Very smart postings.
 

Joshuan

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its pretty easy to be safe if you scan the aera where you are pointing ... i.e -i see a window, a truck a car mirror and a chrome motorcycle, lets avoid hitting those,,
Are windows that much of a problem? Doesn't a big portion of the beam pass through the window or does it bounce off like a mirror?
 

Spooky

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Are windows that much of a problem? Doesn't a big portion of the beam pass through the window or does it bounce off like a mirror?
Angle of incidence, wavelength, properties of the glass, coatings on the glass, to name just a few variables.

Pilkington K series glass reflective and refractive indexes are totally different to float glass, tempered glass again is different. Laminated glass is different.

For example anything that extends to within 800mm of the floor in the UK has to be toughened glass, the high level windows either side of it may well be totally different materials but look the same.

Plastics, 9T30 perspex is close to 100% blocking of 1064 CO2 gas laser beams, however Black 9T62 that looks exactly the same passes IR in the same range at almost 95%.

Asking the same essential questions re-wrapped different ways rarely changes the answers.

cheers

Dave
 

Joshuan

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Asking the same essential questions re-wrapped different ways rarely changes the answers.
Not the same question at all, otherwise you wouldn't have answered it and attempt to explain it. Thank you.
 

EpicHam

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Angle of incidence, wavelength, properties of the glass, coatings on the glass, to name just a few variables.

Pilkington K series glass reflective and refractive indexes are totally different to float glass, tempered glass again is different. Laminated glass is different.

For example anything that extends to within 800mm of the floor in the UK has to be toughened glass, the high level windows either side of it may well be totally different materials but look the same.

Plastics, 9T30 perspex is close to 100% blocking of 1064 CO2 gas laser beams, however Black 9T62 that looks exactly the same passes IR in the same range at almost 95%.

Asking the same essential questions re-wrapped different ways rarely changes the answers.

cheers

Dave
Not the same question at all, otherwise you wouldn't have answered it and attempt to explain it. Thank you.
He just did.

He said , in order to answer your question you'll need to provide him with the variables as stated to give you an accurate solution.

The main problem with glass is that not all of that incident rays will be transmitted through the medium .
And due to the beam diameter , we might even end up with some incident photons exceeding the critical angle and end up hitting yourself .

We aren't even diving into the possibility of the laser diffusion.

All that uncertainty is precisely why we MUST be careful when using lasers indoors
 

Spooky

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Hammy Said
HOWEVER!
Despite of the diffused reflection due to microscopic imperfections of the surface , within a short distance the laser's photons still reflect by generally the same method as one hitting a smooth reflective surface , albeit now with a cone .

Its still dangerous.
That pretty much covers mirrors, glass, wood, paint, water etc, everything has a reflective index (even dirt)

Some items have a high index, some have a low to almost non existent index, if the laser is above 5mW and it hits something reflective then there is a risk of backscatter or direct reflection causing injury to the eyes. The risk will vary between very high (gold plated flat surfaces) to very low (Dirt).
In effect everything is a danger, it's just the degree of danger that changes. It has been medically established that above 5mW of lasing radiation hitting the eye is dangerous, how that 5mW gets to the eye is academic and should be considered an unknown variable.

cheers

Dave

ps: For the sake of accuracy, Black Holes don't in theory have a reflective index.Everything else does.
 
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EpicHam

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Hammy Said

That pretty much covers mirrors, glass, wood, paint, water etc, everything has a reflective index (even dirt)

Some items have a high index, some have a low to almost non existent index, if the laser is above 5mW and it hits something reflective then there is a risk of backscatter or direct reflection causing injury to the eyes. The risk will vary between very high (gold plated flat surfaces) to very low (Dirt).
In effect everything is a danger, it's just the degree of danger that changes. It has been medically established that above 5mW of lasing radiation hitting the eye is dangerous, how that 5mW gets to the eye is academic and should be considered an unknown variable.

cheers

Dave

ps: For the sake of accuracy, Black Holes don't in theory have a reflective index.Everything else does.
Black holes.
Now thats interesting .
Black holes in the end is still just a cluster of massively compressed matter.

What is black holes made of anyway?
Quantum gravity can't explain the situation of our everyday life , vice versa.

Ideal black body doesn't exist irl , so black holes HAS TO reflect some portion of incident radiations , but it doesn't .
Its just ... gone forever , adding to its mass .
Its siphoning matter and photons of our world and sinking it away :O

WHAT THE HELL ,BLACKHOLES?
 
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Joshuan

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He just did.

He said , in order to answer your question you'll need to provide him with the variables as stated to give you an accurate solution.

The main problem with glass is that not all of that incident rays will be transmitted through the medium .
And due to the beam diameter , we might even end up with some incident photons exceeding the critical angle and end up hitting yourself .

We aren't even diving into the possibility of the laser diffusion.

All that uncertainty is precisely why we MUST be careful when using lasers indoors
I understood. What I said was that I didn't ask the same question rewrapped as a different one, I was genuinely trying to understand the physics with windows and lasers which you have explained.

By the way, thank you EpicHam for your intervention on my different threads. I always appreciate your thorough input.
 

EpicHam

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I understood. What I said was that I didn't ask the same question rewrapped as a different one, I was genuinely trying to understand the physics with windows and lasers which you have explained.

By the way, thank you EpicHam for your intervention on my different threads. I always appreciate your thorough input.
:beer:
 

Spooky

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What I said was that I didn't ask the same question rewrapped as a different one
Hi Josh,

I wasn't complaining :)

The easy way to look at it all as an overview,

LaserBeams of over 5mW are dangerous if they enter the eye, anything between the source of the beam and the eye is a variable. There are likely billions of possibles right down to variations of the material structures of two materials that "appear" to be the same. Take float glass, the thickness of that varies over the basic sheet size, the surface finish varies over a single sheet, it's optical purity varies over a single sheet, it's atomic structure varies over a single sheet, it's cleanliness varies over a single sheet so just on one piece of glass you have 5 variables, now multiply that by two sheets and 5 variables. Just for fun calculate 30 different grades of glass from 10 different batches, all from different suppliers / manufacturers, chuck in a few other fun things to make it interesting such as surface imperfections and specular reflections and the numbers get big, VERY big as to their reflective / refractive indexes. That's just for "windows" so imagine when you have the myriad of other potential reflectors involved it gets mind numbing.
Most of the time the guest readers here don't have Masters Degrees in Particle Physics or Electromagnetics (or on occasion the interest sufficient to read further) so the chances of them bothering to read the intrinsic physics behind a statement is very low.Far more likely is that they will gloss over the bits that give them a headache and pick out the best bits that they believe will suit their question and then repeat that as fact without the variables or qualifiers.
For the benefit of those people it is better to say "A laser of above 5mW is potentially dangerous to the human eye" to ensure they have full command of the most basic of facts rather than risk dilution of that ending with little Jonny and his Ebay overwatt doing damage to the kid next door then his mom blaming everything except her own negligence in buying him the dam thing in the first place.

I see it in my industry every day, people ask why their laser tube that was advertised as having a life of "10,000" hours only lasted 200 hours, they have often neglected to read the details that say "UP TO 10,000 hours IF the current is kept below 28mA and temperature below 22 degrees" no, they ignored that bit and pumped the tube with 32mA and didn't bother checking the water level in the chiller so when the internal mirror was nice and hot they turned the water flow on and cracked the mirror killing the tube. Within hours they are on various forums slamming the "crap Chinese tube" they "Paid a lot of money for" and calling the seller a "Conman and rip off merchant" to cover for the fact they didn't bother reading the instructions.

hope that makes some kind of sense :)

cheers

Dave
 




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