5mW limit too conservative?

[EpicHam]

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

Well.... you don't need to go into THAT sort of technicality,dave.
That'll scare off newcomers .

But since you opened that cans of worms , I might as well as join in
In the construction biz we have a little thing called the Fresnel equations which helps us determine the light reflection and transmission in glass.
(Oh wow , so cross discipline DOES happen :thinking: )

So if you have all the refractive index info.
You can calculate the reflectance of your window

When the angle of incidence is 0 , about 8% of the incident rays are reflected back at the source when the rays strike a common soda lime glass surface.

For your window however , you'll need to input your own numbers

:beer:

 

[Spooky]

(Oh wow , so cross discipline DOES happen )

Indeed it does my friend I used to do the structural calculations and noise control stuff for Salisbury Glass (a commercial window manufacturer) the corner cutting some companies do is astounding (claiming 6mm glass will attenuate to 63Db 43 Db (A weighted)) just to keep costs down Cheshire Street this was the last place I worked on.

On the upside, now I just cut stuff up with lasers it's more fun, less stress and pays better

cheers

Dave