# At what point does one need to protect their eyes?

#### Abemas

##### Newly Enlightened
Assuming that you using a laser appropriately, (i.e. not shining the laser into your own or someone elses eyes) at what mW or wave length is eye protection neccesary to safely operate a green laser?

5 mw and above.

#### Cornkid

##### Flashlight Enthusiast
really.... thanx!

-tom

#### comozo

##### Enlightened
A better place to post technical questions are Google's Groups alt.lasers and possibly sci.optics. Most that use this forum lack the highly technology information you seek, that includes me. I do remember reading an Army document awhile back, somewhere on the web that the minimum threshold for permanent eye damage was 25mw. I cannot remember what the wavelength was or the beams divergence or even when those tests were conducted. Do some keyword searches, see what you can find.

#### comozo

##### Enlightened
It's not 5mw and above it really depends on what the beams divergence is.
This was copied from Wickedlasers. Post titled "Impressive".
"It is important to note that extremely high power densities are achieved at the focal point of a concentrated laser beam. A 10-milliwatt beam focused to a diffraction-limited spot 0.22 micrometers in diameter results in a power density of approximately 30-million watts per square centimeter. Such high energy levels can rapidly degrade or destroy lens and filter coatings, as..."
1 square centimeter =
100 000 000 square micrometers. microns. As you can see, in a space less that 1 micron you have a power density of 300mw,
30 million divided by 100 million.
I think the calculations and conversions are right.

#### hank

##### Flashlight Enthusiast
Re: At what point does one need to protect their e

Remember those little bright and dark spots you see in a room illuminated by a laser?

Those are the places that -- after traveling various distances -- the peaks and valleys of the wave happen to overlap or cancel out.

Any reflective or refractive bending is also going to concentrate the energy.

On the ocean, you get "rogue waves" and with electronics you get "electromagnetic smog" -- points where the energy moving around sums up to a LOT, unpredictably.

You don't want your retina to be focusing any very bright spot, whether it's the direct beam, or a reflection off some flat or curved surface, or the beam refracted through some transparent material -- or even the place where a whole lot of peaks of the wavelength, after traveling various distances, happen to overlap.

From the work on ocean 'rogue waves' here
http://at.yorku.ca/i/a/a/h/51.htm
see this example of waves summing to make a high energy spike

http://at.yorku.ca/i/a/a/h/51.dir/wavetrain.gif

#### DarkLight

##### Enlightened
[ QUOTE ]
comozo said:
It's not 5mw and above it really depends on what the beams divergence is.
This was copied from Wickedlasers. Post titled "Impressive".
"It is important to note that extremely high power densities are achieved at the focal point of a concentrated laser beam. A 10-milliwatt beam focused to a diffraction-limited spot 0.22 micrometers in diameter results in a power density of approximately 30-million watts per square centimeter. Such high energy levels can rapidly degrade or destroy lens and filter coatings, as..."
1 square centimeter =
100 000 000 square micrometers. microns. As you can see, in a space less that 1 micron you have a power density of 300mw,
30 million divided by 100 million.
I think the calculations and conversions are right.

[/ QUOTE ]

And how would that apply to a hand held unit?

It doesnt...

You are talking thousands of dollars of optics to do what you are describing on a test bench..

For all practical purposes you arent going to do this with a pointer..ever.

#### comozo

##### Enlightened
You don't need to spend thousands, just one lens to focus the beam to a spot that can do damage. The same result you get when you focus the sun rays with a magnifying lens. You won't get 30 million watts unless you have a diffraction limited spot but you can greatly increase the power density. That's the point I was trying to make.
Does it matter whether 10 milliwatts of light is produced by a laboratory grade diode laser or a handheld unit?

#### Raccoon

##### Enlightened
To the original poster's question... don't forget that the eyes are damaged every day by normal use. Frequent exposure to lasers that aren't powerful enough to pop your eyeball in one exposure may be enough to wear away your vision after a hundred exposures. Even a normal (lessthan) <5mW laser can do this when used in the dark (while the pupil is fully dilated)

Just like going outside without sun glasses, but a little worse.

#### 2dim

##### Enlightened
Re: At what point does one need to protect their e

[ QUOTE ]
hank said:
Remember those little bright and dark spots you see in a room illuminated by a laser?

Those are the places that -- after traveling various distances -- the peaks and valleys of the wave happen to overlap or cancel out.

[/ QUOTE ]

...and here I was thinking it was all the dust from my poor housekeeping!!!