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# Is Time Really A Factor?

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#### BrittanyGulden

##### Guest
Here:

Assume I have a laser diode optical output of 1w, & a pupil size of .25" in my eye (@ Night). If I shined that 1W Laser directly into my eye, I will be blind instantly. However, what If I put that 1w through a lense & disperse it out into a circular area. I should only need to have a large enough circle so that the fraction that would go into my eyes is a safe amount, correct? Area of a Circle = Pie * (R)^2, So I can just figure the fraction of the pupil's area to the larger circle's area. I only need an "Eye-Safe Level," which I'll call 1mW. So I have (Diode Power) x (Pie * (.25")^2) / (Pie x r^2) which equals 1mW. Hence, 1000 x (1/16) / (r^2) = 1. Radius = about 8" or 16" in diameter. This means that If I have 1 watt of light, going through a lense , & spreading out into a 16" circle of light, and that shines directly into my Eye, my eye will see 1 milliwatt correct?

Is this correct & if so how or does Time play a part?

-Thank You

#### lasersbee

##### Well-known member
That is basically correct if the larger circle's power is identical
at every location in the circle.

Time is a integral part... the faster a laser would scan past the
eye the lower the total power that enters the eye.

Hence the max allowable MPE ratings that need to be respected
to do a Laser Projector Show...

Jerry

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#### BrittanyGulden

##### Guest
this is the response I was given:

I'm not on any laser pointer forums, but I'm an engineer.

Exposure time is critical with diode emissions. A 1-second exposure of a 1mW source is the equivalent of a 1/1000s exposure of a 1W source. Permanent damage from a 1W source can occur in just a few milliseconds, so any prolonged exposure to your diffused beam is going to be problematic. In some studies, exposure to 445nm wavelengths for just a few seconds would cause temporary deformation of the rods/cones and you'd see everything through a green filter for the next 4 to 6 months at less than 3ms exposure time.

Similarly, you are wrongly assuming the function of a lens. You'll have to choose very carefully the type of diffuser you end up using. Keeping in mind that the beam is already being diffused through it's diode emitter. Use the wrong type of lens, and you'll find yourself with a central hotspot as intensity dithers towards the edges. This may or may not be apparent visually. Not all lenses are created equally, and finding one with perfect diffusivity is going to be extremely costly.

You are playing with fire trying to project a Class 4 laser into your eye. I'd say it's borderline stupid.

I have no idea what you are trying to do, but whatever it is, I wouldn't.

^ If this guy that gave me this response is incorrect, please note on it because he is kind of a dick

Thanks Tyler

#### ElektroFreak

##### New member

If you expand the beam as you suggest the hazard is reduced, but I'm not sure by how much. I didn't check your math, but assuming you're correct a direct eye exposure to 1mW will not cause damage. Obviously safety is paramount, so double-check your math, both the methods and the calculations themselves. If you're uncertain about any part of this, DON'T do it.

#### justinjja

##### New member
I'm fairly confident this is not correct: "A 1-second exposure of a 1mW source is the equivalent of a 1/1000s exposure of a 1W source"

I would much rather stare at a 1mW laser for 1 sec, that have a 1W flash in my eye for a ms.
For one you would blink if a 1mW laser was shinned in your eye for 1 sec,
and two i believe that over the time of 1 sec, the heat being aplied to your eye could be dispersed to some extent.

However he is is probably correct is saying you will have hot spots in your ring, 445 diodes just dont put out a nice round dot

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#### BrittanyGulden

##### Guest
It's just a "theory" I came across. Of course I will be doing plenty of research as I have done already.

The guy that posted that response basically said my "theory" was incorrect & I was a dumba** for leaving out the "time Factor."

I do not know what his reasoning was on the "In some studies, exposure to 445nm wavelengths for just a few seconds would cause temporary deformation of the rods/cones and you'd see everything through a green filter for the next 4 to 6 months at less than 3ms exposure time."

^I would be using a Diode W/ a wavelength of around 900 or so. How is his post relevant to my 900 nm diode?

All I am trying to figure out is what type (how strong) of safety glasses to get. Now obviously I am going to get the appropriate ones, but I am just a bit curious to see if my "theory" actually applies. I only assume the more powerful the glasses the more expensive & as long as I am "dispersing" the beam, what's the point in buying more "safety" than I really need?

^Yes it sounds dumb, but my "theory" is just a question & I will be buying the appropriate eyewear

#### ElektroFreak

##### New member
Don't skimp with goggles. Most likely you'll end up getting more and more powerful lasers (that's kinda how it works for most of us) so you'll want the best protection you can afford.

#### Guyfromhe

##### New member
Why not just buy a less powerful laser or buy a bright led if you want a giant spot.

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#### BrittanyGulden

##### Guest
Let me elaborate:

For a while now, I have been wanting to build a DIY Night Vision Device. Some of you may have already came across my thread a while back. Here is what I was thinking of doing:

Take a Digital Camera & use it as a "View Source." I have a few newer digital camera's laying around. They all are equipped W/ IR Stop filters. All I would have to do is take out the IR Stop Filter to allow IR to be passed so I can view it on the LCD Screen.

Find an IR Laser Diode & use this as a means of "illumination." I want a Diode W/ a Wavelength of around 900 so it's invisible to humans & I want this device to be able to reach out as far as possible so I am choosing a very high Power.

Now the Laser Diode will be used to illuminate an area, so obviously it needs to be dispersed. I was thinking about doing this W/ the appropriate lens.

My question is Safety. If I am going to be "dispersing" this beam so much, why bother spending so much \$\$\$\$\$\$\$\$\$\$\$ on Eye Wear that I wouldn't need.

Yes, I know sounds dumb but my theory that I posted in Post #1 sounded pretty darn accurate & I just wanted to see if it applied & from the responses I have been getting from other people, it doesnt?

-thanks

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#### BrittanyGulden

##### Guest
^ I have done that in the past & they work great for distances of less than 25 ft.

...I am looking to illuminate an area of several hundred yards W/ a width span of several hundred yards. I came across a Post a while back & a guy accomplished this using a Diode W/ a mW rating of around 250 and was able to illuminate an area of 100 yards W/ a width span of nearly 50 ft.

Here, so Post Number 96 : http://legacygt.com/forums/showthread.php/poll-all-women-stupidi-178441p7.html

& this is the response I was given: I'm not on any laser pointer forums, but I'm an engineer.

Exposure time is critical with diode emissions. A 1-second exposure of a 1mW source is the equivalent of a 1/1000s exposure of a 1W source. Permanent damage from a 1W source can occur in just a few milliseconds, so any prolonged exposure to your diffused beam is going to be problematic. In some studies, exposure to 445nm wavelengths for just a few seconds would cause temporary deformation of the rods/cones and you'd see everything through a green filter for the next 4 to 6 months at less than 3ms exposure time.

Similarly, you are wrongly assuming the function of a lens. You'll have to choose very carefully the type of diffuser you end up using. Keeping in mind that the beam is already being diffused through it's diode emitter. Use the wrong type of lens, and you'll find yourself with a central hotspot as intensity dithers towards the edges. This may or may not be apparent visually. Not all lenses are created equally, and finding one with perfect diffusivity is going to be extremely costly.

You are playing with fire trying to project a Class 4 laser into your eye. I'd say it's borderline stupid.

I have no idea what you are trying to do, but whatever it is, I wouldn't.

^ This guy referred to me as a "sh*t head" & basically said my "theory" didn't apply. I think it does & I would be more than happy if anyone could "wrong him."

Thank you JustinJJ for noting this: I'm fairly confident this is not correct: "A 1-second exposure of a 1mW source is the equivalent of a 1/1000s exposure of a 1W source"

I would much rather stare at a 1mW laser for 1 sec, that have a 1W flash in my eye for a ms.
For one you would blink if a 1mW laser was shinned in your eye for 1 sec,
and two i believe that over the time of 1 sec, the heat being aplied to your eye could be dispersed to some extent.

So I already have a bit of dirt on him. I just don't understand how his 445nm statement has anything to do because the diode of my choice is around 900

So does a diode W/ a wavelength of 445 nm may not put out a nice round dot, but how about a diode of 900 nm ?
-Tyler

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#### justinjja

##### New member
hum,
how about building a spotlight with like a 400w halogen, then putting a visible light filter on it?

like half of the light output by a halogen is IR, so that should be brighter than any IR laser.

just a thought...

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#### BrittanyGulden

##### Guest
^ There's still usually a "Dim" light visible & I'd rather carry around a "handheld laser" VS a huge spotlight.

LOL, I'm just trying to get some dirt on this guy that is calling me a Sh*thead for stating my "Theory."

So a Diode W/ a Wavelength of 445 nm MAY have Hot Spots which is fine & dandy, but I am using a Diode W/ a wavelength of 445 nm. Will that still have "hot spots" & does Power play a part in this?

#### ElektroFreak

##### New member
Not sure where you're finding protective eyewear that is so expensive that you're bothered by it. You should be covered by some run-of-the-mill OD3-5 IR glasses such as these: 808nm safety goggles [OLSF808] - \$36.99 : Zen Cart!, The Art of E-commerce. They say 808nm, but they cover from about 780nm-950nm and perhaps further..

Unless you're 110% sure that your power levels are what they should be across the beam profile I wouldn't chance looking directly into the laser source (even if expanded and diffused) without at least some protection. Why take the chance just to save \$20-30?

He refers to 445nm because these days 90% of the time people talk about 1W laser diodes they're referring to the 445nm blue diodes that are abundantly available for a dime a dozen. You probably aren't aware of the fact that these are so popular and numerous.

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#### justinjja

##### New member
fair enough,
if i recal correctly most high power IR laser diodes have a fairly signicant fast axiz,
meaning you will get a square or line like output from the laser.
so it will still be hard to get an even circle of light