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basic optics question

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milkmandan

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in researching an idea i came across this page below.

http://laserpointerforums.com/f48/laser-through-toslink-fiber-optic-49068.html

if you had a lense to make the beam go wide (toslink cable) immediately after coming out of the pointer, then a second lense make the beam narrow to a focal point (last pic) would the only dangerous part to an eye be at the focal point. is there a way the beam would only be concentrated and thus dangerous at 1 point, a focal point? on a 2W laser for instance. or a 1W maybe.
 
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Yes. All else being equal, a laser is far less hazardous with a wider beam. Basic geometric analysis shows it follows inverse square function - twice the diameter is 1/4 of the power density, 4 times the diameter is 1/16 the power density, etc.
 
then could you tell me what type of lenses would be needed? could you give me a general idea of how to build this thing? e.g. distance between lenses etc. i don't want to use a fiber cable to widen the beam cause that post says the cable degrades the beam bigly. would i just need to get a focusable laser for the widening part and a lense for the focal part? sorry i'm so stupid my knowledge of lasers begins and ends at pink floyd laser light show.
 
my idea would be to have the beam travel 4-6 inches forward, than bounce off a 45 degree mirror (this part would be jacketed to protect eyes) then travel ~2inches hit the widening lense then 1inch then hit the narrowing lense. would this work? also after the focal point would the beam continue to diverge thereby becoming harmless? at least theoretically would you need to wear glasses if this was 100% designed right? rep'd
 
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We can help you a lot better if you actually tell us what you're trying to build.
 
a laser that can burn only at 1 point (focal point) instead of along the whole length and one that you don't have to wear glasses around because it has been engineered to be safe (unless you just shine the focal point in your eye).
 
You are talking about power density, that you want the power density to be so low having it flash in your eyes can't harm them. One thing to keep in mind, which you indicated being aware of, is that even if the power density is low due to dispersion, the lens in your eye will take that dispersed power and focus it down to a point inside your eye so then you are right back to where you were before the power was spread out over a larger area. How much of that power your eye can collect due to its aperture and how much power is present within that amount of aperture, if known, will yield the answer you seek.
 
thank you alaska for following my ramblings and cyparagon. rep'd you both! 1 more question cause this is feeling like it wouldn't be realistic w/o glasses which ends it right there.

let's say the final narrowing lense brought the beam to a focal point really quickly/aggressively like <1". and let's say the focal point, which is at the end of this device, is at arms length pointed away from you. ~2-3' from your eyes. then let's say the focal point hit something reflective and bounced back right at your eye. well, the beam might be focused at that one spot, but it hasn't been collimated. i understand what alaska is saying but if that 2-3' distance is always maintained, if we start with a laser that is right on the cusp of being damaging to an eye, say 50mw, then this process makes it so where this laser could only be damaging to another's eye. only on the business end of this thing.

is this correct?
 
The light reflected (even diffusely) from the focal point itself is hazardous for a class 4 laser. What you are asking cannot be done.
 
Im curious why we have multiple people signing up and asking questions about lasers in the eye and how to build one safely to hit someone in the eyes.... :wtf:


Bottom line, if it's over 5mW there is a possibility whether it's 1 : 100 or 1 : 1 doesn't matter; the problem is you should wear your safety goggles for that respective wavelength if it's over this power and it can get into your eyes. If there is a focal point coming from a laser pointer period, and it's over 5mW just wear goggles, and people around you should wear goggles. Especially being new to lasers, if you start playing with 50mW laser and mirrors, you are gonna get this laser in your eyes. WEAR GOGGLES!
 
Im curious why we have multiple people signing up and asking questions about lasers in the eye and how to build one safely to hit someone in the eyes....
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i'm curious why the dude (who's a 3 month old member) that complains about this behavior answers these questions.
 
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