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Old 04-28-2013, 03:29 AM #1
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Default Making laser safe to eyes?

I wanted some opinions on a project I'm trying. I have no idea if it is going to be safe or not. Basically I want a laser focus assist light for a camera. Years back sony used such a system with some of there cameras, so I would assume that it can be done safely (the camera I own that uses it is about 8 years old).

I know green is worse for eyes, but its also easier for cameras to see (the sensor on the camera it will be used on has twice ans many green sensors as red and blue ones). I could go with a red laser but it would have to be more powerful to get the same effect.

This could be for use only when not photographing people (low risk of hitting someone in the eyes), or it could be for use on people too (the idea would be to never aim it at their face when the laser is on but much higher risk of accidentally hitting someone in the eyes).

I have a green laser that was supposed to be 5mw (off ebay, it was supposed to be new but looks used and has no warning label). It looks identical to many on ebay that are 5mw. I am aware that these cheap Chinese lasers can be more powerful than they state. It uses a lens to create an adjustable pattern. The pattern has an 11x11 grid of dots so 121 dots. When adjusting the lens, each of those single dots opens into a 5x5 grid for 25 dots from each dot. In theory, that should be the laser getting split up into 4235 dots.

Does that mean that that 5mw / 4235 = 0.0012 mw per dot? Is that correct? About 20 percent of the dots are dim (around the outside of each group) and some are brighter than most, in particular one right in the center (not a huge amount brighter but brighter). There are maybe 2 dozen or more dots that are the brightest with the center dot being a hind brighter than those.

Lets assume that the brighter dots are 10 times brighter and the 5mw laser is actually 30mw. That is still only 0.07 mw per dot. Am I calculating this wrong?

Fyi, the dots do not seem to enlarge at distance but they get noticeably dimer. The whole grid expands a lot though about equal to distance (at 3 inches, the grid is 3 inches, at 5 feet, the whole grid is about 5 feet).

It would seem these dots are way below what is safe. If that wrong? If it is wrong, here are additional plans/possibilities.

I plan on using something with a hole to block most of the grid (I only need a couple or even 1 of the 25 dot grids in the center).

A circular polarizer seems to reduce stray light and reduce intensity some. I'm not sure if circular or linear polarizers are appropriate for lasers (I probably only have circular). In theory, I can reduce the power as much as I want with neutral density filters. With the single beam a diopter type macro filter will spread the beam (not only enlarge it but defocus it so the dot gets bigger the further away it is). I have various groups of lenses (partial assemblies out of camera lenses) some of which do similar things. I could experiment with those to have expand and weaken the beam. With the pattern lens it behaves a little different (at closer range the dots actually get smaller but past a couple of feet the spread out and keep spreading the further they get). Some lenses that work with a single beam just horribly defocus the bean with the pastern lens on but I'm sure I can find something that could work.

So could anyone give any input on the safety of this? Is the laser alone with the pattern lens safe if someone accidentally gets hit in the eye? If its intensity is reduced a little, and the beams are spread/defocused just a little, would that make it safer or would I need to do it a lot? Should I switch to a red laser, even though it would need to be more powerful to do the same job?



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Old 04-28-2013, 04:59 AM #2
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Default Re: Making laser safe to eyes?

I am quite positive that you would be safe doing that type of focusing laser but please consider the following, auto focus usually searches for lines not dots, that is why sometimes it's not easy to make the autofocus camera work well in making a panorama picture of a city from a hill top, where all the camera sees are the dots of the city lights. You would be much better off using lines, in that case vertical lines work best for focusing. If you were able to find a lense to make that than you would be set.
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Old 04-28-2013, 05:21 AM #3
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Default Re: Making laser safe to eyes?

Quote:
Originally Posted by norbyx View Post
I am quite positive that you would be safe doing that type of focusing laser but please consider the following, auto focus usually searches for lines not dots, that is why sometimes it's not easy to make the autofocus camera work well in making a panorama picture of a city from a hill top, where all the camera sees are the dots of the city lights. You would be much better off using lines, in that case vertical lines work best for focusing. If you were able to find a lense to make that than you would be set.
Are you saying that the laser grid of dots by itself is fine (with the one lens that came with the laser that makes the grid of dots), or that it is fine with the addition of filters to expand the beams and or reduce their intensity?

As far as the camera liking lines, not dots, I'm aware of that, but I haven't came up with a god idea yet. The laser was supposed to come with 5 tips, 2 of which do a circular line pattern and one of which does stars (so at least they have edges, not just dots). One was lines of dots rather than a grid. I was going to play with them all and see if the camera liked any of them. The seller only sent the one grid tip though (I guess I got taken, he sent one message that the one tip did the 5 patterns which it doesn't and now is not responding). It wasn't expensive though. I have considered buying another one with all 5 tips from a different seller. I have also thought about expanding the beam, then passing it through a clear filter painted with black lines. I'll just have to experiment and see what works. The grid of dots do work to a point but I think you are right, lines would work much better.

One thing that seems to work best so far. On the smaller grid of 25 dots, the spacing between dots is more in one direction than the other. If I focus it down so the dots are close to each other in one direction, it is almost like 5 lines of dots. That seems to work best so far. I would still like to get the other tips to play with though.

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Old 04-28-2013, 03:14 PM #4
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Default Re: Making laser safe to eyes?

I don't have a clue astro how auto focus works, or what you're trying to accomplish... But laser color has nothing to do with safety. It's all about power of laser, distance, and focus of beam.
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Old 04-28-2013, 11:34 PM #5
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Default Re: Making laser safe to eyes?

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Originally Posted by chipdouglas View Post
I don't have a clue astro how auto focus works, or what you're trying to accomplish... But laser color has nothing to do with safety. It's all about power of laser, distance, and focus of beam.
I was under the impression that your eyes could absorb more green light than other visible colors so green lasers were more dangerous but its quite possible that I have been misinformed.

The camera in question uses contrast detect auto focus. Imagine a piece of paper that is black on one side and white on the other, with a sharp line in the middle (the transition from white to black is a sharp line). If the image is out of focus, there is no sharp line (the edges of the black and white areas are blurred together. When it is in focus, that line becomes sharp. The camera detects this. A laser provides bright, sharp contrast (and a laser line has a long line of contrast as opposed to the tiny spot of contrast on a laser dot).

I just got to thinking, what if I managed to get two lines side by side, one green and one red? That might be hard to accomplish though.
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Old 04-29-2013, 12:43 AM #6
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Default Re: Making laser safe to eyes?

So point at something and the camera auto-focuses on the dot? Nothing else is happening here? You shouldn't need much power at all, and greens are unstable. Try a 1mW red first and move up from there.
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Old 04-29-2013, 01:01 AM #7
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Default Re: Making laser safe to eyes?

I second the 1mW red suggestion. Completely eye safe (if it really is 1mW!) and most cameras should see it well. Line generating diffraction grating would probably work better than dots as pointed out above. If you use a LGO (line generating optic) and feed the output from that in to a dot grid or 13500line diffraction grating it should do what you want.
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Old 04-29-2013, 04:08 AM #8
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Default Re: Making laser safe to eyes?

While I have found a few that say 1mw, the have class 3a warnings on them? I have no issue using a 1 mw. I already have a few red ones that are much less powerful than the green one but still say class 3a, < 5mw. Am I looking for something like these

Line and Cross Line Generator Lens | eBay

3pcs Lot Dot Line Cross Laser Lens 200nm 1100nm with Lens Frame M9 and Metal Cap | eBay

Ore even something like this

Multifunction Laser Level Leveler with Tripod Vertical Horizontal Line Tool | eBay

That last one has a tripod mount so I could screw the big ugly thing right onto the camera (all I would need to do is wire a remote button). Of course it looks like junk but I'm trying to stay cheap, at least til I see what works.


Cyparagon said greens are unstable. What does that mean? This particular camera's sensor has twice as many green sensors as red and blue so it can see a green light or laser much better. I was thinking of using the 5mw green with neutral density filters. I already have found a nd4 (25 % transmittance) and an nd8 (12.5 % transmittance) and they can easily be combined. I very likely have more around somewhere. What are the problems with using green.
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