75mW & Retroreflective Street Signs

I've been playing with lasers for awhile now, though my highest power laser is 15mW. I'm interested in purchasing my first dangerously bright laser at 75mW from NovaLasers ( http://www.novalasers.com/NOVAstore/pc/viewPrd.asp?idcategory=3&idproduct=85 ). When showing my lasers to friends, even a 5mW 532nm laser appears very bright at night when hitting a retroreflective surface, such as a street sign or license plate. I only want to use my lasers with care, and am unsure about how safe it is to view a 75mW laser bouncing off a retroreflective surface. I'd rather not damage my eyes for a lifetime for a few minutes of looking at bright lights.

Since retroreflectors point the beam back in the direction the light came from, the 5mW looks extremely visible even from 300 meters away, would doing the same with a 75mW be essentially shining the laser directly into my eyes and damage my eyesight? Off of these surfaces it's never a perfect mirror, everything because relatively diffused. I'd like to be able to have some fun with my laser to justify spending $150 for a nice one. Wearing 532nm blocking goggles prevent me from seeing the beam at all, which seems like a real waste unless I'm setting up a project to be viewed without the goggles later (like my spirograph or sound-reactive diffraction grating display).

Second question, how bright does a 75mW actually look? Any company that demonstrates pictures of their lasers will either tweak the camera exposure/shutter settings or use only their most powerful lasers for photography, making it impossible to tell what it would actually look like. Video cameras are no where near as sensitive as a human eye is to laser light. How bright should I realistically expect my 75mW to be? The beam itself visible in daylight? A brilliant, solid, green beam in the mountains at dusk?

Thanks for any information, it is greatly appreciated.

You wont damage your eyes, to be safe shine it on a 30 degree angle or something, i have shined my 300mW red on a stop sign and my 100mW green many times and have not had any eye damage.

you'll want to be careful with the flourescent green/yellow signs that are posted around schools. Those seem to MAGNIFY (they don't really) the light. it's very impressive.

A while ago I pointed my X100 at a stop sign that was about 2 blocks away and it was still incredibly bright. Be careful when shining your laser on anything even remotely reflective. Btw the nova Xseries lasers are awesome. You'll love it ;D

I'll make a quick video while it's still a little dark out. Signs reflected with lasers

The first is a 330mW blu-ray hitting one of those white speed limit signs and a stop sign. the difference between the shots is how much I focused the beam. The second in the short clip is with a 30mW green but I still can't believe it, supposed to be 125mW I tested it at 30mW. Rule of thumb probably shouldn't shine at green at signs in the dark. The reflections are mild but enough to annoy the eye, I needed goggles to do it. The distance I was at about 100ft and 35ft I zoomed in for all the shots. No tripod as this was a quick visual.

For the love of god stop pointing them at signs, I noticed you speak of your own eyes but what about other people looking at that sign while driving? Be careful with that.

Sounds a little like all you want to do with your lasers is point them at road signs...

You cant really describe what the beam of a 75mW green is like apart from saying its 75 times brighter than a 5mW. As for wearing goggles, I would get some but if your star pointing or anything like that they are pretty pointless, they are good if you are going to be burning things or perhaps using the laser in a small room where you are close to the beam though.

The point of these reflecting signs is that the light gets reflected back where it came from. So if you point it at one of them, only you get the reflection, not somebody driving by. Of course there is some scatter as these aren't optically perfect triplet reflectors by a long shot; but this in turn means that the reflected beam won't be dangerous any more. A couple dozen or hundred milliwatts are blindingly bright only if you got them in a narrow beam.

lamborgini8 said:

For the love of god stop pointing them at signs, I noticed you speak of your own eyes but what about other people looking at that sign while driving? Be careful with that.

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OK, I'll stop pointing my lasers at signs on an empty street that leads nowhere at 5AM with absolutely no traffic. Thanks for letting me know that lasers aren't toys.

Don't make broad assumptions without anything to assert them.

I did it once and it almost flashed me back... i am now scared to flash signs again.

rangedunits said:

I did it once and it almost flashed me back... i am now scared to flash signs again.

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Did watch the video link? That's why I needed goggles on, lots of reflection there.

XCreedX said:

You wont damage your eyes, to be safe shine it on a 30 degree angle or something, i have shined my 300mW red on a stop sign and my 100mW green many times and have not had any eye damage.

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It's a retroreflector, the whole idea is that most of the light bounces directly back no matter what angle you hit it at. It probably isn't safe to look at the reflections for too long, even if they are scattered somewhat. It certainly wouldn't be comfortable.

I would also watch out for vehicles, it may be distracting to a driver to see a large green dot infront oh him/her while trying to read a sign.

-Adam

diachi;

I would still be very cautious about the "retroreflectivity" on some of the better signs.

According to industry standards, a full-cube sheeting has 58% retroreflective efficiency.

http://www.minimumreflectivity.org

So your 75mw laser could return over 43 mw back at you.

I have seen some very bright flashes off retroreflective road signs myself, with some of my Red OC lasers firsthand.

LarryDFW

lamborgini8 said:

You cant really describe what the beam of a 75mW green is like apart from saying its 75 times brighter than a 5mW.

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Probably just 15 times brighter...

Wow, thanks for the quick replies. I think I will purchase the xSeries 75mW Green then, just when I point it at a street sign I'll give it at least 75 meters distance for a there-and-back distance of 150m, making it 'safe enough'.

Dr. Ebert is correct, the point of a retroreflector is the light comes back to the light source, so when your car headlights are driving at night, the sign becomes highly visible to the driver, rather than being dimly lit like everything else. Though there is some scatter, laser light is unlikely to mess with any drivers, especially if you are off to the side.

A reason for wanting the 75mW green is up in the Sierra Nevada mountain range (where I work at Hartland Christian Camp during the summer), the view of the stars is absolutely amazing. Not to mention the mountain biking and hiking, and cool fog and weather effects we get (snow, low clouds, whenever there are California fires the sky turns orange and the sun becomes red). I bet I'm going to have a lot of fun with this. Thanks a bunch, and thanks to laser83 for the video demonstration. I've got an excellent night-time camera, when I get the laser I'll adjust the exposure and shutter settings (though it could be deceptive, I'll do it until the pictures match what my eyes see) for others.

LarryDFW said:

diachi;

I would still be very cautious about the "retroreflectivity" on some of the better signs.

According to industry standards, a full-cube sheeting has 58% retroreflective efficiency.

http://www.minimumreflectivity.org

So your 75mw laser could return over 43 mw back at you.

I have seen some very bright flashes off road signs myself, with some of my Red OC lasers firsthand.

LarryDFW

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Even at 43mW returned, it's going to be scattered. So say the scatter is over a 1m square area. and the area of your pupil is 1cm squared , that would make the power that reaches your retina 43/100 = 0.43mW which is safe.

-Adam

Actually the typical dark-adapted pupil has more like 5mm diameter, i.e. 1/5 cm squared (hmmm... no way to get sub/superscripts here?).

Also an area of 1 sq m is 10.000 sq cms, so the actual amount reaching your eye with the numbers supplied would be below 1 uW (i.e. 1 microwatt. Hmmm... no greek symbols either?).