Dangerous reflection Materials and burning power.

[SOG]

Hey Guys, Actually I did look around the Forum, but I can't find similar Titles, or maybe I am not using the correct keywords when searching.

But I do wonder about two questions.

1. I have heard the shorter the wavelength the more powerful it is. so does that mean a 400nm burn better than a 700nm? I think it also had something to do with how the color is being absorb ?


2. I have been wondering which material/thing gives the most top 10 dangerous laser reflection?

a. Mirror Like Materials (clearly this is dangerous...)
b. Reflective Materials e.g. road signs (I wonder how much power they reflect though?)
c. Window
d. I am running out of examples, maybe computer screen?



I am sure it has something to do how much % of light it reflect, but do we have something more solid that will certainly give dangerous reflection when you point your laser at it?

 

[Fenzir]

As for burning, I can't give you a definite answer. Some people say 405nm is best for burning others say 808, sometimes and argument sparks and we get nowhere. However, I know that trying to burn a black object (like black construction paper) is A LOT easier than a white object. As far as reflectiveness, mirrors are designed to have high amounts of reflectiveness, so yes, they will probably be the most dangerous. Although there are first surface mirrors that don't have a glass pane infront of them that reflect more than your normal mirror, which would be more dangerous. Road signs will g et your eyes pretty bad, again an item that was ment to reflect. Windows are interesting though, the windows I have at my dorm are triple pained and they slide, so I can get six panes over one another. I've shined a 5mw green into it and it kinda looks like a laser show, but I only did that once or twice because I know of the dangers. I can't help too much, but if an item is created to reflect back to you, it probably does a pretty good job of doing just that.

 

[Tech_Junkie]

Road signs are designed to directly reflect light back to the source. Which means your eyes, and possibly the laser. If it hits the laser directly the diode will pop. So avoid road signs at all costs.

 

[Fenzir]

And a popped diode means that your laser is busted. The diode is what actually emits the light.
Just thought I'd clarify.

 

[Tech_Junkie]

To be even more precise, its causes COD (Catastrophic Optical Damage).

 

[SOG]

Um... wait, I didn't know shining at road sign will get light back to the diode and burn it? I have heard COD, it's like the lens cannot take the power of the lens, so it went black, or something like that, but I didn't know it can burn the diode?

But then what happen to those lens that are not coated internally? I mean the laser will also get reflected by that glass, right ?

 

[qumefox]

Er. I illuminate road signs all the time and i've never killed a diode. Yes they are designed to reflect light back towards the source, however they just reflect 'most' of the light back 'in the general direction' of the source. You don't get a low divergence tight beam coming back at you. The warning against illuminating them is moot unless your just at point blank range to one. Though I advise against doing it when other vehicles are around. People drive badly enough in most places without strange distractions like that making them drive even worse.

 

[InfinitusEquitas]

@SOG

Be especially paranoid about mirrors and street signs. If you have a powerful flashlight, shine it a street sign sometime from a close distance of say 10' and see just how bright it will appear to you. Retroreflectors are very dangerous when it comes to lasers.

A regular mirror will reflect a lot of the light back at you... from 12" distance a reflected beam of a 1.1W laser was down to ~950mW - more than enough to do serious damage.

Aside from that obviously any glossy or shiny surface is also bad news... so watch out for glossy TVs, monitors, etc...

With glass, it's a bit more complicated, most glass will pass through 90%+ of the light, the rest is absorbed or reflected. I've been meaning to do some reflection testing with an LPM, but need a few samples of common glass.

With specular reflection overall, at distances greater than 3 feet from the eye, trust your eyes... another words if it's uncomfortable to look at... close your eyes. Even though the mW directed at your eyes would be well below 5mW, you can still overload your retina.

Personally I find the dots of blue lasers to be the most unpleasant to look it (10ft+ distance), and experience an afterimage even though there is no actual danger.

With regards to burning... most materials absorb shorter wavelength after, so per mW typically 405nm is best for burning, but this will vary depending on the target material.

The argument could also be made that IR is best for burning since you can burn any material with no need to worry about the color of the material you're burning.

EDIT: I would worry a lot more about catching a quick reflection in the eye from a street sign then the diode...

 

[JBTexas]

qumefox; agreed/ditto

About road signs...

The reflective coating disperses light quite a bit to promote visibility from a wide range of angles... like the reflectors on the back of a car. Some kind of metalflake paint?

When I was in Augsburg, Germany, I had a ~ 5mw red HeNe... one of my favorite things to do was to go to the top floor of the barracks (4th floor, storage area, a fairly TALL 4 floors) at night and aim the laser at a monument on a hill overlooking the general area... maybe about 4 miles away. (It was a monument to one of the Bismarks... I forget which one... hopefully his ghost didn't mind). There was a stop sign up there, situated "just right" to see REAL well from the army base when illuminated by that laser.

I've also hit road signs up close... the reflection is not a tight beam.

"Start Small" and experiment first. Hit the sign at an angle, but from several feet away, to make sure there is NOT a tight, bright, "hot" beam coming off of it; play it so that any reflections hit, say, on pavement close by and observe the reflection(s). If the best you get is a widely dispersed and distorted "spot", then hitting roadsigns are probably safe, especially from a distance.

 

[Fenzir]

Road signs are designed to directly reflect light back to the source. Which means your eyes.

Alright, you just rephrased what has already been said

 

[JBTexas]

Again, I would say that one can experiment... hit the roadsign at some distance away, at an angle (non-perpendicular), and "holding the laser high"... so that any reflections hit the ground/pavement. Lets also say "when no one else is around".

If a SAFE distance can be determined (from the size/dispersion/distortion of any reflected spot); then why not? IF, say, it is relatively safe to hit a roadsign from, say, at least dozens of feet/meters away, then why not?

As long as one is careful.

(EDIT) After all, TECHNICALLY, taking your laser outside and hitting a neighbors house with it to make a YouTube video or whatever... or hitting snow... TECHNICALLY has SOME risk to it.
The idea is to have fun with lasers while using caution and common sense.

IF done SAFELY, a roadsign makes a good target. I remember my HeNe was pretty dramatic looking from 4-8 miles or so (round trip), when reflected back from a roadsign.

OK... then... IF it is focusable, then say, widen the angle a bit to make a larger spot.

 

[Tech_Junkie]

Sorry but I dont take chances like that with high powered lasers, thats why I recommended not to do it. I used to light up a stop sign on the next block with my ~1mW key chain laser, and it was very bright. So how bright will it be using a laser at 1100 times the power?

I have some of the same material they use on road signs. I'll dig it out. If anyone want to do some experiments, all the power to you.

 

[JBTexas]

It might be informative and useful to estimate how much of the energy is dispersed. That TYPE of material (roadsigns, car/bicycle reflectors) seems to consist of fine (metallic?) reflective particals suspended in something transparent/semi-transparent. So what we get back is millions of little lasers; but those particles can't be "all the same" or "all reflecting in one direction"; it's just that there are so many of them, it SEEMS that way. And you know the "spot" thrown back (if there is one) is distorted.

If a *ROUGH* estimate (by (goggled)eye/guess/meter) of a percentage of energy can be determined, energy that gets thrown out in some direction other than "as if off of a mirror", that might be useful in judging just how much danger is involved in lasering roadsigns.

Here is a bottom-line thing though - HAS anyone actually popped a diode/DPSS from a roadsign, car reflector, etc? If so under what conditions?

(EDIT)

Please don't assume I am irresponsible... of course I advocate caution, as noted above in previous posts. I would also add; insure there are no CARS headed for any targeted roadsigns, preferably NO CARS on said road.... I know that I would probably freek out if I was driving at night and some stop sign seemed to say "BEHOLD! LOOK UPON MY BRIGHTNESS, AND BE BLINDED BY MY LIGHT! I AM THE ALL POWERFULL GOD OF ROADSIGNS!"

 

[qumefox]

Road signs are similar to high quality projection screens. They're designed to reflect light back in the 'direction' of the source, but not 'at' the source. They might behave like mirrors to a coherent light source at a few inches, maybe even up to a foot, but the divergence of the reflection is going to be fairly large. Its the same as having a defocused laser shined at you from a distance. Sure it looks hella bright, but the actual power densities are so low you'd have problems separating a power reading out of the background noise even with a high end laboratory grade LPM.

Though like I said in a previous post, and JBTexas did as well.. Don't illuminate street signs with traffic around. It's not an eye hazard but it will be a hell of a distraction. And most people drive terribly to begin with without odd external stimulus like that.

 

[BShanahan14rulz]

Road signs are designed to directly reflect light back to the source. Which means your eyes, and possibly the laser. If it hits the laser directly the diode will pop. So avoid road signs at all costs.

Road signs are designed to directly reflect light back to the source. Which means your eyes, and possibly the laser. If it hits the laser directly the diode will pop. So avoid road signs at all costs.

Also, Road signs are designed to directly reflect light back to the source. Which means your eyes, and possibly the laser. If it hits the laser directly the diode will pop. So avoid road signs at all costs.

Retroreflectors like on sneakers and clothing are made of tiny glass spheres that bounce the light back forward. Neat stuff, but can be dangerous to glance some retroreflective material with a laser.

 

[CapeClover]

One day I pointed a 5 mW red laser at a STOP sign several hundred feet away (at the end of the block) in broad sunny daylight. You couldn't see the beam or even the dot in all that bright sun but, as soon as it hit the Stop sign it lit up like a neon sign. The reflection was extremely visible.