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Clarify the "dot viewing" fear?

rhd

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I'd love some clarification on this point - why do some people talk about dangers associated with viewing the dot of a high powered laser unless it's xyz number of meters away? Apart from the anecdotal evidence that I have in fact viewed the dot of a laser at closer ranges without any obvious detrimental results, here's my main conceptual roadblock

If the dot of a 2W laser is 10 feet away, hitting a surface that is diffusing the light, why is that dot any different than a 2W flashlight outputting similarly diffuse light?

I can understand obvious concerns with respect to reflections if the dot is on a shiny surface. That's a no-brainer. I can also understand the related concern that if you're viewing a 2W dot on a surface that is 10 feet away, you should have goggles on for other reasons anyway - particularly the danger of accidentally moving that dot across something reflective in the process.

But if we're really talking about the danger of looking at the dot itself, and not these related dangers that have to do with beam reflections, why is the danger any greater than viewing any other ~2W source of diffuse light?
 
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Krogith

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How do you know that said surface is perfectly defusing the light? Couldn't a tiny reflective spec technically shoot that focused beam into your eye? Just my 2 cents as i'm not a expert and just guessing and would also love a Technical explanation :beer:
 

jakeGT

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I do now that black cloth diffuses good, but also burns. I would never do it inside a room or anything though, from myroom to my living room is close to 100 feet, and that doesnt bother me, but as he said, I'm no expert
 

millirad

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The 2 watt flashlight is divergent, and the laser is collimated. Edit: the light coming from the laser is also "coherent", and the light light coming from a flashlight is not. Because the light leaving the flashlight is not coherent, the photons collide and scatter easily while the coherent light does not.
 
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rhd

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The 2 watt flashlight is divergent, and the laser is collimated.
It's not though - at least not after diffused by a matte surface.

Again, absolutely no objection to the concerns re: a reflective surface, or any of the valid related concerns regarding using these lasers indoors. I just really want to boil down whether our concerns regarding looking at the dot are actually concerns related to the dot itself, or to something tangential. IE - hitting a reflective surface that bounces the beam back into your eye, etc.

It may be a distinction without a practical difference, but I'd really like to understand whether there is an inherent danger in looking at the dot produced by a high powered laser on a non-reflective matte surface.
 

sharp

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I am new to this forum and new to lasers as a hobby, but let me give this a shot:

My understanding about the ratings of lasers is that a 2 watt green laser emits 2 watts of green light from its aperture (regardless of the energy it consumes.) The light then hits a surface, at which point the light diffuses and goes in all directions (which is why you are able to see the dot.) So, essentially, at the dot you have a light source giving out 2 watts of green light in all directions.

How is this different than a 2 watt incandescent lamp? Well, incandescent lamps are rated by how much power they use, not how much is radiated as light. Incandescent lamps are inefficient and give of only about 10% of the energy they consume as light. The light they do give off is not concentrated in one wavelength. Finally, lightbulbs are not as small as laser dots. The energy from a tiny dot from a laser beam refocused as a tiny dot on the back of your eye is more dangerous, because with a bigger bulb, the same amount of energy could be entering your eye, it just wont all be focused on a smaller area.

Lets pretend your 20 watt light (of which 10% of the energy used is now given off as light) was able to give off green light at the same wavelength. (Impossible for an incandescent bulb.) And lets also shrink the 20 watt bulb to the size of the dot, so that when your eye focuses on it, all the energy from these light sources are more densely focused on a point on the back of your eye. The light given off by the dot from the beam of the 2 watt laser and by the 20 watt light are now essentially the same.

Now we get to why it helps to stand back. As you move back from these sources of light which are both emanating light in all directions (not in one direction like at the aperture of the laser; the laser beam has now hit a diffusive surface) less of the light coming from these sources is entering your eye. Google: inverse square law.

But I think you got that. Perhaps the main point is that a 2 watt bulb is not equivalent to the dot projected by a 2 watt laser, as you assume.

As I said, I am new here, sort of new to lasers, and possibly unknowingly talking over my head -- so someone please correct me if I am -- but I think I'm on to something here. I see some differences between a laser dot and a lightbulb which I think matter.

-sharp

I'd love some clarification on this point - why do some people talk about dangers associated with viewing the dot of a high powered laser unless it's xyz number of meters away? Apart from the anecdotal evidence that I have in fact viewed the dot of a laser at closer ranges without any obvious detrimental results, here's my main conceptual roadblock

If the dot of a 2W laser is 10 feet away, hitting a surface that is diffusing the light, why is that dot any different than a 2W flashlight outputting similarly diffuse light?

I can understand obvious concerns with respect to reflections if the dot is on a shiny surface. That's a no-brainer. I can also understand the related concern that if you're viewing a 2W dot on a surface that is 10 feet away, you should have goggles on for other reasons anyway - particularly the danger of accidentally moving that dot across something reflective in the process.

But if we're really talking about the danger of looking at the dot itself, and not these related dangers that have to do with beam reflections, why is the danger any greater than viewing any other ~2W source of diffuse light?
 

willpine

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I would have to vote yes on a danger. yes on wearing goggles or improve your distance away. First, I would think light behavior from a full spectrum scattered light traveling with different wavelengths behaves differently than laser light stimulated emissions, photons scattering quanta, if you will, some still aligned in monochromatic color with still narrow beams scattering. You ever see a bright light when burning? or when shinning on something black and really look into the dot? It can hurt. I dont know if I am correct for reasoning this way, i hope someone would politely correct me if so. but one thing for sure safer than sorry. You can get goggles with high vlt so you can see the beam and dot easily
 

aryntha

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Sharp is right, and so is millirad.

While the light is *diffuse* it is still *coherent*, yet uncollimated, Meaning that it is much more easily *recollimated* than "flashlight light".

You cannot really "decohere" laser light, you can only scatter it. The light coming directly out of a diode is just as diffuse of that coming out of a small LED. That same light pattern can be collimated far more easily than that of the LED, because the light coming out of the LD is truly coherent (though diffuse and uncollimated), whereas the light coming out of the LED, or flashlight, is diffuse, non-coherent, and uncollimated.

So I think you were confusing collimation and coherency a little: Once it hits a diffusing surface it is no longer *collimated* but IS still coherent.

The light, being coherent, is much easier to *recollimate*, including via the lens in your eye. Additionally, a nondiffused area (or less diffused area) could reflect a semi-collimated beam.

Adding to this is the fact that a 2W laser is 2W of light output, and a 2W flashlight is 2W of LED consumption; much of this (even in the case of LEDs) is lost through filters, phosphor layers, inefficient reflectors and of course heat. 2W of laser light is 2W of light. So you're talking a lot more power actually coming out of the aperture.
 

rhd

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Ok - I'm starting to come over to your side of the fence.

Having said that, we can still talk about LED emitters like the XM-L etc, where they're gobbling up 12W. I'm certain that, like an LD, we're getting at lease 2W of actual light production (if not, then LEDs are incredibly inefficient!)

My remaining conceptual hesitation is this -

2W focussed on a single dot, 10 feet away.

- By the time you're 10 feet away, you've got the equivalent of less than a mW beam even hitting your eye. Even if it IS still coherent light, that your eye can re-collimate, why would that be any worse than a 0.5 mW pointer beam hitting your eye directly?
 

DrSid

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Generally smaller the light source is, more dangerous it is, as it can be focused into smaller point on the retina. Laser dot is very small. Light from 2W LED flashlight is several times bigger, as plenty of the energy comes from the reflector. Still 2W LED flashlight is pretty painful to look at.
 

123splat

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What about the issue of back-scatter? You are not only getting the reflection of the dot (which is why you can see that there is a dot), but also the reflected light of the glow (halo) around the dot. The way we see things ig by our eyes capturing the reflected light from the object observed. The back-scatter is more widely spread and so deminishes more over distance (kinda' like gravity). Not So?
 

123splat

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So, what is the end result (Beyond speculation)? The math presented tends to soften the issue some, but not entirely. The common sense 'If it hurts, don't do it' seemd most logical, but then there is the retort that 'if it hurts, the damage is already done'.
If beam reflection is more a concern than back-scatter, not much (if any) mention if 'angle of reflection equals angle of incidence' . IE: it may not be your eyes hurting....as opposed to someone else's.
 
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Krogith

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So, what is the end result (Beyond speculation)? The math presented tends to soften the issue some, but not entirely. The common sense 'If it hurts, don't do it' seemd most logical, but then there is the retort that 'if it hurts, the damage is already done'.
If beam reflection is more a concern than back-scatter, not much (if any) mention if 'angle of reflection equals angle of incidence' . IE: it may not be your eyes hurting....
The Math points out even in the most perfect scenario Your at risk. And so in the Real World Application you would want goggle at all times when viewing the dot close to your self.
 

BShanahan14rulz

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12W LED means LED consumes 12W of electrical power.
1W Laser Diode means diode puts out 1W of optical power.

My 10W LED flashlight puts a spot on the wall about 6" around, while
my laser puts a spot on the wall 1/8" around.

My flashlight outputs Po Watts of optical power, most of which ends up in the spot.
My laser outputs Po=1W of optical power, most of which ends up in the spot.

The image size directly relates to the size of the image on your retina. Smaller the image, the smaller it is translated onto your retina. Now it becomes more about power density.
 




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