Is an unfocused 4W laser do dangerous at 5 meters?

[Alaskan]

At some power level a flashlight should be able to produce a power density strong enough to do the same damage of 5mw laser could do, if focused upon long enough. It cannot focus down to as fine a spot, but no doubt a power density equal to that can be achieved, but at how much power when coming from a uniform 45 degree spread, for example.

 

[bluestars]

The eye is designed to focus onto the retina. Since the parallel nature of a laser beam represents an object a very, very long distance away, the resulting spot on the retina is potentially very, very small. This concentrates the power and therefore greatly increases the danger as compared with an LED.

This definitely seems to be true for a laser beam, but the output of a multimode laser diode without a focusing lens is not a beam - it is a cone with a high divergence, just like a power LED. If the rays were already parallel, we wouldn't need a collimating optical assembly to form a laser beam.

If you add a lens to the situation, I strongly agree that lasers represent a much more significant eye hazard than high power LEDs. However, without a lens, I still think my analysis stands.

I don't want to claim that 4mW is above or below MPE for any particular wavelength and exposure time. I am not a laser safety officer and I'm really engaging in this discussion for my own edification. I believe the OP's question has been well answered by the members who say "don't do that".

Interestingly (and a bit off topic), since all optical processes must be reversible, it's actually impossible to focus light from any real light source to a point. Instead, the best you can do is an (increasingly small) image of the light source. XKCD explains well:
https://what-if.xkcd.com/145/

 

[diachi]

This definitely seems to be true for a laser beam, but the output of a multimode laser diode without a focusing lens is not a beam - it is a cone with a high divergence, just like a power LED. If the rays were already parallel, we wouldn't need a collimating optical assembly to form a laser beam.

If you add a lens to the situation, I strongly agree that lasers represent a much more significant eye hazard than high power LEDs. However, without a lens, I still think my analysis stands.

I don't want to claim that 4mW is above or below MPE for any particular wavelength and exposure time. I am not a laser safety officer and I'm really engaging in this discussion for my own edification. I believe the OP's question has been well answered by the members who say "don't do that".

Except, even unfocused laser light is still coherent and will still be focused to a point at the back of your eye. Read up on "coherent light". You are misunderstanding the meaning of parallel in this context. Your eye adds the lens to the situation.

 

[bluestars]

Coherent light just means that the light waves are in-phase and have the same frequency:
https://en.wikipedia.org/wiki/Coherence_(physics)

The laser diode rays are definitely not parallel (or even nearly so) or the size of the spot on my wall would not be ~1 inch at a few inches and 42 inches at 3 feet!

If I look at an active laser diode from 10m away with no optics near the face of the diode, my eye will not be able to somehow focus all the light that didn't hit my pupil down to a single point. However, I have a hypothesis for the relevant difference between power LEDs and laser diodes here: the light that does hit my pupil will be relatively parallel, because the ratio between the size of the diode's emitting area (on the order of 100µm²) and the distance to me is quite large, so it will be focused to a pretty small region. An LED has an emitter size of something more like 10mm² (2 orders of magnitude larger), which means that the light rays will be "less" parallel. I'm not sure if that makes any sense from an optical point of view, and I don't have any drawing tools at work so I can't quite work the problem. If anyone could chime in to help me understand the relevant difference between uncollimated laser light and LED emitters, I'd much appreciate it!

 

[Cyparagon]

Interestingly (and a bit off topic), since all optical processes must be reversible, it's actually impossible to focus light from any real light source to a point. Instead, the best you can do is an (increasingly small) image of the light source.

That's not off topic at all. It is another way of stating the dangers of laser diodes. The emitting area ("image") of your typical high power LED is several mm or even several cm. By contrast, a typical laser diode has an emitting area about 2 µm for single mode, and 15-150µm for multimode. In a way, you answered your own question.

Even if we could somehow get an emitting area approaching size zero, the focus would still be limited by diffraction, and is roughly equivalent to the wavelength of light. ie 405nm light cannot possibly focus onto a smaller diameter than 405nm.

The laser diode rays are definitely not parallel

Except they are, and as you alluded to it's because of this: the rays not entering your eye can be ignored... because they're not entering your eye. The EFFECTIVE angle is from one side of the pupil to the other. This is still very near parallel.

 

[bluestars]

Even if we could somehow get an emitting area approaching size zero, the focus would still be limited by diffraction, and is roughly equivalent to the wavelength of light. ie 405nm light cannot possibly focus onto a smaller diameter than 405nm.

Oh, that's interesting! Can you recommend any good sources to learn more about optics like this? Undergrad was a while away.

You're wrong. And I'll tell you why. It's because the rays not entering your eye can be ignored... because they're not entering your eye. The EFFECTIVE angle is from one side of the pupil to the other. This is still very near parallel.

That makes sense, but why is this different for an LED? The rays that manage to hit my eye should still be pretty parallel, right?

 

[diachi]

That makes sense, but why is this different for an LED? The rays that manage to hit my eye should still be pretty parallel, right?

Light from an LED is going all over the place, essentially traveling in different directions, it's an unorganized jumble with a wide range of wavelengths (Different wavelengths will focus slightly differently due to varying indices of refraction at different wavelengths).

Light from a laser is coherent, all traveling in the same direction, organized and for all intents and purposes "parallel". All of the energy is also contained in one narrow wavelength "band".

This may be useful reading:

https://en.wikipedia.org/wiki/Diffraction-limited_system - The Wiki article talks briefly about lasers, but covers more than just lasers.

 

[Cyparagon]

but why is this different for an LED? The rays that manage to hit my eye should still be pretty parallel, right?

The emitter size dictates the minimum focus size. You cannot focus the beam from a 5mm LED source any smaller than 5mm, no matter your optics. Google ray diagrams to understand why. The ray coming from one edge of the LED will be focused to a different spot than the ray coming from the other edge.

 

[Alaskan]

Cyparagon,

The thread is now off of what I believe is the original heart of the question, at least, just my interest in this subject; how much power from an incoherent light source such as a high power flashlight LED is needed to equal the power density an unfocused (edit: except by the eye) laser diode can produce within the eye at 5mw of power, using the originally stated parameters, less the 1000 lumen stated limit? Assume a 3000 um x 3000 um LED emitter such as the SST90 has.

http://www.luminus.com/products/Luminus_SST90C_Datasheet.pdf

Is the power output of the 2000+ lumen flashlight SST90 LED at a 5 meter distance below the power density produced in the eye from the output from an unfocused 5mw laser diode at that same distance with a small 100x10um emitter, for example? I suppose I could use a smaller emitter size for a 5mw laser diode, certainly when single mode, but for example, I think it is close enough. Although when referencing this LED, the angles are different than a laser diodes radiation angle, so all things are not equal and require further number crunching than the simple example in the question given earlier. Also, I will have to reference a specific laser diodes characteristics if comparing with a specific flashlight LED, I think.

I'm also curious - what makes bright flashlights safe but (unfocused) laser diodes dangerous? Assume a 1000 lumen flashlight with a 15 degree spot, 75% of power in the spot...

A rough mental comparison of the emitter sizes and power levels alone if the radiation angles are similar to one another is an easy answer, but I will crunch some numbers for a specific 5mw laser diode for both fast and slow axis to be sure and post later.

 

[bluestars]

Sam's laser FAQ has a good page discussing laser safety, although it almost entirely deals with collimated beams. It does say this:
"A collimated beam is the most dangerous since it can be focused to a microscopic spot on the retina. A highly divergent beam - even a high power one - is much less of a hazard unless something is very close to the source."
Sam's Laser FAQ - Laser Safety

Staring at an active laser diode certainly seems like a bad idea.

The emitter size dictates the minimum focus size. You cannot focus the beam from a 5mm LED source any smaller than 5mm, no matter your optics. Google ray diagrams to understand why. The ray coming from one edge of the LED will be focused to a different spot than the ray coming from the other edge.

I must be misunderstanding what you mean by "focus the beam". The moon is about 2000 miles in diameter, but my eye focuses light from it down to between 1mm² and 1000mm² on my retina. Of course, that's not _all_ the light from the moon (since most of it didn't hit my pupil) but I am certainly able to focus what does hit my eye to something smaller than 2000 miles in diameter!

 

[Cyparagon]

ALL the light being the difference here. No one makes a flashlight or laser and throws 99% of the light away. If you take a minuscule portion of the light, you can get better results. The final intensity is still proportional to emitter size, and the intensities involved in lasers and moonlight aren't really in the same ballpark, are they?

 

[Drivinfast247]

If the FDA calls 5mW safe for direct exposure, and this is equivilent to 4mW, wouldn't that mean the FDA would call this situation safe? :thinking:

I'm also curious - what makes bright flashlights safe but (unfocused) laser diodes dangerous? Assume a 1000 lumen flashlight with a 15 degree spot, 75% of power in the spot, and 250lm = 1W output power.

At a distance of 1m, the light would have spread to a circle of radius 100cm * tan(7.5degrees) = ~13cm. (75% of 1000lm) / (250lm/W) = 3W light power. 3000mW / (pi*(13cm)^2) = 5.65mW/cm²

That's about half as much as the calculation for the OP's situation, but still in the same order of magnitude. You can easily buy a 3800lm flashlight for around $50. How is this not dangerous?

Admittedly, the power density of a collimated and focused laser, even something with only a few hundred mW, is much much higher than either situation given in this thread, which is where the severe danger of lasers can be found.

Edit: Just to add, empirically and anecdotally, I have pointed my 1W 445nm laser at the wall without its lens (so unfocused). From a meter or two away from a white wall, the beam is not particularly painful to view. It certainly isn't as bright as the reflected spot from my 2500lm triple Nichia 219C flashlight, which leaves a faint afterimage when I look away. Leaving aside blue light effects, which is more irresponsible with my vision - looking into the beam of a ~3000lm flashlight or looking into the (unfocused) output of a laser?

Where is this $50 3800lm flashlight?

 

[bluestars]

Where is this $50 3800lm flashlight?

The Convoy L6:
https://www.fasttech.com/products/1653/10002364/5184101