What power level is needed to clearly see the beam without fog at night?

[LightBeamConcentrate]

I've watched videos on youtube, and I'm talking LOTS of videos to try and get an idea of how powerful the lasers need to be before you can actually see the beam clearly, and not just see the dot when it hits something.

But.........Many of the videos are shot with A. a crappy video camera or cell phone camera, and B. There is often some fog or smoke.

Obviously fog, either natural and outside, or artificially made with a fogger bought off Ebay for $40, will dramatically increase the visibility of the laser's beam.

I've now ordered the Amazon.com 3 pack of over spec lasers in 405nm, 532nm, and 650nm, and unfortunately before I was aware of those 3, I ordered another inexpensive 100mW laser(650nm red) from Olike.

I have not received any of them yet, and since I ordered the 100mW from Olike first, 3 days ago from China, I will probably receive it in 4-6 months!

But right now, the only laser I have is a cheap 1mW red key chain laser. However, when it was dark, and foggy outside, I could go out and shine it across the yard, and clearly see the beam for 150 feet or so! But only if I was looking straight down the beam. If I was looking at it from an angle, then I couldnt see it. But the fact that a little 1mW can be seen in foggy darkness proves how much fog will skew and exaggerate the beam.

What I want is a laser that can be seen clearly at dark, without any fog or smoke. I've now read enough, and watched info-videos from some guy "styropro" on youtube, and both there and here I've learned that the 532nm green laser frequency is the color that the eye is most sensitive to so you can see it easier than the other colors. But I dont know which frequencies are #2, #3 and #4 as far as human eye sensitivity goes.

I've heard that 532nm is 4 times a bright to the human eye as 650nm, but dont know if thats accurate(?)

So it's hard to tell what power level is actually needed for easy viewing of the beam in each color/frequency range. Some of these unclear videos suggest that maybe 50mW is all you need in 532nm. But other videos seem to show that even 150mW in 532nm might not be enough!

So how much power do you really need to see the beams clearly on a reasonably dark night, in the following color frequencies?

405nm:
445nm:
532nm:
635nm:
650nm:

Thanks, and sorry if this is a really stupid, basic question, it's just that the accurate answers seem hard to come by! Plus they fluctuate.:thinking:

 

[lman]

It depends
On many things with dust in the air you can see a 5mw lasers beam but with no dust you can't see a 1w green lasers beam so there is no real answer for this question.

 

[Wolfman29]

Another useful comment from lman.

Again, lman is not correct. Even if you have very clean air (i.e. just nitrogen, oxygen, some helium and CO2) in the air, you can still see the beam. While some of the results of a beam's visibility is due to dust in the air, those are clearly different from the main cause of visibility called Rayleigh Scattering. Rayleigh Scattering is the scattering of light off of particles much smaller than the wavelength of the light - that is, single molecules of various gases like nitrogen, oxygen, etc.

This can occur wherever there is light. While yes, it is pretty easy to see a beam when it's really dusty, obviously it's not a beam - it's just the same thing you get with a fairly bright flashlight - just the floaties, and not a fully filled in beam.

A fully filled in beam occurs when you are receiving a decent amount of power (hard to calculate) to your eyes from the Rayleigh Scattering.

The interesting thing about Rayleigh Scattering is that the amount of scattering you get is proportional to the square of the frequency (inverse of wavelength) of light. So if you have a laser that is half the wavelength, you will have four times as much Rayleigh Scattering.

It is for this reason that 445nm lasers often seem to be only slightly darker in the sky than a green laser. While our eyes are much more sensitive to green light (peaking at around 555nm), the blue laser will have significantly more Rayleigh Scattering.

All the science done with now, here are some estimates (from experience?):

405nm: Probably around 300mW. It's pretty hard to see, and it varies A LOT for people because it is just on the edge of human vision. I think it was Tom Steele who said he could barely see it at all? However I see 405nm as a bright violet.

445nm: Probably around 100mW for a clear beam in a dark sky. I've never had a 445nm of that low power (weakest was around 400mW I think) but that still had a nice beam.

532nm: Really, you could probably go with 30mW or so. This is because 532nm lasers typically have a smaller beam diameter, so they are more concentrated and easier to see. Also, they are closer to the peak of eye sensitivity.

635nm: These require about 150mW or so from a multimode for decent beam visibility.

660nm: Pretty hard in general. I would put it at around 300mW on a dark night. These are approaching the edge of human vision. Not quite there, but the Rayleigh scattering here is also really limited.

Note that these numbers are all really approximated, and are for looking straight down (parallel to) the beam. For beam-viewing perpendicular to the beam's path, it requires a lot more power. This is because the majority of light from Rayleigh scattering is in either the direction of the light's path or anti-parallel to it.

 

[ryansoh3]

For comparing brightness as the human eyes see it, you can use rhd's handy tool here:

Beam: (482nm 1mw) vs. (501nm 1mw)

 

[LightBeamConcentrate]

Another useful comment from lman.

Again, lman is not correct. Even if you have very clean air (i.e. just nitrogen, oxygen, some helium and CO2) in the air, you can still see the beam. While some of the results of a beam's visibility is due to dust in the air, those are clearly different from the main cause of visibility called Rayleigh Scattering. Rayleigh Scattering is the scattering of light off of particles much smaller than the wavelength of the light - that is, single molecules of various gases like nitrogen, oxygen, etc.

This can occur wherever there is light. While yes, it is pretty easy to see a beam when it's really dusty, obviously it's not a beam - it's just the same thing you get with a fairly bright flashlight - just the floaties, and not a fully filled in beam.

A fully filled in beam occurs when you are receiving a decent amount of power (hard to calculate) to your eyes from the Rayleigh Scattering.

The interesting thing about Rayleigh Scattering is that the amount of scattering you get is proportional to the square of the frequency (inverse of wavelength) of light. So if you have a laser that is half the wavelength, you will have four times as much Rayleigh Scattering.

It is for this reason that 445nm lasers often seem to be only slightly darker in the sky than a green laser. While our eyes are much more sensitive to green light (peaking at around 555nm), the blue laser will have significantly more Rayleigh Scattering.

All the science done with now, here are some estimates (from experience?):

405nm: Probably around 300mW. It's pretty hard to see, and it varies A LOT for people because it is just on the edge of human vision. I think it was Tom Steele who said he could barely see it at all? However I see 405nm as a bright violet.

445nm: Probably around 100mW for a clear beam in a dark sky. I've never had a 445nm of that low power (weakest was around 400mW I think) but that still had a nice beam.

532nm: Really, you could probably go with 30mW or so. This is because 532nm lasers typically have a smaller beam diameter, so they are more concentrated and easier to see. Also, they are closer to the peak of eye sensitivity.

635nm: These require about 150mW or so from a multimode for decent beam visibility.

660nm: Pretty hard in general. I would put it at around 300mW on a dark night. These are approaching the edge of human vision. Not quite there, but the Rayleigh scattering here is also really limited.

Note that these numbers are all really approximated, and are for looking straight down (parallel to) the beam. For beam-viewing perpendicular to the beam's path, it requires a lot more power. This is because the majority of light from Rayleigh scattering is in either the direction of the light's path or anti-parallel to it.

Ok, so as you said, the diameter of the beam plays a role in its ability to be seen, and the thinner, tighter, more concentrated the beam is the easier it is too see. So I guess that means that power isnt the only criteria for a given frequency rating, as a cheaper 532nm laser with a more scattered and wider beam would be more difficult to see, even compared to another 532nm laser with a tighter beam?

The explanation you gave about why its much easier to see a laser beam from behind it as opposed to perpendicular to it makes sense. I didnt realize the technical reason why laser beams are visible until now, and it makes sense that the beam is easier to see along its plane/parallel because its basically lighting up particles. The same is basically true when you point flashlight at something. Its easier to see when you're facing the object thats being lit up in the same direction the light is coming from, than it is to see the object from another angle perpendicular to the object.

But whats most important: What mW power rating were the phasers in Star Trek, because you could always see the entire beam when they were fired, even from another angle? Even the red Phasers as well as the blue and green ray guns used by the enemy aliens were very visible, and not just when it was foggy on the alien planet, or smoky in the damaged space ship.

 

[Wolfman29]

Glad to hear it makes sense. Regarding Star Trek phasers... those were Special FX Sorry to ruin your innocence :cryyy:

But for lasers like that to be realized now, in broad daylight, we are probably talking 8W or so of 445nm, 5W of green, 15W of red. Just REALLY high numbers

 

[LightBeamConcentrate]

Glad to hear it makes sense. Regarding Star Trek phasers... those were Special FX Sorry to ruin your innocence :cryyy:

But for lasers like that to be realized now, in broad daylight, we are probably talking 8W or so of 445nm, 5W of green, 15W of red. Just REALLY high numbers

Plus I'd imagine that even at 5-15W, if you shot it at someone, they wouldnt just dematerialize when hit.:crackup:

That would be convenient though, you could get rid of your garbage just by shooting it with a laser(or your ex-spouse), and it would just disappear into nothingness!

 

[Dam]

I can see the 5mm diameter beam of 600mW 532nm laser in the room without smoke during sunny afternoon. Once, I clearly saw the beam in broad sunlight outdoors, when my neighbour made some smoke from hay.

Another experiment I made with my friend - he clearly saw the beam from my bigger laser over the sky from 1.5km distance whenether it was foggy or clear at night.

I can't see my 80mW 532nm pen's beam in that conditions at all despite it's 1,5mm diameter, if this helps.

 

[deathstarlaser]

i have a simular question,,,how strong of a 405nm laser would have the same brightness as a 5mw green...the dot not the beam?

 

[scopeguy20]

Answer will vary fro person to person, for me about 450 mW of 405nm will be ~ equal to about 5 mW of 532nm -GH

 

[Atomicrox]

Just a small correction, Rayleigh scattering is inversely proportional to the 4th power of the wavelength, not the square.

The problem with those numbers is that they're very subjective and also vary from place to place.

I used to live in a city where 200mW 650nm was *very hard* to see even on a completely dark night. Here it's still very weak but I can always see it at night from a very small angle. I don't know if it's the pollution or something else but it sure makes a lot of difference.

I can see 200mW 405nm as well. Not the brightest thing around but not that hard to see either. Way brighter than the 650 but way weaker than my 10mW 532 (which I suspect is overspec).

Edit: 415mW 635nm is almost as visible as the 10mW 532nm for me.

 

[tsteele93]

Light pollution might make a big difference in perceived brightness. Good point!