Amateur Astronomers Flash the Space Station with 1W Blue Laser

[GeorgeHelio]

Dollar per mW it is, but green is far brighter than blue, mW to mW.

This is confusing to me because the reason can be found in one of several corners.

1) Are we using input wattage into the diode for comparison? If so, then efficacy may be a big explanation for a green advantage over blue.

Speaking of this, I guessed that the Arctic blue laser might be in the range of about 150 lumens per watt. This value is important in calculating apparent magnitude of the light from space, of course, Is this value a fair guess?

2) I suspect that our eyes ability to see color is best modeled using photon counts rather than spectral energy levels. Blue photons require more energy to produce than do the other colors of the spectrum, ignoring violet. [E =hf.] This would give green -- red would have an even greater advantage here -- an advantage. But, I suspect, the lumens per watt efficacy rating would reflect this issue.

3) Is there something special about 445nm that reduces our reception to it that is not seen in the spectral sensitivity distributions for our color cones? This seems unlikely.

4) Color contrast? This can cause us to see one color better given certain background colors. Yet, given a black background of a dark sky, this shouldn't matter, though a green light might stand out nicer than a blue one if shone into a blue sky.

5) Scattering. Though I'm about to appear to contradict something I just posted in a prior post, a green beam may, perhaps, appear brighter due to the better receptivity to green light, not just the cones but the rods also. [I think I've read that the rods do contribute, though only slightly, to color determination even at photopic levels of brightness, and the rods like green best, as do the cones, but with separate peaks.] In other words, what few photons we gain by Rayleigh scattering of "blue" photons we more than make up for with the improved receptiveness of "green" cones. An analogy might be in comparing the weight disadvantage of a bigger racing engine to the h.p. advantage, both must be taken into consideration, else most racing engines would be huge.

That's the old way of doing it, with an AOM (AcoustoOptic Modulator). I've built a voice-over-laser transceiver that uses direct amplitude modulation of the current going to the laser diode. It's only 12mW but could easily be scaled up to a full 2W quiescent output level with funds thrown at it. You could easy send AFSK (audio freq. shift keying) data through it. I've also built from scratch a CW (morse) over laser transceiver as well. It's only at 2.5mW output but the transmitter side could control any diode based laser of any power.

That's cool! No surprise, someone around here has already been doing the things that would make a great follow-up ISS lighting event.

For a "pointer" without safety features, 5mW is the limit. If you add FDA required safety features there is no power limit. If you build it yourself, there is also no power limit, instead the legality falls on application and intent.

The Arctic blue laser is rated as a class IV laser. I duplicated the "Danger" decal used for these lasers and put it on the "gun". Wicked Lasers sent me three safety glasses with the laser, and these glasses allowed the wearer to look at a bright day-lit landscape and feel like they were on Mars. Impressive glasses. We didn't use them, to be attornistically incorrect, I fear, but we had three people watching for aircraft and Keith ("The Shooter") was very cautious to keep the beam well overhead of anyone. [In the video of the event, Keith is seen to be shooting down the laser near the end of the event, though everyone else is still shining their light and blocking it per Robert's directions. Keith told me that the ISS altitude was getting a bit too low for him to be assured that no one could accidentally look into the beam, so he shut it down early.]

I would like to learn more about any requirements for class IV, or greater, laser safety. Do you have a thread on this or other reference?

 

[Sigurthr]

This is confusing to me because the reason can be found in one of several corners.

1) Are we using input wattage into the diode for comparison? If so, then efficacy may be a big explanation for a green advantage over blue.

Speaking of this, I guessed that the Arctic blue laser might be in the range of about 150 lumens per watt. This value is important in calculating apparent magnitude of the light from space, of course, Is this value a fair guess?

2) I suspect that our eyes ability to see color is best modeled using photon counts rather than spectral energy levels. Blue photons require more energy to produce than do the other colors of the spectrum, ignoring violet. [E =hf.] This would give green -- red would have an even greater advantage here -- an advantage. But, I suspect, the lumens per watt efficacy rating would reflect this issue.

3) Is there something special about 445nm that reduces our reception to it that is not seen in the spectral sensitivity distributions for our color cones? This seems unlikely.

4) Color contrast? This can cause us to see one color better given certain background colors. Yet, given a black background of a dark sky, this shouldn't matter, though a green light might stand out nicer than a blue one if shone into a blue sky.

5) Scattering. Though I'm about to appear to contradict something I just posted in a prior post, a green beam may, perhaps, appear brighter due to the better receptivity to green light, not just the cones but the rods also. [I think I've read that the rods do contribute, though only slightly, to color determination even at photopic levels of brightness, and the rods like green best, as do the cones, but with separate peaks.] In other words, what few photons we gain by Rayleigh scattering of "blue" photons we more than make up for with the improved receptiveness of "green" cones. An analogy might be in comparing the weight disadvantage of a bigger racing engine to the h.p. advantage, both must be taken into consideration, else most racing engines would be huge.

I was using OUTPUT wattage for comparisson. It's just a matter of how expensive the very electrically efficient blue diode is compared to a very (electrically and photonically) inefficient a DPSS laser is. A blue diode that will ouput 2W when connected to a $1 driver is only $50. A green dpss module that will output 300mW is often more than $200 (in fact, that price would be a STEAL!). The blue diode directly converts electrical energy to blue light; typically 1mA @ 4.5V = 1mW light out. The green dpss laser converts the electrical energy to 808nm IR light, and is then fed to a Neodymium crystal which converts maybe 50% of it to 1064nm IR light. That 1064nm beam is then fed to a KTP crystal which converts maybe 50% (if lucky!) of the 1064nm to 532nm green light. So just from a photonics perspective you're losing 75% of the light generated. efficiency aside the process is very expensive to manufacture, where as a simple diode probably costs Nichia a penny.

I base my statement that mW to mW green is more visible than blue on a few factors:

1) rayleigh scattering; the more light scattered back towards the laser the less light that ends up at the target. With the laser being aimed at the astronauts' eyes they shouldn't see any beam anyway, and I doubt the beam could be seen when not aimed at their eyes due to how dispersed it would be that far out from the laser. Green scatters less so more photons hit the target.

2) color sensitivity; while blue light is easier to see in a scotopic scenario, the astronauts were just being blinded by the sun's glare a few moments before being able to see the earth, I doubt their eyes would have fully dark adapted to scotopic vision. The green would have a higher sensitivity in mesopic or photopic vision. There may have been enough time for them to experience the perkinje shift, but who knows for sure.

Somewhat related:
Intense blue light also tends to overload your rods and set back your dark adaptation, this is something often observed by those with high power blue lasers and LED lights. In this respect, when wanting to preserve the eye's dark adapted state, light above 640nm should be used as the rods and blue cones have no reaction to it and thus cannot be bleached by it. Though the very reason you would use it, also requires you to use an awful lot of it since out eyes aren't ever very sensitive to such long wavelengths.

3) beam characteristics between high power multimode blue and dpss green lasers:

DPSS lasers generally have a perfect TEM00 gaussian profile with excellent even spread of energy within the beam. Very low divergence angles are easily attained with this kind of laser. With a beam expander, typically 0.1mRad are achievable.

Multimode blue (>150mW) diodes have a fast axis and a slow axis to their profile and many "hot spots". Overall the distribution of energy is pretty uneven in the beam and without exceptional corrective optics the beam will have a pretty poor divergance for even the slow axis. The fast axis can have divergence angles greater than 6mRad when fully collimated.

Because of this the energy density at very large distances between equally powered blue and green lasers can vary a very large amount. Usually the tigher beam of a green laser combined with the increased sensitivity in all but truly scotopic environments makes it brighter than a much more powerful blue laser. We have a tool which helps estimate the comparison of wavelengths and power levels in terms of visibility, it is based on the CIE charts. I have found it to not be 100% accurate, at least not to my eyes, but it is quite hard to truly test. Plus, I have increased sensitivity in to the UV and IR than most so I may just be optically odd, haha.

That's cool! No surprise, someone around here has already been doing the things that would make a great follow-up ISS lighting event.

Thanks! I really enjoyed building it an the local HAM clubs absolutely love it. I've been doing presentations on it once a month for months now and I'm always asked to do it again somewhere new.

The Arctic blue laser is rated as a class IV laser. I duplicated the "Danger" decal used for these lasers and put it on the "gun". Wicked Lasers sent me three safety glasses with the laser, and these glasses allowed the wearer to look at a bright day-lit landscape and feel like they were on Mars. Impressive glasses. We didn't use them, to be attornistically incorrect, I fear, but we had three people watching for aircraft and Keith ("The Shooter") was very cautious to keep the beam well overhead of anyone. [In the video of the event, Keith is seen to be shooting down the laser near the end of the event, though everyone else is still shining their light and blocking it per Robert's directions. Keith told me that the ISS altitude was getting a bit too low for him to be assured that no one could accidentally look into the beam, so he shut it down early.]

I would like to learn more about any requirements for class IV, or greater, laser safety. Do you have a thread on this or other reference?

Right, class 4 is anything over 500mW. Basically anything over a Class 3a
which does not have multiple lockout and safety features built in, is illegal to SELL. It's legal to own or build though. I don't know all the requirements but it's usually:
1) aperture shutter
2) warning label
3) key lock
4) safety interlock pin
5) delayed start
6) indicator light

For any future use of high power lasers grab a pair of certified laser goggles for protection. The WL goggles are not actually the protection they claim to be, I have a pair and it barely qualifies for OD2. OD4 is usually required for Class4 lasers.

 

[GeorgeHelio]

I was using OUTPUT wattage for comparisson. It's just a matter of how expensive the very electrically efficient blue diode is compared to a very (electrically and photonically) inefficient a DPSS laser is. A blue diode that will ouput 2W when connected to a $1 driver is only $50. A green dpss module that will output 300mW is often more than $200 (in fact, that price would be a STEAL!). The blue diode directly converts electrical energy to blue light; typically 1mA @ 4.5V = 1mW light out. The green dpss laser converts the electrical energy to 808nm IR light, and is then fed to a Neodymium crystal which converts maybe 50% of it to 1064nm IR light. That 1064nm beam is then fed to a KTP crystal which converts maybe 50% (if lucky!) of the 1064nm to 532nm green light.

Wow, I didn't know green lasers required stages like this. Interesting.

So a blue laser might have an "overall themral efficency" of roughly 22%. This seems to make perfect sense because maximum efficency is 683 lumens per watt, so at 22%, the efficacy for a blue laser becomes about 150 lumens per watt. This is the value I used, but it was just a guess on my part. [I had seen one site claim an efficacy of 170 lumens per watt for a laser, though I don't recall the beam color.]

So just from a photonics perspective you're losing 75% of the light generated. efficiency aside the process is very expensive to manufacture, where as a simple diode probably costs Nichia a penny.

Yes, I see your point, though it could be worse assuming a loss in the original light producing stage.

But, as you say, these aren't issues if we use output wattage instead of input. Being a bit of novice with most of radiative terminology, I am a little more comfortable with input wattage and efficencies simply because I feel I can get something more objective than to have to rely on what others claim is output wattage. Perhaps I should be less skeptical, but lasers may be one area to remain skeptical of output claims, and the advertising I've seen strongly suggests such a view.

I base my statement that mW to mW green is more visible than blue on a few factors:

1) rayleigh scattering; the more light scattered back towards the laser the less light that ends up at the target.

Yes. William Strutt (Lord Rayleigh III) showed that it is an inverse 4th power rule. So the 4th power of the ratio of 532/445 produces double the scattering for blue light over green.

Yet, as they say, the devil is in the details. As particle density increases, the scattering, will eventually begin to decrease because the sub-wavelength size particles must be treated as something different, more like a crystal instead.

The best way, I think, to determine the atmospheric scattering difference is to compare the spectral irradiance of the Sun from space (Air mass of zero, or AM0) with an AM1 or AM1.5 (roughly 45 deg. above the horizon) level. Here is a plot of those differences.

It reveals that blue scattering is greater than green, but not near as significant as the inverse 4th rule would suggest.

With the laser being aimed at the astronauts' eyes they shouldn't see any beam anyway, and I doubt the beam could be seen when not aimed at their eyes due to how dispersed it would be that far out from the laser. Green scatters less so more photons hit the target.

Right, the best we could hope for is that they would see our tiny point-source of light (laser or search light) as significantly brighter than the background lighting, which was quite dark, thanks to Robert Lozano's observatory location.

2) color sensitivity; while blue light is easier to see in a scotopic scenario, the astronauts were just being blinded by the sun's glare a few moments before being able to see the earth, I doubt their eyes would have fully dark adapted to scotopic vision.

Actually, green is the winning color in scoptic vision. Many emergency vehicles are now painted a tint of green for this reason, as I understand.

Nevertheless, you make an interesting point because their time aboard the ISS from seeing a bright Earth to our little light was just a few minutes, so this is just a little amount of time for dark adaptation for the rods to do their thing at full power. Yet, the first two minutes for adaptation are fairly effective, I think. This is one reason I liked to compare our light with known celestial objects magnitudes. Vega was the first goal at 0 magnitude. If an astronaut looked out the window and could see Vega easily, even with only photopic vision that was typical given all their internal lighting, then I felt Don Pettit would be able to see us equally as well if we could produce a 0 mag. level of brightness. Fortunately, the search lights put us above -6 in brightness, not counting the laser, which came in at -3.9 mag. assuming the Arctic blue laser produced 800mW of light.

Intense blue light also tends to overload your rods and set back your dark adaptation, this is something often observed by those with high power blue lasers and LED lights. In this respect, when wanting to preserve the eye's dark adapted state, light above 640nm should be used as the rods and blue cones have no reaction to it and thus cannot be bleached by it.

Right, amateur astronomers require only red light for illumination. I dread my headlights coming on accidentally at any of these events. I haven't quite figured out how to fix this problem on my newer vehicle (F150), so I park a fair distance away from the viewing area.

3) beam characteristics between high power multimode blue and dpss green lasers:

DPSS lasers generally have a perfect TEM00 gaussian profile with excellent even spread of energy within the beam. Very low divergence angles are easily attained with this kind of laser. With a beam expander, typically 0.1mRad are achievable.

Wow, that's incredible. Such a dispersion would bump the magnitude level for the ISS observers at 350km altitude to better than -10 (equal to the total light of a quarter moon)! But, this narrow beam becomes much more difficult to hit an astronauts eye. Today's high tech tracking programs could be used to overcome this targeting problem, no doubt.

Because of this the energy density at very large distances between equally powered blue and green lasers can vary a very large amount. Usually the tigher beam of a green laser combined with the increased sensitivity in all but truly scotopic environments makes it brighter than a much more powerful blue laser. We have a tool which helps estimate the comparison of wavelengths and power levels in terms of visibility, it is based on the CIE charts. I have found it to not be 100% accurate, at least not to my eyes, but it is quite hard to truly test. Plus, I have increased sensitivity in to the UV and IR than most so I may just be optically odd, haha.

I too am a little skeptical of the CIE charts, especially when celestial lighting is involved. [One reputable site claims the Sun is .... peachy pink in color. Ug. I think they are using the 6500K standard, which would be an erroneous comparison, but one to consider for monitor coloring for the Sun. I still disagree with their result.]

Right, class 4 is anything over 500mW. Basically anything over a Class 3a
which does not have multiple lockout and safety features built in, is illegal to SELL. It's legal to own or build though. I don't know all the requirements but it's usually:
1) aperture shutter
2) warning label
3) key lock
4) safety interlock pin
5) delayed start
6) indicator light

Thanks. That makes sense, but I have no key lock for the Arctic.

For any future use of high power lasers grab a pair of certified laser goggles for protection. The WL goggles are not actually the protection they claim to be, I have a pair and it barely qualifies for OD2. OD4 is usually required for Class4 lasers.

The glasses for the Arctic do a great job of attenuation of the beam and spot, but there is no certification stated anywhere that I see.

 

[Sigurthr]

Yes. William Strutt (Lord Rayleigh III) showed that it is an inverse 4th power rule. So the 4th power of the ratio of 532/445 produces double the scattering for blue light over green.

Yet, as they say, the devil is in the details. As particle density increases, the scattering, will eventually begin to decrease because the sub-wavelength size particles must be treated as something different, more like a crystal instead.

The best way, I think, to determine the atmospheric scattering difference is to compare the spectral irradiance of the Sun from space (Air mass of zero, or AM0) with an AM1 or AM1.5 (roughly 45 deg. above the horizon) level. Here is a plot of those differences.

It reveals that blue scattering is greater than green, but not near as significant as the inverse 4th rule would suggest.

Thank you for that, that is actually very interesting! I did not realize there was a derivation from the inverse 4th rule in play. I love when intelligent folks like you come to the forum, haha, it's great to learn something new!

Right, amateur astronomers require only red light for illumination. I dread my headlights coming on accidentally at any of these events. I haven't quite figured out how to fix this problem on my newer vehicle (F150), so I park a fair distance away from the viewing area.

I hear ya. I don't do a lot of stargazing (though I'd sure like to, the night sky is bright and I can almost see 160deg of sky from my location, there is some light pollution from a town 20miles north, and another 15miles south, but treeline helps to hide it, we get a TON of overcast nights though so a starry night is rare because of that) but I do rely on my dark adapted eyesight a lot when I'm out at night. My wife has mild night blindness and lights the place up like times square sometimes, heh. I tell her I might as well be night blind since all the flood lights knock out my adaptation. I've wondered why we don't use red headlights on cars, to be honest. Sometimes when I'm driving at night the reflection from the pavement for the first few meters out is enough to make my pupils contract enough to give me headaches. Not to mention it sure makes it a lot harder to look for deer (a major issue here) when your adaptation gets knocked out everytime an oncoming car goes by.

I have a 200mW waterproof focusable 658nm laser with unlimited duty cycle at full output that I use for dark adapted lighting. It works fantastic for distant throw lighting and preserves my adaptation perfectly. It's just not great without a diffuser for anything closer than 30ft or so. I get about 8hrs run time with it on a single charge, and if I ever need to, I can start a survival fire with it if I bring some flashpaper (which I keep sealed up in the flashlight holster with the laser).

It should be noted by the way, that with enough optics a high power blue laser's output can be fast-axis-corrected to give an immense improvement in divergence, however losses will be present even in AR coated high end optics, and it would probably cost you a pretty penny. Even then, the beam specs would be poor since it is still multimode at heart. I just didn't want to give the wrong impression, it CAN be done. We have some members who are experimenting with fast axis correction for the multimode 635nm high power red diodes, as they have even worse beam characteristics then the high power 445s. Oh, and nitpicking here, but it's also interesting to know; these "445" diodes can actually vary from 435nm to 460nm. Currently 458nm is the highest found, but it is quite rare. RHD here on the forums is binning them with a spectrometer. Low power blues that are single mode TEM00 (clean gaussian round beam) are available at 450nm <150mW for nearly the same price, too.

You're right also about scotopic green sensitivity over blue, I was thinking of circadian vision for some reason, heh, gotta love those neural hiccups! My mistake. I had a mental image of the responce chart, and had the mental equivalent of my thumb over the word.

 

[GeorgeHelio]

Oh, and nitpicking here, but it's also interesting to know; these "445" diodes can actually vary from 435nm to 460nm. Currently 458nm is the highest found, but it is quite rare.

That might explain why my so called 445nm wavelength beam appears so blue. It is my limited understanding that 445nm is on the violet side of the blue-violet line. A 455nm beam would likely appear blue. [There is not definitive color chart that nails it for me, which may be due to the assumption that we are all slightly different in our color determinations. The Wiki "visible spectrum" section seems to be about the best I've found, though there may be some significant papers on this that would be more accurate.]

You're right also about scotopic green sensitivity over blue, I was thinking of circadian vision for some reason, heh, gotta love those neural hiccups!

For dark adaptation, it is interesting to note that the descriptions of color change is often described as a shift toward the blue, meaning a shift toward the blue end of the spectrum, though the peak is in the green. This same story happens to be, IMO, a principal reason that most people think the Sun is a yellow star, though it most definetly is not (ignoring atmospheric effects). In the 1800's, stellar spectrums were recognized as a huge benefit to astronomy -- they are even more valuable today -- so much so that star colors became based on their spectra relative to a reference star, Vega, which is considered to be white, and is hotter than the Sun. Therefore, like the favoring of the blue-end of the spectrum during dark adaptation, the cooler stars like the Sun were color shifted toward the yellow regions (red for the really cool stars).

 

[TheReddish]

royal blue is what my 800mw looks like to me, the higher power ones at a range of 1w to 1.6w look like a very bright blue with a dark blue glow, but I think its my eyes being overwhelmed that make the beam look a bright white blue. I would very much like to make a spectrometer but I have projects on top of projects to do I think my diodes vary quite a bit in the way they look. going to get one of the newer diodes in a few days been having a hankering for a new build and I have no m140s yet. and +1 for the influx of knowledge I have revived from you. thanks.

 

[Ghostchrome]

Article posted March 26, 2012
by Jessica Nimon
International Space Station Program Science Office
NASA's Johnson Space Center
Beaming Success For Station Fans - Space News - redOrbit

Astronaut Don Pettit takes photographs of the Earth as part of the Crew Earth Observations investigation from aboard the International Space Station. (NASA)

Woah, check out the picture in the article, you can really see that thing! Thanks for sharing.

 

[GeorgeHelio]

That's a great shot of Don in the Cupola!

Here is a shot from Don showing some of the glare he talks about:

 

[Encap]

I am surprised that Wicked Lasers has not used this to shamelessly promote the S3 Arctic.

I would have guessed they would put something up on their web site hailing that the Spyder 3 Arctic is so powerful it was seen from the International Space Station 310 miles away while orbiting the earth.

So far they have not done anything like that---maybe they are changing their marketing tactics--go figure.

 

[jcranmer]

What an interesting thread this has turned out to be!

Welcome to the forum George.

 

[Keith Little]

It was the 1 wt "Arctic Blue" from Wicked Lasers.
I'm the person that "shot the station"
I'm one of the organizers of the project, and the Marketing Director for the San Antonio Astronomical Association

 

[Keith Little]

Hi George,
I just joined today. Everyone seems real interested in our accomplishment.
I didn't know that you joined this form. I just introduced myself as the "Shooter"

 

[tsteele93]

Welcome aboard Keith! Really neat accomplishment you guys did there!

 

[Keith Little]

Thanks for the welcome to the group. We really had no idea that this would get as big as it has become. We are getting traffic from around the world. We just wanted to see if we could do it. Having the contact on the Station made it worth a try.

It took about three weeks of planning and a little logistics challenge to make it happen. The real "ace in the hole", were the unbelievable search lights that we used. The video can be seen on many sites now by goggling "flash the station"

Details of the "Flash" project can be heard in the "podcast" at 365 Days of Astronomy. Search Flash the Space station.

Visit our club web page at San Antonio Astronomical Association: iS.T.A.R. | Welcome to iS.T.A.R.

I'll be back here, it looks like a good forum

 

[lasersbee]

Nice thread (and forum)!!

I was the number cruncher, whether they wanted one or not. It isn't that hard to calculate how bright an object will appear from space, once you see how to compare one standard (e.g. Sun) to whatever light we chose to use. [inverse square law and lumens per watt]

George

Hi George,
I just joined today. Everyone seems real interested in our accomplishment.
I didn't know that you joined this form. I just introduced myself as the "Shooter"

to the Forum guys....
Don't forget to read the FAQs...the Stickies
and the Forum Rules..

If you plan on buying anything on the Forum....
PLEASE read this first...

6 Steps To Prevent You From Getting Scammed

and PLEASE don't forget to read to this....

39 Reasons NOT to Buy From Wicked Lasers

If you do get a Laser or already have one be sure
to get appropriate Laser Safety Goggles/Glasses...


Enjoy your stay...


Jerry

 

[GeorgeHelio]

Hi George,
I just joined today. Everyone seems real interested in our accomplishment.
I didn't know that you joined this form. I just introduced myself as the "Shooter"

Hi Keith, I mean "Shooter"!

I wish I could have been up the hill with you while you shot the station. The gun that Paul and Fred (from my work) had made, including a unique stand, needed you to have a little practice shooting since it had an off-axis pivot point. Unfortunately, you really had no time at all to get comfortable with this, so I am even more impressed that you seemed to be on target with the ISS almost contstantly. I think I recall seeing you hold on to, and move, the mounting brace rather than hold the wood stock. That was the wiser choice and I had wanted to suggest that grip, but ran down to help Ron get set-up instead. [Ron told me one of the search lights was brand new and would be brighter. Guess which one I got.]

It would be cool if I could go over the calculations with Don or another NASA engineer because I am curious if those equations, though straight forward, were on the mark. It seems that my guesses on some variables were a little too optimistic, but the image seems to show our light as quite bright. Maybe I was fairly close after all.

George