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US: YouTuber makes 200 watt car-mounted laser, aims into sky
The video was done by 29-year-old Sarasota, Florida resident Kevin Kohler, who uploads as TheBackyardScientist on YouTube (4,500,000 subscribers) and who is on Twitter as @ChemicalKevy.
In the video, he states "As normal, you should never shine a laser into the sky. But we've checked the flight radar and there's no airplanes in 100 miles that direction."
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There were never any planes in danger and I can prove it with science. First of all, we never turned the beam on more than 4 amps (50 watts) because we were having cooling issues and couldn't really monitor it up on the car.. (we tried 6 amps once but it was too bright.)
So assuming the beam stayed perfectly collimated, the beam would have an area of 430 square centimeters. If the laser was running at 25% power (4 amps/50 watts) that would be about 110mw/cm2 from the telescope. However, your pupil is only half this size fully open, so a pilot could theoretically receive a 50mw/cm2 exposure. A decent amount, but you have to remember even the smallest plane is traveling 150 miles per hour which would limit total exposure time to only 3ms (time it takes to cross a 9 inch beam). That puts it at the cusp of the maximum exposure limit, which is defined as "about 10% of the dose that has a 50% chance of creating damage under worst-case conditions." Here is the chart im referencing https://en.wikipedia.org/wiki/Laser_s...
So even under perfect conditions, this would still be 10x less than the exposure that carries a risk of damage. That's assuming perfect conditions, but in reality it was a hazy night, the laser would not be perfectly collimated, the atmosphere would distort the beam, the windows in the airplane would deflect some of the beam, unless the pilot was staring directly at the beam they would receive a smaller dose, I tried to keep the beam moving when up in the sky, and most planes are moving faster. all of this meaning the pilot would be exposed to a much lower amount of laser energy in the real world than under perfect conditions. even at full power this laser is still within the safety factor of the maximum exposure limit. Plus do you know the odds of hitting a plane with a laser when your [sic] trying not to? just pick a random star in the sky and wait for a plane to fly over it.
After 4 days, the video had 500,000 views and 4,300 comments.
A few of the comments addressed the "flight radar" such as: "I'm a controller. There are definitely more aircraft flying around than the public can see on any app." and "FlightAware and the like do not show all aircraft." [LaserPointerSafety does not know how true these statements are, or how relevant they are to the safety of the video's nighttime usage.]
Other comments referenced YouTuber Styropyro (Drake Anthony) who makes similar videos with homemade high-powered lasers; for example, a 100 watt handheld laser.
COMMENTARY AND ADDITIONAL SAFETY INFORMATION FROM LASERPOINTERSAFETY.COM
In an August 2016 interview in SQR magazine, Kevin Kohler is quoted as saying the following “In my videos I don’t say, ‘Don’t try this at home.' I’d like to see more experimenting and people trying new things in general.”
We believe there are times to say "Don't try this at home", and this is one of them.
To give just one example, Kohler "tests" supposed safety glasses by demonstrating that an IR camera cannot see the beam through one of the lenses. For such a powerful laser, the glasses should have been selected by contacting a reputable laser eyewear manufacturer and getting glasses specifically for that wavelength and power.
If you really want to test laser safety glasses, very carefully have the laser beam shine near the edge of the lens to see how long it might take to burn through. THIS is a realistic test of whether the glasses can protect eyes. (Though the test may compromise the glasses' safety, so it is best performed on glasses that won't be worn. Also, the angle of the laser to the glass surface is important if the glasses use thin-film or holographic techniques which do not protect as well against beams coming in at an angle. Dye lasers do not have this characteristic; a beam coming in at an angle will be attenuated as much or more than a beam coming straight onto the glass surface.)
Given LaserPointerSafety.com's focus on outdoor laser safety, we asked a laser safety expert to review the outdoor part of the video. This is the expert's analysis:
My first impression is that this is somewhat irresponsible, not so much with the experiment itself, but with the widespread dissemination of how this was done. If we get a few copycats or bad eggs who intentionally misuse this technology, then lasers will quickly be considered as weapons, and will become greatly restricted.
While one might argue that the invisible nature of this beam (at a suspected 1064 nanometer wavelength) makes it less of a distraction/dazzle/flashblindness issue, the one case I know of where a retinal injury was suspected also involved 1064 nm. If you can't see it, you can't avoid it.
I am not sure what make or model of laser, but from observation I can suggest a few things:
- The laser is probably a diode-pumped solid state (DPSS) as it is coupled into a rather small fiber (100 µ), and it would be challenging to get that much diode power into that size fiber.
- The laser is not visible to the human eye according to the video, but is somewhat visible to the camera, suggesting standard Nd:YAG at 1064 nm (infrared)
- The 200 watt power specification does not make clear whether this is average power (continuous wave) or peak power (pulsed). Guessing it might be continuous wave based on the comments about cooling challenges.
- The various burn tests are basically similar (in results) to the tests I did years ago with the Master Blaster (a 50 W continuous wave argon-ion laser)
The YouTuber's analysis presented is not particularly helpful. He touts a 9.25 inch Cassegrain telescope, so about 23.5 cm diameter. If the laser beam emits from a 100 micron fiber (0.01 cm diameter), this represents an expansion of 2350X. In other words, this reduces the divergence proportionally when collimated.
I don't think the direct beam from the fiber would 'fit' into the optical train (overfilling) so a collimator was probably used to reduce the divergence from the fiber somewhat. Bare 100 µ fiber may have a divergence of 10-30 degrees, so I am guessing that the collimator reduced the beam to about maybe 2-5 degrees (34-85 milliradians), and then the expansion. The answer I arrive at is about 36 microradians.
Since the telescope has adjustable focus, I suspect that it was used more in a focusing form, as you get more exciting results than when you simply have a collimated large beam. Certainly the "burn tests" are with a well-focused beam. Once the beam passes the focal point, then it will diverge at a commensurate angle.
The irradiance calculation premises an area of 430 cm², which is close enough, but assumes that the beam is fully filling the primary mirror. Since this is a variable focus system, the focal point can be anywhere from a few tens of feet to infinity, with the latter being the collimated condition. With the stated running power of 50 Watts, this would be an irradiance at the output of about 116 mW/cm². If I recall correctly, the Maximum Permissible Exposure (MPE) at 1064 nm is about 10 mW/cm², so this around 10 times over the MPE (e.g., 10x more hazardous).
The YouTuber's the analysis at this point starts to be questionable.
The suggestion that the "pupil is only half this size full open" is not meaningful. Yes, a smaller pupil will allow less total power or energy to enter the eyes, but the issue is the corneal irradiance (mW/cm²) of that entering light — not the total power transmitted through the pupil aperture. With a beam of a given power and divergence, a smaller pupil allows less light to enter. But if that beam is strong enough, then even 1/2 or 1/4 of the light will be enough to cause injury.
The next problems relate to claims about the beam exposure time to an aircraft and pilot.
First, the YouTuber asserts that time of exposure would be small due to the velocity of the aircraft. But the angle of incidence is a significant factor. If the aircraft is flying towards the beam location, the time of exposure would be very long. Essentially the laser is flying right into the beam.
Second, assuming a 9" beam diameter is only one case. Assuming some distance to the aircraft, most exposures will be longer. The larger beam diameter, the longer the time of exposure.
Third, the YouTuber has seemingly forgotten about helicopters. They can be at zero forward velocity.
[Also, as noted above, the app that Kohler used may not have data on all aircraft within the laser hazard distance.]
The only thing in favor in this outdoor "experiment" has to do with the low probability of striking an aircraft when projecting into the sky. Of course, this is only a probability matter. Things can change quickly when an aircraft suddenly appears in the field of view ("the plane came up from behind the hill, building, etc.") While I can calculate the odds of the laser beam and the aircraft intercepting, this is not a very good safety measure.
If the YouTuber wishes to provide more information about the specifics of the setup, I can do a more detailed analysis.
Finally, this is relatively minor but shows how this video has more "backyard" than "scientist": At around 11:00 Kohler says that the beam divergence is measured in "millimeters".
He is using the wrong technical term. Laser beam divergence is measured in milliradians, not millimeters. A milliradian is an angle; specifically 0.0573 degrees. (A radian is 57.3 degrees. Scientists use radians since they make some calculations easier.)