Tuesday, December 15, 2015

Are Christmas Laser Lights Really a Danger for Airplanes? Probably Not.

Are those holiday laser lights like the Star Shower really a danger for airplanes and pilots?

Probably not.

One pilot does not think so and posts
"We have had a couple of local private pilots also report on these lasers, and one issue with them is that they are LOW intensity lasers, as opposed to the HIGH intensity ones that have been blinding the pilots. These will still be reported by some of the pilots, especially civilian, but are not as dangerous, and should not cause any major problems with commercial aircraft."
And another pilot reports,
It is not a problem at these distances. These use a 5mW laser with a divergence of roughly 8.13mrad or more would give you a spot size of a bit larger than 200ft. and a luminosity that would be lost among lights of even a small town....
These are not high powered lasers in these displays and while maybe annoying, equating them to the higher powered lasers with wattage that could cause damage is ridiculous These are little more than overpowered flashlights with cheap diodes, and reflected off mirrors. These are not going to harm anyone.
As for me, I first saw a Christmas laser light at a home show last summer.  So last November I decided to check them out.  After watching some review videos on YouTube, and scouring Amazon, I decided that Seattle based laserchristmaslights.com had the ideal solution.  Their website says they are used at Six Flags.  But these high quality laser projectors were sold out and I put myself on the waiting list.

Meanwhile I bought a couple of the inexpensive Star Showers at Bed Bath and Beyond.  They were just $39 each.  As the box said, in just 10 minutes I lit a 30 foot tall cypress tree at the top of my driveway with thousands of green and somewhat dim red dots.

The Star Shower Iisn't Going to "Blind" Anyone
At 80 feet across my back yard the green laser was no brighter in my eye than a common flashlight.  How can this be?

 Both the green and red lasers in the Star Shower pass through a glass diffraction grading which splits the beam into about 1000 beams.  This not only creates the star effect of hundreds of dots, it also means each little beam is only about 1/1000 as intense as the original beam, which was only 5mw (considered safe) to start with.  

Add to that the divergence of the laser beams in the Star Shower and similar holiday lights.  "Divergence" means how much is spreads out as it gets further from the laser source, just like a flashlight beam that gets bigger as you shine it on more distant objects.  And it gets much weaker. (See below)

The little 1/16 inch beam that hits your house would spread so much as to be over 75,000 times weaker at 1000 feet.

But you hear one pilot on the news (who has not actually flown over a holiday laser light) speculate to a reporter that if the beam is "much wider" up in the air He tells the reporter it could "fill the cockpit" and he says "blind" the pilot.  Only if physics didn't apply. It can't spread without getting proportionately weaker. Just like a flashlight right against your eye is blinding, but at 200 feet seems very dim.

The effect is amazing, if not all that bright.  From the street, about 100 feet away, I can barely tell my tree is lit.  But get within 30 feet of it and it looks great.  A guest asked if I had put thousands of tiny red and green lights all over it.  It is a very subtle look on the tree.  And the beams are way, way dimmer than any cat toy or other $4 1mW laser pointer.

Star Shower Power:  Searching online it seems the Star Shower uses a 20mW green laser and a 30mW red.  These are literally split into about a thousand small beams by a glass diffraction grating.  The resulting beams seem way under 1mW, in fact the brightest, at the center, are probably less than .1mW.  That stands to reason:  20mW / 1,000 = .02mW on average.  The 30mW red is significantly dimmer despite the higher powered laser.  The eye simply does not see that color as brightly as green.

Beam Spread is Very High:  I measured one of the brightest green beams from the Star Shower to be 1mm diameter at 1ft from the projector.  I was able to trace that same beam to a tree about 80 feet away in my back yard where it measured over 1.5 inches in diameter. That is quite a divergence in the laser beam!  And needless to say it was hardly "blinding" at that distance.  Certainly no brighter than a medium powered flash light at about the same distance. 
The approximately 1.6mRad divergence of a Star Shower laser beam means a beam that is 1 mm in diameter 1 foot out would be 20" in diameter at 1,000 feet and only 1/75,000 the light per square inch.

Using www.pseudonomen.com/lasers/calculators/diameterCalculator.html I was able to roughly calculate that the divergence of that beam was about 1.6 mRad.  That is very high.  A typical 1mW cat toy or low end laser pointer I am told is usually around 1.2 mRad.  The diffraction grating used to split the beam into a thousand tiny beams is probably causing the divergence to be so high. 

At 1.6 mRad divergence the beam would be approximately 3.8" diameter at 200 feet.  That's almost 100 times wider than at 1 foot and the brightness per sq mm would be reduced by about 3,000 times!

At 1000 feet that one beam would be nearly 20" wide, 488 times wider, and would be extremely dim, on the order of 1/75,000th of it's original power per sq mm.

At 13,000 feet that same feeble beam would have diverged to a whopping 249" or almost 21 feet in diameter!  That is 6340 times wider than at 1 foot and 13 million times dimmer per sq mm, since it is covering about 13 million times more area (31,563,000 sq mm vs. 2.46 sq mm at 1 foot)

Let's say that beam was a full 1mW (which it was not even close!) at 1 foot from the Star Shower.  Then at 13,000 feet it would be the equivalent of 1/13,000,000 mW in a given area.

This is just theory based on the spread at 80 feet.  But certainly it seems unlikely the Star Shower hit a jet liner 13,000 feet in the air at the FAA's "distraction" level.  Someone needs to do real testing.

With millions of Star Showers out there it stands to reason we would be hearing hundreds of reports, not a grand total of about 4 or 5 over two week's time if they were so powerful they could blind or distract a pilot at 13,000 ft.  Figure that there are over 3,000 intentional laser aviation events reported each year anyway.  It seems more likely a bad guy was intentionally hitting the Dallas bound aircraft with a laser pointer, but that when police searched the only laser they saw was a lowly holiday laser projector and they jumped to conclusions.  By that time the bad guy would have put the laser away so as not to be discovered.

Laser Christmas Lights, a Seattle company specializing in high quality holiday laser projectors, notes in its FAQ section that "the laser beams from our projectors diminish and become non existent after 850 feet."  Hmmmm.  I note that they have taken this off of their FAQ page this week.  One hopes they are testing their units and will report.

One YouTube reviewer measured this unit and the output of a single beam was so low that it did not show up on his laser power meter, being a small fraction of 1mW.   And, the Laser Christmas Lights are about 5 times brighter than the Start Shower.

In any case, it seems that it would be easy to make a safe holiday laser projector by adjusting the lenses to intentionally diverge the laser beam so that by 1,000 feet it truly did diminish well below the FAA's "distraction" standard.

Blisslights is another laser projector vendor.  Their site reads:
Some people are worried about our outdoor units and there interference with airplanes. When using our BlissLights in your back yard the laser can only travel about 100 to 200 feet into the sky before it dissipates due to the divergence of the laser. The average flight altitude is 18,000 feet, which can be almost 90 times the distance that our lights will travel.

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