Laser threat defence
 

In light of the news on Monday about a laser attack on a Virgin 744, a few of us have just spent our lunch hour chewing over the issue.

Apart from the usual sentiments involving locking up the perpetrators and throwing away the key, we were asking ourselves whether there's any technology that could be used to protect flight decks against laser light?

Offhand, I can't think of anything - filtered lenses exist, but they're only effective if you know what frequencies you're designing for. Block the windscreens and go IFR immediately? - would work but degrades lookout immediately so contra safety for other reasons.

Does anybody have any bright ideas?

G

How about modifying existing side window sunshades? Not sure how many attacks are aimed at the forward windows as opposed to the side views though....also how much visibility the crew would have once the shades are in place?

My favourite tactic is to swich the external lights off as soon as I see someone fiddeling around with this things. Yes, it is against the ANO but as a commander I can take all measures.... you know the stuff. Worked a treat so far. What the warrior can't see, he can not attack.

Maybe something using the technology of this welding helmet would do: https://www.youtube.com/watch?v=L5bMlI9ZHyY

In a previous job, hovering over and illuminating the perpetrator with the 30 million candlepower Nitesun used to give them a bit of a hint.

Blue flashing lights outside the front door was the second hint that a slight misjudgement on their part had been made.

Seeing as we live in an age of laser guided weapons would it not be possible to install reflective devices on the aircraft which would reflect the beam straight back at them?

Maybe. But that might give the miscreants some feedback on their aim. And some people might just get a kick out of being 'flashed back' by a passing airplane.

If that works reliably. What is the arrest rate for l@ser incidents? Unless you are dealing with some abject morons who keep lighting up successive planes, by the time one is reported, they have turned their pointer off and gone inside.

Something needs to be done to ensure that detection and arrest is nearly certain. Possibly a detector (camera) and GPS position that could automatically forward source location data to authorities on the ground. An iPhone/Android app on a phone positioned to look out the cockpit window might be sufficient.

I'm sure that something along the lines of an electronic auto-dimming welding mask glass could be made to work in cockpit windows. But it'd have to be totally reliable; after all you wouldn't want it going dark by mistake, might make the landings tricky...

It would be sad if something like that became the only solution to the problem (instead of import controls, prosecution of vendors and manufacturers, policing, etc).

It's well known that pilots are getting hurt. Are there any reports on passengers and cabin crew getting hurt? I guess that side on illumination isn't going to be as steadily aimed, but would probably still be damaging.

I'm sure there are systems in the military that can pinpoint the origin of laser light. Of course, civil aviation isn't warfare so the Geneva convention does not apply; you could have automatic gun aiming/firing integrated with such a system. It could be a specific variant, e.g. 747-400-130...

Illustration of the problem

Someone I worked with was involved in buying items like laser pointers from manufacturers in a certain large Far East Asian country. At one place he was shown a selection of laser pointers, and the labels on them (an indeed the demonstration) made it quite clear that the power output was far in excess of what was safe or legal in Europe.

When he explained this to the company and said he couldn't possibly buy them, their response was "no problem, we'll change the labels". Says it all really.

I'm pretty sure I've seen laser protection glasses that work by blocking a very specific wavelength. Because they take such a small chunk out of the spectrum, they have little effect on other light sources. Something like that might work for windshields. (I guess you might need to block more than one frequency based on the types of laser that need countering, but probably not more than a few.)

Or maybe you could use the welding helmet technology with a trigger that responds only to very specific frequencies, so the windshield (hopefully) wouldn't go opaque with each lightning flash.

Laser Pointers - The Maths
 

I tried to post some analytical numbers about this in the other thread, but I mixed in some of my conjectures and opinions and it seems they upset people, so my posts were deleted and I was locked out of that thread.

I think the analysis is still worth an airing, and I’ve had some PMs asking for the details again. So I thought I would have another go, but this time sticking solely to the analysis and leaving my opinions out of it in the hope that no one feels it necessary to censor it again. So for what it’s worth…

Laser pointers – the maths:

Assume airliner flying at 8,000 feet.

Assume that to be visible to the pilot the ground level source must be no more than 30 degrees below the horizon.

Therefore the distance between source and target is 8,000/Sin(30) feet = 16,000 feet: let’s call it 3 miles.

Laser dispersion of 2mrad (0.06 degrees) at 3 miles gives a beam width at the target of roughly 10 metres (generous number).

That same number tells us that to keep any part of the beam impinging on the eye of the pilot needs the laser to be within 0.06 degrees of the nominal target. For those with target shooting experience – this is the equivalent of a six inch grouping at 100 yards (not hard to achieve with a rifle while lying prone, but rather optimistic with a pistol held in one hand while standing up).

The target is moving, of course, but at that range it moves quite slowly.
Of course if we consider the low-level case on approach at 300 feet the range becomes 600 feet and the beam diameter becomes four and a half inches, but the targeting accuracy (where the “target” is a small point like the pilot’s eye) is actually the same, although the target is moving much faster across the sights.

According to the open sources [lots of them; the wiki page is the most easily found and digested] the power density required to cause retinal damage for lasers in the visible spectrum (say 400-800nm) is around 2mW/cm^2 for a 10 second exposure, 4mW/cm^2 for a 1 second exposure or 7.5mw/cm^2 for a 0.1 second exposure. So the question becomes what source power is required to achieve those power levels.

The beam diameter at 3 mile range was 10 metres. So the beam area is 785,500cm^2. If we assume even power density across the disk that means that to achieve a damaging power density for a 10 second exposure needs a 1500W laser, for a 1 second exposure this becomes a 3000W laser and for a 0.1 second exposure it would need a 5,800W laser. Note that all three of these are more than three orders of magnitude (ie a thousand times) more powerful than the worst of the “illegal” laser pointers being discussed, and six over orders of magnitude (ie over a million times) more powerful than the standard laser pointers people like me use in lecture theatres. KiloWatt lasers are not hand-held devices.

For the 300 foot case it’s a bit different. At this range the beam diameter is only 11.5cm, so the disk area is 104cm^2. The laser power to achieve the damaging exposures then becomes 0.21W, 0.42W and 0.78W (or 210mW, 420mW and 780mW if you prefer) for the 10 second, 1 second and 0.1 second exposures respectively. “Illegal” laser pointers with *claimed* powers up to around 1watt can be bought from overseas [from the UK], so if someone can target them accurately enough they certainly could represent a risk of eye damage to the pilots of aircraft in the final few hundred feet on approach, and less so on departure (simply due to the angles involved).

These rough calculations ignore the attenuation effects of the cockpit glazing and the atmosphere. And the latter should not be overlooked, because visible lasers are just light – haze, cloud, dust/smog will block a laser just as much as the reduce visibility. Visible light lasers don’t “burn through” haze and cloud until you get to power levels of tens or hundreds of kilowatts, that’s why jet fighters still have guns and missiles instead of laser weapons and even the anti-satellite laser concepts need lasers so large and powerful that they need an airliner to lift them and they need to be carried up to over 30,000 feet to get to air that is clear enough not to disperse the beam.

Finally, this analysis only considers the danger of eye-damage from lasers. The distracting flash will still be visible, assuming the target can be held accurately enough, at both the ranges considered.

PDR

Beam diameter.
 

PDR- From where does the beam angle spec come? What class of lasers have the .06 degree angle. Do cheaper and more expensive models have different angles?

PDR,


Without going into details that may help the bad guys, I think you need to take a look at the actual beam parameters from a moderate cost 1 watt laser pointer. Your required laser power estimate for damage at higher elevation is off to the high side by a huge factor..


I just ran your numbers thru a professional beam safety program, and if one small beam detail is improved by a factor of 10, the 1 Watt laser has a Nominal Ocular Hazard Distance far longer then your giving it credit for.


Especially if the inadvertent viewer is wearing eye glasses, and if the pointer user has added, or purchased a certain common modification.


However, I'm glad that your attempting the math instead of just repeating the common assumptions and rumors that are out there.



I work with laboratory lasers, and laser display projection systems for a living.


The real answer is to convince the Chinese government to stop the dumping of extremely low cost lasers onto the international market.
These low cost, hand held, lasers are a huge cash cow for Chinese optics companies. Most of which have some fairly shady owners, and have long been dumping optical parts at below market value.


Most of the time they are imported under fraudulent labeling to get around Customs...


As I work with Quality American and European systems, I can assure you these low cost lasers have taken a toll on the income, and sanity of legitimate laser users. Now they are slowly creeping into the cosmetic medical device market, via on-line marketing.


And Pilots, one tip from a laser professional. Avoid a possible secondary effect. Do your best NOT to rub your eyes after an inadvertent laser strike. A bright laser strike may strain muscles in the eye. Then if you drag a dirty hand thru the eye, you run a strong risk of pulling a muscle or irritating the soft tissue.


I got my start in lasers doing laser shows. The low cost lasers pushed many of us who had professional gear, who obtained the required permits, and insurance for outdoor shows out of business. Most of those low cost laser users are totally unaware or ignoring the fact that they need permits for un-terminated laser shows in airspace. Filing the paperwork generates NOTAMs, and there is a lot of math required to ensure safety compliance. That means that the companies who still try to be legal must turn down a lot of potential customers, as there is a delay, and mandatory review period with the local agency.


Most of the time, if the green 532 nm wavelength is blocked, that takes care of 90% of the offenders. However when you start getting into multiple wavelength blocking filters, the optical transmission of non-laser light goes way down. If you block the needed red band, you loose viewing much of the aviation standard red and green lighting.


I've been reading PPRUNE as a outsider for years, sorry that my first post has to be on such a disgusting topic.



Sim

Excellent analysis and gives serious food for thought. Given the number of attacks reported (and I have a strong suspicion there is actually significant underreporting) it would be helpful if the authorities issued some guidelines. PDR1's numbers suggest the risk decreases with height and above 10,000 feet on his numbers it looks low. But on final approach it could be significant. It appears you cannot reliably assess the power or exposure. But one option might be for a recommendation to get a medical check if targeted on final approach etc. Either way despite a little ill humour/stupidity that always seems to occur I think the wider discussion on this topic has been very helpful. Thanks to PDR1 and thumbs up to Scroggs.

I like Hahn's idea. I have been lasered five times so far, twice in the cruise. Trouble is these idiots can look on flight radar and listen to our RT and score a 'bingo' when we report the laser.

I was once lasered on the ramp by a passenger sitting in the aircraft next to us !?! I kid you not. :ugh:

Thanks PDR1, very interesting analysis. The chances of retinal damage would appear to be low?

Just to caveat this:

I am an engineer - not a doctor and not a magician, so all I will ever talk about is whether there is a risk that received power levels approach or exceed the values that SOMEONE ELSE has published papers saying they are dangerous.

So I cannot say "The chances of retinal damage when lasered above x,000 feet are low" - all I can say is that "The indications are that when lasered above x,000 feet the exposure won't exceed the values which the published papers say will cause retinal damage".

Now based on the sums I did last night the indications are that the risk of the retina receiving power levels above that threshold from a ground-based laser pointer while flying above (pick a number - let's say 5,000 feet) is probably very, very low, but the risk at low level (again, pick a number - I choose 500 feet) is probably high enough to be a concern. But it would still require very clear air and probably require illegal devices and very accurate targeting.

It got late last night and I didn't finish what I wanted to say. So to continue - why did I mention the 10sec/1sec/0.1sec exposure times? Well when I first raised the issue of the targeting accuracy needed[1] in the thread I'm now excluded from it was countered by the suggestion that the beam wasn't "aimed" but "waggled about" [with some gratuitous suggestions about the waggler being drug-crazed, and maybe even a Trump supporter]. So again I did some math to evaluate this which was deleted because I included opinions. So I'll try again:

If someone has a pointer and is just "waving it about" the angular rates are likely to be of the order of more than 60 degrees per second (try it). Based on data from targeting trials (not public domain - sorry) for someone trying to aim at a device without any sights the "shake rate" is likely to be in the region of 20 degrees per second, dropping to 10 degrees per second for plain sights (people can get much better than this with telescopic sights and lots of practice, but neither has been suggested for morons lasering aeroplanes).

So what does that mean? Well for the aeroplane at 8,000 feet and three miles range a 10deg/sec "waggle" translates into the beam moving from side to side at 2,800 feet per second. We said the beam would be 10m (lets call it 33 feet) diameter, so any single point (like an eyeball) will only be within the beam for 12 milli-seconds: 0.012sec or one eighth of the fastest "flash" case I discussed in the previous post. So even if we pick some very pessimistic assumptions (clear air, high power, good laser and an aimed laser with sights) we are still many orders of magnitude too small to get near these published damaging power values.

Looking at the 300 foot case it suggests that the seemingly "dangerous" values mentioned above need to be divided by eight for a real-world case - or in other words the danger threshold power values are closer to 1.7W, 3.4W and 6.25W (or 1700mW, 3400W and 6250mW if you prefer) for the 10 second, 1 second and 0.1 second exposures respectively. We're start to get up to power levels which are harder to find on the open market (or even ebay!) even for the low-level case. Although the flash-distraction would still be an issue of course, and I can't find any particularly digestible authoritative data on the power levels at which flash after-images become persistent for more than a few seconds so I can't include that in the anlysis.

PDR

[1] Where someone said "laser pointers are used to point at stars, so obviously it isn't a problem" - huh? If you pointed a laser pointer at the nearest star it would be 9.4 years before you saw the dot even assuming you were pointing it in the right place! Do people never think about what they are saying??

PDR, you seem to be hooked on actual damage to the eye rather than temporary blinding/light spots and distraction. I can temporarily blind someone with a torch.

I will say again when I was subject to a laser attack I got a brief flash across the eyes and that was enough to render my eyesight essentially useless for several minutes. Don't you consider that dangerous?

Chesty

What height were you at?

I have been flashed with these things at 500ft in a helicopter and had no loss of sight at all, just annoyance.

Twice. First at about 3000' on departure. My FO essentially became single pilot as I couldn't discern the ASI properly and would've retracted the flaps too early if he hadn't stopped me.

The second at 36000' somewhere over Greece.

Both at night.

Chesty

Whatever hit you and caused you visual problems at 36000' was not a laser pointer held by a chav, believe me.

It definitely behaved like a handheld laser.

I didn't really care who was holding it at the time though.

Really.

And how do handheld lasers behave? Do they have good table manners?

If you were at FL360 and flying straight & level then the source must have been at least 14 miles away, and even on a remarkably clear, undercast-free day that's one heck of a laser...

PDR

Yes, really. I'm sure you're clever enough to work out the difference between a hand held laser and a stabilised laser. And 14 miles? No.

I note that you ignore the more serious incident and my first question to you.

In this day of precision guided weapons, surely a detachable device could ride the beam back down to the source.

Perhaps a javelin type implement with GPS. Pin the bastards to the floor until the cops arrive!:ok:

In the spirit of hopefully good humoured and constructive exchange of views, I think PDR1 you are guilty yourself of not thinking things through regarding pointing at stars....or was your comment about 9.4 years an attempt at humour? Laser pointers are advertised for this purpose, what they reflect on (presumably something in the atmosphere) someone more knowledgable can no doubt tell us.

Your maths and experience give the debate more substance, but some of your assumptions regarding waggling do not fit with my perception. I may be incredibly unlucky, but I have been targeted many times and it certainly seemed for longer periods than you are suggesting is possible. Just humour me....if these idiots can target the beam reliably (I know you think this unlikely) at what range does your maths suggest it becomes dangerous with the 90mW green laser we discussed earlier?

Your last post went some way to convincing me on the 8000 foot case, but I missed a step explaining why the 300 foot case needed to be divided by eight, probably me, but I missed your logic.

Bringing a bit of maths in myself.

If you were at FL360, let's say that you were impacted at 45° angle below the horizon, as from directly below you wouldn't enter the cockpit. So from surface, that's about 15km in a straight line.

Firstly atmospheric losses would need something rather bigger than the 5mA handheld I use in a lecture theatre. When I've played with it on a clear day, a few km is all that works at.

Secondly, you were flying I'd guess at around M=0.75, that would map to a groundspeed of about 220m/s. Side on then, to track you, that would need some device that moves the laser at a rate of about 0.8 degrees/second. Achievable, but would require some kind of aiming and tracking device - say a rifle sight attached to the laser, mounted on the top of a tripod.

Let's say that the tracking is achievable to a steadyness of ±1%: I suspect that this would actually be pretty good. At the aeroplane, that would map to a back and forth wobble of ±2.2m: or a couple of times the size of an aircraft side window. In practice, it probably wouldn't be that good and probably back and forth nearer the length of the aeroplane is more likely.

My guess then is, that from where you were sat, that would look very like a wobbly handheld device when, in reality, it was incredibly steady.

G

As I am having a rest (not a ban) from the other thread... :)

As others have pointed out, optical filters to cut red or green laser light will also affect the visibility of aircraft navigation lights, and ground based lighting. So it is effectively impossible to block laser light while still allowing a pilot to keep an unimpeded visual look out.

However, reflected laser light can be just as damaging as a direct hit, the pilot doesn't need to be looking out the window, a reflection off an instrument could be just as bad.

My suggestion would be for antireflective coatings to be applied to all cockpit glazing and for other materials in the cockpit to be dark or light absorbent matt colours and finishes wherever possible. Micro scratches on the outer surfaces of the cockpit screens also cause issues when back lit by bright light. A hardened coating or polish capable of surviving the rigours of flight could be used to try and smooth out these scratches.

These suggestions wouldn't stop a direct laser hit, but by eliminating reflections the dazzle effect in the cockpit could be reduced significantly.

Alternatively, pilots could take to wearing Raybans 24/7. Wouldn't do much to cut down the laser light, but they would look cool. On a serious note, dark glasses would likely make the problem worse because the eye pupils would be open wider. One of the reasons daytime laser attacks are seldom reported is that the pilot doesn't even notice them against a daylit background, although eye damage could be being caused and going unnoticed.

Interesting figures on the strength and spread of beams from the ground. The first 6000 feet of air above ground is stuffed full of dust, moisture and other particles and attenuates laser light considerably. If a beam is aimed straight up, it appears to cut off rather abruptly after a rather short distance. This is where the quantity dust in suspension from the ground diminishes. As the beam is swung down towards the horizon, it appears to lengthen. This effect is due to the dust being illuminated by and refracting and attenuating the beam. In clear air, a 200mW green laser has a naked eye visible range in excess of 80 miles (demonstrated to 84 miles between mountain tops). At sea level altitudes a range of 10 miles is more likely. A 200mW laser will cause instant and permanent eye damage at ranges of less than a couple of hundred yards. The potential for permanent vision loss diminishes with distance. At a mile or so, the blink reflex and beam spread will prevent permanent injury, although the dazzle effects will be severe and vision will probably be affected for several minutes.

As for an outright ban on lasers? Well the average household has at least five fitted to domestic entertainment systems, CD and DVD players, laser printers, laser levels, laser pointers, etc, etc. These things are not going to disappear.

Ghengis, on the other hand it could've been somebody getting lucky with a powerful handheld.

Hellishly lucky!

The longest lens on my camera is 1200mm, which will fill the frame with a large aircraft a mile or so away. It's not much short of a miracle if I can keep tracking an aeroplane accuratelyat anything much beyond that - and that has a viewfinder, and I've been taking photographs like that (and before that was a pretty good rifle shot on the range) for several decades.

G

I repeat, you do not need to track an aeroplane.

I don't mean track electronically, I mean use some device to follow the aeroplane's track.

I can track an aeroplane with my camera, or a rabbit across a field with a rifle.

G

One thing PDR is missing is that up-collimators have been shipping on high end laser pointers for a long time. The two milli-Radians he uses in his example are a low end toy at this time. Its easily possible to achieve 0.1 mR with a few external optics.


Laser pointer companies like to cater to their mainly teenager and 20-something customers, and those customers are often into "burning", ie seeing what they can burn with a focused beam, Power, which equates to the teenage desire for more horsepower in muscle cars, and Distance. Distance is becoming more and more popular, which is a problem.


When we do Laser in Airspace Calculations, we look not just at the Nominal Ocular Hazard Distance, but the Sensitive Zone Exposure Distance and the Critical Zone Exposure Distance. CZED and SZED are more about what pilots need to worry.


This is not just about laser shows accessing airspace legally, there are Lidar Systems, and Laser Guide Stars for astronomy, and Laser Wind Profilers. The cloud height sensor at most airports is a pulsed infrared laser of very low power. There are also laser based glideslope systems for a variety of applications, including some that are directly visible to the pilots eye (Those are intrinsically safe, btw)


So lets start with NOHD. NOHD is a statistical likelihood that there will be large, observable damage to the retina for a visible laser. Not all retinal burns readily show up in basic ophthalmic testing. It usually takes a retinal profiler to detect small damage, and those machines do not grow on trees. The famed Amsler grid referenced earlier pretty much only shows large scale or very severe laser injury to the eye.


For the record, it is often possible to discolor the proteins in the back of the retina and do damage that heals, as well as permanent damage.
So each case must be evaluated on a case by case basis.


A quick approximation for NOHD in feet for a continuous wave laser is


NOHD = (32.8/theta ) times the Square Root of (( 1.27 * average power)/0.00254)) Where theta is the 1/e divergence of the laser.


1/e Divergence is basically the 10% and 90% points in the power distribution curve across the beam in the laser. This curve is most often a Gaussian distribution, although cheap laser pointers often have horrible beam profiles, most of the high power green pointers are starting to ship with much better beam quality.


So lets move away from the damage threshold, which is a probability of damage occurring, not a absolute guarantee that there will be damage if you observe a beam that is over the NOHD. After all these beams are moving, and greatly disturbed by atmospheric turbulence.


Next Post, CZED and SZED.


Steve

I know! You don't need to do that.

I dont think "anti laser" coatings to windows or glasses work by pilots would be practical. Ones that would work reliably would have lenses so dark, you would inhibit normal vision.

A reflective (partially mirrored) coating to the inside of windows would work better. It wouldn't be 100% effective, but would provide a measure of assistance.

Indeed. The sudden availability of low cost easy to use lasers has been a double edged sword to the laser industry. Many promoters seem reluctant to spend proper money on a well run, safe show, when they can buy something cheap, nasty and dangerous off ebay for far less.

If Sim is who I think he is, his knowledge on the subject of lasers is quite legendary. :ok:

Back to the OT...

I was zapped by a laser at night a few months ago whilst downwind to a major airport.

Fortunately recognising what was going on, control was handed over to the other guy, whilst looking away from the source.

It seems that an ability to pull over a completely blacked-out blind on the side windows would be a better solution than anything we have at present.

It is less likely that someone could be dazzled through the front windows unless the beam is directly in front, such as during final approach.

Notwithstanding trying to pinpoint, apprehend and prosecute the perpetrators, are the aircraft manufacturers looking at this, since this is becoming a more frequent occurrence?

What are the regulators doing?

Well I took my numbers from BS-EN60825 (aka " IEC60825"), the recognised standard for laser safety. Specifically the "Maximum Permissible Exposure" (MPE) data it contains, as summarised in this graph:

http://i925.photobucket.com/albums/a...25_MPE_J_s.png

[this is the wiki version which has the plots in colour so they're easier to see]

Now it's worth noting that "MPE" is defined as "The highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe, i.e. that has a negligible probability for creating damage", and the typical value it produces is 10% of the dose that has a 50% probability of creating damage in the 95th percentile population (by sensitivity to laser injury), and includes all eye damage modes (corneal, retinal etc). So it's actually well BELOW the value that would be certain to cause damage - exposure AT this level is actually deemed "safe". So where does your suggestion that I'm underestimating the number by a factor or 10 come from? I've given traceable, authoritative sources - what are yours?

PDR

Am I? is that why I explicitly talked about this at the end of the post immediately above this one of yours?

PDR

Yes, and further you talk about aimlessly waggling at 60 degrees per second. Why don't you consider a more concerted waggle in the general direction of the aeroplane with the intention of actually hitting it.

Indeed it is, and while that significantly increases the power density in the beam by a factor of 400 (remembering that the analysis suggested we were 3-6 orders of magnitude below the MPE value) it also reduces the size of the beam for the 8,000 foot aeroplane from a 10m disk to a 50cm disk, and the probability of that disk illuminating the pilot's eyes (the required targeting accuracy and/or the duration of illumination for a "waggle" swipe-pass) become much more significant, so it doesn't change much. For the aeroplane on finals at 300 feet the 0.1mRad beam diameter drops to about a quarter of an inch. Are you really suggesting that a person standing 600 feet away with a laser pointer can hold a 1/4" diameter beam onto a pilot's eye, when he probably can't even SEE the pilot through the cockpit window?

But I am also led to believe that the sort of lasers used as toys by idiots aren't the ones with particularly effective collimators fitted to them, although the source for that isn't particularly authoritative.

PDR

He wouldn't need to "hold" it on the eyes.