Laser / lightning risks

[marting]

There is currently much media talk about pilots being temporarily "blinded" by land based lasers. A technician was arrested and confessed to pointing a fibre optic testing green laser he bought on ebay for $100 at an aircraft.

Green lasers certainly can affect night vision which peaks in the green range but what is the intensity of this? How does it compare to nearby lighning strikes? Has anybody gone through the numbers to determine the degree of risk this really represents?

I know pilots revert to instruments when their night vision is supressed by floodlights or lightning at night.

-m

 

[Iconoclast]

Green lasers certainly can affect night vision which peaks in the green range but what is the intensity of this? How does it compare to nearby lighning strikes? Has anybody gone through the numbers to determine the degree of risk this really represents?

I thought the risk with lasers was that the beam can actually burn holes in the retina, I might be talking out my a** though.

 

[richardm]

Green lasers certainly can affect night vision which peaks in the green range but what is the intensity of this?

I suppose the answer to that is "it depends on the laser". Some of them are certainly powerful enough to do permanent damage to the retina, but I think in this case the problem is with pilots becoming dazzled at a critical part in the landing sequence.

 

[TonyL]

Re: Re: Laser / lightning risks

I thought the risk with lasers was that the beam can actually burn holes in the retina, I might be talking out my a** though.

That depends on the power of the laser. I did a quick scan of ebay and all of the currently listed visible lasers have an output power of less than 5 mW. At this power a laser can dazzle you and temporarily ruin night vision if you get a brief flash, or can damage your retna if you stare at it for several seconds. (depending on the diameter of the beam when in hits your eye, staring at a 1 mW laser can be comparable to, or worse than, staring at the sun) Since most people blink, turn their head, and don't stare at it, it's rare that such a laser causes permanent damage, although a brief glimpse will leave a bright spot in your vision for a brief period of time.

Now, if your eyes are hit with a more powerful laser (in the visible range), yes it will deliver enough energy to damage the retina, or even rupture it before you can respond by blinking and turning away. Those lasers are available, and not too expensive, so I would assume it is somewhat of a concern. (although, I don't know how you'd go about aming such a thing in a way that it is likely to hit the eye of a pilot who is thousands of feet away and probably not looking at you.)

So, in short, there are several possible concerns.

 

[Iconoclast]

Thanks Tony.

 

[marting]

If we assume 1KW/m^2 incident from the sun, and about 10mm^2 pupil dilated then the energy incident on the retina whould be about 10mw. Obviously, in the daytime, the pupil would not be dilated so much.

A red pointer laser spreads about 20mm/100 ft. At 1 mi this is about 1m diameter. The max energy entering a dilated pupil would be less than 20^-9 W. That's pretty small. This is also why I thought it might be useful to compare it to the flash of nearby lighning - which would also be small.

It probably takes very little light to supress night vision and I think this is the real problem - though not as high risk as implied in media reports.

I do not know what the power and spread of the fibre testing green laser was but it seems likely it is more a temporary distraction threat than a serious bodily injury threat.

-m

 

[marting]

Found a site selling 15mw green lasers. This is pretty high power. Divergence is spec'ed at < 1.2mrads.

http://www.megalaser.com/gallery.htm

I found FAA discussions of approx 200 incidents of red penlight lasers being reported. There was particular concern that helecopters flying low where the pilot has near vertical gnd visibility are most at risk. Gang members in LA are reported to use so called "6 packs" of bundled pointer lasers. Apparently these are red lasers.

The above green ones are much more of a threat to night vision. I think since low light retina rods are not very responsive to red light (pilots use red light reading maps to keep their night adapted vision).

-m

 

[elgarak]

It's fairly easy to order high-power green laser pointers on the internet. Those are modified to emit up to ~100 mW and cost ~$200-300. Those are serious high powered lasers, and illegal to operate in public (limit is 5mW, however, higher ones may be sold and owned and operate in non-public areas, such as your bedroom or in a University lab, at least in th USA).

I doubt that you can use even those to permanently damage with the visible red and green light alone an eye from a fairly large distance. The damage to the retina takes some time, and it's difficult to hit the small target eyeball, especially if the target is in a moving plane. The pilot might still be blinded temporarily, though. Not a good thing during landing.

The problem with green lasers is the principle: They do not operate as single solid state diodes as the red ones. Instead, they use a high power infrared laser to optically pump the green emitting crystal into lasing. The IR light should be confined to the laser pointer, but people can remove the filter if they want. The light is not visible, therefore people don't react when they are hit with it (like moving their head, blink, or block the light with their hands). Damage is then much more likely.

 

[CurtC]

Found a site selling 15mw green lasers. This is pretty high power. Divergence is spec'ed at < 1.2mrads.

1.2 mrads would be a beam 6 feet (2m) wide at a mile distance. It's strange to me how that beam is lighting up the air that it passes through so much.

A couple of years ago, I came across a web site where a guy was trying to get as many people as possible to shine their laser pointers at the Moon at a given time. He even said *where* on the Moon to point the beam. He was hoping that you'd be able to see a red glow, or something, when looking at the Moon. I emailed him and asked if he had run any calculations on this - it had apparently never occurred to him to do so. Maybe it's the engineering school training, but I just don't see how it's possible to have an idea, go to all the trouble to promote it, and never even think about running some calculations to see if it's possible/practical.

 

[marting]

I found a story by a DC3 pilot describing a lightning hit on the nose that blinded the pilots so long they though the cockpit/instrument lights had gone offline. Turning on a flashlight they still couldn't see anything and realized they were blinded. Fortunately their vision returned enough to read the HSI and so they could keep the wings level.

The plane wasn't hurt at all by the strike.

I think the lasers are a real threat, but somewhat exaggerated in the public mind by the impact of culture and scientific illiteracy. "Laser's on stun, Scotty"

As for bouncing off the moon, that's a tough one. I did once test a pointer for distance with a night vision scope. It was visible bouncing off hills up to about 5 miles in a remote area without city lights. It was only visible for a 1000 ft or so otherwise.

One of the more interesting possibilities is that the night vision microchannel tube voltage can be modulated. It may be possible to detect a laser moon bounce by modulating the signal and de-modulating the signal detection to "filter" out the high background light much like old chopper designs were used to detect sub-microvolt dc levels.

 

[TillEulenspiegel]

I started to post a reply , but it became a mini-course in physics and electronics, so I didn't.
To address a few issues.................

Laser is not like "regular" light , it's called coherent, meaning that all the photons march in lock step, in one direction .

It still has the limitations of any electromagnetic/light/radio Freq radiation. I.E. it spreads and loses power in distance .
The constraints it operates under are power, freq, beam spread ( due to freq and collimation) and media conditions ( weather-fog, rain ,etc. AKA scattering ) and others that don't matter in a topical thread.

You can up the output power of a laser by charge pumping or pulsing.

NASA had an experiment sent up on one of the moon landings ( Apollo 11) that used a ground based laser and a corner reflector on the moon to gage distance. The outcome was a measurement of 4 Cm a year that the moon is receding from the earth. Good ole c. ( the experiment continues)

so it's not Majik or even curious, rather a quantity that is well understood. I think if anyone has an interest in the nature of lasers , they should research and enjoy, It is really cool stuff the cuts across many disciplines.

 

[balrog666]

I do not know what the power and spread of the fibre testing green laser was but it seems likely it is more a temporary distraction threat than a serious bodily injury threat.

-m

The real question is whether or not this is a dry run for terrorists planning on using laser guided missiles against aircraft.

 

[marting]

I started to post a reply , but it became a mini-course in physics and electronics, so I didn't.
To address a few issues.................

Laser is not like "regular" light , it's called coherent, meaning that all the photons march in lock step, in one direction .

It still has the limitations of any electromagnetic/light/radio Freq radiation. I.E. it spreads and loses power in distance .
The constraints it operates under are power, freq, beam spread ( due to freq and collimation) and media conditions ( weather-fog, rain ,etc. AKA scattering ) and others that don't matter in a topical thread.

You can up the output power of a laser by charge pumping or pulsing.

NASA had an experiment sent up on one of the moon landings ( Apollo 11) that used a ground based laser and a corner reflector on the moon to gage distance. The outcome was a measurement of 4 Cm a year that the moon is receding from the earth. Good ole c. ( the experiment continues)

so it's not Majik or even curious, rather a quantity that is well understood. I think if anyone has an interest in the nature of lasers , they should research and enjoy, It is really cool stuff the cuts across many disciplines.

Bouncing radio waves off the moon has been a popular hobby for some time and radio waves, like lasers, are normally coherent.

What's the smallest divergence achievable with visible lasers? That would seem to be more important than power.

Didn't mean to imply any "majik" was involved with lasers.

 

[Mr. Skinny]

If we assume 1KW/m^2 incident from the sun, and about 10mm^2 pupil dilated then the energy incident on the retina whould be about 10mw. Obviously, in the daytime, the pupil would not be dilated so much.

A red pointer laser spreads about 20mm/100 ft. At 1 mi this is about 1m diameter. The max energy entering a dilated pupil would be less than 20^-9 W. That's pretty small. This is also why I thought it might be useful to compare it to the flash of nearby lighning - which would also be small.

It probably takes very little light to supress night vision and I think this is the real problem - though not as high risk as implied in media reports.

I do not know what the power and spread of the fibre testing green laser was but it seems likely it is more a temporary distraction threat than a serious bodily injury threat.

-m

I'm in the safety business, and work with lots of lasers of different power and wavelength outputs. As a very general rule of thumb, just about any laser with an output of over 0.5 mW can potentially cause eye damage. This includes invisible beams, like CO2. They just damage different parts of the eye.

There are many, many factors that affect the beams ability to damage the eye. I have software that calculates the safe eye-exposure distance (SEED) for give wavelength, output, and other factors.

Till is right though. Laser light is coherent and quite unlike a lightening strike/flash.

There is lots of info on lasers on the web. I pulled up this one on google and scanned it rather quickly. Looks like it may have good info (not endorsing it though).

 

[neutrino_cannon]

The real question is whether or not this is a dry run for terrorists planning on using laser guided missiles against aircraft.

Wholely unlikely. Laser-guided munitions are very expensive and are air-to surface weapons, not surface to air weapons. Terrorists would most likely use cheap (relatively) easy to use infra-red ("heat-seeking") shoulder fired SAM's anyhow, supposedly some were fired at an Israeli airliner a while ago.

According to Pater_cannon who is an occasional pilot and gets all sorts of wonderful aviation magazines, lots of pilots have reported this kind of thing for years, and it's only getting attention now. Based on that it would not appear to work that well.

Since pater_cannon is a lawyer full time, I took a little time with google to see that he wasn't just spreading the fertilizer, if you will.

I came up with this, so I'm inclined to believe him.

 

[marting]

The real question is whether or not this is a dry run for terrorists planning on using laser guided missiles against aircraft.

We should be so lucky that they are that stupid.

-m