Monday, December 20, 2004
Don't go towards the light!
Yet another break in holiday programming to bring you the following exchange of emails regarding the highly publicized possibility that terrorists are hoping to use lasers to blind airline pilots during landing. It all sounded pretty menacing, so one of my brother's friends wrote him to ask if it could be true. Joshua, said brother, and his friend Rob are students at the Center for Research and Education in Optics and Lasers at the University of Central Florida. A technical but less scary than conjecture explanation:
From: Drew Thoeni
Sent: Friday, December 10, 2004 4:38 PM
To: Joshua Duncan
Subject: Some help needed here
OK, so there's this story in the news about terrorists using lasers to blind pilots in the cockpit (from five miles away) while they are landing a plane in hopes of blinding the pilot and crashing the plane. I have a lot of theoretical problems with this that you may be able to confirm or, if I'm wrong, set me straight on.
1) These would have to be very powerful lasers to have any real affect at five miles out. Such lasers are hard to come by.
2) Assuming you had one, it seems it would require more than just a household 110 volt plug, so you couldn't just plug it in anywhere and start burning out pilots' eyes.
3) Even if you could get one, and power one, you'd have to hit a pretty small target.
4) Even if you hit that target, you'd have to *focus* on that target toget to concentration of power needed to burn their retinas. Of course you could just get a more powerful laser and have to worry less about focus, but then we start over on point 1.
Your thoughts?
Drew
From : Joshua Duncan
Sent : Sunday, December 12, 2004 3:05 AM
To : Drew Thoeni
CC : Rob Bernath, Sarah Duncan
Subject : RE: Some help needed here
I'm going to have to side with you on this one. (Rob, correct me if you see an error.) On your points...
(1) True enough. Not only hard to come by, i.e., regulated, but also expensive. A cheap 5-Watt solid-state green laser will run you about $35k new. There's always eBay, but you still probably wouldn't find one say below $15k or so. And I'm not even saying that 5 watts would be enough ... just an example.
(2) Invariably so. We used a small 10-Watt green laser last year, and still the head is about the size of a shoebox. Then there's the power supply, which is probably about 5U of rack space, and a chiller the size of a medium toolchest. And you need probably a good 30 Amps of 110 to run it all. And it takes half an hour to warm up.
(3) A small *moving* target, no less.
(4) Not necessarily focus; your eyes will do the focusing. But you would have to have a sufficient power density at that large a distance. A good laser will give you a divergence angle of about a milliradian (probably can do better, but that's typical).
Some basic calculations:
tan(theta) = y / x -> y = x tan(theta)
So the distance multiplied by the tangent of the angle will give us an indication (ballpark) of the beam's size at 5 miles' distance: (5 miles)(5280 ft/mi)(12 in/ft)(tan 0.001) = 316 inches
The power density of a 10-Watt laser at that distance would be (10 W) / (pi * (316 inches) ^ 2 / 4) = 127 microwatts / square inch.
Pretty darn small. Not going to hurt you. The sun dumps about a kilowatt per square meter on earth, so that works out to 657 milliwatts / square inch (4000 times higher), if I'm doing the math correctly. I'm not figuring numerical aperture and all those fun things that you have to take into account for careful calculations, but it should be a decent estimate. And of course, that's a $100,000 laser.
I've heard stories of spy planes that flew over Russian ships where supposedly the pilots got their eyes zapped by lasers, but still not to the point of blinding them, and it's a lot more feasible to get a big laser on a ship. Now, of course, it's a lot easier to do damage with a pulsed laser system, and my calculations are for continuous, but even still ... it's going to be a big system that could cause someone eye damage at 5 miles, and it's still a small target moving a few hundred miles an hour.
Oh, and I might also note ... there are co-pilots for that sort of thing. And, though it's not widely known, with the instrument landing systems that are currently in use, planes can land themselves. The main reason they don't is so that the pilots get more practice.
The short answer ... umm, no.
-Joshua
From : Robert Bernath
Sent : Sunday, December 12, 2004 1:54 PM
To : Joshua Duncan, Drew Thoeni
CC : Sarah Duncan
Subject : RE: Some help needed here
Sounds like a good estimate to me. You could always go more advanced into a channeling regime but that would require a half million dollar laser and probably a few million dollar tracking system (the things only governments like to build).
My brother has told me stories that he heard about this (for those who don't know, he is a pilot for USair) and he was saying that what people were trying to do was to blind the pilot's night vision (in the version that he heard). For night flying the cockpit is very dark so that you can see outside and just a very bright flash would cause temporary "blindness". My brother was saying though (as you noted Josh) that that is why you have two people. Also, you can just turn up the lights and fly it in with instruments. He didnt think it would cause any problem with landing, more just a big irriation to have.
Rob
From: Drew Thoeni
Sent: Friday, December 10, 2004 4:38 PM
To: Joshua Duncan
Subject: Some help needed here
OK, so there's this story in the news about terrorists using lasers to blind pilots in the cockpit (from five miles away) while they are landing a plane in hopes of blinding the pilot and crashing the plane. I have a lot of theoretical problems with this that you may be able to confirm or, if I'm wrong, set me straight on.
1) These would have to be very powerful lasers to have any real affect at five miles out. Such lasers are hard to come by.
2) Assuming you had one, it seems it would require more than just a household 110 volt plug, so you couldn't just plug it in anywhere and start burning out pilots' eyes.
3) Even if you could get one, and power one, you'd have to hit a pretty small target.
4) Even if you hit that target, you'd have to *focus* on that target toget to concentration of power needed to burn their retinas. Of course you could just get a more powerful laser and have to worry less about focus, but then we start over on point 1.
Your thoughts?
Drew
From : Joshua Duncan
Sent : Sunday, December 12, 2004 3:05 AM
To : Drew Thoeni
CC : Rob Bernath, Sarah Duncan
Subject : RE: Some help needed here
I'm going to have to side with you on this one. (Rob, correct me if you see an error.) On your points...
(1) True enough. Not only hard to come by, i.e., regulated, but also expensive. A cheap 5-Watt solid-state green laser will run you about $35k new. There's always eBay, but you still probably wouldn't find one say below $15k or so. And I'm not even saying that 5 watts would be enough ... just an example.
(2) Invariably so. We used a small 10-Watt green laser last year, and still the head is about the size of a shoebox. Then there's the power supply, which is probably about 5U of rack space, and a chiller the size of a medium toolchest. And you need probably a good 30 Amps of 110 to run it all. And it takes half an hour to warm up.
(3) A small *moving* target, no less.
(4) Not necessarily focus; your eyes will do the focusing. But you would have to have a sufficient power density at that large a distance. A good laser will give you a divergence angle of about a milliradian (probably can do better, but that's typical).
Some basic calculations:
tan(theta) = y / x -> y = x tan(theta)
So the distance multiplied by the tangent of the angle will give us an indication (ballpark) of the beam's size at 5 miles' distance: (5 miles)(5280 ft/mi)(12 in/ft)(tan 0.001) = 316 inches
The power density of a 10-Watt laser at that distance would be (10 W) / (pi * (316 inches) ^ 2 / 4) = 127 microwatts / square inch.
Pretty darn small. Not going to hurt you. The sun dumps about a kilowatt per square meter on earth, so that works out to 657 milliwatts / square inch (4000 times higher), if I'm doing the math correctly. I'm not figuring numerical aperture and all those fun things that you have to take into account for careful calculations, but it should be a decent estimate. And of course, that's a $100,000 laser.
I've heard stories of spy planes that flew over Russian ships where supposedly the pilots got their eyes zapped by lasers, but still not to the point of blinding them, and it's a lot more feasible to get a big laser on a ship. Now, of course, it's a lot easier to do damage with a pulsed laser system, and my calculations are for continuous, but even still ... it's going to be a big system that could cause someone eye damage at 5 miles, and it's still a small target moving a few hundred miles an hour.
Oh, and I might also note ... there are co-pilots for that sort of thing. And, though it's not widely known, with the instrument landing systems that are currently in use, planes can land themselves. The main reason they don't is so that the pilots get more practice.
The short answer ... umm, no.
-Joshua
From : Robert Bernath
Sent : Sunday, December 12, 2004 1:54 PM
To : Joshua Duncan, Drew Thoeni
CC : Sarah Duncan
Subject : RE: Some help needed here
Sounds like a good estimate to me. You could always go more advanced into a channeling regime but that would require a half million dollar laser and probably a few million dollar tracking system (the things only governments like to build).
My brother has told me stories that he heard about this (for those who don't know, he is a pilot for USair) and he was saying that what people were trying to do was to blind the pilot's night vision (in the version that he heard). For night flying the cockpit is very dark so that you can see outside and just a very bright flash would cause temporary "blindness". My brother was saying though (as you noted Josh) that that is why you have two people. Also, you can just turn up the lights and fly it in with instruments. He didnt think it would cause any problem with landing, more just a big irriation to have.
Rob
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