infra-red and mirrors

Hello, I am designing an aircraft for a university project. The specification states the aircraft needs camera capable of recording IR video, PAL format, 10cm rsolution. I am considering using a kind of periscope in stead of those big spherical "gimball" jobs. If I used a series of mirrors and the camera was embedded deep within the aircraft, would the infra-red camera still work?

Any help would be greatly appreciated. Best Regards, Adam

Reply to
Adam Chapman
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sounds like fun, but suggest you try an optics newsgroup, say sci.optics

martin

Reply to
martin griffith

Thanks, will do

Reply to
Adam Chapman

Consider how much your airframe will vibrate, and how that will translate into image motion.

Then consider how much radiation you are going to lose in your optical path.

If, at the end, you have adequate performance -- you're not trying very hard. They don't hang those gawdaful gimbals on the outsides of otherwise sleek aircraft for nothing.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

conditions?

Tim Wescott's prescient comments about problems due to vibration, and image attenuation and alignment problems due to a longer optical path, should not be dismissed lightly. At lower altitudes, in a maneuvering high-performance aircraft, there could be extreme vibration. And, every part of the airframe and the stores could be, and probably would be, flexing in at least two directions as well as twisting, in multiple vibrational and other, lower-frequency structural modes. That, alone, does not bode well for a long, complicated optical image path. It would also add weight and more "plumbing" problems. Unless there were some huge benefit from that approach, to make it worth trying to surmount all of the potential problems, and suffer the other downsides, it wouldn't be tried, at least not for low-altitude warplane use. But, if you're talking about only high-altitude cruise conditions, then, maybe you should continue to investigate it.

- Tom Gootee

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Reply to
tomg

On 24 Oct 2006 15:41:06 -0700, "Adam Chapman" Gave us:

Yes, it would. Your mirrors all need to be "first surface" type mirrors (flats). Ellipticals are the most efficient use of area for round tube periscope building. Your field of view would be severely narrowed by the periscope path windows. Bad idea if you want the type of pictures a fully exposed camera could take.

Get one of those fully gymballed creative labs cameras that can be positioned thru the interface. Mount your IR camera head in the movement.

The ideal reflective surface for IR

Reply to
JoeBloe

You can get IR mirrors, but most normal mirror surfaces are suprisingly transmissive to IR. Beware also of the other problem, that at this sort of wavelength, 'glass' is very opaque.

Best Wishes

Reply to
Roger Hamlett

yep:

The first prototype for this:

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EWMS, "Electronic Warfare Management System" in an A10 lasted exactly 1 (one, less-than-a-second) burst of the GAU-8 cannon. The leaded components were nicely sheared off the boards and could be poured from the enclosure ;-)

Surface mount and many iterations eventually fixed this (The A10 was the most "vibrating" plane available so of course the final tests was done there).

Reply to
Frithiof Andreas Jensen

It really depends on which kind of IR the OP means. Near-IR (eg 'night vision' CCTV cameras with IR illuminator) is transmitted adequately by normal glass. Mid-IR (thermal imaging) isn't. That helps greenhouses work - shorter wavelength radiation from the hot sun gets in, longer wavelength radiation from the earth-surface-temperature stuff inside doesn't get out.

Tim

Reply to
Tim Auton

Thanks everyone. The main reason I suggested the mirror periscope idea is because ny aircraft needs to have a very low radar cross-section. A spherical gimbal would have the same radar cross section when the radar is looking at the aircraft from all directions, where as a simple flat plate mirror pokking out of the bottom would only have a significant radar signature over a very short range of directions.

Reply to
Adam Chapman

Any new ideas would be appreciated

Reply to
Adam Chapman

How about a faceted gimbal? Look at an F-117 -- it's all flat plate, but there sure are lots of them.

If you're looking to reduce radar cross section you'll have difficulty with the necessarily straight lines around the gimbal axes, and you'd have to treat the windows very carefully to make sure that the radar bounced off the outside instead of getting thoroughly diffused by the inside of the thing.

I wouldn't write off your periscope idea -- I'd just look at the kind of stabilization you expected to achieve, and ask myself if I could do it with mirrors (but hopefully no smoke).

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

If you don't need to be able to look sideways put the gimball inside the body of the aircraft.

If you do need to be able to look sideways, how about a fairing around the gimball, so that instead of seeing the side of a sphere (which will always present a 'face' directly back towards the radar) the radar sees an angled surface, which would reflect the beam into the ground instead of back at the radar. The fairing would need an optical window/slot and to rotate with the gimball.

Something like this going all the way round, but with a much lower angle (hard to do in ASCII!). View with fixed-width font, eg. Courier:

____________________ < airframe \\ ____ / \\ / \\ / \\ | () | / < fairing \\\\____//

^gimball

If that's not clear, this image shows the kind of shape I mean. The picture is actually of the aeroshell for the Deep Space 2 probe. The shape of the white bit is what I'm getting at, but you'd probably want a flatter cone - as flat as you can manage given the diameter of your gimball and width of your fuselage:

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Tim

Reply to
Tim Auton

On Tue, 24 Oct 2006 19:48:29 -0700, JoeBloe Gave us:

Where did the rest of my post go?

I was saying the ideal reflective surface for IR is gold. Gold faced first surface mirrors make the best IR optical systems.

Put it (the camera) in the nose of the plane, and put the plane's engine up top like a sea plane.

Reply to
JoeBloe

Don't think that is true. Very wavelength dependent. Near ir (1-2microns) I suspect Al will beat gold (assumeing no tarnish). Arround 10 micron Cu is commonly used I understand.

M Walter

Reply to
mark

On Fri, 27 Oct 2006 23:14:30 GMT, mark Gave us:

Gold is used as the foil on spacecraft. There is a reason.

Do you know what that reason is.

Gold is used on the mirrors in the IR thermometers a company I used to work for builds. Do you know why they chose Gold when Aluminum "silvered" mirrors are much cheaper?

Reply to
JoeBloe

First-surface gold mirrors have reflectances of > 99% from very long wavelengths to about 1 micron. They deteriorate gradually from there until they fall off a cliff about 550 nm. Silver holds up very well throughout the visible--it's the best mirror material if you can keep it from tarnishing.

Aluminum is actually quite poor from the near IR to the visible range--there's a big intraband absorption feature near 800 nm that reduces the reflectance of first-surface Al (Al-air) to as low as 88% and second-surface (i.e. glass-Al) relectance to about 80%. The reflectances of gold and aluminum cross at about 600 nm--at longer wavelengths gold is better, and at shorter ones aluminum is better.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Please see the spectral data from this mirror manufacturer:

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--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
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Reply to
joseph2k

Frithiof,

Thanks for posting that.

Looking at the link you provided, it looks like you've been having some great "fun".

Does the 3D Sound help the pilot know which direction to look? That would be cool. They do often seem to have the pesky habit of wanting to actually SEE what is about to kill them, before they'll dispense any countermeasures. ;-)

Your first-prototype-component-shearing story was great! I'd not seen that type of thing happen, before. Vibration is an amazing thing, sometimes. That made me think about the little circular-blade vibrating saws that are used to remove casts from people who have broken a bone: They can touch flexible skin with absolutely no possibility of damage. But the rigid cast material gets cut through quite quickly. I assume those little saws are operating at 60 Hz/50Hz. The GAU-8 goes at 100 Hz, if I recall correctly.

So, maybe someone should invent FLEXIBLE leads for through-hole components? Or maybe flexible solder? ;-))

Thanks again.

- Tom Gootee

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Reply to
tomg

most

The pilot usually cannot see anything at the point of the warning and there is no point: an anti-aircraft missile is too small and much too fast. Instead at the first warning the pilot will "throw" the plane in the escape direction indicated by the EWMS display. He is then likely to be blinded by G-forces and perhaps his head is too heavy to move - but he can still hear!

The idea is that the sound is projected as coming from the chasing missile so he can escape by "running" away from the sound. It is supposed to be pretty intuitive. The EWMS is an automatic system so it will dispense whatever it "thinks" is the best (or a happy mix of everything it's got).

In a modern fighter aircraft the pilot the odds for the pilot is good; I could not find any public data on the F16 (which is the one I know a little about) but it is probably ok to say that the difference between fuel consumption at cruise speed and full throttle shows that the F16 can *move*.

At TERMA they have a decent test area where one first bakes the prototype assemblies in a large oven - under power, while running test - and then the assembly get the same treatment on the vibration table. To see what breaks.

In vibration proving, apart from reducing mass that can resonate, the next best thing is applying Hot-Melt glue in industrial quantities; it's just flexible enough to absorb a lot of energy from vibrations ;-)

Reply to
Frithiof Andreas Jensen

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