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Thermal imager versus foil

  1. Nov 2, 2007 #1
    Just been looking at a video of a bunch of people at night, showing up
    glowing white, through FLIR imager. Would wearing an aluminised cloak
    make one invisible to it? In other words, would the IR sensor see the
    heat emitted from the cloak (at body temp) or the reflection off the
    cloak of the cooler surroundings?

    --
    Dirk

    http://www.transcendence.me.uk/ - Transcendence UK
    Remote Viewing classes in London
     
  2. jcsd
  3. Nov 3, 2007 #2
    In article <5ovascFos2ptU1@mid.individual.net>, Dirk Bruere at NeoPax
    <dirk.bruere@gmail.com> writes:

    > Just been looking at a video of a bunch of people at night, showing up
    > glowing white, through FLIR imager. Would wearing an aluminised cloak
    > make one invisible to it? In other words, would the IR sensor see the
    > heat emitted from the cloak (at body temp) or the reflection off the
    > cloak of the cooler surroundings?


    I'm assuming you're talking about an infrared-sensitive camera. IR is
    simply light at a longer wavelength. If we see something, then either
    it is reflecting the radiation (normally the case for visible light, at
    least where people are concerned) or emitting it (normally the case for
    infrared, at least where people are concerned). Think about "reflection
    off the cloak of the cooler surroundings" in visual terms, i.e.
    reflection from DARKER surroundings. Obviously, if something is visible
    and is surrounded by darkness, this doesn't make it less visible (in
    practice, it makes it more visible due to greater contrast). The same
    would apply in infrared, i.e. cool or dark surroundings would not make
    something invisible that is otherwise visible.

    If you wear an aluminised cloak, at first it will reflect your IR
    radiation back towards your body, making you invisible to some extent,
    at least at first. However, with time it will probably heat up to the
    same temperature as your body, and you will become visible again.

    Of course, if there are other IR sources nearby, then your cloak could
    reflect the radiation into the camera, rendering you visible.
     
  4. Nov 3, 2007 #3
    where the foil touches your skin it will conduct the heat and radiate. you
    need to insulate your body from the outer surface, just be warned that doing
    it properly results in all the body heat being retained inside the suit so
    it gets hotter and hotter the longer you wear it.


    "Dirk Bruere at NeoPax" <dirk.bruere@gmail.com> wrote in message
    news:5ovascFos2ptU1@mid.individual.net...
    > Just been looking at a video of a bunch of people at night, showing up
    > glowing white, through FLIR imager. Would wearing an aluminised cloak make
    > one invisible to it? In other words, would the IR sensor see the heat
    > emitted from the cloak (at body temp) or the reflection off the cloak of
    > the cooler surroundings?
    >
    > --
    > Dirk
    >
    > http://www.transcendence.me.uk/ - Transcendence UK
    > Remote Viewing classes in London
    >
     
  5. Nov 4, 2007 #4
    Phillip Helbig---remove CLOTHES to reply wrote:
    > In article <5ovascFos2ptU1@mid.individual.net>, Dirk Bruere at NeoPax
    > <dirk.bruere@gmail.com> writes:
    >
    >> Just been looking at a video of a bunch of people at night, showing up
    >> glowing white, through FLIR imager. Would wearing an aluminised cloak
    >> make one invisible to it? In other words, would the IR sensor see the
    >> heat emitted from the cloak (at body temp) or the reflection off the
    >> cloak of the cooler surroundings?

    >
    > I'm assuming you're talking about an infrared-sensitive camera. IR is
    > simply light at a longer wavelength. If we see something, then either
    > it is reflecting the radiation (normally the case for visible light, at
    > least where people are concerned) or emitting it (normally the case for
    > infrared, at least where people are concerned). Think about "reflection
    > off the cloak of the cooler surroundings" in visual terms, i.e.
    > reflection from DARKER surroundings. Obviously, if something is visible
    > and is surrounded by darkness, this doesn't make it less visible (in
    > practice, it makes it more visible due to greater contrast). The same
    > would apply in infrared, i.e. cool or dark surroundings would not make
    > something invisible that is otherwise visible.
    >
    > If you wear an aluminised cloak, at first it will reflect your IR
    > radiation back towards your body, making you invisible to some extent,
    > at least at first. However, with time it will probably heat up to the
    > same temperature as your body, and you will become visible again.
    >
    > Of course, if there are other IR sources nearby, then your cloak could
    > reflect the radiation into the camera, rendering you visible.
    >


    Then let me rephrase the question:
    Are all surfaces and materials at a given temperature equally good
    radiators? Is Al foil at 37 degC as bright as skin at 37 degC to a
    thermal imager?

    --
    Dirk

    http://www.transcendence.me.uk/ - Transcendence UK
    Remote Viewing classes in London
     
  6. Nov 4, 2007 #5
    Dirk Bruere at NeoPax wrote:
    > Just been looking at a video of a bunch of people at night, showing up
    > glowing white, through FLIR imager. Would wearing an aluminised cloak
    > make one invisible to it? In other words, would the IR sensor see the
    > heat emitted from the cloak (at body temp) or the reflection off the
    > cloak of the cooler surroundings?


    Probably not. I couldn't find a measured reflectivity of aluminzed
    mylar between 8-12 microns, but let's assume it's highly reflective.
    This document had some data:

    http://sti.srs.gov/fulltext/ms2000921/ms2000921.html

    The wearer will reflect the environment- including the cold sky (-50 C).
    See Figure 4 in the above document. This will still register as an
    anomaly on the IR detector.

    Plus, if one is wearing a reflective cloak, unless they don't need to
    see where they are going, they will need holes to see and breathe, both
    of which will be easily visible unless additional measures are taken to
    cloak the leaks (as is done on the B2 stealth bomber)

    --
    Andrew Resnick, Ph.D.
    Department of Physiology and Biophysics
    Case Western Reserve University
     
  7. Nov 4, 2007 #6

    marcusl

    User Avatar
    Science Advisor
    Gold Member


    The property you are asking about is called emissivity, and tables are readily available on the web. Most natural substances have e = 0.9 to 0.98, including cloth, skin, dirt, snow and ice. The surfaces of lakes and the ocean are much lower, and the lowest e's are for polished metal surfaces (in the range of 0.05, if I remember). Aluminized mylar depends. The aluminized side is probably low if it is new and not overcoated, but the plastic side has a high e just like all organic materials. If you are shiny side in, then much body heat will be reflected but the film will be warm due to the air (you know how hot you get inside of a non-breathable raincoat) so the plastic side should radiate at body temp. Al side out will radiate less.
     
  8. Nov 6, 2007 #7
    On Nov 3, 7:23 am, Dirk Bruere at NeoPax <dirk.bru...@gmail.com>
    wrote:
    > Then let me rephrase the question:
    > Are all surfaces and materials at a given temperature equally good
    > radiators? Is Al foil at 37 degC as bright as skin at 37 degC to a
    > thermal imager?


    Good absorbers are good radiators, while good reflectors are terrible
    for both absorbing and radiating. Rule of thumb.

    >From playing with FLIR cameras, my experience is that aluminum

    usually looks much darker than the outdoor surroundings, since it's
    reflecting the cold sky or at least the distant landscape. If an
    aluminum
    cloak was warmed up to 40C, I don't know if it would radiate enough
    to offset the 'cold' environment reflections. The camera might see
    a black object moving against the background rather than a white
    object.

    To become "invisible" to ground-based cameras you'd do the same as
    you
    do for visible light: hide behind a large mirror which is motionless,
    vertical,
    and reflects the horizon. But for 10u, a matte finish on aluminum
    looks
    like a highly polished mirror. So any piece of sheet metal would
    probably
    work. Bend it into a cylinder and hide within.
     
  9. Nov 8, 2007 #8
    billb@eskimo.com wrote:
    > On Nov 3, 7:23 am, Dirk Bruere at NeoPax <dirk.bru...@gmail.com>
    > wrote:
    >> Then let me rephrase the question:
    >> Are all surfaces and materials at a given temperature equally good
    >> radiators? Is Al foil at 37 degC as bright as skin at 37 degC to a
    >> thermal imager?

    >
    > Good absorbers are good radiators, while good reflectors are terrible
    > for both absorbing and radiating. Rule of thumb.
    >
    >>From playing with FLIR cameras, my experience is that aluminum

    > usually looks much darker than the outdoor surroundings, since it's
    > reflecting the cold sky or at least the distant landscape. If an
    > aluminum
    > cloak was warmed up to 40C, I don't know if it would radiate enough
    > to offset the 'cold' environment reflections. The camera might see
    > a black object moving against the background rather than a white
    > object.
    >
    > To become "invisible" to ground-based cameras you'd do the same as
    > you
    > do for visible light: hide behind a large mirror which is motionless,
    > vertical,
    > and reflects the horizon. But for 10u, a matte finish on aluminum
    > looks
    > like a highly polished mirror. So any piece of sheet metal would
    > probably
    > work. Bend it into a cylinder and hide within.


    The question arose from a video I was looking at of four guys at night
    being blown away by a 30mm cannot from an AC130 at an estimated range of
    some 3km.
    I suppose I could post a URL to it if anyone is interested, but it's
    pretty graphic. Anyway, they showed up against the ground and nearby
    buildings as bright white on black.
    I was wondering if they would have been so visible had they been wearing
    one of those popart aluminised raincoats that is almost cloaklike with a
    hood.

    Anyway, it might be an interesting experiment for your site if you can
    borrow a thermal imager and such a raincoat and view it from various angles.

    --
    Dirk

    http://www.transcendence.me.uk/ - Transcendence UK
    Remote Viewing classes in London
     
  10. Nov 9, 2007 #9
    Is in not also true one could use a laser based light source operating in the same wavelength region as the thermal imager to illuminate and increase the contrast of the individuals trying to cloak themselves.
     
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