infra-red and mirrors

Discussion in 'Digital Photography' started by Adam Chapman, Oct 24, 2006.

  1. Adam Chapman

    Adam Chapman Guest

    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
     
    Adam Chapman, Oct 24, 2006
    #1
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  2. Adam Chapman

    Paul Rubin Guest

    "Adam Chapman" <> writes:
    > If I used a series of mirrors and the camera was embedded deep within
    > the aircraft, would the infra-red camera still work?


    If you mean near-IR, then probably yes. Simplest experiment is get
    hold of a camcorder with nightshot and point it at your bathroom
    mirror on IR and see what happens.
     
    Paul Rubin, Oct 24, 2006
    #2
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  3. Adam Chapman

    Adam Chapman Guest

    The aircraft need to watch the ground from a height of 300m. I do not
    quite understant what you mean by near-IR
     
    Adam Chapman, Oct 24, 2006
    #3
  4. Adam Chapman

    Paul Rubin Guest

    "Adam Chapman" <> writes:
    > The aircraft need to watch the ground from a height of 300m. I do not
    > quite understant what you mean by near-IR


    IR refers to a range of frequencies. The higher IR frequencies are
    near the frequencies of visible light and are called Near IR.
    The lower frequencies are further away and are called far IR,
    thermal IR, etc. The ones in the middle are called medium IR.

    Imaging with thermal IR has traditionally required chilled sensors
    though I gather there's ways around that these days. Near IR imaging
    is not much different than ordinary photography.
     
    Paul Rubin, Oct 25, 2006
    #4
  5. Adam Chapman

    Adam Chapman Guest

    Thanks for the help. I really appreciate it.
     
    Adam Chapman, Oct 25, 2006
    #5
  6. Adam Chapman

    Scott W Guest

    Scott W, Oct 25, 2006
    #6
  7. Adam Chapman wrote:
    > 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


    Yes, but you may need to watch the material used for the mirrors. For
    near IR normal mirrors should work. For thermal IR, gold is better,
    and the mirrors MUST be first surface ones. Glass and aluminum oxide
    protective overcoats will strongly absorb thermal IR. Aluminized
    mirrors, even first surface ones, will oxidize and eventually dim in
    the thermal IR so they are not really recommended. The amount of gold
    needed to coat a metal or glass substrate isn't really that much so is
    not prohibitively expensive.
     
    Don Stauffer in Minnesota, Oct 25, 2006
    #7
  8. Don Stauffer in Minnesota wrote:
    > Adam Chapman wrote:
    >
    >>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

    >
    >
    > Yes, but you may need to watch the material used for the mirrors. For
    > near IR normal mirrors should work. For thermal IR, gold is better,
    > and the mirrors MUST be first surface ones. Glass and aluminum oxide
    > protective overcoats will strongly absorb thermal IR. Aluminized
    > mirrors, even first surface ones, will oxidize and eventually dim in
    > the thermal IR so they are not really recommended. The amount of gold
    > needed to coat a metal or glass substrate isn't really that much so is
    > not prohibitively expensive.
    >

    I use aluminum coated first surface mirrors, standard ones off the
    shelf, uncooled in spectroscopy from 0.2 to 150 microns,
    that covers UV to far infrared. Uncooled mirrors are used
    in the thermal IR all the time, in the lab, on aircraft,
    on telescopes, and on spacecraft.

    Roger
     
    Roger N. Clark (change username to rnclark), Oct 26, 2006
    #8
  9. Adam Chapman

    Adam Chapman Guest

    Thanks everyone,

    I was hoping i could coat the mirror in gold anyway because it will
    reduce the chance of being detected by radar
     
    Adam Chapman, Oct 26, 2006
    #9
  10. Adam Chapman

    Adam Chapman Guest

    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.

    Any new ideas would be appreciated
     
    Adam Chapman, Oct 26, 2006
    #10
  11. Adam Chapman wrote:
    > 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.
    >
    > Any new ideas would be appreciated


    Depends on the size of the mirror and the wavelength of the radar. If
    the size of the mirror is anywhere near the wavelength of the radar,
    you will not get a specular reflection, you will get a diffractive one
    (physical optics, not geometrical optics, and the return will be over
    much wider range of angles.

    For mininum radar return from the mirror it could have a glass
    substrate and a multi-layer interference coating (reflective at desired
    wavelength). Coating materials selected to minimize response at radar
    frequency. Unfortunately, such a mirror will take a fairly
    sophisticated effort to design. For that matter, the radar return from
    a rectangular metal mirror at a given wavelength is not trivial to
    compute either, especially when combined with a complex structure. One
    way out is to use a gimballed structure shaped not spherically, but
    some other low cross-section shape, with a window coated with metallic
    or at least conductive coating that is transparent to IR. Such windows
    have been developed for some time for use in stealthy aircraft.
     
    Don Stauffer in Minnesota, Oct 26, 2006
    #11
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