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Need help focusing IR using a fresnel lens

  1. Oct 8, 2008 #1
    Hello everyone,

    I want to focus IR on a small rectangular area using a Fresnel lens. I tried trial and error to find the focal point of the lens but can't seem to be able to get the exact point. The frensnel lens is a multiple fresnel lens. Can anyone please help?

    Thanks =)
     
  2. jcsd
  3. Oct 9, 2008 #2

    Redbelly98

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    Welcome to PF.

    What wavelength (range) of IR?

    Try using a visible source to find the focal point. It will not be very different for IR.
     
  4. Oct 13, 2008 #3
    I tried using a laser pointer to do that. It was about 600nm. But it was too focused to let me see any difference. Can u suggest any other light source?
     
  5. Oct 13, 2008 #4

    Redbelly98

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    You can take it outside and use the sun. Or inside, if the sun is visible through a window.

    Less accurate, but maybe reasonably accurate, is to use a ceiling lamp or other lamp, at a distance at least 10 focal lengths away from the lens.

    1/f = 1/(focusing distance) + 1/(lamp distance)

    Hope that helps.
     
  6. Oct 18, 2008 #5

    Redbelly98

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    Note:
    BE VERY CAREFUL if using the sun as a light source. Only focus the sun onto non-flamable surfaces!
     
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