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How to get information on low frequencies of a signal using FFT?

  1. Jun 25, 2011 #1
    Hello. I've made a lab experiment of interference and with a CCD camera photographed fringe pattern which one can understand as a periodic signal.
    My question is: When I apply an FFT to the image is there any limit to the amplitude information I may get from lower frequencies?
    I read in the internet that for a 1 sec signal, one may be able to obtain information about only freqs above 1 Hz.
    Why do I ask this? Because the width of the fringes is of about 1/10 the size of the total image, and I am interested in low frequencies.

    Now I am processing the image in ImageJ in order to get some information.

    Thanks in advance.
     
  2. jcsd
  3. Jun 26, 2011 #2

    MATLABdude

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    Welcome to PhysicsForums!

    I'm not quite sure how you're getting temporal measurements (hz) from the FFT of an image (usually in vector space), unless you're asking about contributions from low-frequencies in general.

    When you mention fringes being 1/10 the size of the image, you may have things reversed: the contribution from that should actually have a spatial frequency of 10, not 1/10 (think 10 fringes per image):
    http://sharp.bu.edu/~slehar/fourier/fourier.html#harmonics [Broken]
     
    Last edited by a moderator: May 5, 2017
  4. Jun 26, 2011 #3
    The lowest positive frequency represented in the FFT output will be 1 cycle over the length of data. So, for a 1 second signal, there would be information for ..., 0 Hz, 1Hz, ...

    1 * fs/N = 1/(Ts * N) = 1/(Total time)
     
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