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How to photograph a diffraction pattern?

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  1. Mar 8, 2015 #1
    I want to take a picture of a diffraction pattern directly. If I project it on a wall I see a clear pattern, but when trying to get the pattern on the sensor, I only record a bright green spot. I don't know what the problem is.

    I'm using a ~50mm focal length lens focused at infinity. Is this the right setup? Should the photo be taken without the lens?
     
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  3. Mar 8, 2015 #2

    mfb

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    Can you change the exposure time or make an HDR image? The central spot can be very bright relative to the others if single-slit interference is relevant.
     
  4. Mar 8, 2015 #3

    blue_leaf77

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    The sensor might have already been saturated. The huge difference in the zeroth diffraction order than the others is normal, that's why sometimes diffraction pattern image is displayed in log scale.

    Reducing exposure time or placing a filter might help. Remember that if you use filter, its thickness might slightly change the overall size of the diffraction pattern.
     
  5. Mar 8, 2015 #4
    Hi jinawee, I've done this at home myself.
    I'd recommend you experiment with
    • different exposure times
    • different camera settings (i.e. if you can change exposure compensation, white balance, ISO)
    • project the pattern on a white wall or white paper/screen
    • turn off all unnecessary room lighting and/or do it in the evening/at night, i.e. exclude all superfluous light
     
  6. Mar 9, 2015 #5
    Thanks, I'll give it another try with a longer exposure.
     
  7. Mar 9, 2015 #6

    Drakkith

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    How are you producing the diffraction pattern? Are you using a prism or a diffraction grating? Any focusing/collimating lenses in your setup? (Other than your imaging camera)
     
  8. Mar 9, 2015 #7
    I'm strudying Fresnel diffraction, so:

    Laser pointer -> ~-10mm diverging lens -> Slit -> 2m -> Camera
     
  9. Mar 9, 2015 #8

    Drakkith

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    Is there any chance you could upload a picture of the diffraction pattern on the wall and give us an indication of its size?
     
  10. Mar 16, 2015 #9
    The image I see on the screen is something like this:

    6itAKZ1m.jpg

    The central line about 1mm thick. The diffraction pattern covered the whole lens, so the size shouldn't be a problem.

    I used higher exposition times and open and closed diaphragms but got nothing. For example,

    jxkSY0Am.jpg

    those points might be from the sensor, but it's not what I was looking for. It seems that the bright spot from the lase has some structure:

    5aANK8wm.jpg

    7JfBtyRm.jpg

    Would this be part of the diffraction pattern from the slit?

    I was recommend to focus the camera at infinity, but since this is a divergent beam shouldn't I change the focus distance. Anyway, I don't think that's the problem because I tried several distances.
     

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  11. Mar 16, 2015 #10

    mfb

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    Do you have a sketch of the setup?
    I was assuming you make a picture of a screen, but the last post doesn't sound like that.
     
  12. Mar 16, 2015 #11

    sophiecentaur

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    It seems to me that the problem with photographing the diffraction pattern could be the ratio of intensities you are trying to record. You probably have only, effectively 8 bit level resolution, which is 256 levels and also, unless your optics are very good, the flare of a lens could be limiting the resolution. Why are you not using a projection on a screen? It could be a good idea to 'mask' the centre spot by using a small hole / slot in a screen to let the peak of the pattern through. You could also double or triple the throw of your projection. The loss of light can easily be dealt with by a longer exposure.
    There could also be a problem with the JPEG processing of the image. Could you set the camera to TIFF or RAW format?
     
  13. Mar 16, 2015 #12
    Perhaps HDR might be worth investigating. That is, combining multiple images taken with different exposure.
     
  14. Mar 16, 2015 #13

    Drakkith

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    I don't think you want to be in focus. It seems to me that being in focus just means you're imaging the aperture of the slit. The particular diffraction pattern produced on the wall is a result of the light diverging. If you haven't already, you could try going waaaay out of focus and slowly coming back in and observing what happens to the pattern.
     
  15. Mar 17, 2015 #14

    sophiecentaur

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    If it were only a matter of ratio of light levels then that would be a good idea. I suspect HDR could just reveal artefacts of the optics, though.
     
  16. Mar 17, 2015 #15

    sophiecentaur

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    Yes. You really need to be 'focussing' on a real image that's at the position of a notional projection screen. I can't get my head around the consequences of that and, personally, I'd be inclined to put an actual screen in that place! You should get a wider part of the diffraction pattern that way, I think.
     
  17. Mar 17, 2015 #16

    Andy Resnick

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    If you put your camera at the screen (no lens on the camera), the CCD should directly record the diffraction pattern.

    Alternatively, try putting your camera (with lens attached, focused to infinity) as close to the aperture as you can.
     
  18. Mar 17, 2015 #17

    sophiecentaur

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    I would think that using a camera sensor directly could limit the size of the image.?
     
  19. Mar 17, 2015 #18
    First of all, I'm trying to record the image directly because my professor said that was the correct way to measure the intensity. I should compare the measured intensity with the predictions.

    Now, HDR doesn't seem to be relevant because it consists on "fusing" the highlights and the shadows in a single image. But I can't get the shadows to start.

    I'm confused why it's so easy to get an image of the pattern projected on a screen, but not directly. Why do I observe a bright point from the laser through the camera, but none on the screen?

    It seems that placing the screen modifies the relative intensity, but why? Shouldn't it preserve the dynamic range?

    I think that the lens itself is part of the problem, since there might be internal reflections. Random phase shifts might be important too. I haven't tried without the lens to prevent dust getting into the sensor. I could try with just a piece of glass covering it. Or using a webcam without the lens, although the sensor would be really small and I would need some mechanism to move it along a line.

    My professor has commented that we could drop the laser and use a home-made spectrometer as a light source. I don't think this is a good idea because it would have quite low intensity and it's hard to calibrate.
     
  20. Mar 18, 2015 #19

    Andy Resnick

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    Yes, but since the diffraction pattern is a function of angle, just move the sensor closer to 'see' more of it. To be sure, there are limits (pixel size, spurious patterns from the Bayer filter, etc.), but it's quick and easy.
     
  21. Mar 18, 2015 #20

    Andy Resnick

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    Because, as far as your camera (or eyeball) is concerned, the screen is an object that emits light. Another way to see this is to have your camera (with lens) facing the laser/aperture, about 10 feet away. Place a thin sheet of paper somewhere in between the laser and camera, then focus on the paper- you'll see the diffraction pattern (or whatever). Then, without re-focusing, remove the paper. What do you see now?
     
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