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Thread: Physics question: interference pattern?

  1. #1
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    Default Physics question: interference pattern?

    I've recently turned my Chinese projector into a lumia box using some textured plastic from Home Depot that's used for diffusing florecent light fixtures and I've noticed a strange feature in the projection. When the image fans out there are noticeable bands producing evenly spaced black lines. It seems to be most apparent in the green and red lasers. Is this an interference pattern produced by the light waves or is this a product of some characteristic of the lasers themselves? The green is a dpss laser and the red is a direct diode. Anyone have any insight?

    Ignore the two or three horizontal bands. They are actually lines in the wall that i was projecting on. The bands I'm referring to are visible in the projection itself and follow the contours of the light as the image changes. When the light is focused on a smaller area the frequency of the bands increases and they become smaller but as the light becomes bent outward over a greater area the frequency becomes less and they appear larger.
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    Last edited by ZeroPoint; 11-16-2014 at 22:02.

  2. #2
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    The fact that it is seen using lasers with different wavelengths and methods of generating light makes me think it is a property of the optics they have in common. You say it is more apparent in the green and red, so I assume it is still apparent, but less so with the blue. Try some experiments. Measure the frequency and width of the bands as you vary power and with each of the colors. If you provide some images, that will help a lot in deciding what they may be or what would need to be checked to narrow it down.

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    updated with photos. it seems that the same effect is present with the violet but its harder to see due to the laser itself being less visible.
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    Upon inspecting my avatar photo I have realized that the bands are present there as well. My avatar photo was taken years ago using different lasers and textured glass instead of plastic. There were no special optics in the original lumia other than the collimator lenses and the textured glass. This new lumia goes through a series of dichros and a single galvo mirror.
    LASERS!!

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    If I had to guess, I would say that it is a somewhat of a prismatic effect on par with lenticular plastic. I think it is mostly caused from the machining artifacts on the metal roller that presses these diffusers. Similar to how a mill will leave cicular lines in metal. If that metal is used as a mold to press acrylic, those machining lines would be present in the plastic too. If you run a beam through it, it would act like a lenticular diffuser but not as clean and uniform.
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    I had always assumed that those were the interference fringes.

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    A typical lumia is about 50% interference pattern and 50 caustic optics. "Caustics" is the name lens designers apply to bad aberrations. The name was picked up by optical artists in the 70s for patterns caused by distortions in lens like materials, and is the proper term.

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    I suspected it may have been the plastic but then I noticed the same effect from my first lumina that used glass with a different texture pattern. I suppose the same process may be used for textured plastics and glasses though which would cause the same aberrations. Is this a common occurrence in everyone's lumias then? Are there any techniques to minimize the bands? They aren't really bothersome but I would like to try for a more fluid projection. I'll have to sample some different diffusers.

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    To be honest, my first lumia pieces were plastics like the ones you're using. I didn't like the patterns at all. I swtiched to rolled glass. You can't beat it. It is way cleaner than plastic IMO and the patterns shift more gradually when rotating them. It does seem like the bands of light are fewer when using glass, as well.
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    Quote Originally Posted by Mattronium View Post
    I had always assumed that those were the interference fringes.
    That's what they look like to me.

    These can be seen in Michelson interferometer set ups or with a 50mw 532nm Chinese laser from 1999 with obscene amounts of speckle aimed at a suspended droplet of water collected from the soil of a potted tomato plant and projected onto one's bedroom wall:

    https://www.flickr.com/photos/raytra...57624842473450

    Interference patterns in motion:

    https://www.flickr.com/photos/raytracing/4942214591/

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