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Strange optical effect

Discussion in 'Multimedia, Projection, and Show Control' started by Andrijana, Mar 13, 2018.

  1. Andrijana

    Andrijana New Member

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    Hi,
    I've been using two fisheye lens projectors for rear projection (one connected through VGA and other through HDMI) to screen one image (edge blending I did through Resolume Arena).
    Now, what is really strange is: when you look at the screen from the front, the image is fine, as you move to the right, the image from right projector becomes darker, and as you move to the left, the left side of the screen becomes darker. It is quite annoying, especially when I don't know why does it happen and how to solve it. Both projectors are the same type and intensity, and the same thing happens when they are not connected to the computer.

    Please if you have any idea why it might happen, I'd be endlessly thankful...
     
  2. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Andrijana Hello; I'm so old all of my projection experience relates to 35 mm Kodak Carousel slide projectors but I suspect you're looking at the limitations of what screen material manufacturers call "viewing angle". Regardless of what type of projector, when you project from a greater distance with a narrower projection lens the image from your projector is hitting the screen more or less straight on. I'm thinking primarily of rear projection but the same holds true to a degree with front projection as well. When you use an extremely wide angle lens to throw a comparatively short distance to your screen, you are counting on your screen material to bend the image to face out equally to all patrons. The bulk of my experience was with Da-Lite and Rosco Laboratories screens. Rosco had a black screen which was DRAMATIC when used for rear projection as you could produce bright, brilliant, vividly colorful images on a totally black screen. Its major limitation was an extremely narrow viewing angle with patrons seated on the center line seeing the full image and those seated off to one side seeing literally only half the screen lit with the opposite side remaining visually black and unlit. At the time, approximately 30 years ago, Rosco Laboratories manufactured screens in black, dark grey, light grey, white and twin white with their viewing angles getting wider and wider in that order. Their twin white was promoted for simultaneous front and rear projection. I never personally had any of their twin white to deal with.
    I very much suspect you're seeing the limitations of your screen material's included viewing angle.
    With apologies for inadvertently hitting post while in the midst of composing my post.
    EDITED: To correct a spelling mistake, I omitted the 'n' in brilliant.
    Toodleoo
    Ron Hebbard.
     
    Last edited: Mar 13, 2018
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  3. ruinexplorer

    ruinexplorer Minion CB Mods Premium Member Fight Leukemia

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    I agree that this is a result of the screen viewing angle.
     
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  4. Andrijana

    Andrijana New Member

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    @RonHebbard @ruinexplorer
    Thank you for your quick answer!!
    We are using NEC light gray wide angle rear projection screen, dimensions 6 x 2.25m, and the company we ordered it from said they never had such a problem... Maybe the problem might be in the viewing angle of the projectors?
     
    Last edited: Mar 14, 2018
  5. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Andrijana I doubt it. I'm still suspecting its a problem with the viewing angle of your screen. @ruinexplorer What're your thoughts?
    @Andrijana Do you have any other projector / lens options that would let you run a test in which the projected image would hit the screen at closer to 90 degrees purely as a test?
    Again, only with testing in mind, can you place your screen further downstage and project on it with a narrower lens to see how the image appears? Is your screen hung on a fly system pipe and weighted across its lower edge or is it supported on four sides and stretched on a frame? I'm NOT suggesting this alters viewing angle, only asking questions and thinking in terms of testing options. I'm still strongly suspecting your isssue is your screen's viewing angle.
    Toodleoo!
    Ron Hebbard.
     
  6. ruinexplorer

    ruinexplorer Minion CB Mods Premium Member Fight Leukemia

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    @Andrijana, I apologize for the delay in answering. I went to USITT and am just catching up. I have a couple of pictures as to what I think you are seeing. I got these from FB from someone who seemed to be experiencing something similar. In addition, I have added a chart from a screen manufacturer showing the optimal gain for a rear surface with short throw lenses.

    Is this kind of what you were seeing?

    center.jpg left.jpg right.jpg short_throw_optimal_gain.jpg
     
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