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  1. #91
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    Re: 2 Place Collimated Display

    Amazing, when are you planning to sell it?

  2. #92
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    Re: 2 Place Collimated Display

    We're not. At least until after 2017 when a patent that the design relies on expires.

    Even then, it may only be a plans-only kind of thing. Anything more wouldn't be able to bridge the hassle/income gap.

    g.

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    Re: 2 Place Collimated Display

    Quote Originally Posted by geneb View Post
    We're not. At least until after 2017 when a patent that the design relies on expires.

    Even then, it may only be a plans-only kind of thing. Anything more wouldn't be able to bridge the hassle/income gap.

    g.
    Ah OK You mention in your original post about a vendor and a CD being made available. How do i get those?

  4. #94
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    Re: 2 Place Collimated Display

    You don't. Not until after 2017.

    Rockwell-Collins would turn me into a greasy smear on the pavement if I start selling that thing before the patent expires.

    g.

  5. #95
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    Re: 2 Place Collimated Display

    *headslap* Of course, Wayne, I hadn't even realized that the BP screen needs to be in the focal point of the mirror! Thanks for pointing that out. Seems I should still be able to upsize the mirror and just move my existing BP screen, no?

    When you speak of the pilots and the mirror center point forming an equilateral triangle, are you saying that the center point of the mirror is the center of the sphere that the mirror is (theoretically) part of? That would mean the corners of a 180* mirror would not be directly to the right and left of the pilots. Am I misunderstanding?

  6. #96
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    Re: 2 Place Collimated Display

    Matt -
    No, you can't simply upsize the mirror and move your existing screen, as the screen size needs to scale with the mirror.

    When talking about the centerpoint, I'm actually referring to the vertical axis of symmetry shared by both the mirror and the screen. Ideally, the eyepoint would be along this vertical axis as well, as this would make the optics identical in all directions. Practically, however, this defeats the purpose of a cross-cockpit collimated display, as you can't have both pilots in the same spot at the same time.

    Since the mirror is a sphere, there is spherical aberration in the image - the collimation is not perfect. You could move the pilots straight outward from the center, but that would cause the point of maximum distortion to lie essentially directly in front of each pilot, with the points of minimum distortion at +/- 90°. In order to improve the visual, the pilots are moved forward at the same time. Putting them at the corners of an equilateral triangle is not an optical necessity, merely a convenient compromise. As you said, this places the corners of a 180° mirror behind the pilots, but due to the collimating effect, the pilots will still see a nearly correct image when they look left and right, just as they still see a nearly correct image when they look forward, even though they're not directly in line with the center of the mirror.

  7. #97
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    Re: 2 Place Collimated Display

    Ah yes, I see now why I can't just upsize the mirror. I keep thinking in terms of front projection (what I'm using now) and not a mirror. As many women say, mirrors are not very forgiving.

    Thanks for the further explanation on spherical aberration. I have considered that being off-center from the mirror would throw off the image a bit, but I hadn't given a lot of thought on how best to handle it. I was misunderstanding your comment previously, I was thinking of the spherical origin of the mirror, and moving the pilots *backwards* from there (so the mirror would actually be IN FRONT of the pilots). But you're saying the pilots move forward as they move from the centerline of the mirror, which explains why Castle is so close to the mirror's surface.

    There is one thing I don't understand: how is this system collimated? As I understand collimation, it simply refers to light rays being redirected so that they are parallel to each other (ergo, the Fresnel lens). What we're doing is simply making a mirrored image that is curved so that we see it in 180* (or so), and putting the reflected image in nearly the same plane as the pilot, so that the image "moves" with the pilot as he moves in the cockpit (that it, we're not looking at a fixed point on the physical mirror in front of us). I recall at Flight Safety once I used the word "collimation" in reference to the screen above the cockpit, and the sim tech was quick to point out that it was not a collimation screen, but simply a back-projection screen. Does bouncing the image off the front-projection screen in your setup cause the light rays to run parallel? That *would* be desirable, I'd think, as it would make the image look further away.

    What I need to do now is work out how big my mirror should be for my BP screen size...time to look up those patents from Rockwell, it seems. Now I'm worried that it might come out too small for my cockpit, especially if the pilots have to be inside the mirror. I would think it's going to come out quite large, as the sim it came from was large...I think it even had the complete nose section intact. But, if it's too big, it won't fit into the limited space I have to build my sim shop....arrrghh! I suppose in the worst case I will just build a cylindrical (or spherical, if I get ambitious) "wall" in my shop, and shoot projectors onto it directly. That would still be awesome'ish.

  8. #98
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    Re: 2 Place Collimated Display

    The mirror is the part that makes the collimation happen - the shape of the mirror in combination of the shape of the screen is how it works. Wayne can describe it better, but that's the gist of it.

    See here: http://en.wikipedia.org/wiki/Full_flight_simulator

    Read the section titled "Collimated Cross cockpit displays"

    g.

  9. #99
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    Re: 2 Place Collimated Display

    You mention collimation with a fresnel lens, so I'll start with that. With lens-based collimation, the screen is placed at the lens focal length, so that light rays diverge from a given point on the screen, pass through the lens, and continue on parallel to each other on the other side of the lens. As with a lens, a concave mirror has a focal point. More accurately, a concave parabolic mirror has a focal point. A spherical mirror does not have a single focal point, but if you assume a small enough aperture and a large enough mirror radius, the focal region becomes acceptably tight, in this case smaller than a pixel on the screen.

    As Gene points out, the collimation happens at the mirror, not the screen. The screen does, however, play a very important role. As seen in the image below, each sightline from the pilot reflects off the mirror and intersects the screen. In order to allow the mirror to produce a collimated image, the screen geometry is chosen so that rays from a given point on the screen are parallel when they reach the pilot. In actuality, the calculation is performed by casting multiple parallel rays from the pilot's eyepoint and slightly above and below the eyepoint, and determining the intersection of these rays after they've reflected off the mirror. If the screen is too close to the mirror, the image will appear too close to the pilot, and if it's too far away, an image won't form properly (this is known as overfocus), and will quickly cause eye fatigue.
    PilotView.jpg

    Let's take a look at a bundle of rays coming from the screen.
    Collimation.jpg

    In this case, I've selected the horizon point on the screen. Notice that at the circle representing the pilot's viewpoint, the rays are nearly horizontal (ok, not quite, as I couldn't get the rays to cast from exactly the horizon point). They remain nearly horizontal if the pilot moves his head upwards or downwards. This effect holds true in the horizontal plane as well, so that the pilot will still see a point directly in front of him with both eyes. Because both eyes see the image at the same angle, the image appears far away. It holds true well enough that the pilot can move his head quite a distance, and still see the same point in the image as directly in front of him. For a small display, the acceptable distance may be several inches; for an airliner-sized display, the acceptable viewing volume can be several feet across, accomodating both pilots.

    If you take a close look at the above image, you may notice that the rays aren't perfectly parallel; if the pilot is above the design point, the horizon appears very slightly high, and if he's below the design point, the horizon appears slightly low. This is the effect of spherical aberration. As seen in the next image, if you go too far away from the design point, this aberration, in the form of severe displacement (wrong angle) and overfocus (rays diverge), becomes more apparent.
    aberration.jpg

    If the screen is too close, the image does not appear at infinity, but rather at some finite distance. In the image below, the light source is moved forward of the screen to produce underfocus. I've moved it a large distance forward to exaggerate the effect, but this effect is present for any point in front of the screen. Notice that the rays reflected from the mirror, if traced back through the mirror, would appear to converge at some nearby distance. In this case, the image would appear very close, defeating the purpose of the collimating mirror.
    tooclose.jpg

    In reality, the collimating optics system is a compromise. Moving away from the design point, as is mandated by having two pilots which cannot not co-located at the exact vertical axis of the mirror, ensures that there will be spherical aberration, and thus some overfocus. Fortunately, an image doesn't need to be at infinity to appear distant; At a distance beyond 30 to 50 feet, stereopsis (our ability to judge depth due to stereo vision) plays less of a role, and other cues such as perspective, shadow, and contrast begin to play a much stronger role. Fortunately, in these areas, current graphics do pretty well. This allows the optics designer (in this case, me) to place the screen slightly forward, eliminating or at least strongly reducing any overfocus, while still keeping the nearest image well beyond 50 feet.

    If you can give me the specs on your existing screen, I can crunch some numbers to see if I can fit a mirror to it. I'll warn you ahead of time, though, if it came off a large sim, it will need a large mirror.

    -Wayne

  10. #100
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    Re: 2 Place Collimated Display

    Wow, you are a wealth of information, Wayne! I had to read your post several times....it made a lot more sense after I waited a day or two and came back to it. I still don't fully understand the optics, but you've made it a lot clearer for me. Now I see why the sim tech was not calling the screen a collimation screen, as it does not do the collimation...the mirror does.

    Well, no wonder these big-sim systems look so good...getting the image out near infinity must be HUGELY beneficial for the immersion factor.

    I got some numbers on my screen..it's a big ol' thing:

    Height: 48"

    top bow radius at center: 65.5"
    bottom bow radius at center: 60"

    top bow diameter: 134"
    bottom bow diameter: 124"

    Seems it's not a *perfect* circle, but slightly oblong. What other dimensions do you need? I assume you also need to know the radius and vertical location of the screen's deepest point (largest radius), but I'm not sure how exactly to measure that accurately. I ball-parked it best I could with a tape measure:

    Vertical location: 20" from top bow
    Radius: 67"

    Since the "equator" is not at the vertical center of the screen, I'm not really sure how close I am to a perfect spherical section. And I highly doubt I can get the tech data from the manufacturer.....

    Another note on this screen: the inside surface is very smooth and shiny, the outside surface has a frosted appearance. I *think* there are at least two layers to this thing. Presumably, light passes through the shiny layer, and is diffused by the frosted layer. I have tested it with a projector from both sides. Putting the projector inside the screen (as it was intended) yields a sharp, gorgeous, very bright image on the outside of the screen, and actually shows up fairly well on the inside surface (but with lots of weird reflections and highlights). Beaming from the front of the screen gives a fairly usable image on the inside surface, but it's a bit fuzzy. Again, there are lots of funky reflections and highlights being thrown off by the shiny inner surface. Seems I recall the outside image was not as clear when beaming onto it from the outside, but I can't recall exactly (been a few months since I had it set up). Perhaps the inside layer actually has some optics to it that help it look sharp when diffused.

    Matt