Question about collimated display systems.

[mikesblack]

http://science.discovery.com/videos/...simulator.html

Found this link that has a good view of the mylar parabolic mirror.

Is it always the case that the view the crew actually sees come directly from the parabolic mirror? From this presentation, it seems that first thing that occurs is the projectors generate a 2d image on a curved ( semi spherical) screen, that I assume is at 0.5 the radius of the Mirror. The mirror then reflects this image to the crew.

Can it also be the case that the projectors beam to the parabolic mirror first and the reflected light illuminate a viewing surface that the crew would subsequently see?

Thanks.

[castle]

Unfortunately, the physics of optics in this case is not "reversible". The light rays relected from the mirror are collimated and the focus is at infinity. Illuminating a curved screen will result in the eyes focusing at the surface where the image appears. If that worked that would be a far less costly solution and one the sim manufacturers would have employed long ago.

JW

[wledzian]

The view seen by the crew is always a reflection of the screen in the spherical mirror. Due to the shape of the mirror and location of the image on the screen, the virtual image seen by the crew is at or near infinity.

The projectors do not beam to the mirror.

[mikesblack]

Thanks castle,

Why am I to understand that it is improbable a hobbyest could build such a vacume Mylar screen successfully? The materials are seemingly not a cost barrier, and so do you know what process may be beyond reach? I have done a lengthy search and it is quite obvious not too many diy examples abound. If I didn,t know any better, I might think it could be done with favorable results, but given the evidence, or lack there of, I,m inclined not to begin and to seek some other approach. That said, I still am not at that point of totally ruling out the possibility.

Any more info related to this subject would be greatly appreciated.

[castle]

I'm kind of the same mind set regards a DIY collimated display. BTW the link to the short video was cool.

Part of the problem is in the precision required to shape and hold the Mylar in position, then you need to construct the curved screen on which to back project the image(s), then a projector lens to keep things in focus by accounting for the changes in the throw distance projecting a flat image onto the back of the curved screen. You might be able to use a front projection system -- easier to build a solid opaque curved surface but still need to find an appropriate lens system.

Engineering and building the curved mylar mirror is the greatest challenge. Looking at the video portion showing the mirror was informative as to the shape but very little on the vacuum system. Need to come up with some numbers to describe the mirror's geometry. Also speculating the mylar as to quite flexible to stretch in two directions so as to create a snug fit against the spherical surface. Try wrapping some mylar or any sheet of material around a sphere and you get the idea.

As shown in the video the size of the visual system also requires a little more expertise than simply throwing up a 4x8' white board

Maybe a good project to tackle in 2011....

JW

[Mike.Powell]

Most cross-cockpit collimated displays are rear projected. Here's a link to a marketing video showing the general layout: http://www.q4services.com/images/supravue001.mov The company hosting the video, Q4 Services, provides maintenance for Mylar mirrors for just about all types of simulators.

RSI Visuals is a small company that offers a front projected collimated display. The projection path uses a spherical-section fold mirror just above the collimation mirror. http://www.redifun.com/r/products.ph...=3&category=14 Front projection has the disadvantage of requiring a second mirror, but can offer higher contrast and brighter images for the same projector light output. Rear projection has significant issues with back surface reflections.

[mikesblack]

Thanks you castle and thank you Mike. Mike I appreciate the links, especially the "Supraview" movie.

I have perhaps way too many questions. I wonder if either of you or others can address the following.

If one were to start such a project, would it make most sense to first construct the visual mirror with the idea that the other components, be they front or rear projection screen projector/ first mirror, etc, being easier to make the neccessary adjustments . I assume there would be no final adjustments that could be put in place to vary the viewing mirror. So the mirror would have to be done correctly.

I have yet to experiment with N Thusim/ Sol 7, but I can only imagine an image from a parabolic mirror that is not buit with accuracy and neccesary precision could not be compensated with the pre warp process of the 2 d image this software provides.

So then the mirror.
First: Size: I can only afford a 6.5 foot radius. So would the first image need to be created on a 3.25 foot radius spherical screen? Is this even possible with Sol 7?

If this size is not a problem, what must the proper geomety for viwing mirror? I imagine a perfect sherical shape would be constant radius horizontally and vertically? I would want 220 degree viewing and I haven't considered the vertical amount yet.

Orientation of mirror to preimage. It seems there must be some way to prevent the mirror from capturing the simulator shell or any other non image related visual artifact. The examples I have seen so far are of a mirror designed to curve up and out, like a bowl. This way the orrientation is up away from the shell and only on the first projection surface.

Consistancy of mirror surface. Obviously no rinkles. This then is the role of the vacume pump. So some questions about this. Are there any other materials with good mirror surfaces that could be glued on, even painted or moulded so that the vacume pump part could be avoided? Not sure how this was done, but this is an interesting image I found. http://www.dailymail.co.uk/sciencete...ve-mirror.html
If not, how would one need to prepare the mylar sheet. Before applying to a vacume box shaped accourdingly. How would the the mylar be cut to accomidate the shape? would it be glued down? Taped? Tacked?

Thanks so much.

[Mike.Powell]

I've been seriously researching collimated displays for inclusion in an upcoming book on simulator/computer-gaming display systems; however, I don't have all the answers yet. (I sporadically post progress reports on my website.) Here are some thoughts:

The best source of information is searching patents though Google. Patents include a section on prior art to illustrate the shortcoming the new invention is supposed to overcome. Then, of course, the patents describe the new ideas. The writing all seems to be twisted 19th century legalese. Fortunately, there are a lot of pictures. Good search terms are "infinity display", "film mirror", and "collimated display". Check out patents #3,432,219, #3,659,920, #3,785,715, #5,253,116 for starters. There are lots more.

The collimating mirror is a spherical-section of radius R. For proper focus and collimation the image must be located near or on the mirror focal surface which is a spherical-section surface of radius R/2. The mirror focal surface has the same center of curvature as the mirror. The pictures in the sales literature looks otherwise, but they really do have the same center. The odd look is because cross-cockpit collimated displays use off-axis optical paths.

R can be small or huge. Collimated systems have been built for head mounted displays and for wide-body aircraft simulators. You can scale it to meet your needs.

The field of view details can be worked out with ray tracing and high school trig and geometry. Ray tracing with a CAD drawing system works well. I've been using TurboCAD. DoubleCAD XT would probably work and it's free.

Metalized Mylar can be used to make film collimating mirrors. Up to about 40 degrees of vertical field of view, building one is merely difficult. Building one with a 60 degree vertical field of view apparently requires magic. SEOS figured it out a few years ago. Rockwell Collins was so impressed they acquired SEOS in 2008.

I agree. Start with the mirror. If the mirror doesn't work out, nothing else matters.

[castle]

Some other technique other than a vacuum system to hold the mylar will not work. I tired glue with very poor results.

Built a small frame, 5' radius, 60hx45v degrees FOV, just no way to get a smooth application of glue and no matter how thin I spread the glue, the mylar always had bumps and irregularities, and it was impossible to "stretch" the mylar to make it conform to the surface.

So you need to devise a manufacturing process to precisely create a concave surface that is very smooth and uniform. I imagine you could tolerate some variation in precision but it would not be much. And don't forget the projection screen has to be quite precise as well.

Another problem to solve would be some non-uniformity of the mylar attach points along the edges which will result in variations in the tension applied to the mylar. This would be a problem if the mylar was "free standing" and relying on the vacuum to control the shape. If you suck the mylar onto a solid surface, imagine that should not be a problem.

I don't have the links handy but the technique of deforming mylar and glass mirrors with a vacuum has been used by astronomers to produce optical collection mirrors starting in the '80s. The mirrors were much smaller and they actually used the vacuum as a control function to achieve dynamic focusing of the mirror.

My approach would be to pick some arbitrary size as noted above, maybe even smaller, and focus on the mechanical and manufacturing details. That is one question I'm still puzzling over --- does the vacuum actually shape the mylar attached to a frame or simply suck it onto a surface that defines the shape?

One other consideration I've not researched yet is the lens requirements and what f-stop is required to project the image onto a curved screen that is "nearly" in focus everywhere or some other lens arrangement to handle the problem.

JW

[mikesblack]

I just bought a roll, so can't wait to experiment.

My thoughts are to use phenolic or other soft flex material cut into long narrow strips that would act to clamp down on the Mylar film. The strips would be set front and back along the edge of the Mylar sheet. I would need to figure out the interval for spacing drill holes. The idea is to not only hold the Mylar in place, but also to prevent wrinkling at the edges, so there would need to be a tight enough, but also fairly uniform holding pressure all along.

These strips would fit into a track that would be set in the semi sphere frame along the mirrors border. I imagine this track would be made of the same material. The track would need to be snug, but also air tight for suction. I imagine perhaps using a router so that a grove can accommodate the phenolic track housing . The track groove would need to be set square and consistent along a precise arc.

The material should slide into place. I would also need to have these strips set along the vertical edge. Perhaps the track groove would be set so that the track would lie into the frame, set back down.

I need to figure out how much play there is in the Mylar and how and how cut and bunch the material prior to affixing the track. From the movie link that I provided, seems the Mylar prior to suction shows wrinkles at 45 degrees to vertical. Sort of reminds me of those old instant popcorn tin tops that were twisted or rotated so that when the tin expanded with the popping it would expand and rotate out. This is an exaggeration as the wrinkles on the mirror are very slight, but perhaps telling none the less.

Lots of unknowns, so I too will start with smaller models, e.g. 1-2 foot radius. Perhaps in time we'll get it figured out. I know that it would be a terrific enhancement. We shall see what challenges lie ahead. I'll keep you all posted as I progress, or not.