Mirrors and Curved Screens

[castle]

Hey, what can I say? MyCockpit is just a great site to hang around!


I find collimated displays very interesting. While the FAA/CAA/etc. require them for level C/D sims because they present the same perspective view throughout the sim cabin, I think the hobby community would benefit because collimated displays can provide an expansive view within a restricted space. The optical design is straightforward, but as I'm sure many know, the physical implementation is a real handful. I've been researching the topic for a possible new book.

Not an optics guy, so could be all wrong on this....

I was under the impression that the visual systems in high end sims use curved mirrors to reflect a virtual image back to the viewer. That is not the same as viewing an image on a curved screen; i.e. bouncing an image off a curved mirror to a curved screen is not the same as viewing an image of a curved screen on a curved mirror. Or is it?

And that is what makes the problem so difficult for the hobbyist not to mention the cost and precision required.

As you've noted, the rays of light from the viewing surface hitting the viewer's eye must be parallel or collimated which is easy to understand from a math and geometric point of view ( ooo, -2 for bad pun) but hard to build.

JW

[Neil Hewitt]

Hi neilh,

I'd be intersted to know more about the curved mirrored you are using and more generally about your approach. I am considering all options related to creating true stereoscopic vision. I plan on building a semi spherical surface that will surround my fov to 200 degrees.

My thoughts are using Wideview, rather than T2go to accomidate a true circulair FOV, Nthusim for image distortion and 3 or 5 projectors. I've been considering ways other than collimated mirrors as a of the considerations mentioned here(cost and dificulty of precision fabrication), but have come up short.

If you can add more detail, would be greatly appreciated.

Mike

Hi Mike.

I'm a long way from the visuals part of my project. Still on the shell build, probably not even halfway done there yet. It's slow going this year.

That said, my tentative plan, because of a lack of space, is to rear-project onto a curved screen positioned relatively close to my windows via an intermediate mirror to give me the throw length. I was planning on creating a simple curved mirror surface to match the curvature of the projection screen. NTHUSIM to adjust it all nicely. I briefly entertained crazy ideas about CAVE-style visuals but I don't see that working. I know that with this setup I'm not going to get anywhere as good an impression of depth as I might if I could put the screen a few meters away, which is why I was interested in collimation as a topic.

Having followed this thread and others, I am wondering what could be done with mirrors, even non-spherical, non-collimated mirrors. It seems like a way to get a large display in a small space. But I'm a long way from truly understanding what would need to be done. More study time called for, I think.

[Neil Hewitt]

Not an optics guy, so could be all wrong on this....

I was under the impression that the visual systems in high end sims use curved mirrors to reflect a virtual image back to the viewer. That is not the same as viewing an image on a curved screen; i.e. bouncing an image off a curved mirror to a curved screen is not the same as viewing an image of a curved screen on a curved mirror. Or is it?

And that is what makes the problem so difficult for the hobbyist not to mention the cost and precision required.

As you've noted, the rays of light from the viewing surface hitting the viewer's eye must be parallel or collimated which is easy to understand from a math and geometric point of view ( ooo, -2 for bad pun) but hard to build.

JW

Yes, absolutely. Mike Powell's site has a great section on exactly this. Commercial sims put the mirror in front of the pilot's seating position as a display. The use of curved mirrors to reflect a projected image onto a projection screen is something different that some builders have used to get over not having a long enough throw length or enough room in their sim room. I'm in precisely that boat. The use of the curved screen itself is (as I understand it) to lessen the horizon-distorting effects that you can get with the traditional 'three screens at 45 degrees' approach. You can't (as far as I know) do a collimated display that way. I suppose you could put a collimating fresenel lens in-between the projector screen and your viewpoint, but making it work would be, erm, hard.

I should shut up now. Plenty of people here know more than me

[mikesblack]

I wonder what the most dificult part someone without "profesional equipment" would encounter in the attempt at building a diy parabolic mirror. Matt I believed mentioned he had been working with this and ahd made an attempt. I'm currious what difficulties he had encountered and what things he thought made this project improbable.

Also, I am quite new to optics as well, and so I may be somewhat nieve with my ideas I'm expressing, but I wonder about Wideview. Given enough seperate windowd views, could we theoretically converege on a collimated type view.

Say for example we had 10 networked PCs or 20 or some crazy number, let's say for example 180 projectors each beaming at 1 degree change along the circumferance screen would we in effect be doing the same thing a curved mirror would.

[Mike.Powell]

A collimated display creates an image which appears to be at a great distance from the viewpoint. I am unaware of any way to create a collimated image directly from a nearby projection screen. A distant projection screen will approach collimation. Lacking that, light bending optics are required. Given the required size of the optics, mirrors are the approach of choice.

If you look at you own reflection in a flat mirror you see an image of yourself which is as far behind the mirror as you are in front of it. If you could slowly curve the mirror into a spherical, concave shape, your image would appear both to get larger and to move away from you. At some point the image appears "infinitely" far away and "infinitely" large. The light ray paths from each image point are parallel. This is collimation.

In this example, the image also becomes quite distorted. A spherical-section mirror can collimate, but also distorts because rather than having a flat focal plane, it has a spherically curved focal surface.

In simulator collimated displays, the real image is created on a spherically curved surface. Generally this is an acrylic rear-projection screen. If the screen has a radius of curvature that is one half the curvature of the mirror, AND if the mirror and screen share the same center, the virtual image (what you actual see) will be both collimated and undistorted.

The reason that the mirror is a spherical section is that spherical mirrors are easier to make, and don't have a required or preferred viewing axis.

Most, if not all, collimation mirrors used in contemporary, cross-cabin collimated display are Mylar film mirrors. Reflective Mylar is fastened to a carefully shaped framework with a closed back. The air pressure behind the Mylar is reduced and the Mylar is forced back into a curved shape. If the Mylar is stretched just right, the curved shape is spherical.

Managing the distribution of tension across the Mylar surface is key to controlling the mirror shape. It gets more difficult as the vertical arc of the mirror gets larger. Mirror size limited the vertical field of view to about 45 degrees for some time. Eventually SEOS figured out a way to pre-tension the Mylar, and got the vertical field of view up to the 57~60 degrees needed for helicopter and aerial refueling sims.

[mikesblack]

Hi Mike,

Thanks for that. Very interesting indeed.

Mike

[lineman55]

Here is what I did when I had my 737 pit running.I no longer have this setup but have switched to the Dodo Bell 206.It worked well for me although not perfect.And still affordable.
http://biagettis.com/737simproject/7...roject_008.htm

[mikesblack]

linemann55,

I had seen your set up before. Very interesting.

Can you answer a couple of questions? I was wondering how you managed to allow for the FO position to crab free as you mentioned on your website. Could you imagine a way to do this with a curved screen of say 220 degrees, and multiple projectors?

I was wondering about sintra material. I am currently experimenting with EVA or running shoe midsole material. It comes in sheets of various color and density. What I like about it is that it is smooth, light and extreemly flexible. The issues I see are finding a method to blend the seems with glue cleanly. Also, considering it is a porous matt serface, I wonder if I will get good relectivity. I did test with what seems good results, but I don't have much to compare with.

[lineman55]

Well. what I was able to discover is by curving the screen just enough without distorting the image to much it gave the Captains view a more realistic view.As for the FO ..it was better but not as good as the Captains side.I was never lucky enough to have anyone in the area to share my hobby so I wasnt to concerned with what the FO saw or how.
I believe by curving the screen and setting the zoom just right and shifting the projector image over to where my view was more realistic it satisfied my want for a noncrabbing view.
Sintra or cintra ,not sure how its spelled..worked fantastic for this pupose.Sturdy and flexable and strong enough to drill,sand and still light enough to easiliy move about.Plastic suppliers for sign companies usually will stock this.You can even use it for creating a shell surface for your project.
Im not familiar with the material you are interested in using for your screen.It sounds workable by your description.
The curve I implemented in my project was the maximum for use with 1 projector.
Any more then that and the images on the sides tend to run or blur somewhat.
James

[Sean Nixon]

As far as I can tell, the FO crabbing effect only applies when sitting close to, say a monitor. The effect just isn't there with a large screen placed way out in front of you.