Mirrors and Curved Screens

[Ferrino]

Hi - I'm looking to use a single 1080p DLP projector with a rectangular curved screen (not settled on desired curvature yet, but hoping for something along the lines of 135 degrees: only curved left-to-right). Have the space for rear or front projection, although I am looking to test with front projection only initially (due to cheaper screen material).

I am trying to establish if a curved mirror (matching the curvature of the screen) offers any benefits other than doubling the effective throw of the projector for small spaces?

Does it also improve the focus at the sides (compared to projecting directly onto the curved surface) and can it also remove the need for image-warping software when set up correctly to match the screen?

Thankyou!

[riche543]

Hi ferrino

I have not yet got to my visual setup, But im in a very small space with no height for a projector, so i hunted around & found this regarding corved mirror http://www.flightdecksolutions.com/f...er=asc&start=0 I have only run breif test,s with mine . But have the projector low down in front of the sim pointing back to a curved mirror fixed roughly behind MIP . I fixed each corner of the mirror so they can be ajusted to approx the same curve as screen which is made from sintra. I found though if you curve the mirror to much the edge,s of picture stay focused but the centre goes blurry as you are streching the mirror so to speak. Im guessing you would get better results wiyth short throw projector as you wouldnt have to bend the mirror as much. then use sol7 or similar software to correct image
Good luck with it, like i said im really no expert on this as im not at that stage with mine & havnt experimented enough. Keep us posted , im interested in the results you get.
Cheers
Richard..

[Neil Hewitt]

I doubt it would be practical to use the sorts of mirrors within the range of the average cockpit builder to optically correct for all the distortion introduced by a curved screen. It's certainly physically possible, but you'd have to make a mirror precisely shaped and ground to correct for the focus and image distortion. This is what they do in commercial simulators, which use a collimated mirror arrangement that both precisely corrects for the distortion of the image and also re-focuses it at infinity to avoid parallax issues (where two viewers see an object at different positions due to their angle relative to the screen, which is a problem for two-pilot simulator operation). Such mirrors are custom-made and approximate a section of a sphere, which you couldn't do with ordinary acrylic mirror sheet.

What you certainly can do is use a curved piece of acrylic mirror to reduce the distortion when reflecting onto a curved screen and then do the final corrections using NTHUSIM. Optical manipulation of the image is clearly better than digital manipulation.

I'll be using a curved mirror arrangement in my own project and am planning to use NTHUSIM as well. Make sure you buy 'first surface' acrylic mirror - this has the mirrored surface on the front, rather than on the back as with a traditional glass mirror. Otherwise you get imperfections and distortions on the image as it has to pass through the acrylic on its way back to you.

Alternatively you can use reflective mylar sheeting and glue this down to a backing material of your choice. You're likely to get lots of little imperfections this way, of course, so I figure acrylic mirror sheet is better. It's pretty cheap too.

[Matt Olieman]

Neilh is 100% on the point. This been a topic ever since the beginning of Cockpit building. This is the one item of cockpit building that has not been conquered by amateur builders. Believe me, I've tried.

Matt Olieman

[Ferrino]

Thanks for the replies. OK, so I guess what I was getting at was:

Given a rectangular projection screen which is moderately curved, is it there a significant improvement in picture quality (focus on the side views, distortion) when reflecting off a similarly curved first-surface mirror (+ finishing touches with NTHUSIM) compared to projecting directly onto the surface? Assuming a single viewer and a single projector.

The main thing that is putting me off directly projecting onto a curved surface with one projector is the blurry/out-of-focus side views, which I guess vary with severity depending on how aggressive the curve is.

Thankyou!

[Neil Hewitt]

A well-aligned and suitably curved mirror will certainly help a little. How much it helps is implementation-specific. The shape of your curve should never exceed that of a simple circle, certainly. Some people have tried to stretch the picture from a single projector right round the sides, but invariably it doesn't work well.

Acrylic mirror is cheap and easy to obtain. I'd say buy some, and give it a try. You can do it on a small scale to test it, and then scale up to the real thing when you're ready.

[Mike.Powell]

I would not expect the curved mirror to help with focus.

Focus depends on the optical path length and the depth of field of the projection lens. Most projectors are designed to focus on a flat screen. When the screen sides curve toward the projector, those portions which are moved closer will lose focus. The smaller the lens depth of field, the more defocusing you get.

Lens depth-of-field depends on the exit aperture of lens and the optical path length between the lens and screen. Think of a cone of light with the cone base at the lens exit aperture and the cone point at the screen. If the screen is exactly at the point, the image is in focus. As the screen moves closer to the lens, the screen intercepts a larger cross section of the cone, the light spot size grows, and the overall image appears less focused.

Depth of field or "being in focus" is simply the range of very nearly focused that is acceptable to the viewer. There actually is a loose camera industry definition of depth of field based on spot size, but it comes down to looking at the image and saying "That's okay",... or Not!

Most consumer LCD and DLP projectors have small exit apertures and provide much greater depth of field than did the older CRT projector with their massive lenses. You can boost the depth of field by choosing a long throw distance. That will give you a longer, skinnier light cone.

A nice thing about mirrors is that the optical performace is simple. The angle of incidence equals the angle of reflection. That means you can easily and accurately map out the light paths for different mirror/screen geometries on paper. You can get an idea of what will happen without building a complex test setup.

[Neil Hewitt]

Hi Mike! Didn't realise you hung around these parts (of course, had I bothered to look around a bit I'd have spotted Mike's Tips). I've been using your site among several others for inspiration over the past few months. Your stuff on collimated displays in particular is fantastic.

[Mike.Powell]

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.

[mikesblack]

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