Question about collimated display systems.

[Mike.Powell]

Neilh has a good point about a WAC style collimating display being easier to make. Here's some hopefully useful information.

WAC stands for wide angle collimating display. The name is something of a misnomer today, but when introduced it was a step up on the competition.

Imagery is produced on a CRT mounted above an angled beamsplitter. Light reflects off the beamsplitter to the collimating mirror before finally getting to the viewpoint.



Depending on design choices, this approach provides on the order of 50 degrees horizontal field of view and 35 degrees vertical field of view. Adjacent units, angled in toward the viewpoint can provide panoramic imagery.

The mirror has a spherical curve. It should be much easier to make (or buy) than the film mirrors used in cross-cockpit collimated displays.

Glass Mountain Optics makes large optical elements for simulators. Several years ago they advertised on their website that they made lower cost mirrors for museums and hobbyists. Since then they were acquired by FlightSafety International. I don't know if GMO still offers products to the general public, but maybe they do.

A few years back I ran across a posting about having a spherical section mirror made by a firm that makes the hallway safety mirrors. It was a special order, but all they did was put the reflective layer on the other side, i.e. making a concave mirror rather than their normal convex products. If I can find more details I post them later. [edit: I think this was the company: http://www.campuscrafts.com/]

US patent #3,549,803 "Virtual Image System for Training Simulator" describes a method for making spherical film mirrors using reflective Mylar, urethane foam and fiberglass.

US patent #4,822,155 "Mirror Assembly with Flexible Membrane" describes the construction details of a large diameter, spherical-section film mirror. University of Strathclyde built a number of these mirrors for their use in research projects. They use reflective Mylar. As Matt pointed out, Mylar is tough stuff that takes a lot of force to stretch. The method of mounting the film is an important factor in the optical quality of the finished mirror. This patent details one approach which has been shown to work well.


It may be possible to use sun control window film as a low cost beamsplitter. A more expensive option is available from Teleprompters.ws They sell properly coated glass beamsplitters with the partially reflective coating on one side and an anti-reflection coating on the other to reduce ghost images.

To avoid image distortion and other problems, the faceplate of the CRT should has a spherical shape of one half the radius (or slightly more) of the collimating mirror. It also needs to put out a lot of light. CRTs of this shape are no longer available. A rear projection screen is the contemporary replacement. If you project on-axis with the proper throw ratio, the majority of the projection-related distortion vanishes.

Because of the multiple reflections and the 50% beamsplitter, at most 25% of the light from the screen makes to the viewpoint. 10~15% is more likely. You need a lot of light. The old CRTs were special units pushing 50 to 100 foot Lamberts. A pico projector won't do, but a regular projector should.



I think the value in a collimated display is the promise for greater immersion. It makes the world outside the sim window look bigger, and adds realism. I think the cross-cockpit off-axis collimated system would be best because it provides an unobstructed view with considerable allowable head movement. The WAC system can provide a panoramic view by sticking multiple units side by side, but this leaves structure in view and limits allowable head movement. That said, the WAC approach may be the best, workable approach available at present.

[Mike.Powell]

Acrylic mirror material does not tolerate heat well. Once you get it hot enough to bend, the reflective coating is toast. I've tried.

[Neil Hewitt]

A few years back I ran across a posting about having a spherical section mirror made by a firm that makes the hallway safety mirrors. It was a special order, but all they did was put the reflective layer on the other side, i.e. making a concave mirror rather than their normal convex products. If I can find more details I post them later. edit: I think this was the company: http://www.campuscrafts.com/

Yes - their rectangular convex mirror looks like a section of a sphere; get a 36" one of those and have them silver the other side, you might be in business. That'd create a WAC unit wide enough for one window of a B737 sim. Make the join directly at the edge of the mirror, assemble a few units together, and you'd have something useful. Not much vertical FOV, though.

I'll see if I can find someone similar in the UK. Got to be worth a try, even just as an experiment.

[Matt Olieman]

Please keep in mind, this works well with curved monitors not flat screen. Flat screen will cause blurry edges. Also, some CRT's do not work well when the screen is facing down.

Matt Olieman

[Neil Hewitt]

True, but the standard replacement for the CRT in a WAC (or cross-cockpit display) is to use a projector + curved rear-projection screen as the picture source.

Alternatively, perhaps a correctly-focused collecting fresnel lens in front of a TFT could create the same optical effect as having a curved CRT screen.

[wledzian]

I've been reading through several patents related to design, manufacture and installation of cross-cockpit collimated displays. A few things I've learned:

• The mirror is typically made from Mylar film (we knew that already)
• The film is held in shape by vacuum. It is supported only at the edges, and is not glued to any backing.
• The applied vacuum is typically dynamically controlled via a feedback loop and position sensors located behind the mirror.
• A certain amount of distortion will be present due to uneven stress within the mylar film. Most of this distortion occurs near the corners, and can be partially mitigated by methods described in several of the patents.
• While high-school level physics usually teaches that "the focal plane of a sphere is at R/2", this only applies for significantly on-axis rays. In a cross-cockpit display, the eyepoint is far off-axis, and aberration must be taken into account. Due to this, the radius of the projection screen is significantly larger than R/2.
• Due to the details of the optics, vertical FOV over 40 degrees is difficult to achieve. The vertical FOV distribution (ex. 20 deg up/ 20 deg down or 10 deg up/ 30 deg down) can be set by design choices..
• Horizontal FOV can be as wide as you like, limited only by the available width (length is practically unlimited) of the mylar needed for the large mirror.
• Mylar film in widths greater than 56" is hard to find.

[Matt Olieman]

Excellent article by Mike Powell on the home page regarding Collimated Displays.

Thanks Mike

Matt Olieman

[Mike.Powell]

I've been reading through several patents related to design, manufacture and installation of cross-cockpit collimated displays. A few things I've learned:

• The mirror is typically made from Mylar film (we knew that already)
• The film is held in shape by vacuum. It is supported only at the edges, and is not glued to any backing.
• The applied vacuum is typically dynamically controlled via a feedback loop and position sensors located behind the mirror.
• A certain amount of distortion will be present due to uneven stress within the mylar film. Most of this distortion occurs near the corners, and can be partially mitigated by methods described in several of the patents.
• While high-school level physics usually teaches that "the focal plane of a sphere is at R/2", this only applies for significantly on-axis rays. In a cross-cockpit display, the eyepoint is far off-axis, and aberration must be taken into account. Due to this, the radius of the projection screen is significantly larger than R/2.
• Due to the details of the optics, vertical FOV over 40 degrees is difficult to achieve. The vertical FOV distribution (ex. 20 deg up/ 20 deg down or 10 deg up/ 30 deg down) can be set by design choices..
• Horizontal FOV can be as wide as you like, limited only by the available width (length is practically unlimited) of the mylar needed for the large mirror.
• Mylar film in widths greater than 56" is hard to find.

Excellent summary. In particular, you're spot on about the radius of the screen in an off-axis display being greater than R/2. I got my facts scrambled in the latest Mike's Tips article and will have to get with Matt to correct it.

[mikesblack]

wledzian,

Great post!

Can you post a link to the patent site that describe the methods for correcting corner distortion?

I'm also interested in the method of vacuum regulation via feedback. I wonder if this is critical for maintaining of the image on a flight on an ongoing on demand basis or for keeping the Mylar tension and thus the image from degrading over a longer time. If it is the later, I wonder if one can build in some mechanism to regulate tension, like tuners on a guitar. I imagine one would need to have an idea of the practical requirements for keeping that Mylar in tune and set these adjustments at the correct intervals.

Thanks,
Mike

[castle]

Great posts from Mike and wledzian. I was wondering about that R/2 question as well.

The original video from the discovery segment seemed to show a projection screen that was (at least based on a visual observation) larger than the R/2 ratios and extended beyond the top of the cabin windows so as to avoid any shadowing problems.

If you go back to the basic physics of optics and collimated mirrors, anything inside the focal point will appear upright and larger than the object, in this case the projection screen, and will be a virtual image.

As Mike noted, the need to warp the image to handle the distortion requires aspheric lenses and those are not cheap, as in thousands of dollars. This is going to be a major hurdle.

JW