Question about collimated display systems.

[Mike.Powell]

Thanks, Jordan.

Both timely and appropriate, IMHO.

[mikesblack]

Weledzian,

Spent the last few days considering your diagrams and comments. I have also been reading the patents, and also Mike's additions.

While considering these many elements, the first and possibly most important is if I have the space required to do this. The thing I have done so far that gives me some hope is cut 6 curved pieces of plywood that is meant to fit just below the lowest point of my cockpit windows. This is intended to be the lower edge base for the mirror. My goal is to have this curve clear the entire structure to allow the rear windows to have full view. It turns out this curve is 5 feet 6 inches radius.

The arc radius will be greater than this 5ft 6, presumably 7 feet and as much vertical to allow for max 55 inch Mylar width limit. I'm thinking of 30 degrees. I'm considering as much adjustment as I can build in the better. I will use some tracking system that will allow adjustment of the mirror forward - aft and up and down, perhaps by several inches either way. I plan to do the same with the projection screen.

Beside the issues we are trying to better understand, e.g. clamping Mylar, I will need to be sure that I can walk through the narrow gap between the wall and the mirror and that the mirror can be set so as not to reflect the fuselage and but not set at such an angle to restrict passage.

Additionally, I was considering Mylar material. Do you know if there is much variance in quality? I was considering reflectivity and durability( scratch resistant).

Again, I want you to know how much I appreciate your help.

Mike

[wledzian]

mikesblack,
No problem - this is fun!
I need a few more measurements:
I need to define the vertical axis of the screen and mirror. Where is your eyepoint with respect to the center of the 5'6" curved base?
I need to define the lower eyeline. Measured in a plane defined by the eyepoint and the mirror axis, what is the angle between horizontal and the edge of the lower edge base?

[Mike.Powell]

I understand that the quality of mirror Mylar does vary. However, I've had very good luck with 2 mil Mylar from Tap Plastics, and I've just received a roll of 1 mil Mylar from Nielsen Enterprises which also looks good. It's coated on both sides and is nicely reflective. I haven't unrolled it, but what I see so far looks to be high quality. Nielsen sells in 56 inch widths.

Mylar as a base material is strong. The coating is rather fragile. If you crease the Mylar, which happens very easily, you get a permanent mark. The surface also scratches very easily. Your best bet cleaning it is with a soft cotton cloth. A paper towel will scratch.

[crashdog]

just found this document about plexiglas forming.
http://www.plexiglas.com/literature/pdf/135.pdf

and this one
http://www.iss.infn.it/cusanno/publi...io/mirrors.pdf

Gery

[Matt Olieman]

Your best bet cleaning it is with a soft cotton cloth. A paper towel will scratch.

In addition, do not use cleaning products. If you need to wet the cloth use distilled water.

Matt Olieman

[castle]

@mikesblack

Some one can check my math, here are a few numbers...

For a base radius of 5.5 feet and a vertical arc of 30 degrees down from the equator requires a radius at the equator of

5.5 / cos(30) = 6.35 feet

increase the vertical arc to 45 degrees and the radius at the equator is

5.5 / cos(45) = 7.78 feet for the same base radius

Working backwards, for a radius at the equator of 7.0 feet if you go down ~38 degrees the radius at this point is 5.5 feet.

Hope this helps you with your sizing.

JW

[wledzian]

@castle
your math is right, but the geometry is wrong. Your numbers assume the eyepoint is located at the center of the mirror sphere. In the case of mikesblack's cockpit, the eyepoint will be offset both horizontally and vertically, thus my need for this information before I can give an answer.

[castle]

Got it! But for a side-by-side seating you would still have to account for the right seat and make the compromise that both seats would be a tad off and just use the center of the TQ for the horizontal point?? Then vertically, the distance from some "average" eye point to the top of the mirror assuming that is the equator.

Is my understanding correct?

As you said, this is really neat stuff. Just in passing, looks like a short throw lens for the projection system can be had for around $600 in the el cheapo models. Don't have the numbers as to depth of field, focal length etc. For the really good stuff the price is a bit steeper; from $1700 to $2500 per lens. in either case you still have to deal wth keystoning for off-axis and image merging at the boundaries, front and rear projection systems.

I'm trying to get some estimates for bending acrylic for those of us not as skilled as geneb with tools. Will keep you all informed...

JW

[wledzian]

Got it! But for a side-by-side seating you would still have to account for the right seat and make the compromise that both seats would be a tad off and just use the center of the TQ for the horizontal point?? Then vertically, the distance from some "average" eye point to the top of the mirror assuming that is the equator.

Is my understanding correct?

For the most part, yes. There is certainly accounting for the right seat. The center of the TQ does not have to correspond to the center of the mirror, and if space allows, should not correspond to the center of the mirror. Ideally, the eyepoints should be somewhat forward of the axis. This results in reduced distortion for both pilots, but leads to a larger mirror. For a cross-cockpit display on a 767 cockpit, the eyepoint is at least 21 inches off-center horizontally, assuming the mirror axis is directly between the pilots.

Vertically, the FOV is restricted by the optics. If you take a look at the raytrace plots I posted way back on pages 6 and 7, you can see that the lower eyeline in essence defines the upper eyeline by virtue of locating the lower screen edge, and that the upper eyeline does not necessarily correspond to the equator.

In Mikesblack's case, the design eyepoint is fixed by the cockpit. The horizontal offset is fixed by the 5'6" arc he's planning as his lower edge, and the lower eyeline is fixed by the angle to this arc. The mirror radius is a variable, limited by the 7' distance to the side wall, with the vertical offset defined by the selected mirror radius.
Once I have the limiting geometry, I can play with mirror radius to get the maximum vertical field of view. If the geometry allows, I may also be able to assume a larger base radius in order to move the axis rearward.

As you said, this is really neat stuff. Just in passing, looks like a short throw lens for the projection system can be had for around $600 in the el cheapo models. Don't have the numbers as to depth of field, focal length etc. For the really good stuff the price is a bit steeper; from $1700 to $2500 per lens. in either case you still have to deal wth keystoning for off-axis and image merging at the boundaries, front and rear projection systems.

There are short-throw projectors available in the $700 range already that would likely work well for the rear-projected screen. No need to spend $$$ on aftermarket lenses. If you're talking wide-angle (fisheye) lenses, your wallet may be better off projecting onto a security mirror, at least as a first attempt. Keystone correction is best done entirely in software, as correction will be necessary and it's better to do it all at once, rather than run the image through two extra processing steps.

I'm trying to get some estimates for bending acrylic for those of us not as skilled as geneb with tools. Will keep you all informed...

JW

Please do. The topic of forming the rear-projection screen came up on Saturday, and there was discussion of building a forming oven just for that purpose.

@geneb -
If it does come to that, I'd be more than willing to share the cost with you. It's looking like a screen for a 48" mirror could be formed from a single 4x8 sheet of plexi.