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post #121 of 144 Old 04-24-2005, 06:29 PM
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Quote:
Originally posted by AaronNWilson
When you have your projector turned on and then turn on the fan to tighten the screen, does the brightness suddenly increase dramatically? I would be very interested in seeing this happen, would it be at all possible to make a video of the tightening/brightening?
Well, the gain factor is a function of the material so there is no real difference between fan on and off. What the curvature does is focus the light such that the gain is even across the surface. Normally a high gain material will be brightest in the middle (assuming you are perpendicular to the middle of the screen). The curves actually point the edges toward you a bit so that you are also more perpendicular to the edges, in essence pointing all the hotspots at you. I hope I am making sense!

In person it's hard to see the screen actually pull in, so I'd imagine it would be hard to capture on video!


James

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post #122 of 144 Old 04-25-2005, 06:04 PM
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Quote:
Originally posted by bob2010
James,

From the looks of your screen it did't seem as though there was insulation
on the edges. I remember the original DIY torous having a rather large rolled
edge. Maybe due to thicker plywood and insulation? I just realized, that screen was done with full depth radius, plus some bigger insulation, thats why I remember the thing looking huge. It was. It had to be 12" deep at the corners,
your's is almost half that, thats what makes your's look so slick.

Yes, some movie shots would be great.



Bruce
OK, I took some shots. It is HARD to take a good screen shot! I ended up just setting my camera to auto with no flash and took some pics... The tripod helped quite a bit. The camera is a 2 MP Canon Powershot S110 Digital Elph. Some shadow detail is lost in the pics, and they look more grainy and dark than in real life, but what the hey... Ha, after looking at other screen shots of similar scenes, my fleshtones are too red! Of course now that I have the Torus I will finally try to do a real setup.

The pics are in my gallery.


James

p.s. Don't critique my convergence, greyscale and such too much... I'm still learning!!

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post #123 of 144 Old 04-25-2005, 07:34 PM
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James, with the 7.5" & 3.5" curves, how far back are you watching it, and how far is your pj?

I'm wondering if my small room (~10' throw distance & watching distance) is too short to be practical for a torus. Would I have to use such short radii that it would be impossible to focus on the corners?
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post #124 of 144 Old 04-25-2005, 08:11 PM
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Quote:
Originally posted by garyfritz
James, with the 7.5" & 3.5" curves, how far back are you watching it, and how far is your pj?

I'm wondering if my small room (~10' throw distance & watching distance) is too short to be practical for a torus. Would I have to use such short radii that it would be impossible to focus on the corners?

Ummm... projector is approx 149 inches and viewing position about 140 inches.

The first Torus I saw (Atom's) used the same depth curves, but was a bit smaller at 96" wide. I would guess throw and watching distance was around 10', maybe a bit more. Atom posted a bunch of stuff on Torus screens a little while back...

One great thing about the Torus is the wide and varied good viewing positions!


James

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post #125 of 144 Old 04-25-2005, 08:28 PM
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Hm, OK. If Atom's worked at 10' with the same curves, I guess I'd have no problems. I'd only be looking at about 88" wide.

'Course, come to think of it, "curve depth" adapts to different screen sizes. A smaller screen, with the same depth, will actually have a shorter radius.
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post #126 of 144 Old 05-15-2005, 08:09 AM
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This was a very informative thread. I would love to try a Torus. If i do, does that mean i need to move my projector? Or can it be done relatively easily with the projector in the same place as i have it now projecting onto a flat screen? Also, can somebody describe that string method in a bit more detail? I'm not sure how it would work. How do you go from using a string "attached" to your projector to a curve on a piece of wood that you can cut? Thanks!

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post #127 of 144 Old 05-15-2005, 12:00 PM
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If you can't or really don't want to move the CRT back, you could still do a torus and not move the CRT..depending on the range left on the CRT size controls the image may be a tad bit narrower..

We designed mine knowing that I was going to move the CRt back because I went a foot wider..Mine is on a stand and the legs alone brings it 5" closer..

Drop the stand - mount the screen directly to the wall - you'll still need some depth to suck the screen back but you should be ok..

John
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post #128 of 144 Old 05-15-2005, 12:00 PM
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The Torus is essentially a concave surface that reflects more of the light to the viewer. (With the viewer being the focal point.) Or - If you were an ant, the torus screen would be the magnifying glass.

The "string method" is a basic method for obtaining the needed curvature on each of the Torus side panels. It's straight forward and it works. (P.s. the focal point should be the viewer, not the projector).

Or, here's an excel program to give you the dimensions of the torus panels. The program takes into consideration the different curvatures for the vertical vs. horizontal panels. And it'll adjust the focal point of the screen to accommodate a wider viewing area (such as the width of a couch).

You'll need to save the zip file to your hard drive, then you'll be able to open it.

 

Torus calculator v2.zip 8.62109375k . file

 

Torus calculator explained.zip 20.1943359375k . file

-Nothing relevant to add.
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post #129 of 144 Old 05-15-2005, 12:29 PM
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I haven't read all seven pages or paid much attention to torus screens, but
I'd like to ask why torus screens are considered to be better than spherical
section screens? (I presume that's the case.)

It would seem to me that a spherical section screen would be optimal as a torus
mandates different edge focus corrections for the horizontal and vertical axes,
at least in theory, while a spherical section screen would need just one edge
correction, and it would be the same in both the horizontal and vertical axes.

Either way, the radius of curvature of the screen needs to be exactly equal
to the center lens front to screen distance for perfect focusing. In my case,
that would mean a 120 inch radius.

CJ
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post #130 of 144 Old 05-15-2005, 01:04 PM
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CJ, a spherical screen section would be perfect if your eye and the projector were at the same point. Since they aren't, you need a different radius to properly focus the light from the pj to your eye.

Paul, I looked at that torus spreadsheet. It shows incredibly shallow measurements. I tried an 88x48 screen with a 10' viewing distance and 5' wide viewing area, and it showed a torus with a max top/bottom panel depth of 2 9/16, and max side panel depth of 3/4". Most of the people building torii are using panel cuts 5x or more deeper than that!
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post #131 of 144 Old 05-15-2005, 01:25 PM
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Yes, that would likely happen. It's mostly due to the wide viewing area.
You've got a 4 foot wide screen, and you want the screen to focus on a 5 foot wide area. You're going to have a pretty flat screen.

Take note of the included pictures. As the viewing area width approaches the screen width, the torus dimension will become closer to a flat screen.

-Nothing relevant to add.
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post #132 of 144 Old 05-15-2005, 03:59 PM
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I always read that the curve of the Torus contributes to the gain factor. Am I the only one who believes all the curve does is focus the viewing cones, thereby making the gain uniform?

James

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post #133 of 144 Old 05-15-2005, 04:17 PM
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Gertjan,
Run don't walk to Chuchufs house. He has a Torus screen.

James,
Yeah, I think that is all it is doing.


The zip file did not open correctly.

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post #134 of 144 Old 05-15-2005, 05:10 PM
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Wow, Chuchuf is here in GA as well. Never realized that during all this time i've been here on this forum (over 4 yrs now) :O Hmmm.. i'll have to see if he's got some time for a fellow HT enthusiast...

Pjackso - I understand what a torus is supposed to do, and i know of the string theory (hah), as in, i have seen it mentioned. Thing is, i haven't seen a description of how the string method would actually be done. Where do you put the string and what do you do with it and how do you end up with a curve on a piece of wood that you can cut?? The spreadsheet seems a lot simpler method, but i like the practical hands-on measure the real thing kind of approach because that way i'll know for sure it's right :)

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post #135 of 144 Old 05-15-2005, 05:27 PM - Thread Starter
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you guys are over thinking things...follow atom's plan and you will stop talking about tori (??) and start watching one!
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post #136 of 144 Old 05-15-2005, 05:43 PM
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Gertjan
I will build a torus soon , possibly 112 " wide and 4:3 aspect .
planning on my vertical sides to be 4" deep and 6" deep horizontal...

Will mark the boards in the center for depth , place the pencil in the string.
Place the pencil on the mark in center while the other end of the string is secured far enough back so that when I make the arc . The arc will "make" the center and both ends of the boards . it will take the correct placement of the secured end of the string to get the correct arc...

maybe wrote this not too great but hope it is clear

The radius will be quite big to touch the marks ...

different aspect ratios and sises may need different depths , but from reading others comments its not too critical within reason to have different
depth of the arcs...

let us know how you do and give tips as I have not done it yet...

later

Bob

keep at it , results are worth it !!
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post #137 of 144 Old 05-15-2005, 08:26 PM
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Paul, the screen I used was 4 feet HIGH, not 4 feet WIDE. It's 88" wide, over 7'. Maybe 5' is still a wide viewing area for that size of screen, but others have built sucessful torii in that size range that used MUCH deeper curves. The curves in the spreadsheet were only about 2.5" deep on an 88" chord. If I'm figuring correctly, that works out to a radius of about 388" = over **32 FEET** !! How can that be right!?

Wait, I see. I guess the spreadsheet is aiming the screen such that the angles from the edges of the screen to the focal point bisect the required viewing width at the specified viewing distance. The attached not-to-scale picture will explain it better than my words can. Looking back at your spreadsheet, I see you do show this in one of your pictures -- I just didn't think it through.

That's a lot different from what I think most people are attempting, with their approximations of the string method. The string method focuses the whole screen on one spot, with a radius from that spot to the screen. That's effectively the same as specifying a zero-width viewing area with your spreadsheet -- which produces a top/bottom curve depth of almost 22 inches! Your spreadsheet is spreading the "sweet spot" across the whole requested viewing area. Makes sense.

I wonder what kind of gain curve results from this approach vs. the string approximation that everybody seems to be using? The ideal string approach would produce a huge gain spike right at the sweet spot, and it would drop off VERY rapidly as you move to the side. The "longer than string method radius" approach that most people use would probably produce a lower gain in the sweet spot but a reduced dropoff. I suspect your spreadsheet approach would produce fairly uniform gain across the width of the specified viewing area, but a much lower gain. Sound right to you?

Interesting spreadsheet! Thanks for sharing it.
Gary
LL
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post #138 of 144 Old 05-16-2005, 05:44 PM
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Garyfritz,
You pretty much figured it out, and you explained it very well.

Take note: The spreadsheet gives the depth of curvature for the top/bottom/and side panels. But take note that the curvature depth given is for the top and bottom panels. The greatest curvature depth is at the dead center of the screen. So be sure to give yourself a few inches of extra room in the back of the panel. (was this confusing?)


Semisentient,
By focusing the entire "hot spot" to the viewer, isn't that how you make the hot-spot look uniform? (and hence the increased brightness...)

BTW, I see that your torus screen is 110" x 62" with a 7.5" curve (top/bottom panel) and 3.5" curve (side panels). In a old post you said your viewing distance is about 140" back.
That corresponds fairly close with the spreadsheet with a viewing width of 35 inches wide.

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post #139 of 144 Old 05-16-2005, 05:56 PM
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Quote:
Originally posted by -Pjackso

Semisentient,
By focusing the entire "hot spot" to the viewer, isn't that how you make the hot-spot look uniform? (and hence the increased brightness...)

BTW, I see that your torus screen is 110" x 62" with a 7.5" curve (top/bottom panel) and 3.5" curve (side panels). In a old post you said your viewing distance is about 140" back.
That corresponds fairly close with the spreadsheet with a viewing width of 35 inches wide.
My point is, any point of the Torus would not be brighter than the brightest part of the same material on a flat screen. Of course, the edges would be brighter than they would be normaly, but the centre would be about the same.

I am happy with the curves of my screen. I do believe the first Torus I built with slightly deeper curves is maybe a bit more uniform in brightness. Of course I have only seen that one in a different room with a different projector (same model though).

I love my Torus! I just wish I had gamma control on my Momitsu as I find my whites a tad too bright. Not too bad a problem to have though!

James

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post #140 of 144 Old 05-16-2005, 09:46 PM
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I'm just finishing off the box for my 3.5m (11.5ft) wide torus, and she is pretty damn deep. We will be sitting about 4-5m (13-15ft) back from the screen. Now all I gotta do is find me some big fabric. (All posts about the intelligence of building a box without first having the fabric are hereby acknowledged... build it and they will come)

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post #141 of 144 Old 05-16-2005, 09:50 PM
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The first torus I made use the string method, and yeah, the viewing cone was pretty tight. I can't remember exactly, but if you sat at the edge, you ended up with a noticably less uniform/darker image. Sit in the middle though and it was blinding.

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post #142 of 144 Old 05-16-2005, 09:58 PM
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How about hanging a piece of 1.3 material in front of your torus for comparison and taking a movie shot?

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post #143 of 144 Old 05-16-2005, 10:54 PM
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If anybody near Seattle wants to see a Mocom 20 gain curved screen, just let me know, as I got a 100" diagonal version not too long ago. I only have digitals to show on it, but of course somebody is welcome to bring a CRT if they want. :) I'm guessing the screen is close to 10 gain in actuality in the middle, but haven't done exact measurements. I've dimmed a digital down quite a bit, so it isn't like the images are real bright even with this screen.

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post #144 of 144 Old 05-16-2005, 11:01 PM
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Yowza!
Is that a hard screen, or do they sell the fabric?
If you ND filter the digital down to the point where the greys are black (or damn close) with that screen , how badly is the rest of the picture affected?

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