Dual Fresnel

[dracul]

though at the moment im deciding weather to use white or mylar for the inside

Hey ace, I thought mylar burns big time. What about a pyramid with mirrors or does that screw up light angles?
Where can I find links to your pyramid or is this still in the work?
And this pyramid angles out to the first fresnel?
Also someone built a trapezoid tunnel going from top fresnel or lcd to objective? You planning on this?

[ace3000_1]

Hey ace, I thought mylar burns big time. What about a pyramid with mirrors or does that screw up light angles?

Nope it dont burn up bigtime if u are carful in your design, this has been proved by cruser, with mirror it will be too heavy, fragile and exspensive for a large setup.

Where can I find links to your pyramid or is this still in the work?

Still in the works buddy, waiting on some friggin parts to get here that are late as usual.

And this pyramid angles out to the first fresnel?

Nope to the last frensel its all tapperd.

Also someone built a trapezoid tunnel going from top fresnel or lcd to objective? You planning on this?

Nope i dont have the room and i dont need to, there is no light leaks from this area, all that is for is to stop the light from lighting up the room, it will not give u a better performance as in brightness and getting more light through your projection lens as the first field frensel should be focused to your projection lens.

Trev

[freeupyermind]

Originally posted by Hezz
Freeup,

what you are proposing sounds a little unrealistic without a very wide angle lens with a short focal length. How big of a LCD are you planning to use. A lens that could do this without much distortion would be very costly because of it's high end and critical engineering and multiple lens with aspheric surfaces.

Hezz

I plan on using one of those 15.4 inch widescreens, and I have ABOUT 4 foot to get a 125 inch diagonal image, is that reasonable?

[dracul]

And this pyramid angles out to the first fresnel? Nope to the last fresnel. Its all tapered.

Let me get this straight. Lets assume I dont use split fresnels just one before lcd. The trapezoid will extend to the one and only fresnel only. If you are splitting than you take the trapezoid to the last fresnel. Correct?

I plan on using one of those 15.4 inch widescreens, and I have ABOUT 4 foot to get a 125 inch diagonal image, is that reasonable?

We are already pushing things with 8 feet image at 8 feet. Come on, you want a decent image right. Ideally you want longer and longer throw. The best lenses are normal lenses. That is, not long nor short.

[Hezz]

Brainchild,

While what you are intending is possible I don't think that you will be able to find a lens that will do what you want. It would have to be an expensive custom made lens system. It is really much better to go with a front projection system if you want a screen that big.

Hezz

[freeupyermind]

Brainchild hasn't even posted in this forum, are you talking to moi?!?!?!?! And I was thinking of using multiple lenses i could salvage from a few old ohps, i don't see how it wouldn't work?!?!?!?!?!

[Dwiel]

free:

I think that it is possible, but you must realize that if the picture is going to get that big in that short amount of space, then the lenses will have to get, relatively, equaly as big. Also, the more lenses you introduce the more chromatic abberation and loss of focus you will get. The optics if you will use as many as you are wanting I'd say at least 3 or 4, will need to be very high quality.

I wonder though, if you could find a rear-projection TV lens from a surplus store or something. If you can find a lens that was made to do what you are trying to do, you might be in luck. Otherwise be ready for a distorted picture.

Or, you might surprise us all and have a sweet picture in a very small space, but I doubt it. You might also find that you will get hotspotting because the light that is going to the edge of the screen has to travel so much furthur from the PJ lens... the closer you get to the screen, the bigger the difference between light distance at the middle and corner will be. your center will be 48in away, and your right/left edge almoast 80" Your corners will be even further. I'm not sure, but I'd say that the light going to the corners will be much darker due to the fact that it has to travel nearly 2x as far. (from the PJ lens)

Hope I've cleared things up some!

[ace3000_1]

i don't see how it wouldn't work?!?!?!?!?!

Try focal and magnifacation, also angles, there is a way u can get it to work i have posted it in the rear projection thread and its proven to work by the big guys, its very simple, cost efective and u can obtain a big image.

Trev

[freeupyermind]

would this work if iused a more traditional rptv setup, that would give me about 11 feet projection space for a 125 inch diagonal screen, but what size mirror would i need and would i have to do some keystone correction in my setup...

ps i know my angles arent exactly right, and nothing is to scale, but its a crappy diagram bare with me

pps im sorry for being a thread jacker