If you have an adjoining room to your home theatre going spare or have some space in your home theater room..and perhaps you would rather not have your projector either hanging from the ceiling or in front of you..or your rooms just to lonnnnngggg for front projection then check this article out it may give you some ideas..

Fell over this article explains it all..it's aimed at using CRT projectors however I see no reason why we cant use it with LL and the like projectors..




http://www.cinemasource.com/articles/about_rear_screen.pdf

My next build will be a RPTV. I have been thinking that the standard RPTV conversion by adding a LL pj behind a Fresnel type screen is wrong.

I think the fl of the standard triplet is to long to match with the fl of the fresnel screen and there is no need for it in a fresnel system any how.

By using a triplet between the LCD and screen it is no different than adding a single lens between the LCD and Fresnel screen. Any time you add positive fl lens to a system the FL will become shorter. This will mean you will have to have the LCD a lot closer to the screen than planed.

I believe that a Fresnel type screen needs no lens at all between the LCD and screen. The screen focuses on the LCD and you’re done.

The light system need to project its light cone to cover the Fresnel screen. This could be any condenser system with a short FL to match the angle needed.

The setup would be lamp/ condenser lens/LCD/ Fresnel screen. This is a simple setup and should work fine.

I have never liked the way the Fresnel screens looked much so I may take a look at some of the diffusion screens too. It is also easer to make custom sizes with the diffusion screens.

Nice find.

AV

Nice post..if what you are saying turns out correct then in theory the RPTV should end up brighter...
It's a shame that there seems little interest in some kind of RPTV..
yours I noted was the only reply to this topic.
seems people think about it then stop..I guess this is highlighted by only a few RPTV successes .. which is puzzling..your theory might be part the reason

It so happens Ive been trying to fix a currently unfixable RPTV so if it really is unfixable then it is a candidate for a diy rptv...it's a shame as everything works except the blue..I would love to be able somehow to use the tuner..sound system (leave it in) would be neat ..it also has a video output.. so that could be connected to the LCD.. anyways I doubt if I would have the room unless I were to put the whole thing in a new shell...almost to much hassle then...probably look to gutting then seeing if we can use the existing..making it fit..

Cheers

I would like to see the LCD or picture tube arangment. There should be some kind of lens in front of it too.

AV

Just wanted to check your idea/theory for an RPTV without a triplet or fresnels (except for the screen)

Please have a look at pic attached.


What condensor lens in your opinion would be suitable for either a 15 or 17 inch LCD and a 50 inch fresnel screen...that is if Ive got this right

TIA

For any projector, including rear projection, to focus an image an objective lens of some type is needed. The only way to do away with an objective lens would be to use a true point source or lasers. Fresnels and lenticular lens used in rear projectors are there to steer the light toward the viewer, similar to a front screen having gain or a viewing cone. That’s why allot of RPTVs look so bad from the side.

DJ

Daz I think your right ..it's most likely i misunderstood AV's idea

You understood me correctly.

Your drawing is exactly what I was saying.

I set my 680mm fl RPTV fresnel lens in front of a CRT and it looks like it has an image that you can see. I will put a diffuser screen in front of it and put it in front of a LCD at around 600mm with a condenser light system like you drew. I have a lot going on right now so it may take a while.

There may still need to be a second fresnel infront of the LCD but I don't see why.

AV

I gave it a go myself with the 50 inch fresnel RPTV screen and a 14 inch flat CRT..

Couldnt get a a picture approaching anywhere near focused..plenty of light and bright but no discernible image..

When I add a triplet to the mix .. then the image is there all focused.

On your RPTV screen (with triplet), how is the overall picture quality? Do you have problems with corners? And wide angles? Thanks

funny you should mention the corners..they are somewhat blurry,,seems the fresnel screen accentuates the slightly less focused outer areas formed by the triplet..

I have another lens to try and will post if this makes it any better...

I often thought about doing that with a gutted RPTV

Hello all,

New here.
I've been reading the site & getting interested in using an old RPTV instead of your standard projection method.

I just picked up an old 47" RPTV.
The picture is basically blue, blurry & so dim you can barely see it.
I don't fix TV's & since this is from 1992 & was free, gutting it to try a rear projection retrofit seems like a good idea.

I've got a 15" LCD ready to go.
I just need a fresnal & a triplet.

Some have said the lumen labs triplet has too long of a throw & i think I agree with that.

I've seen 82mm triplets online for $20.
Using the focal length calculator, theoretically it should work & allow me to keep the bulk of the projector from sticking out too bar in the back.

but i'm really not sure.

Yah, I really would like to eventually build an RPTV, but it is as you say; there are so few people that make them. My thought is that the reason why people build projectors is for the size which is much greater than an RPTV, also an RPTV takes up a lot more space than the standard projector.

I've been trying to conceive of a way to make the size of an RPTV much smaller. My latest thought was that a larger lcd (like a 22 inch or something) might work, and since it's hidden it might even be doable. Also I'm still waiting out some of the other more experimental stuff, like the LED's, to see if they might work better in this kind of set up.