Hey, I was going to do the split-fresnel design, but it was just to hard to get everything center. Im going to do un-split now, but I have a few questions.

1) Do the fresnels need to be bigger then the LCD? I know you should cut them bigger, but in the frame they would end up having the same viewing area as my LCD. Should I rebuild the fresnel frames so that they are larger? Any rule of thumb for this?

2) Dos anyone know the advantage of an unsplit versus a split set-up? Which one produces a better quality picture? I know you cant key-stone with an unsplit, but thats how the story is gonna have to play out. Any thought here?

Thanks!

Well, ive been useing the seach function for a while trying to find answeres, but I have not come across anything. Anyone have a clue on this?

1) My understanding is that if you don't make your fresnel frames slightly bigger, then you might have a problem with edge brightness.

2) An unsplit fresnel setup is supposed to be sharper... That may partially be because keystoning has the potential to make parts of your screen slightly out of focus (like mine.. >10 degrees keystoning) but I read that you have to center the fresnels very precisely, or you'll get hot spots or visible fresnel rings in your projection.

Thanks for the help!

hang on a sec, I need to get my terminology right. Split means a fresnel on either side of the lcd and "parallel" light going through the lcd, whereas unsplit means the fresnels are before the lcd meaning it's already going at an angle, right?

and split allows you to keystone more effectively because you can change thwe angle of the forward fresnel, right?

Yes and Yes.

I thought I read that you want your fresnels about 1/4"in bigger than the LCD.
I need to bookmark more.

If you oversize, will it create problems?

Yeah, you do want your fresnels to be larger than the LCD. It doesn't matter how much larger (some people make their projectors without cutting the fresnels; that way they can change to a different LCD easily later.

I think there are 3 reasons for this...
1. It ensures that you have enough light to cover your LCD. It makes it quite a bit easier at least.
2. It's possible to condense the collimated light a little by adjusting the collector fresnel (I think you move it away from the bulb to do this). This'll give ya a light collection area that is slightly bigger and therefor you'll get a slightly brighter picture.
3. The edges of the lenses sometimes (always?) produce "chromatic aberrations" ("color distortions"). The light going through the edge sometimes comes through as a different tint (yellow usually I think). That yellow tint will be projected if you let it hit your LCD.

Both
Split
Unsplit...
Reflector -> Bulb -> Pre-Condenser -> Collector/Collimator Lens -> LCD -> Condenser Lens -> Triplet -> Projection Screen
Reflector -> Bulb -> Pre-Condenser -> Collector/Collimator Lens -> Condenser Lens -> LCD -> Triplet -> Projection Screen

Keystone correction is possible with either setup, but it is easier with split fresnels. I'm using unsplit lenses and have keystone correction using what is called "lens shifting". This takes more math/planning but the end result is better in almost every way (IIRC).

Oh, and yes, when you use the unsplit design the light is already being condensed (converging toward the triplet) when it goes through the LCD.

Do you need an extra lens with the unplit keystone? If not, can you explain it more to me? Thanks!

Nope!


I don't really wanna get into it now (it's kind of complicated).
More or less you just move, or "shift" the lenses up or down (left and right would work too).

Google Search: site:lumenlab.com "lens shift"

Here's a post from my plog (of my first real projection)... notice the projector's orientation compared to the ceiling, then check out the orientation of the projection to the ceiling.

Here's an early design for my projector, notice the placement of the triplet and the cone coming from the condenser fresnel...