Why are the fresnels split? Is it because both collector and collumnator normally come bonded together to protect the grooves?

I've done a couple searches but can't seem to come up with an answer.

I'm assuming the lenses in the kits are split. Can they be purchased this way normally. or does someone always have to split them?

Sorry, noob questions...

Tony

They come in seperate package when you buy them they are call split when you have one in front of the lcd and the other one behind. on the unsplit they are both behind the lcd screen

The lenses are split to eliminate seeing rings in the projected image, and also allow the projector to have a keystone correction.
So if you want to have your projector mounted from an angle, using keystone correction will tilt your front fresnel to compensate for the tilt you have the projector set to.
If you are looking for simple design go with the fresnel combined, if you are looking for image clarity, and keystone correction, go split design.
What ever design you chose, be careful with the fresnel lense, a split fresnel can be very hard to clean the inside grooves, wear gloves, and keep fresnel in protective material, the lumenlabs lenses come in a cloth sleave, for the most part if you use a box to project you will use split design, if you use an overhead projector, you will use an unsplit, however it is possible to do either one you like with either style.
Personally I asked the fresnel question, to figure out why my overhead image wasn't right, and after looking further into it, I moved the top lense closer to the triplet, and found it would have worked, if I had a way to set the top frensel 2-3 inches above the lcd on my overhead projector. also you must determine which lense is the collector(bottom one closest to the light source) and the collaminator*not positive this is right word,but hey i'm trying to be helpful at least (top lense with rings facing towards lcd, and light source) there is a thread to show you how to set an object in front of your split fresnel pieces with rings facing the object you can determine bottom, and top lense easily, it worked for me anyways.
I am excited that my new lense is going to be here next week, and i'll post a plog after i'm finished
good luck