Joe is right on in his comments about upstage space.  Also keep in mind that professional theatrical lights are different than professional architectural lights, with the theatrical lights being easier to get special effects with.

 

1.) The kind of projector for rear projection is one scaled to the environment.  So don't expect to use a conference room projector in a large auditorium.    Use a projector that is sized to get the image size you want in the environment you are using it.  The strength of the projection fades the larger the image and the further the projector is from the screen.  The image also fades with ambient light.  So if you strongly light what is in front of the projection, you might fade the projection itself--just like when the house lights are turned on during a movie viewing.

 

There is nothing particularly special about a rear projector vs. a front projector for the projector itself.

 

2.) But there is something special about the scrim used for rear projection. I suggest using Chameleon scrim (but that is what I sell. See: www.studio-productions-inc.com)  because it has two aspects that are critical to projection quality.  First as opposed to ordinary scrim  (sharkstooth), Chameleon has a denser surface, so you don't lose a lot of projection through the holes in the material weave.  (Sharkstooth when stretched out has holes about the size of those in a screen window, so lots of projection is lost there).  Second, for rear projection a key thing is the transmission of light through the screen as a medium.  Chameleon is thinner and transmits more light, giving it a key advantage in rear projection.  You can see how thin might be better by imagining projecting on a thick wool blanket from behind--not much projection would get through!

 

Chameleon also hangs square allowing this light weight material to be hung in a rectangular shape without heavy seams.  This can be important where you don't have a fly space and are considering building frames.  The negative to building frames with a rear projection is that you see the shadow of the frame silhouetted in the projection--also you see any cross bracing, which is hard not to have in a large screen.  But if you hang a 15' x15' Chameleon scrim it weighs just about a pound.  If you keep the pipe as aluminum on the top and a pVC pipe on the bottom you have something light enough to hang from simple hooks and heavy duty fishing line.  This costs less time to construct than frames, and less money too.

 

Now a note on projection of scenery.  You have several choices... 

 

If you decide that you don't have the upstage space for rear projection, you can front project by moving the horizon of the projection up higher on the screen (thus avoiding shadows of actors downstage). 

 

If you choose to do rear projection you can do full scenic projections (the entire witches castle), or even shadow projections.  Shadow projections can be done of scenery in small scale without a projector.  Light the back wall and use luan (or even cardboard) cut to shape of the witches castle in the distance to be the silhouette.

 

You can also do a reveal of that same scenery in painted form.  Now your painters are only painting a small picture of the witch's castle, or Narnia battle scene, leaving the rest as sky.  That scenery is easy to remove with one stage hand carrying it off.

 

Scenes can be played in silhoutte behind the scrim, or revealed behind the scrim--for example fight scenes may be best to stage in silhoutte to disguise some gore.

 

There are several articles on the Studio Productions, Inc. website about getting these effects and also projection itself.  Take a look at this list and links to articles:  http://www.studio-productions-inc.com/white_papers/wp_home.html

 

Also feel free to contact me through IM if you need more specific advice, or call 800-359-2964 and ask for Susanne.  (In Canada or outside USA call 812-579-5063)