The oldest 3D effects from the 1950's used Red/Blue color seperations and Red/Blue glasses. Quite primitive but interesting in the sense that these 3D effects could (kinda) be broadcast over the air or recorded onto video tape - as long as the viewer used the glasses to view the two images, the 3D effect was present on your TV screen.
Later color 3D systems were very costly to use - two side-by-side 35mm cameras with synchronized shutters, twice the expense in film and processing and film distribution and requiring synchronized dual projectors. The two images were projected using oppositely polarized light, and viewed with polarized glasses that effectively dimmed the R image in the L eye and dimmed the L image in the R eye. Your brain overlapped them by causing your eyes to go slightly crosseyed in the theater under the glasses.
In practice 35mm 3D was more than 2X as expensive as filming flat 35mm, because you can make mistakes like unsynchronized cameras that require re-shoot, and the optical special effects used on film must be done twice - but slightly differently, to preserve the 3D effect.
RealD 3D is an entirely digital process. The RealD 3D camera is actually two smallish digital cameras mounted side-by-side. Two seperate images are captured for the R and L perspectives at 1080p24, but recorded into the same video file. Digital special effects are simple, and are inserted into the digital master using the same software and workstations used for the animated film Beowulf (the very first RealD 3D film). RealD 3D offers the same advantages of digital distribution (inexpensive and entirely secure) but because of the two seperate 1080p24 image streams, the digital files occupy twice as much space on the theater server as flat digital films.
RealD 3D projection uses two polarized light sources, one for R and one for L images, and they simultaneously illuminate two DLP mirror arrays. Both R and L image streams are projected from the same lens and projector. The same polarized glasses used for the dual 35mm 3D system can be used to dim the image intended for the other eye. In fact the two image streams are triple clocked to 72Hz which is even faster with less flicker than double-flashing 24fps 35mm film to 48Hz.
RealD 3D projectors can be made compatible with earlier 35mm 3D films (by capturing digitized frames from both films) and RealD 3D digital movies could be printed onto R and L 35mm film prints for earlier 3D theaters. But using the RealD 3D technology from beginning to end is the best and least expensive plan. Note that flat movies are also compatible with the RealD 3D projector if they are digital "prints", and the glasses are not required.
I thought the first RealD 3D film Beowulf was OK, and the 3D technology interesting enough to make it enjoyable - but I was never tempted to watch it again at home in the flat version. Likewise the second film was a Hannah Montana concert film of no interest to me - even though it was the first live action photography in RealD 3D. Journey to the Center of the Earth being the second live film. In fact I have seen several flat digital movies in that same theater, the first being the Disney film Eight Below.
I'm going to see "Journey" in RealD 3D on Tuesday night and I'll be comparing it to 35mm and IMAX 3D systems I have seen in the past.
