Three-color photography with a 2-color camera
I've been toying with the mechanics of trying to build one... Making a two color system at home isn't trivial either, or I'd be all over it. But it isn't outside the realm of hobbyist possibilities either. And a plate beam splitter and front surface mirror is a lot less expensive than getting the tri-color prism.
Ok, so I don't know why it took me so long to put these two pieces together... but there's a very interesting idea presented by F.E. Ives that would make a two-color camera, as suggested by Michael, possible of creating three color images.
Here's the paragraph from JS Friedman's 'History of Color Photography'... (page 145, available on Google books)
"There is one field, however, that appears to be ideally suited for the screen plate. This is the use of a screened bipack in which the front element is a screen plate which registers two of the primaries, and the rear element registers the third. This procedure was first disclosed by Ives. The screen units in the front element were colored yellow and magenta, and these were coated with an emulsion sensitive only to the blue & green primaries. The blue densities would be registered only behind the magenta elements, while the green densities would register only behind the yellow lines. By the use of filters, it therefore becomes possible to copy the images behind these lines upon separate plates. Both the yellow and the magenta lines transmit the red rays w/ great efficiency. Since the front element is completely insensitive to this section of the spectrum, it will register only upon the rear element, and in the form of a continuous tone negative."
Ok, so the difference would be that a bipack would be unncessary, as we'd theoretically have a camera capable of holding two plates/films. This would greatly simplify things.
The logical application would be to make this two-color screen-plate and to place orthochromatic film behind it. This would go in one of the holders. From this you'd make your blue & green separations. The typical drawbacks of a screen plate (visible "grain", inability to enlarge much) would be greatly diminished as it only has 2 colors as opposed to 3. Therefore it would be higher resolution than all other screen plates.
Then in your other film holder you'd have panchromatic film with a typical (29 or 25) red sep. filter.
Since the red separation would be continuous and your screen-plate finer than ever, the overall fidelity would be far and away better than what we are used to with "screen-plate" photography.
Once you obtained a well balanced screen plate, matching its sensitivity to the red sep. would be merely a matter of placing neutral density where it's needed.
So, the concerns as I can see them would be.... is commercially available orthochromatic film the right sensitivity to be used for this kind of separation? And of course, making the screen-plate; but using a printer on transparency paper would certainly be easy, plus the integrity of the dyes used would be inconsequential, that is, they don't have to be "archival" really, since you could make your separations soon after, and then if the screen-plate faded over time it wouldn't really matter.
Just something to put in your pipe and smoke....