Printing using RGB filters and multiple exposure rather than CMY?

[grainyvision]

I had a trial run at this last night by cutting 2 sheets of 8x10 in quarters and using a tiny 5x7 tray with just enough developer and blix to cover the paper (didn't have much time for printing, so knew I wouldn't be able to make much more than 8 sheets).

The results are not awful, but definitely not proper color balance yet. My best (bottom right) was printed (in R/G/B) 4/9/6 at f/11 and with the enlarger very close (4x5 paper). And second best I think for color though way too light (bottom middle) was printed at 2/2/4 at f/11.

Trying to figure out how to correct casts is definitely not trivial. Does anyone know of any guidelines or other instruction on proper adjustment for each color of cast? I stared at a color wheel for a while last night and wrote down some guesses, but not sure if they're right.

 

[DREW WILEY]

I have two true RGB customized enlargers, one an 8x10, the other 5x7. It takes a lot more light to punch a big additive color print than a subtractive one. These colorheads are designed for simultaneous printing; but if I want to, I can use individual RGB channel sequentially. One of the most precise ways of doing this, in
terms of actual color reproduction, is to take sequential black and white shots through tricolor RGB filters, then punch them in register for use in a matching registered neg carrier, then sequentially print each of these RGB onto RA4 color paper. Or separation negatives on black and white film can be made from color transparencies and printed in the same way. Usually, corrective masks must also be generated. I'm equipped to do all this, as well as dye transfer printing, but it's quite a headache
getting everything calibrated. I got as far as generating several sets of perfectly matched separation negatives from 8x10 chrome shots then got diverted onto other projects. One of these days ...

 

[darkroommike]

Additive printing filters are "hard cutting" transmit only a tiny amount of the white light from the lamp, so exposures run very long. Subtractive filters transmit a broader spectrum and only remove as much as needed to balance exposure. The other issue that has not even been mentioned yet is reciprocity departure, most modern color papers are optimized for relatively short exposures, say under a minute. Long additive exposures need to be even longer due to reciprocity departure.

 

[DREW WILEY]

Additive printing requires either long exposure with reciprocity risk OR a more powerful light source. But RA4 papers are relatively fast anyway.