Specifically I'm thinking about bi-packs & tri-packs, but monopacks certainly apply as well. The azo emulsion that we made at GEH is fairly opaque; creamy & off-white, like a thin layer of gesso or something. So I'm wondering, if I wanted to layer a blue-sensitive emulsion over a green or red sensitive emulsion, how would I ensure that light passes through the blue unscathed? Now, reading an F.E. Ives patent on a tripack he mentions that the front blue emulsion should be of the Lippmann variety (iodide-bromide??), which has to be quite transparent. They are nearly glass-like, judging from some holography plates I have.