[Hologram]

One reason the interference in a Lippmann plate is weak is that your light is incoherent. A laser can fill a thick emulsion with fringes, but incoherent light does much worse, even if it is filtered to be monochromatic. A rough rule of thumb for thermal light (black body radiation, aka sunlight) is that the coherence length is of the order of the centre wavelength. In layman's terms that means you can only expect to get two or three fringes. It's one reason why Lippmann emulsions are thin: there is no benefit to a thicker emulsion, it just wastes silver and absorbs and scatters light.

I agree. By "volume recording medium" I didn't advocate using "thick" recording layers for Lippmann photographs. That's why commercial holographic AgX emulsions, which usually are around 8um thick, are certainly not the best solution for Lippmann work. On the other hand, these materials have been greatly improved. Scatter becomes very negligible with grain sizes <10nm - even for blue and violet radiation.

By the term volume medium I was referring to a recording medium that records the interference fringes within the depth of its layer (and be it only a 2um "thick" layer). That would be in contrast to a surface relief structure, say a photoresist. Maybe I'm wrong but I believe you could not record a Lippmann photograph as a surface relief structure.

By the way, regarding filtered incoherent light, it's possible to make reasonably good contact copies from holograms.

FWIW, the Morpho butterfly scales are 3D on the nanoscale. The remarkable thing about the Morphos is not that they have lovely colours, but that their colour is so strong, and so pure, over a wide range of angles and lighting conditions. If you could somehow reproduce their method of generating colour, and vary it across a substrate, you would be a very happy camper indeed.

Yes, they're truly amazing. See: http://www.opticsinfobase.org/oe/abs...?URI=oe-5-4-87