Science has always been my weak point, John, so maybe someone with more knowledge can address your question
This does raise an interesting question, though: Can interference colors (iridescence) be duplilcated in a full-color hologram, even when using 3 or more lasers?
My guess is no. I think to get those brilliant iridescent effects, you need a recording wavelength that "duplicates" the same wavelength that produces that iridescent effect, which is present in sunlight. It seems very unlikely that a close match would occur at just 3 or 4 specific laser wavelengths.
I am concerned that you need to shoot in a short enough wavelength to get the resolution?
If in fact it is interference, then that means the spacing between the reflecting surfaces needs to be 1 wavelength or multiple of 1 wavelength spacing (1, 2, 3...) for constructive or 1/2 wavelength or multiple of 1/2 wavelength (.5, 1.5, 2.5...) spacing for destructive. If they do in face have .5 to 1 wavelength spacing then for green that would need a resolution of ~250nm wavelength. But maybe the spacing is like 2 to 2.5 or 3 to 3.5 or something. Keep us posted and shoot in blue or green if you can.
Wow, I was just going to ask holo_cyware if he knew exactly what was going on. Thank you Martin.
So, as I understand, the interference part is just to "turn on or turn off" the light at that wavelength, but what gives the multiple colors is basically the same priciple as a reflection hologram made in a clear substrate (like DCG) with index of refraction (air-cuticle multilayer structure) contributing to the reflections/refractions.
So again, I believe that if we are to make a hologram of that structure we need the resolving power. We do not need to make a hologram of the different colors of the reflected light but rather the air-cuticle intefaces.
The second sentence in the article in my last post is the reason I believe we can reproduce iridescence holographically.
Iridescence is simply coloration due to a microscopic physical geometry, rather than the pigments that are usually considered to be sources of coloration.
I've been trying to remember if anyone has mentioned the recording of iridescence in full-color holograms. I know fluorescence has been mentioned many times, but I don't remember anything about the former. Many of us have seen photos of Gentet's butterflies, and they certainly do look brilliant and saturated. But if the butterflies were iridescent, you would have to look at the real thing, and compare it to the hologram, to come to any conclusion.
I still doubt that iridescence will show up in a practical, full-color reflection hologram, or under laser illumination for that matter (except for the specific wavelengths that match the iridescence wavelengths).
Take for example a mirror from a HeNe laser. Say you made a reflection hologram of it (HOE of a mirror). We know that the mirror has a multilayer coating, which relies on constructive interference, to achieve a very high reflectance at 632.8nm. In fact, if it didn't have this interference coating (which I use as an analogy to iridescence) the laser wouldn't have enough gain to lase.
The only way that mirror is going to have that extra "boost" of reflectivity is when it's illuminated with HeNe light (i.e., iridescence). If it's illuminated with another wavelength, reflectance will be lower (i.e., non-iridescence).
Maybe I'm all wet, though. This would be a good question for Kaveh.
Interesting topic. This question has also bugged me since I first saw Gentet´s butterfly holograms; how much of a butterfly´s wing color will show in a rgb color hologram? The pigment colors will be visable, but what about those produced by interference/iridescence? My first thought was that there should be some colors missing, depending on color, but I really don´t know how to think about this. Maybe someone (Kaveh?) has a good description on how a three color hologram of these interference/iridescence forming structures will behave. Comparing the real butterfly to a hologram of it will be one of the first tests I will make whenever I get a working rgb setup.
Don't forget, one of the fundamental theorems of a hologram is that it is the optical double of the original object. Take a look at what the object does under laser light, and that's pretty much what the final holographic image will look like.
So it depends more on what the structure does to the recording light than anything else. Some lambdas of laser light might not be supported in thin film structures, so you have to take a look at the thing under laser light to find the best angle to get some action.
And if the hologram is reconstructed with other than a monochromatic source there's almost always dispersion of some sort so you might get some faux morpho butterfly effects.
I was trying to think if I had ever seen a hologram of peacock feathers or butterflies, negatory on the former, maybe did Gentet did butterflies?
