Iridescence recording ?

Simple answers are here! For Theory look in General Holography.
holo_cyware

Iridescence recording ?

Post by holo_cyware »

I was wondering what would be the result or making a hologram of a iridiscent butterfly like a morpho, or a peacock feather or something else featuring metalic colors. Since the color is generated by the physical structure of the scales on the morpho butterflies wings and not by a pigment, a hologram should in theory reproduce the exact color, regardless the laser used for the exposure (although it may have something to do with the minimum wavelength, meaning the "micropits" in the scales causing the light refraction may not be reproduced by certain lasers in the gelatin layer).

A morpho butterfly: http://upload.wikimedia.org/wikipedia/c ... terfly.jpg

I know Gentet did some amazing holograms of museum insect boxes, but in theory the RGB laser system is not needed to reproduce the iridiscence.

Any thoughts?
Joe Farina

Iridescence recording ?

Post by Joe Farina »

If the colors are caused by diffraction, then you still need multiple wavelengths to get different colors. If you just used red light, for example, and illuminated a diffraction grating, I guess the light would get diffracted into different orders, but they would all be red. Using just two colors, for example red and green, might produce some interesting results.
JohnFP

Iridescence recording ?

Post by JohnFP »

Yes, I would have to agree. You would need all three lasers to get the different sized diffraction fringes, then the replay would have to be with white light also.

But let's think about this a second. I wonder if we could expose the butterfly with with three different angles using the same wavelength laser to get that same effect?
holo_cyware

Iridescence recording ?

Post by holo_cyware »

I see, thanks for the input. Yet another brainfart of mine then...

It's simple: the hologram cannot look differently that what your naked eye sees when the object is iluminated with the laser before the exposure. The result can be color shifted (TEA etc), but that's about it.

A morpho butterfly iluminated with a red laser will apear black, because the difraction grattings from it's scales work by absorbing any light color but the blue one. So at least a blue laser is needed, and it may/may not work, depending on the light spectra reflected by the scales.
JohnFP

Iridescence recording ?

Post by JohnFP »

I love talking about "hair-brain" ideas (as some used to call them). It gets your mind thinking outside the box. :wink:

Your post got me thinking down on the fringe level of how it can be done.
JohnFP

Iridescence recording ?

Post by JohnFP »

Oh oh. Seems iridescence in insects is not what I thought at all.

IN fact,

source: Kevin S. Gould and David W. Lee, "Physical and Ultrastructural Bases of Blue Leaf Iridescence in Four Malaysians Understory Plants," American Journal of Botany 83. 1 January 1996: 45-50.

p. 45: "Iridescence in plants has been attributed to a single physical effect, the constructive interference of reflected blue light," especially the effect created by special structures in cells near the surface of the leaf.

p. 49: These cells have their cell walls "composed of multiple layers of parallel microfibrils.... The orientation of microfibrils in each layer is set at a small angle from that in adjacent layers." Such structures are called "helicoidal structures" and "share some properties of cholesteric crystals, a group of liquid crystals...."

http://www.swarthmore.edu/Humanities/ps ... cence.html

So it is possible. In fact all holograms are iridescent. So now, how do we replicate it? Can we simply make the hologram with the correct wavelength of blue light that is used in the contructive interference? Or is it as simple as making the hologram of the butterfly wing with any wavelength and then shrinking to the correct replay blue wavelength?

Interesting subject.
JohnFP

Iridescence recording ?

Post by JohnFP »

More from that same article.

Each microscopic butterfly scale is made up of not
just one, but numerous angled, ridged surfaces. Rather than having
just one reflecting surface, many are involved. The result of this
multitude of reflecting surfaces is an intense iridescence.....

So if in fact the scale has numerous angled rigid surfaces, I would have to conclude that yes in theory, it could be made with a single wavelength and then illuminated with a white light as long as the wavlength used to record it was considerably shorter then the separations of the angled rigid surfaces. Obviously if this scale is producing this constructive and destructive interferences, then those angled rigid surfaces may in fact be close to 1/2 micron (our laser wavelength) in separation and we need to be able to have the resolution to record those.
Joe Farina

Iridescence recording ?

Post by Joe Farina »

I didn't know that those colors were due to interference rather than diffraction, that's interesting. But it shouldn't make a difference (regarding the question about getting multiple colors using a single wavelength, I simply don't think it can be done, unless we're talking about fluorescence, and fluorescence is supposed to be incoherent). If the butterfly gets its blue color through constructive interference for blue, then any illumination other than that particular blue might produce a fairly dim reflection. But you can't really tell about natural objects, until you illuminate them. If you used red light, then it wouldn't necessarily appear black, it might look quite red, depending on the butterfly's reflectance in the red portion of the spectrum. Now if you used a blue laser that was close to the constructive interference of the butterfly, it might have a very bright and "blue" appearance. But it might not look very good overall, becasue the butterfly will be reflecting other wavelengths as well, which won't be visible under blue laser illumination, or in the final hologram.

Yes, as Kaveh has made clear, what you see under 2 or 3 wavelengths is about the same thing you will see in the final hologram, assuming no shrinkage.
JohnFP

Iridescence recording ?

Post by JohnFP »

Joe, upon further reading I think it is both but let's examine a soap bubble. You get the colors from the different thicknesses of the soap film (refraction). If we could make a hologram with a wavelength short enough to record the front and back surfaces of the entire soap bubble, wouldn't we get multiple colors from the hologram?

Let's simplify even further, if we make a hologram of a prism, do we get rainbow diffraction from the hologram? Even if made with a single wavelength. We just need to record the front and back surface of the prism.

I think the same hold true for the insect. If we can be sure we are recording the different sized surfaces on the scale, then we should be able to see diffraction/refraction or whatever it is in our replay.
holo_cyware

Iridescence recording ?

Post by holo_cyware »

The iridiscence in insects is a vast domain; many studies have been done and SEM microscopy explains now fairly well the phenomena. There are many mechanisms producing it, but it's always interference as far as I know.
Another interesting fact is that the green color in butterfies is ALWAYS a iridiscent color, as they have no pigment to create it. There are pigments for the rest of the colors, but not for the green. Also, the humble white butterflies such as cabbage white and many other do have an interference phenomena just like morpho's, but in UV, so we the humans can't see it. They obviously do it, from miles away apparently.

Anyways, I'll try to see what the results are, as I have got some of these butterflies, including some with interference for all the spectrum (Urania Ripheus).
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