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Dinesh

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Post by Dinesh »

dcgman wrote:I've noticed this effect when processing the recording of a purely conformal mirror- where's the surface relief there?
I'm not sure what you mean by "conformal mirror" Do you mean "conformal' as in an angle-preserving mapping of one surface to another in the complex plane ? I'm afraid I couldn't find any mention of a conformal mirror in the holographic literature.

At any rate, if the mirror is holographically recorded, the pertinent beams will record an interference set. As Ed Wesley has pointed out, the recording beams give rise to subsidiary beams due to spurious reflections within the emulsion, which also record, so there are multiple recordings, albeit the spurious recordings are relatively weak. However, I assume that the emulsion thickness gives you Bragg diffraction as the end product, which is lambda-selective as well as angle-selective. That is, Q>10. However, if the emulsion thickness were reduced for the same recording geometry, there will be some point at which Q<<10. At this point, you get into the Raman Nath regime and the diffraction will be lambda-independant. All wavelengths will diffract. Also, as the emulsion gets thinner, the spurious recordings mentioned above will stand out more, as most of the spurious recordings are in recorded in transmission geometry. Initially, however, before the Bragg planes have begun to amplify from the latent image, surface distortions will cause Raman Nath diffraction resulting in the rainbow observed at the early stages of development. It's also possible that the spurious beams, recorded in a transmission geometry, will predominate at the early stages of development.

Jeff Weil has mentioned holding up the plate in the light in transmission reconstruction.
Jeffrey Weil wrote:While the plate is wet and you see the reflected dim rainbow image hold the plate up to the light and see if there is a bright transmission image, in transmission mode. That might help the dcg pros here figure it out.
Another idea might be to reconstruct the hologram with a single, unexpanded laser beam (in a non-actinic wavelength for the emulsion, of course!). If you see multiple orders, then you probably have a low frequency surface relief grating.
dcgman

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Post by dcgman »

It's just not worth trying to discuss a point with you, Dinesh. OK- so you are the only one on this forum who understands holography, and everyone else should supplicate before you- there, happy?
Dinesh

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Post by Dinesh »

dcgman wrote:OK- so you are the only one on this forum who understands holography
dcgman wrote: happy?
I'm afraid the exact relationship between contentment and comprehension escapes me. Perhaps you have a functional relationship?
Oh, by the way, this is a joke. I'm sure there is an appropriate little yellow face, but I'm afraid the habit of punctuating the obvious with little yellow faces is also still a bit of a mystery.
Ed Wesly
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Post by Ed Wesly »

Even I know what a conformal mirror is, and I thought it was stock in trade for DCG-ers. It’s simply a single beam reflection of a mirror in contact with the emulsion. I remember seeing them presented at a conference by someone from Kaiser or maybe it was Rallison.

Don’t feel too bad about not knowing, I asked Hans Bjelkhagen when he was crashing at my place this past weekend if he knew, but he didn’t!
"We're the flowers in the dustbin" Sex Pistols
BobH
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Post by BobH »

Hey Ed, probably someone from Pilkington. ;) Am I right Dcgman? Anyway, I also didn't know what a "conformal" mirror was until I learned of it here. But that's why I read this forum! :D

I think it would be difficult to see a surface relief grating in a wet emulsion. I know they are there in AgX materials, but those are the very low frequency gratings associated with intermodulation noise coming from the object. It's easy to see their effect by index-matching your transmission holograms and observing the significant decrease in that noise around the image.

A ghost image formed by the reflection of the reference beam during reflection hologram exposure could be prevented by using an antireflection coating on the glass-air interfaces in the plateholder when using glass plate based material. The Fresnel reflections at the emulsion-glass interfaces would be extremely weak compared to the glass-air interfaces, and shouldn't make a visible image. This is an experiment I haven't tried, but I do have glass with AR coating on one side only to give it a shot. Problem is, the AR coating on my pieces is a "V" coating and might not be very "AR" at the reference beam angles typically used. I'll measure that.
Dinesh

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Post by Dinesh »

Ed Wesly wrote:Don’t feel too bad about not knowing, I asked Hans Bjelkhagen when he was crashing at my place this past weekend if he knew, but he didn’t!
A lot of these terms are either historical or they were used at a particular time at a particular place (eg Kaiser or Pilkington) by some specific person, but the term never showed up in the literature. I got into dcg HOEs in the 80's via the research arm of a company (National Technical System, or NTS); none of the research team were holographers. So, I learned my HOE theory and practice from papers by Shankoff, Kubota and Sing Lee, not from Rallison, so I was never exposed to the word "conformal" in this sense. We just said, "on-axis" or "paraxial" for this kind of geometry.

Actually, I think that there is a great danger in someone reacting in anger when anyone is not familiar with a particular word. Each person has their own vocabulary, their own opinions and their own expertise. These personality differences should be respected, not vilified. In a discussion covering some technical aspect of holography, the only things that should be discussed are the validity and the ideas behind the technical discussion, not personal, apparently vindictive, statements that simply inflame the discussion for no reason. No one should be the victim of a personal attack simply because they're familiar with the theoretical basis of holography. If this forum is not interested in theory, I wish someone would say so and so deflect all these personal attacks.
BobH wrote:I think it would be difficult to see a surface relief grating in a wet emulsion. I know they are there in AgX materials, but those are the very low frequency gratings associated with intermodulation noise coming from the object.
I agree, it is very difficult to see these in a wet emulsion. I would say, though, that it may be easier to see in a dcg emulsion than in a silver emulsion, simply because dcg is so much softer than AgX. As a matter of record, I've never seen this rainbow effect in the first water wash. I do know that Don Broadbent showed me such a rainbow effect in the water wash, so I know it's a real effect, but, as I say, I've never seen it.
BobH wrote:The Fresnel reflections at the emulsion-glass interfaces would be extremely weak compared to the glass-air interfaces, and shouldn't make a visible image.
True. But, if you look at a reflection hologram in transmission mode, you see an achromatic transmission effect. This is more obvious in an AgX hologram than in a dcg. In a dcg, this "reverse transmission" image is pretty broadband, but, even more interesting, is the inverse colour of the reflection image (see below)! My conjecture is that the "reverse transmission" image is the light that did not diffract in the reflection direction and so passes through in the transmission direction - a sort of "image zero order". This would explain why the AgX "reverse transmission" image is achromatic (AgX is very narrow band, so what "goes through" is very broad band) while with dcg the "reverse transmission" is of the opposite colour.

The same hologram, one in reflection and the same in transmission.
Attachments
reflection   .JPG
transmission.JPG
transmission.JPG (17.35 KiB) Viewed 4181 times
Martin

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Post by Martin »

Dinesh wrote: But, if you look at a reflection hologram in transmission mode, you see an achromatic transmission effect. This is more obvious in an AgX hologram than in a dcg. In a dcg, this "reverse transmission" image is pretty broadband, but, even more interesting, is the inverse colour of the reflection image (see below)! My conjecture is that the "reverse transmission" image is the light that did not diffract in the reflection direction and so passes through in the transmission direction - a sort of "image zero order". This would explain why the AgX "reverse transmission" image is achromatic (AgX is very narrow band, so what "goes through" is very broad band) while with dcg the "reverse transmission" is of the opposite colour.
Not directly linked to this problem, Bjelkhagen mentions (in his book, on p. 299):
Serov et al. (...) described a technique allowing for drastic changes of the emulsion thickness between recording and reconstruction. With this technique, it is possible to change a reflection hologram to a transmission hologram, and vice versa. To convert a reflection hologram to a transmission type, glycerin swelling is applied. To perform the opposite transition, the hologram is exposed in a swollen condition (using distilled water). After processing and drying, the hologram behaves like a reflection hologram. As regards possible applications of this technique, the UV or IR reconstruction of holograms recorded in visible light was mentioned.
Dinesh

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Post by Dinesh »

Martin wrote:Not directly linked to this problem, Bjelkhagen mentions (in his book, on p. 299):

Serov et al. (...) described a technique allowing for drastic changes of the emulsion thickness between recording and reconstruction. With this technique, it is possible to change a reflection hologram to a transmission hologram, and vice versa. To convert a reflection hologram to a transmission type, glycerin swelling is applied. To perform the opposite transition, the hologram is exposed in a swollen condition (using distilled water). After processing and drying, the hologram behaves like a reflection hologram. As regards possible applications of this technique, the UV or IR reconstruction of holograms recorded in visible light was mentioned.
Yes, I can see how this might work. To change a reflection hologram to a transmission one, you need to tilt the Bragg planes into a more horizontal orientation (assuming you're holding the plate vertically). Swelling the plate will have this effect of tilting the planes back. To go the other way, you need to tilt the planes from a horizontal orientation to a vertical orientation. Shrinking the (swelled) plate would have this effect. However, I suspect that would only work if the planes were all parallel and almost the same spatial frequency. In a real display hologram, the planes are all orientated in many different ways, so that swelling or shrinking the plate would alter the orientation of some of the planes, but not all. I also suspect that some of the planes will twist, rather than tilt backwards or forwards and create a lot of aberrations.
Ed Wesly
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Post by Ed Wesly »

“A lot of these terms are either historical or they were used at a particular time at a particular place (eg Kaiser or Pilkington) by some specific person, but the term never showed up in the literature.”

How about “notch filter”? Does that ring a bell?

For everybody’s viewing enjoyment and educational edification on the topic, I have a little pdf available on my web site, http://nlutie.com/ewesly/RK8.pdf, that starts off with a picture of a Holographic Optical Element kit, the RK-8, that Rich Rallison used to sell, which includes a conformal mirror and other fun toys, some of which have died an early death. Then the very informative instruction sheet. Also in the pdf are some photos of the contents of a box of Rich’s that was left at LFC for his annual demonstrations, including a bag of Kodak Fixer powder, plus a picture of the man himself circa 1982 or 1985, courtesy of Hans Bjelkhagen. (He's the guy on the right.)

As far as when my faulty recollection remembers when I was informed about the conformal mirror, I know I had one in my hand, and I kind of think it was when B. J. Change from Kaiser gave a lecture at the Optical Society of Chicago on HOE’s. Some of his things were super dichroic, meaning the main reconstruction color reflected off to somewhere, leaving a complementary colored shadow! Pretty impressive!

As far as the last line in the post above, that is the problem with triethanolamining pseudo-colors in reflection holograms! But we all know the method works!
"We're the flowers in the dustbin" Sex Pistols
Tony

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Post by Tony »

For what ever its worth Rollison mentions conformal mirrors in this paper (end of page 2)


https://docs.google.com/viewer?a=v&q=ca ... -3A0mfWiow
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