Holographyforum.org / holowiki.org

Actually an interesting discussion although i don't understand all terms used here.

I always wondered about this as when i made my first hologram I looked at it when it was still wet and could only see it when light from the back was passing through, like this:

http://www.youtube.com/watch?v=lTl47uEPicU


The dried one then finally is visible from reflected light:

http://www.youtube.com/watch?v=LUdncm-B6SE

Kind of interesting that the management team at POC didn't keep their ears to the ground and were aware of who was their competition! But then again, that's POC, which I have heard stands for Piece of Cr*p! (Got that from the great Steve Kupiec, who worked there forever!)

I never actually met Rallison, except briefly at an SPIE meeting where he had a booth at the exhibition around about 2000 or thereabouts. I got into dcg HOEs because I was on the team making laser protection goggles, first at NTS, then at POC. These are notch filters (yes, Ed, the word was used by us at NTS!) that deflect laser wavelengths at very high efficiencies, but allow standard white light with very high clarity. The other group(s) making the laser protection goggles were Kaiser and, I think, Hughes Research. There may have been others, but I wasn't aware of them. Anyway, we used Don Broadbent as a technical consultant for a while, but mostly, we figured out all the dcg stuff by trial and (a lot of!) errors. We knew about the small medallions that Rallison had made, but we thought that display dcg had no relevance to what we were doing. At POC, we were shooting some of Augie Muth's H2s on dcg, but again, that was a sideshow. I was the PI for the laser protection goggles and, if memory serves, Chris Rich was shooting the Augie pieces as well as working with me on the laser protection goggles amidst other stuff. However, the real science and engineering was dedicated towards the notch filters and we weren't aware of Rallison as a dcg HOE holographer.

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


https://docs.google.com/viewer?a=v&q=cache:AIyJiQIhZ-oJ:www.dtic.mil/cgi-bin/GetTRDoc?AD%3DADA273768+Richard+Rallison+dcg&hl=en&gl=us&pid=bl&srcid=ADGEESghGaA1qceO55Waj7nq94aKY1ZNngVHDFCquUjliZI0SiZ18IUgt3kzU_m_AzQ6M9_-Ha36z6ebsdRTHxJv-K0e8TqZnRAW87LEha1FqC6LgNh0PUT7nZDW9R3KF8zDAD9XNEA1&sig=AHIEtbQhPT8nc_ZZeJEN4SCi-3A0mfWiow

“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, [url=http://nlutie.com/ewesly/RK8.pdf]http://nlutie.com/ewesly/RK8.pdf[/url], 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!

[quote="Martin"]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.[/quote]

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.

[quote="Dinesh"] 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.[/quote]

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

[quote]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.[/quote]

[quote="Ed Wesly"]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![/quote]
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.

[quote="BobH"]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.[/quote]
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.

[quote="BobH"]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.[/quote]
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.

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.

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!