Why why why would a transmission holo only replay when wet!?

[Justin W]

Hello all!

OK, here is the story - I shot a test strip last night with the goal of discovering how index-matching film to glass with only water would effect things.
The film in question is VRP-M on triacetate substrate. I lase at 532nm.

I have been to this point indexing the film to a single sheet of glass by taping the edges with a thin layer of mineral spirits between film and glass. This kinda sucks for various reasons and so I decided to try something different; I want to get a system of sandwiching the film between two sheets of glass to work. My original plan was to use mineral spirits as before, but I've grown leery of what spirits do to my emulsion so I next considered using lamp oil...

Lamp oil doesn't seem to wash off the film as easily as some say it does, so I put a pin in that idea in lieu of using simple water instead. I realized, of course, that wetting the film with water before exposure was a good way to possibly speed up the film, so I gave it a good soak in DI in the dark first, then stuck it between my sheets of glass. The film seems properly exposed at about 1/4 of the exposure that unwashed film would have needed (this discovery being the primary goal of my testing).

I did a cursory check for an image in the original reference beam after my final rinse and saw the image I expected. I then left the film to dry and returned later to view image.

No image. I turned the film this way and that... No image. So what the hell? It showed an image a little while ago. When it was wet? Huh. OK then. On a whim I dunked the film back into a distilled water bath then checked again for an image. There it was. Now that the film was wet, it showed image.

Weird.

So I left it for the night. It occurred to me this morning that perhaps the water had swelled the emulsion and it would only replay properly when swelled... OK, so I gave it a citric acid swell to see if it would view dry then. Nope.

I wetted the film again and sure enough, it showed an image. It's been wet and dried again about 20 times so far today and the unavoidable conclusion is that under white light or laser, it only shows an image (or rainbow smear) when the film is wet.

What is going on here? I am baffled and lost and stumped and just feeling dumb.
I feel like this situation is rooted in having used water to index the film but knowing some of the "why" doesn't help me figure out the "what".

Please anyone with any idea what's going on here please shout something at me.
Thanks!

[dave battin]

swelling the emulsion in a transmission hologram will have little effect on its replay (the replay angle may change a bit), i suspect the glass indexed to the front of the film may be the culprit. its like the oar in the water trick where the oar's visible angle shift's or changes as it enters the glass/water combination. its possible the replay angle may shift enough when dry that it wants to replay on axis ,and the image is there (its just hard to see it looking and illuminating it on axis at the same time)

[Justin W]

That makes sense that the replay angle may just be changing
but doesn't make sense when I go turning the dry piece of film every possible orientation in both laser and white light and see no image anywhere...

[Danny Bee]

That makes sense that the replay angle may just be changing
but doesn't make sense when I go turning the dry piece of film every possible orientation in both laser and white light and see no image anywhere...

ok question what was the refrence angle? because of the diference between swolen and dried might have tweeked your hologram out of view....i did wet trans before but the angle where something like 45 or smaller from 0

[Justin W]

Reference angle 56.whatever to achieve Brewster's.

[Danny Bee]

Reference angle 56.whatever to achieve Brewster's.

i think thats why its tweeked out of view.... lower angle

[Danny Bee]

Reference angle 56.whatever to achieve Brewster's.

also what film did you use?

[Justin W]

The film was VRP-M.

Tonight I will try indexing with water again, but this time we'll jest see what happens when I lose the sheet of glass over the film....

[Kaveh]

I think it is Bragg angle problem. When you use a dry film, the angle of the reference beam *inside* the emulsion cannot be more than about 40°, because of refraction. So assuming your object beam is falling normally, i.e. 0°, the fringes will be only around 20°. Now when you swell the film in water, from memory it swells to some 3 times its thickness. So in this wet film, your fringes will be 20° or a bit less (actually around 17 in your case). Now you dry the film. The fringes tilt more, and will be greater than 20°. So now there is not way to launch the ref beam so that inside the emulsion it is more than 40°. And I guess swelling with another liquid is not anywhere as much as soaking in water.

I delayed this reply as I was hoping to create a little animation to show the effect. I will try to do that.

[Martin]

I think it is Bragg angle problem. When you use a dry film, the angle of the reference beam *inside* the emulsion cannot be more than about 40°, because of refraction. So assuming your object beam is falling normally, i.e. 0°, the fringes will be only around 20°. Now when you swell the film in water, from memory it swells to some 3 times its thickness. So in this wet film, your fringes will be 20° or a bit less (actually around 17 in your case). Now you dry the film. The fringes tilt more, and will be greater than 20°. So now there is not way to launch the ref beam so that inside the emulsion it is more than 40°.

I agree. The emulsion thickness of the dry layer may be around 7um, whereas the wet layer may be around 20-30um. That makes a transmission hologram much more “fragile” vis-a-vis angular shifts (or thickness changes).

And I guess swelling with another liquid is not anywhere as much as soaking in water.

You might use mixtures of water with glycerol, glycols, sugar, sorbitol and alike. That will keep the water much longer within the emulsion than plain water.

[Justin W]

I was hoping to create a little animation to show the effect. I will try to do that.

Kaveh - thank you for your reply. It makes perfect sense that fringes recorded in a swollen emulsion will be properly oriented for replay only when said emulsion is again swollen to the same degree, but I still look forward - as I am a very visual learner - to viewing any visuals you would be willing to create and post. Loosely understanding this on a broad conceptual level will be nowhere near as valuable as seeing these physics laid out technically and specifically, and I thank you for being willing to aid myself and others in visualizing what's occurring here!

As far as index-matching with water is concerned: this was an experiment in simplifying darkroom work, and one that I'm glad only consumed a few scrap strips of film. I guess if index-matching was as simple as using good ol' H2O, then this would be by now a commonplace thing

Oh, well... That's holography as I know it; First learn 1000 ways to do it wrong, then land on one or maybe two ways to do it right

[Kaveh]

Kaveh - thank you for your reply. It makes perfect sense that fringes recorded in a swollen emulsion will be properly oriented for replay only when said emulsion is again swollen to the same degree, but I still look forward - as I am a very visual learner - to viewing any visuals you would be willing to create and post.

Justin, I think humans are naturally "visual" learners, but have had to learn by reading text as it has been the only way to communicate information in detail. Now that we can use multimedia easily we should use it more and more...

I have been wanting to do some online multimedia recordings to put across my understanding of holography, and I thank you for giving me the reason for doing the first one. OK, I have done the visuals, and just need to put some audio. Then upload from a bad line in Iran in the middle of election fever, with rumours that they will cut internet tomorrow while election goes on. So must do today, haha.

Oh, well... That's holography as I know it; First learn 1000 ways to do it wrong, then land on one or maybe two ways to do it right

The important thing is that you have not just thrown the test holos in the bin and forgotten about it, but are determined to understand what happened and why it is not working... That is how you learn a trade well.

[Ed Wesly]

Here's a visual to help see the problem. Think of the fringes being like Venetian Blinds across the holographic plate. With your geometry, the blinds are tilted at about 30 degrees from the normal. This is their orientation while wet, but when the emulsion dries and shrinks, they collapse and close the fringe/blinds so much that no light can sneak in-between them!

[Justin W]

Nice, Ed!
I can dig it and grok it.
I'm hoping Kaveh's multimedia creation helps explain how it is that a beam hitting the emulsion at 56.6 degrees results in baffles cut through at 30 degrees.
Clearly it's an interference thing, but I just can't picture it...
I wish I could shrink down to the size of an atom and watch it happen for myself

[Kaveh]

Here's a visual to help see the problem. Think of the fringes being like Venetian Blinds across the holographic plate. With your geometry, the blinds are tilted at about 30 degrees from the normal. This is their orientation while wet, but when the emulsion dries and shrinks, they collapse and close the fringe/blinds so much that no light can sneak in-between them!

Ed, I don't think that is the right analogy, because it gives the impression that light has to pass through the louvers whereas it has to hit it as the right bragg angle or near. Hope my little graphic will be up soon. I am preparing in Iran, my colleague in India needs to upload it, and you infidels in America will view it. (Just a joke, in case there is confusion!!)