today's DCG

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Joe Farina
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Re: today's DCG

Post by Joe Farina »

Din wrote: Mon Jan 13, 2025 12:56 pm Joe Farina wrote: ↑Mon Jan 13, 2025 9:14 am
I probably underestimated the importance of post-exposure hardening/reducing by the use of fixer.

Din wrote:
It's quite important. As I mentioned earlier, it hardens the surface so that it limits the differential swelling. It's a bit of a balancing act (as most DCG holography is!) because if you over-harden, you get no swelling, and if you under-harden, you get too much differential swelling and so broadband.
If the objective is to harden the surface of the gelatin to limit differential swelling later on, I would think that the amount of time the gelatin is in the fixer would be quite important. Since the fixer is mostly water, it would penetrate easily through the gelatin, and if it's left too long, I suppose that would harden it more uniformly and defeat the purpose. It makes me wonder if a specific "fixer" could be developed for narrowband DCG. Or perhaps a two-part approach to fixing, the first part being specifically designed to cause surface hardening only.

In the past, the hardening substances I've used (and currently have) are chrome alum, formalin, and aluminum sulfate (not the potassium aluminum sulfate, which I haven't used -- maybe it's better, I don't know). As for the aluminum sulfate, I assume it's just a hardener of gelatin, and not a reducer for the dichromate. The reducing agents I have are sodium metabisulfite (forming sodium bisulfite in water) and sodium bisulfate. Lin, in his 1968 paper in Applied Optics, suggested the use of a 2% sodium bisulfate solution as an alternative to Kodak fixer. I assume that these sulfites/sulfates cause hardening by reducing dichromate. I haven't tried the sodium bisulfate yet.

Perhaps it will be feasible to use a solvent with hardener in a fix which doesn't easily penetrate gelatin, but which still hardens the surface of the gelatin.
Din
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Re: today's DCG

Post by Din »

Joe Farina wrote: Wed Jan 15, 2025 9:01 am If the objective is to harden the surface of the gelatin to limit differential swelling later on, I would think that the amount of time the gelatin is in the fixer would be quite important.
Actually, thinking further, I might not have expressed it properly. Bearing in mind that the hardener stiffens the gelatin, while not reducing the dichromate, the idea is not to just harden the surface, but to harden the entire volume of the gelatin, with the upper parts hardening more of the volume. In other words, the hardening should be a function of the penetration of the hardener, decreasing as you move into the depth of the gelatin. In still other words, the surface has to harden more than the body, but the body itself has to harden to some extent. The idea is that as the water enters the emulsion, the upper parts expand less than they would normally have done, while the lower parts have not yet begun to swell. So, the water-generated expansion in the water bath is inversely proportional to the depth. This should mitigate the differential swelling.
But, as you say, the hardener is mostly water, so you have to balance the swelling of the emulsion during the hardening stage against the hardening itself - a delicate balance, as I mentioned all of DCG holography is.
Joe Farina wrote: Wed Jan 15, 2025 9:01 am As for the aluminum sulfate, I assume it's just a hardener of gelatin, and not a reducer for the dichromate.
Yes, I think so. I've seen aluminium sulphate as a hardener in some photographic sites.
Joe Farina wrote: Wed Jan 15, 2025 9:01 am Lin, in his 1968 paper in Applied Optics, suggested the use of a 2% sodium bisulfate solution as an alternative to Kodak fixer. I assume that these sulfites/sulfates cause hardening by reducing dichromate. I haven't tried the sodium bisulfate yet.
I have never tried to harden by chemically reducing the dichromate. But, had I known of the Lin paper, I would not have tried it. The reason is that the chemical reduction of the dichromate has to be uniform throughout the emulsion. However, the reduction of the dichromate depends on the local density of the dichromate. If the density of the dichromate is not uniform, then some parts of the emulsion will harden more than other parts, leading to "greenies".
Joe Farina wrote: Wed Jan 15, 2025 9:01 am
Perhaps it will be feasible to use a solvent with hardener in a fix which doesn't easily penetrate gelatin, but which still hardens the surface of the gelatin.
In 1986, I was researching into "laser protection goggles". These are goggles which are highly transparent - there's a figure of merit for transparency known as the photopic efficiency, and these goggles had to have a very high photopic efficiency - in normal light, but have a high rejection for laser wavelengths. The problem was that when you examined the Bragg plane structure under a spectrometer, you saw the main peak, the planes that are tuned to the laser rejection wavelength, with subsidiary minor peaks on either side of the main peak; these subsidiary peaks were killing the photopic efficiency. This was a problem that needed solving, but all the researchers before me had not solved. It occurred to me that the cause of the subsidiary peaks were due to variations in the Bragg plane spacing, caused by differential swelling, which the fixer had not not completely removed. So, it occurred to me that if I zapped the entire emulsion with uv, there would be partial hardening, since dichromate has a high absorption for uv. Of course, too much uv would make the entire emulsion resistant to water swelling and give a low efficiency to laser wavelengths, while too little would still give subsidiary peaks, but smaller and less pronounced. Anyway, after much trial-and-error, I found the right combination of uv wavelength and exposure energy to keep the main peak unaffected but get rid of the subsidiary peaks. I finally got a very high photopic efficiency with a very high rejection of laser wavelengths.
The point of all this is in 2012, I wondered if I could use the same technique - pre-exposure of the emulsion by zapping with uv - instead of fixer. I had some limited success, but I didn't have the right uv source; the company where I researched the laser protection goggles, National Technical Systems had a much higher budget for research than I did.
Joe Farina
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Re: today's DCG

Post by Joe Farina »

Thanks for the clarification. Regarding the use of fixer, I didn't fully appreciate the importance of the tug-of-war taking place between the post-exposure hardness level, and the hardening effect of the fixer bath. It seems that the holographer (with the fixing bath) is at one end of the rope. The opponent at the other end has an unknown strength. The holographer needs to pull the opponent just to a certain point (just on top of the line) but not pull him too far across. I think, in the past, holographers have used the "age" of the plate to judge the "strength" or hardness. But this doesn't seem reliable enough. There are too many factors which can affect the post-exposure hardness, including environmental factors during and after coating (temperature and humidity during drying and storage, etc.). So, the hardness is pretty difficult to judge in my opinion.

Years ago, I did some experimenting with a pre-fixer hardening bath consisting of methanol and formalin. My intention at the time wasn't to cause more hardening at the surface of the layer, but to try to harden the layer uniformly without swelling it. The reason for this was that I wanted the final emulsion to "spring back" somewhat to the state it was in just after exposure (i.e., its thickness at that time, which would result in more "correct" colors). I thought this hardening step might facilitate this. It did seem to help somewhat in the below hologram, though the layer still shrunk (I was just using 532nm and 633nm). I don't know if the hologram would qualify as approaching narrowband, but the color differentiation was pretty good.
hologram2015.JPG
hologram2015.JPG (452.83 KiB) Viewed 652 times
Thanks for the information about your research with UV exposure. Did you mean UV exposure before laser exposure, or after? I noted your comments about this before, and want to do some testing. The 365nm wavelength is available in old-fashioned "black lights" (the fluorescent tube type, which I have) but the output is quite weak. I'm currently looking at this LED flashlight version on Amazon, which looks more powerful:

https://www.amazon.com/uvBeast-NEW-365n ... hdGY&psc=1
Din
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Re: today's DCG

Post by Din »

Joe Farina wrote: Wed Jan 15, 2025 4:04 pm Did you mean UV exposure before laser exposure, or after?
Before. The most important parameter in efficiency is the modulation, the density difference between plane and not-plane. If the planes already existed in the emulsion - the latent image - then there might be non-uniform hardening between plane and not-plane as a result of uv exposure. If this caused a loss of modulation, the hologram would be weak. I couldn't risk that, so I uv exposed before laser exposure, and hoped to increase the modulation by over-exposing, which seemed to work.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: today's DCG

Post by Joe Farina »

Thanks, I plan to try it.
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