today's DCG

[Joe Farina]

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]

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.

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.

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".

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]

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 (452.83 KiB) Viewed 200318 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]

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]

Thanks, I plan to try it.

[Joe Farina]

I had problems focusing the photo, the figure on the right was focused, the others weren't. This is a two-color DCG hologram done today. Still trying to find a film formula that is reproducible, but I've made quite a bit of progress in the last couple days.

P1010037.JPG (395.94 KiB) Viewed 5863 times

This test hologram was re-processed three times, and is noisy. I used a concave mirror to expand the 488/532 beam, no spatial filter. It looks like simpler may be better with regards to color DCG, if you have enough laser power (which is more doable nowadays). The blue laser was a Sapphire 488-200 and the green was a Genesis 1W. The film is roughly 0.6 gram potassium dichromate, 6 grams gelatin (Gelita Imagel 240 bloom), and 60ml water.

I decided to move the pH of the emulsion from 5.3 to 6.3 using sodium hydroxide. Since the isoelectric point of the gelatin is supposed to be around 4.9, and since that's pretty close to its pH of 5.3, I wanted the final pH to be well away from the isoelectric point. The literature says that gelatin is at its maximum turbidity at the isoelectric point, so my intention was to make the emulsion more transparent. Maybe this pH adjustment had no real effect, I don't know. But the results were good. A problem with sodium hydroxide is that it's hygroscopic, and will cause the hologram to vanish quickly if it's not completely washed out (like TMG). The final hologram has quite a bit of angular variation of color, depending on the angle of the reconstructing light. I might want to harden the emulsion before the warm water wash to a greater extent, using a weak formalin solution.

Since it's easy to copy and paste my notes, the following is attached in case anyone is interested:

Notes on exposure and processing of Plate 21 done 5-24-26

Lab was 63F and 69% (overnight).

A two-wavelength exposure was done. Both lasers were on full (1W and 220mW). Measured power (approximately averaged) was 0.5mW for blue, and 2.0mW for green. 120mJ was tried for blue, and 600mJ for green. 20 minute settle time. Exposure was 300 seconds (5 minutes) for green, done first, and 240 seconds (4 minutes) for blue, done second. After exposure, plate baked at 100C for one hour.

Plate was processed four times (with three re-processings).

Wash at 24.1C for 3 minutes
70% at 24.6C for 5 minutes. Similar brightness compared to plate 20, suggesting that exposure may
have been more than enough.
99% at 25.5C (as buffer for 100%) for 5 minutes. Seemed brighter at this stage compared to #20 in
100% (due to warmer wash water).

Reprocessed prior to 100%, since no trace of milkiness was seen. Note, behavior in 99% may be a good indicator for warm water temperature to be used (and to determine if a higher water temperature should be used), since dendrites only formed during 100% later in the processing.

Wash at 26.0 for 3 minutes with some agitation.
70% at 24.0C for 5 minutes
99% at 29.4C for 5 minutes

Reprocessed again with warmer water temperature.

Wash at 27.0C for 3 minutes with some agitation.
70% at 23.6C for 5 minutes. Definitely brighter than above.
99% at 26.8C for 5 minutes. Apparently very faint glancing milk.
100% at 27.3C for 5 minutes. Very small strip of dendrites formed at top which were gone in about 2
minutes.
Hair dryer. Note: there was a transitional area going up to about 2/3rds from the top. The upper area was substantially brighter, and had a very slight milkiness (barely noticeable). The lower area was dimmer and didn't have the slight milkiness. The transition was abrupt. Apparently the water temperature had reached some kind of transition point.

Reprocessed again.

Wash at 29.0C for 3 minutes with some agitation.
70% at 25.0C for 5 minutes
99% at 32.9C for 5 minutes
100% at 32.9C for 5 minutes. Surprisingly, no dendrites in comparison to above.

Colors seemed to improve with the last reprocessing, with blue, green and yellow, but very desaturated. Placed in oven at about 107F (reduced temperature compared to #20). Colors definitely became better (less desaturated) and possibly brighter. Still substantial angular variation in color going from right to left.

[Joe Farina]

A (still blurry) photo showing blue figure on left. The previous hologram was re-processed today with a higher water temperature. My beam alignment (reflecting off the spherical mirror) was off: too much green on the right, and too much blue on the left.

P1010043.JPG (430.71 KiB) Viewed 5814 times

[Din]

I'm in two minds over the warmer water bath. A warmer water bath increases water penetration. However:

1. An increased water penetration swells the emulsion more. This makes the Bragg planes wider, leading to a recon wavelength shift.

2. On the other hand, increased water penetration, causes the water to penetrate faster, so differential swelling (the major cause of broad-bandedness) is mitigated. Both the upper layers of the emulsion and the lower layers swell at the same rate.

So, I would suspect you'd still get a narrow band colour separation, but each colour would be shifted due to a wider Bragg spacing. But, if the emulsion is hardened by the previous reprocessings, the above factors may be mitigated by the initial hardness of the emulsion before the last reprocessing.

I've often described DCG holography as walking on a high wire, while juggling more than three balls. There are so many variables to take into account, while being careful to maintain accuracy in these variables.

PS. Have you considered the hot alcohol bath I mentioned I used to do? It increases narrow-bandedness and brightness.

[Joe Farina]

Thanks for your thoughts about the warm water bath. So far in my tests, a wavelength shift upward (towards red) is preferable to a blue shift, because there are methods (such as prolonged baking) which can shrink the layer back down. On the other hand, if there is too much shrinkage, it's quite challenging for me to swell the emulsion back up. I've had partial success by diffusing hydrolyzed collagen into the processed layer, but it's difficult. So, a red shift wouldn't be a problem.

I've found that a 70% to 100% alcohol sequence tends to produce more narrowband holograms, so that's my standard practice now. Thanks for pointing out earlier that a 75% to 100% sequence was good for narrowband. As for the hot alcohol, I recall that you mentioned earlier that it could be used for narrowband. I was under the mistaken impression that hot 100% (definitely) encouraged broadband results. So, I tried it (a while back), and it did indeed increase brightness while retaining the same level of (partial) narrowband results. I didn't apply hot alcohol to the above hologram, partly because I was satisfied at that point with the brightness, and partly because my ventilation system isn't perfect, and I try to limit the use of hot 100%.

[Joe Farina]

This is the same hologram reprocessed with 68C 100%. Much brighter, without any noticeable difference in bandwidth. Thanks for reminding me! In transmitted light, the plate is clear. For some reason the camera is picking up some blue glare which is mostly absent when viewing the hologram by eye. It's too bad these new pocket digital cameras have autofocus only, my photos are always blurry. The camera focused on the tray of cups and the figure on the right. I miss the old days with manual focus SLR's (haven't spent the money on a digital SLR yet).

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