Diazo (Speedball) gelatin hologram

Dichromated Gelatin.
Joe Farina
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Re: Diazo (Speedball) gelatin hologram

Post by Joe Farina »

Thanks Martin for your reply and the information about DAS and the Speedball material.

After a 6-year break from DCG, I'm finally making some tests again. The shelf life of materials seems to keep me guessing. My old gelatin, which was culinary Great Lakes, type B, bovine hide, was older than 6 years (kept in an airtight jar), and did not hold up well. I got some new Great Lakes, and also some Knox from the grocery store. That helped. The ordinary culinary Knox is performing better right now. Gelatin manufacturers seem to be quite "squirrelly" about what their gelatin actually is, how it was made, or where it came from. Knox may actually be a mixture of types A and B, but I don't know.

It's harder than I thought to start up again. I'm doing three-color exposures on DCG, and the colors are coming out well. The 200mW Sapphire (488nm) is doing good, I was worried that it might have issues (it's not specified single frequency). But the holograms as a whole are quite poor. Some of my chemicals are old, and for the ones which appear to have a limited shelf life, I bought new ones. The new Gelita gelatins, bloom 240 and 280 (made in Germany?) from France is being shipped now. It seems fairly clear that this is type B bone gelatin, which I believe is the norm for photographic gelatin. I also want to try culinary Superclear gelatin, which is a pig hide type A with a bloom strength of 300, for monochrome (no dye) DCG. One of the forum members mentioned using it (in this thread), and he's making excellent DCG holograms.
Din
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Re: Diazo (Speedball) gelatin hologram

Post by Din »

Martin wrote: Thu May 04, 2023 6:22 am
Isn't it strange that after all these years it's still unclear what we're really looking for in order to get a highly efficient gelatin. What makes it particularly well suited for DCG, FEG etc.? We know certain brands and types are working well while others don't. But we don't know why.
Actually, Martin, a lot of papers exist about the structure of gelatin for silver halide holography as well as DCG holography. Don Broadbent, whose lab we inherited, left behind a very large bound book of his collection of gelatin structure papers and excerpts from other books for silver halide, DCG and other similar materials. Joy tells me that I tried to throw it out when we left Triple Take behind, but she rescued it; she knows it's somewhere, but doesn't know where. Back in the day, there used to be a journal called "Photographic Science and Engineering" that was dedicated to the structure of gelatins and photoresists. I and Chris Rich from POC used to spend hours poring over the papers in what we called "Phot-Sci_Eng" for the ideal structure for DCG holography, since we worked entirely in DCG at POC in the 80's. I was particularly fascinated by the effect of the pH of gelatin on DCG hoolograms for technical applications and designed some gelatins to control bandwidth.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: Diazo (Speedball) gelatin hologram

Post by Joe Farina »

Din, I would like to ask a question, since I know you have a lot of experience with DCG. It concerns bandwidth in relation to the "blurring" effect (so common in DCG). On one occasion, I made a two-color hologram (532 and 633nm), and the image was very sharp, even in the deeper parts. It was comparable to some regular DCG holograms I made earlier (dichromate+gelatin+457nm laser). This only happened once, and all my other two-color (and recent three-color) holograms have substantial blurring of the image at a depth of say 5mm or more behind the film plane. Close to the film plane (almost touching it) the image looks sharp and clear. The processing technique I used was assumed to be narrowband. (These are all Denisyuk holograms.)

My only guess is that I was lucky on that one exposure, and obtained a truly narrowband result. Even though I'm seeing good, saturated colors on two and three-color holograms, apparently that doesn't mean that the image is especially narrowband. This is because the deeper parts of the image are blurry. So basically, my question is whether or not image blurring in DCG is usually (or always) the result of increased bandwidth (chirped fringe structure). Also, can a DCG hologram be made narrowband enough to produce a very sharp image with say three inches of depth, comparable to what silver halide can do?

Thanks.
Din
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Joined: Thu Mar 12, 2015 4:47 pm

Re: Diazo (Speedball) gelatin hologram

Post by Din »

Joe,
The first thing that occurred to me was the size of the reproduction source. But, you say you can get sharp holograms with silver, so this does not appear to be the problem. Joy asked if you used the same source, at the same distance, for the sharp hologram as the blurred ones.
Assuming it to be a problem unique to DCG, other things that occur to me:

1. Scattering of light from nearby objects such as mounts etc, including the table itself. We used to lay down black velvet cloth along the table from the objective to the plate. This matters less to silver halide because DCG is some 100's of times softer than silver, so low frequency gratings from spurious sources are more of a problem. I cannot overstate the importance of baffling, especially on DCG because it's so much softer. DCG, as opposed to silver, will produce modulation from any light, specifically low levels of spurious reflections, since the actinic reaction of DCG is a hardening of the exposed parts. In silver, the actinic reaction is the reduction of the silver ion to elemental silver, and requires a minimum of energy. Not so for DCG, there is no threshold. Another point is to put iris's both behind the objective and in front of it. Light from the mirror behind the objective spreads around the objective and creates noise.

2. The emulsion is too soft. Either there's too much gelatin for the dichromate, or the pre-hardening stage, before the water wash, did not preharden it enough. The result of this is that the emulsion starts to expand as water penetrates it, starting from the top and working it's way downward. This means there's differential swelling, since the upper layers continue to absorb water and expand as the water penetrates deeper to levels that have not yet started to swell. The upper emulsion layers will twist the Bragg planes by a differential amount relative to the planes deeper in the emulsion. This means that the recon angles are variable, and correct only for specific depths of the image. Also, a softer emulsion will give rise to a surface grating which will act as a transmission grating and create noise, which will manifest as blurring.
For what it's worth, we used Knox gelatin (250 bloom for display); our formula was 45 gms of gelatin in 300 mls of water. We used a magnetic hot plate, poured the mix into a beaker, and put in a magnetic stirrer. We adjusted the stirrer control on the hot plate so as not to stir too fast, and set the plate heater control so that the temperature of the 'goop' (Joy called it, the 'goop') was about 50 deg C (important to avoid 'greenies'). We also covered the beaker so as not to lose water through evaporation, but allowed a thermometer into the 'goop' . I checked temperature and stirring every 20 minutes or so. After about 2 hours, when the gelatin was thoroughly dissolved and mixed, we dissolved 8 gms of dichromate in 30 mls of water, added it to the goop, and let it dissolve for a further half hour, still stirring. Afterwards, we filtered the now-sensitive, goop into a second beaker, with coffee filters, adding a few drop of photoflo in the goop in the filter to assure uniformity of the emulsion - again covering the filter to minimise loss due to evaporation. The filtering stage was also on the hot plate to ensure the temperature was kept at about 50 C We used Rapid Fixer, with hardener, for pre-hardening. If the plates were fresh (just a few days old) I fixed for about 2.5 to 3 minutes, if the plates were older (over about 10 days old), I cut down the fix time to about a minute, sometimes even less when I wanted broadband.

3. I passed this by Joy and she pointed out what had not occurred to me: scattering within the emulsion by particulate matter while recording. This would occur either because of pockets of density in the emulsion or to particulates of gelatin within the emulsion that have not completely dissolved, or to particulates of dichromate crystals, also because they have not completely dissolved. Due to the softness of the gelatin, and because there's no threshold of exposure, these 'internal scatterings' will produce spurious gratings. See above my method for making sure everything is completely dissolved and the density is uniform, including dissolving the dichromate in water before adding to the goop.

This is all I can think of now. If anything else occurs to me, I'll post again.
Din
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Re: Diazo (Speedball) gelatin hologram

Post by Din »

Joe Farina wrote: Thu May 04, 2023 12:11 pm Also, can a DCG hologram be made narrowband enough to produce a very sharp image with say three inches of depth, comparable to what silver halide can do?

Thanks.
Yes, this one is about 4 inches deep.
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Joe Farina
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Re: Diazo (Speedball) gelatin hologram

Post by Joe Farina »

Thanks very much Din. Please thank Joy also.

After a 6 year hiatus from DCG, I'm having to re-learn many things. I will be carefully re-reading your posts, and also searching the forum for old posts. There is truly a wealth of information here, if one is willing to search. Regarding scattering in the emulsion, I've made numerous sins lately, including not filtering the emulsion. Another potential error was only heating the emulsion to about 40C (just above the melting temp). If I go higher, say to 50C as you mentioned, I have a feeling the emulsion will be more homogenous. The many potential sources of scattering need to be considered, I think, as you outlined so well in this thread. I also found an old post of yours at:

https://holographyforum.org/forum/viewt ... 3B+gelatin

Today, I bought some silicone molds which I think can be used to make small cubes of gelatin/dichromate. Then I can send a laser beam through the cube, and observe scattering on the sides. This may help considerably with the choice of gelatin, etc. My current DCG formula has a lot in it: gelatin, potassium chromate, TMG, acetic acid, methylene blue, rhodamine 6G, and chrome alum (the latter I've found helpful for bias hardening). All this stuff could contribute to scattering. The cube test may help with this also.

My current plan is to go back to basics for a while, and temporarily forget about color. I just want to do regular DCG (gelatin/dichromate/488nm) and then start to think about color. The most important thing to me is the strength of the image, its clarity, the clarity being maintained over a reasonable depth, say 2.5 inches, and being narrowband overall. Brightness needs to be at a certain level, but it's not the overriding factor to me.

Thanks for the photo, I appreciate it. I've taken a photo of a very poor hologram I made this week (of some painted marbles). There was no attempt at balancing the power of the lasers, the 488nm was turned all the way up to 200mW (much higher than 532nm and 633nm), so the image is quite blue overall. I've gotten so out of practice that I still can't identify the reason for the miserable horizontal splash marks.
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Martin
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Re: Diazo (Speedball) gelatin hologram

Post by Martin »

Din wrote: Thu May 04, 2023 10:40 am Joy tells me that I tried to throw it out when we left Triple Take behind, but she rescued it; she knows it's somewhere, but doesn't know where.
That sounds exciting. Would be nice to see it online some day.

I've been reading a great many papers on AgX, DCG etc. For AgX the specific impact from gelatin (and, later from the addition of sulfur and reduction sensitizers) has been established pretty clearly. I don't see anything alike for DCG. Sometime around the 1990 scientific papers on DCG seemed to have switched from dichromated gelatin to dichromated polyvinyl alcohol. It was pointed out that the less complex chemical structure of polyvinyl alcohol provided better insight into the mechanisms of light-sensitive dichromated systems. But in hindsight it looks a bit like capitulation.

I guess what I've been missing was something similar to what happened to silver halide photography: once the sensitivity promoting gelatin components were identified, they were added to highly purified ("inert") gelatin subsequently. Something alike I naively expected to find for DCG (let alone FEG or diazo gelatin). That would have culminated in adding the chemical(s) XY to some inert gelatin in order to get increased speed/modulation. Various speed enhancing chemicals have been mentioned earlier but to the best of my knowledge there has never been systematic research in that area.
Joe Farina
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Re: Diazo (Speedball) gelatin hologram

Post by Joe Farina »

Din wrote: Fri May 05, 2023 12:19 pm The first thing that occurred to me was the size of the reproduction source. But, you say you can get sharp holograms with silver, so this does not appear to be the problem. Joy asked if you used the same source, at the same distance, for the sharp hologram as the blurred ones.
I forgot to reply to Joy's question. Yes, with the same source and distance, although I haven't done silver since the 90's. But I've noticed, especially with Agfa, that clear and sharp images are typical, with excellent sharpness and clarity in the deeper parts of the image. This is often not the case with DCG. I have a feeling this is partly due to the narrowband nature of silver. Of course, DCG can be made narrowband, low-noise, and sharp in the deeper parts (although this latter quality is something I rarely see). Agfa will probably have strict protocols and quality control measures, unlike many (non or semi-professional) display DCG makers. So, all of the factors you mentioned earlier about possible sources of scatter, noise, etc., are something the professional (HOE maker) like yourself would be aware of, and be able to deal with. For DCG, the burden of producing a quality, low-noise product would rest on the shoulders of the maker. The home-DCG'er is probably not aware of possible noise sources, or even willing to go through the extra trouble to correct the problems. Most of the time, the objective seems to be in making extremely bright (i.e., broadband) holograms, without a great deal of concern about noise.
Din
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Re: Diazo (Speedball) gelatin hologram

Post by Din »

Martin wrote: Sat May 06, 2023 4:34 am
Din wrote: Thu May 04, 2023 10:40 am Joy tells me that I tried to throw it out when we left Triple Take behind, but she rescued it; she knows it's somewhere, but doesn't know where.
That sounds exciting. Would be nice to see it online some day.
Martin, Joy found the book I inherited from Don Broadbent.
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Din
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Re: Diazo (Speedball) gelatin hologram

Post by Din »

Here's the contents page for DCG and other materials. I used the method of carbon printing (chap 2.1b) to show it was possible to use DCG as a resist material. We recorded a thin hologram on DCG with depth < 1u, and then transferred the image to a piece of plastic by casting. We poured Norland onto the DCG, and laid a plastic film on the Norland. After curing the Norland, we peeled it off and showed the image on the plastic film. Processing was simply a warm water bath.
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