today's DCG

[Joe Farina]

I did another exposure this morning, and changed a few processing variables. The plate was 29 days old at room temp, which seems like a long time to me, but it still (apparently) had some softness to it, since the upper left part shows quite a bit of milkiness/frostiness, suggesting softer gelatin in this lightly-exposed area. I relied on the 29-day ageing for bias hardening, but some Kodak fixer is on order and will be tried later. After exposure, room temp water at 21C, 70% at 15C, 100% at 16C, hot 100% at 68C.

Although difficult to see by comparing the photos, today's result seems more narrowband to me. (The lower green dress looks like it's merging into blue near the top, but this is an emulsion coating irregularity.) The middle part of the dress looks bluer (which it is) while in the first exposure, it had some green tendencies at some angles. I was quite surprised that it seemed more narrowband, because it went straight from the water to the 70% and then 100%. It makes me wonder if I made a mistake in my earlier MBDCG by following some of the processing methods described by authors for narrowband MBDCG color holograms.

Thanks Din.

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[Din]

Welcome!
I can see there's a better colour differentiation.
By the way, as I recall, I don't think that I diluted the fixer. As I recall, the instructions on the packet made up 3.8 gallons of fixer, which is what I used.

The fact that, after 29 days, there were still soft parts on the plate indicates that your humidity may be high. Do you have a humidity meter in the room where the plates are drying?

[Joe Farina]

Thanks for mentioning the fixer concentration. It seems that some holographers use it straight, other prefer it diluted. I'm looking forward to trying the Kodak fixer. I tried it once with DCG, a long time ago, but I can't remember why I stopped using it. Today's result proves how important post-exposure hardening is.

I don't know why my plates are keeping so long. The humidity is in the 40-55% range. Maybe it's due to the storage temp of only ~12C. Or maybe it's because the layer is fairly thick, around 22u I guess. There's no reason why I'm using this particular thickness. My habit is to use a certain Mayer bar which results in this thickness.

This hologram turned out brighter than the last few I've done. The illumination for photo was only a 5mm white LED from my keychain. It seems a bit more broadband compared the previous one, though. I'm pretty sure this was due to the fact that I used a 25C water bath (to see what would happen) as opposed to the 21C temp of the last one. To harden the plate after exposure, I used a two-step method which worked previously with MBDCG, but not so well with this one. The first step was to use ~6% formalin in methanol bath in an attempt to harden without swelling, and a second bath to mimic the Kodak fixer with sulfite/alum/formalin/water. This came close to destroying the layer, as you can see from all of the crackles along the outer areas, and the bad processing marks. But if those are ignored, I'm quite happy with the result. I was relieved that the reconstruction angle more closely matched the recording angle (the plate was shot with a side beam at Brewster's). The others, which hadn't been hardened, reconstructed at a much more head-on angle.

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[Din]

Another technique you might try is to pre-expose the plate to UV. We had a low intensity UV source at 365 nm that Joy used to laminate the DCG plate with Norland. I used it on a DCG plate pre-exposure for about 15 minutes and got a narrow band. The idea was to harden the surface enough to delay water penetration into the emulsion.

[Joe Farina]

The idea was to harden the surface enough to delay water penetration into the emulsion.

That's a new concept to me, thanks.

[Joe Farina]

This one done yesterday is dimmer, but the color balance is better. It seems that the ratio of 1:6 in the previous photo (488nm:532nm) produces a too-green result. The ratio of 1:5 is looking better. I used 60mJ for blue and 300mJ for green. My lab was at 40% so I assume the DCG layer was quite slow. If the RH was raised to 60%, the speed should be better. For this one, I baked at 65C for 2 hours post-exposure (the fixer hasn't arrived yet), and then processed with sulfite/alum in water for 2 minutes, washed in water, 70%, 100%, and 100% at 68C. Overall, the result was less broadband than some of the earlier ones, and the colors look better. In the back of the figure is a glass block painted white, with some test colors painted on it. The whiteness came through better than before. I want to work on increasing the brightness without getting too broadband. I'm a bit surprised that in all of these recent tests, the final result had a decent amount of color maintenance, without any special effort on my part. Any emulsion expansion or contraction didn't seem to wreck the final colors. This is in contrast to my earlier experience with MBDCG (which was a more complicated emulsion and process). In those holograms, the colors had the tendency to go all over the place.

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[Din]

Any new results from using the fixer?

[Joe Farina]

The use of fixer is helping, thanks for suggesting it. In the recent tests, I probably underestimated the importance of post-exposure hardening/reducing by the use of fixer. The thiosulphate of rapid fixer is its major component. Since I believe this is superfluous (and the stuff doesn't agree with me either) I've substituted a more concentrated version of my homemade fixer with sodium metabisulfite and aluminum sulfate. I used this with the hologram below, which was done yesterday. Although this is a very rough and broadband hologram, I was testing a new film formula, and because of this was quite happy with the result. It was a dye-sensitized RGB exposure, and all the colors came though (at least initially). It only received 90mJ at 633nm, which is doable with a 35mW SP-127. It didn't have any extra side lighting, except for a small mirror which produced a narrow strip of light at the bottom right of the dress. I was very happy to see how bright this strip was. A small triangular piece of glass, painted red, was also placed in this area. The final color of this came out yellow (as shown in the photo) after the second processing, but just after the first processing, it was red.

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[Din]

I probably underestimated the importance of post-exposure hardening/reducing by the use of fixer.

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.

The thiosulphate of rapid fixer is its major component. Since I believe this is superfluous (and the stuff doesn't agree with me either) I've substituted a more concentrated version of my homemade fixer with sodium metabisulfite and aluminum sulfate.

The function of the thiosulphates in the fixer are to dissolve the unexposed silver halide by forming soluble silver thiosulphates, and you don't need them for your purpose. The hardening agents in the hardening fixer are formalin, chrome alum or potassium alum, which is basically alumuminium sulphate with a little potassium sulphate added, though I suppose sodium metabisulphite is close enough to potassium sulphate.. I tried both formalin, about 10 mls/litre of water, and chrome alum, forget the concentrations. It took a bit of playing around, but I got good results for narrow band holograms with both formalin and chrome alum. I never tried potassium alum, which is what you seem to be using.

[Joe Farina]

Thanks for the helpful comments, I'm making progress.