today's DCG

[Joe Farina]

I'm making a little progress in two-color (488/532nm) DCG, and this hologram was done today. It was first made yesterday, and reprocessed twice today. I was lucky to find a 1W 532nm Genesis laser. And even luckier to have Jody Burns help me set it up, thank you Jody The blue laser was a Sapphire 488-200.

Average power at plate was 1.62mW for green and 0.25mW for blue (both lasers were on full, and the plate size was 3.5 X 5 inches). Exposure was 50mJ at blue and 300mJ for green. This resulted in an exposure time of 200 seconds for blue (done first) and 185 seconds for green. Lab was at 59F and 51% RH. After exposure, dark reaction 50 minutes. I won't describe the first two processings, only the final one. Warm water at ~26C, 50% IPA at 18C, 70% at 18C, 91% at 18C, 100% at 53C, hair dryer, bake at 65C.

Plate was coated with Mayer bar. Formula used potassium dichromate and Superclear (brand) gelatin (8.25-30-350). Plate 24 days old at room temperature when exposed.

P1010034.JPG (64.67 KiB) Viewed 1058 times

Not a very good result, but progress is being made. At least I know that two-color DCG can be done without dyes, etc.

[Din]

I think your DCG is too broadband. The variation in colour is lost due to differential swelling. Imagine you have "green" Bragg planes and "blue" Bragg planes, where the plane separation is based on laser wavelength (if the beams were on-axis the plane separation would be λ/2). If these plane separations are close (for on-axis, the plane separations are green: 266 nm and blue: 244nm - a change of less than 10%) then differential swelling would bring these plane separations even closer together. To get a good colour rendition, the 'green' planes should diffract green light from the (white) source and the blue planes should diffract blue light. However, once you put the plate into water, the water initially penetrates the upper layers of Bragg planes, swelling them, while the lower layers have not yet begun to swell. After a while, the lower layer begins to swell, but the higher layers have continued to swell. The result is a broad band hologram, where the plane separation averages out. In a mono-chromatic recording this simply results in a broadband hologram (the DCG hologram is brighter because it's a broadband material compared to silver). But, in a multi-chromatic recording, both sets of planes average out to an average broadband mono-coloured hologram, which is what you seem to have found. Imagine that the "broadband" blue and the "broadband" green kind of merge to give one broadband colour variation. You should process for narrow band, to keep the plane separation maintained as they were during exposure as much as possible.

Firstly, I notice that you do not harden pre-water wash. This keeps the material soft, and so prone to differential swelling. Most DCG holographers, me included, used fixer before the wash. In my case, the fix time depended on the age of the plate and the desired bandwidth (for technical work). As a rule of thumb, we coated on Friday, and shot on Monday and so I fixed for about 2.5 - 3 minutes - more if I wanted really narrow band. Some DCG holographers resorted to baking the plate pre-wash, I didn't like this method because you have no control over hardening. If the dark reaction is meant to pre-harden, I'm afraid 50 mins is too short, dark reaction takes a bit longer to appreciatively change hardness.

Secondly, if you warm the water for the wash, you increase the possibility of differential swelling. I washed in room temp, about 20 C/70 F.

Thirdly, washing it in the varying IPA concentrations increases the differential swelling because those concentrations have water in them. If I wanted a narrow-band hologram, I skipped the 25% (I started with 25% for display) and the 50% and went straight 75%. You might try experimenting.

Finally, you need 300 bloom for narrow band work. We used 250 bloom for display and 300 bloom for technical.

I never baked, again, because I had no control.

[Joe Farina]

Thank you, Din, much appreciated!

If I can ask, for pre-hardening, did you use the standard Kodak Rapid Fixer with Hardener, and if so, at what concentration?

That's a very interesting point about the warm water wash potentially causing more differential swelling. I will need to try room-temp water. Thanks also for the suggestion of going straight in the 75% and skipping the lower concentrations of IPA. Another thing I didn't know. Some of the earlier papers suggested using lower concentrations for narrowband, that's why I used the 50%. By the way, is there a reason why you used 75% instead of the off-the-shelf 70%? I remember another holographer saying that 75% was useful for color control:

"My method of controlling the color and clarity of master holograms was not disclosed til 1985. I dropped a developed broadband hologram into a certain bath that was about 75% alcohol and when I retrieved it and dipped it in hot dry alcohol it came back as a low scatter blue hologram. From that day in Nov 1975 I had a fast, tunable way to make bright masters that added almost no noise to the copies and so I finally had all the processes I needed to start making masters and churning out thousands of bright dichromates for sale."

The Superclear gelatin is supposed to be 300 bloom.

Thanks again for the valuable feedback

[Din]

Thank you, Din, much appreciated!

If I can ask, for pre-hardening, did you use the standard Kodak Rapid Fixer with Hardener, and if so, at what concentration?

Yes, Kodak Rapid Fixer. I'm afraid I can't remember what concentration, it was 8 years ago! I don't believe we diluted it, if memory serves

That's a very interesting point about the warm water wash potentially causing more differential swelling. I will need to try room-temp water. Thanks also for the suggestion of going straight in the 75% and skipping the lower concentrations of IPA. Another thing I didn't know. Some of the earlier papers suggested using lower concentrations for narrowband, that's why I used the 50%. By the way, is there a reason why you used 75% instead of the off-the-shelf 70%? I remember another holographer saying that 75% was useful for color control:

"My method of controlling the color and clarity of master holograms was not disclosed til 1985. I dropped a developed broadband hologram into a certain bath that was about 75% alcohol and when I retrieved it and dipped it in hot dry alcohol it came back as a low scatter blue hologram. From that day in Nov 1975 I had a fast, tunable way to make bright masters that added almost no noise to the copies and so I finally had all the processes I needed to start making masters and churning out thousands of bright dichromates for sale."

The Superclear gelatin is supposed to be 300 bloom.

Thanks again for the valuable feedback

The reason we used 75% IPA was because I started off using 75%. I first starting making DCG holograms in 1986 at a company called NTS in Culver City (in LA county). They had a research lab working on Head Up Display (HUDs), and I was the physicist in the research group, they also had a chemist and an engineer. They bought lab grade IPA, and their processing scheme was 2 min fix,5 minutes water, then 1 min at 25%, 50%, 75%, 100% - no hot alcohol. Their consultant on DCG was Don Broadbent, who'd been making DCG since the 70's. Later I moved to Physical Optics Corp (POC), and transferred my (NTS's) method over. At POC, in 1988, we had a small research group of three people: myself as the physicist, Chris Rich and a polymer chemist. So, we still bought lab grade IPA and used 75%. Chris had the idea of using hot alcohol as the final wash, which made the holograms more efficient. In 2000, Joy and I met Don Broadbent and worked with him in his lab until, finally, he left, Triple Take was formed and we continued making diffractive optics in DCG using the same methods I used at NTS some 15 years earlier with lab grade IPA.

My suggestion was to start with either 50% or 75%. The 50% increased the bandwidth, but lost a little brightness, the 75% decreased the bandwidth at the cost of efficiency. In technical work, noise is more important than high efficiency.

I used 300 bloom Knox gelatin. My method for technical was:
2 - 3 mins in fixer
2 - 3 minutes in room temp running water
1 min in 50% IPA (which step I sometimes skipped)
1 min in 75% IPA
1 min in 100% IPA
1 min in 100% IPA at 65 - 70 deg C (watch this step, the flash point of IPA is about 80!)

Don gave me the idea of emptying the cold IPA and using the hot IPA to replace it, using fresh IPA for the hot after about 5 holograms. The reason is that at each plate some of the alcohol from the previous bath transferred to the next bath, some 50% would transfer to the 75% etc. So, after about 5 holograms, the 100% was no longer 100%

[Joe Farina]

I used 300 bloom Knox gelatin. My method for technical was:
2 - 3 mins in fixer
2 - 3 minutes in room temp running water
1 min in 50% IPA (which step I sometimes skipped)
1 min in 75% IPA
1 min in 100% IPA
1 min in 100% IPA at 65 - 70 deg C (watch this step, the flash point of IPA is about 80!)

Thank you! I always wanted to know your processing method

Your post has cleared up a few things for me. Regarding narrowband processing, I was always under the impression that a gradual increase in alcohol concentration was preferable, but this seems to be wrong. Another misconception was that the hot final alcohol would have the tendency to produce a broadband result (I don't know where this came from). Maybe if there was some residual water in the layer when going into the hot alcohol, it could result in chirped fringes (?) but since you have a preliminary room-temp 100% buffer bath right before the hot bath, maybe this would take care of the problem. Interesting, thanks.

By the way, as long as you're fielding questions what was your preferred coating method? (another thing I always wanted to know)

[Din]

Well, since I'm now retired and Joy is busy making diffractive optics holograms on polymer, I can come clean

I started with:
45 gms gelatin
300 mls water (DI water).
I put this into a beaker over a heater with a magnetic stirrer and set the temp to 40 - 45 deg C. At NTS and at POC we heated to 60 C, which I started off with, and which didn't affect technical holograms, but created "greenies" (blobs of green in the image) in a display piece. I racked my brain for several months trying to understand these 'greenies', until I realised that heating gel was putting energy into the gel, thus allowing for slight cross linking in the gel itself before the addition of dichromate. The gel will polymerise on its own, albeit weakly. This polymerised, cross-linked gelatin would create pockets of hardness before coating, and will transfer into the coating. This has the effect of localised pockets of hardness within the emulsion which will swell less than surrounding areas, causing a green shift. So, I reduced the gel temperature and got rid of the greenies. Joy made a cover for the beaker with a hole just large enough to insert a thermometer, and I checked temperature every hour or so.

After the gel came to temperature and dissolved (making sure that the magnetic stirrer was not fast enough so as to create bubbles), I separately dissolved 8gms of ammonium dichromate in 38 mls of DI water, and stirred to make sure the dichromate was fully dissolved. Then I added the dichromate solution to the gel, and stirred magnetically a little longer. Finally, I filtered the 'goop' ( as we now called it) using 5 nm filter paper (could have been 1 nm, memory is hazy) in a standard coffee filter. Joy adapted the filter so it could sit on a beaker. While the goop was in the filter, I dropped about 2 drops of Photo-flo onto the goop.

Finally, about 15 minutes before coating, Joy heated some threaded rods at low temperature (about 50 C) in an oven. She then set up a coating station so that there was a sheet of glass on which the (uncoated) plate was placed. She placed the plate, I used a spoon to draw out some goop and poured it at the top of the plate, slowly so as to avoid 'boobles' (Don's words!). She took one threaded rod from the oven, and drew it across the plate in one swoop. The excess gloop spilled onto the sheet of glass below the coated plate. She then placed the coated plate into the drying station I mentioned earlier, dropped the rod into a bucket of warm water, discarded the 'overflow' glass, placed a new "overflow" glass on the coating system,took out a new threaded rod and started again. After however many plates we were coating, she covered the drying station with a plastic sheet, and turned on the overhead air flow (from the cooker hood).

Then we repaired to the "Fish House Restaurant and bar", where I partook of several (well-deserved !) glasses of Merlot, and she had her tea.

[Joe Farina]

Well, since I'm now retired and Joy is busy making diffractive optics holograms on polymer, I can come clean

Thanks for the very valuable info.

My gelatin was melted at 50C and I may have seen the same blobs after the plates gelled. They were perfectly circular and about 5mm in diameter. They seemed to have a different density compared to the surrounding gelatin. I don't remember the exact appearance in the finished hologram, but they were of course very obvious. Thanks for describing your solution to the problem. I will want to try lowering my temp to 40 to 45C.

I always thought that spinning wasn't the ideal way to coat DCG. It seemed too complicated to me, too difficult to manage film viscosity, RPM's of the spinner, and film uniformity issues (some holographers use an air blower directed to a certain location on the plate while spinning to produce uniform results). Also, it would take a lot of practice to know how to change film thickness, seems messy, and introduces additional equipment into the coating area which could collect (or cause) dust over time. It's gratifying to know that I'm not alone in my preference for rods/bars.

[Din]

I mentioned that I didn't have my notes, so a lot of what I said relied on memory. However, Joy had them stored upstairs, so I recovered them. So, I thought I'd post exposure log and coating log.
Here's the exposure log

exposure.jpg (43.76 KiB) Viewed 937 times

[Din]

...and here's the coating log. I must not have mentioned adding Photo-flo, since it was routine. But, it would have been at the filter stage.

coating.jpg (47.5 KiB) Viewed 937 times

[Joe Farina]

Thanks for the log entries. Having a form to fill in is better than my method of taking notes, because without a form, if you forget something, it may not be written down. With a form, the "vacant space" is obvious, and reminds you to fill it in. I like this idea, thanks.