I'm having problems getting my DCG emulsions to work...

Dichromated Gelatin.
dmarks
Posts: 2
Joined: Fri Jan 24, 2020 8:44 pm

I'm having problems getting my DCG emulsions to work...

Post by dmarks » Fri Jan 24, 2020 9:24 pm

Hello Holographers,

Any help you could provide would be appreciated.

I bought a Lasos 50 mW 532 nm single longitudnal mode 1 MHz linewidth laser to do DCG holograms because they look awesome and I finally found a great deal on e-bay to get one. I have been excited to make DCG holograms since I learned about them. I am an experienced optical engineer and so I am frustrated I have problems getting this to work. Any information you could provide would be most helpful.

Anyways, I am completely failing to make DCG holograms. When I do the final 100% alcohol bath, basically I do not get hardly any rainbow fringes at all to speak of. This is even after directly taping coins (as a reflection object) to the gelatin emulsion during exposure to try to minimize the distance between the emulsion and the object.

So let me explain the process I am doing. I have read this forum many times and other people's experiences trying to glean what I could be doing wrong. However, after many attempts, I have no clue.

First, I am coating the plates. To clean the plates, I soak for several says in deionized water with 10% sodium hypochlorite bleach added to it. It seems to do a good job of cleaning and etching the plates. I rinse the plates with deionized water and let them dry by placing them into a box where they are placed at about a 30 degree angle to vertical to drip off and dry.

The gelatin emulsion I prepared is 250 grams deionized water, 30 g gelatin, and 5 g potassium dichromate. I have tried two types of gelatin: 250 bloom photographic gelatin and Knox gelatin that is bought for cooking. They both seem to provide the same results. To prepare the gelatin, I have a pot on a hotplate to which I add the water and the gelatin. I heat and keep between about 40 C and 50 C (usually around 45 C to 47 C) and stir using a thermometer to dissolve the gelatin. This takes about a half an hour or so. Once the gelatin is dissolved, I turn the lights off and use a light source I built from red LEDs. I add the potassium dichromate and stir than in until I see the liquid is clear, though with the red light it can be hard to tell if it's completely dissolved, but it usually takes five minutes of stirring or so.

I then pour this into a squeeze bottle using a funnel. I then coat the plates, which are about 10 by 10 cm. I have tried two methods. The first is the veil coating method, where I hold the plate at an angle and use the squeeze bottle and allow the emulsion to run down the plate. The second method is a Meyer bar I made out of a 10 mm diamater stainless steel rod and stainless steel spring of 0.5 mm pitch. I use the squeeze bottle to spread some on the edge of the plate and then use the Meyer bar to draw it over the plate. It is not perfectly even but I think it would work ok.

After coating, I place each of the plates in a plastic light tight box I prepared which has a rail affixed along the bottom so the plate can be tilted to a 30 degree angle to vertical. I let the plate sit for a day or two at room temperature and allow the emulsion to become firm.

To expose the emulsion, I place a 20X microscope objective in front of the laser to diverge the beam. An object is placed on a honeycomb stainless steel optical table with the glass plate leaned up against it. I remove the beamstop in front of the laser. I have read that the exposure level at 532 nm needed is about 200 mJ/cm^2. This corresponds to about a 5 minute exposure time for the plate size I have. I have tried 1, 5, and 20 minutes exposure time. After the exposure is finished, I cover the beam with the beam stop and allow 5 minutes for the dark reaction to occur.

The development I perform immediately. The first solution is 500 mL of deionized water with 5 g of sodium metabisulfite and about 2 g of potassium aluminum sulfate (potassium alum). I keep the plate in this solution for about 2 minutes to be sure all of the remaining dichromate is reduced to Cr(III). I have also tried sodium thiosulfate as a reducing agent because it is used in Kodak Rapid Fixer, but it did not seem to be effective at reducing the Cr(VI). Then I place the plate into a 70% isopropyl/30% deionized water solution for about a minute or so. Finally, I finish with a 100% isopropyl solution for about a minute. After this, I have tried both allowing the isopropyl alcohol to evaporate slowly and naturally, and using a heat gun about about 100 - 150 C to evaporate the isopropyl alcohol. The result rarely if ever shows any fringes at all. I have only one plate that actually worked, and I'm not sure why.

The emulsion on the plates usually looks a little cloudy after drying, and sometimes there is a "dendrite" crackle like pattern, especially if the plate is dried quickly using the heat gun. But any kind of rainbow fringes are absent.

I taped coins to the emulsion so that the distance between the object and the emulsion is minimized in case the coherence properties of the laser are affecting the hologram. The coins themselves produce no hologram, but where the tape was on the gelatin sometimes has a little rainbow interfence to it.

I used a Michelson interferometer to test the laser coherence length, since I bought it off of e-bay and so I could not be sure it was working properly. I had a 25 mm delay in one arm of the interferometer and was able to observe stable interference without any problems. So I would think the coherence length would be well within the distance needed if I was taping coins directly to the emulsion. I don't really have a way to test the full 100 meters or so of coherence length but my guess is that would be overkill for this application. I leave the laser on for about 15 minutes before I take my first hologram to ensure the wavelength has stabilized as well.

If any of the DCG gurus here could help me figure out what mistake I have been making, it would be most appreciated. I have tried most everyhing I can think of and have not been successful yet. Even if my plates were poorly made I think I would observe some kid of interference fringes.

Thank you,
Dan
profdc9@gmail.com

lobaz
Posts: 280
Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: I'm having problems getting my DCG emulsions to work...

Post by lobaz » Sat Jan 25, 2020 6:06 am

Hi, Dan,
it seems you are not completely sure if the problem is in DCG or somewhere else. I think it would be worth buying a pack of photopolymer (e.g. from Geola or Litiholo) and test the setup with it.

Petr

Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: I'm having problems getting my DCG emulsions to work...

Post by Joe Farina » Sat Jan 25, 2020 7:11 am

A cloudy layer of DCG or one with dendrites suggests too-soft gelatin. For good-quality DCG holograms, cloudiness, white opacity, and dendrites should be absent, for the most part. It's not always easy to get a handle on gelatin hardness (prior to exposure and during processing) and it's really the main issue with DCG in my opinion. It might be worthwhile to age the plates somewhat (slowly in a refrigerator, or more quickly at room temperature) to provide some bias hardening before exposure.

In my experience, drying the plates (after coating) in a sealed container can cause problems. I prefer to dry my coated plates under a gentle air flow (a fan) overnight.

The main source of the problem may be the taping of the objects to the plate. I would strongly suggest not doing this, as the tape may be moving around during exposure. The relationship between the objects and plate must be rock-solid. Resting the plate directly on the coins might work.
Last edited by Joe Farina on Sat Jan 25, 2020 7:15 am, edited 1 time in total.

soda
Posts: 38
Joined: Mon Oct 08, 2018 4:05 am

Re: I'm having problems getting my DCG emulsions to work...

Post by soda » Sat Jan 25, 2020 7:14 am

I had making DCG a long time ago, and my opinion that your problem shoud be in processing. After fix bath to reduce/wash out the rest of dichromate you !must use warm water bath! Without it, DE is low, the holo is barely visible. The water bath should be 35-42C (perhaps much higer) for rougly 2 min. You need to determine the temperature/time experimentaly - warmer and longer time increase DE dramatically, but also noise, optimum should be just before emerges milkiness of the exposed part of the plate. Unexposed plate should be totaly white. Do not hurry with dark reaction and with last IPA bath, which needs to be really 100% free of water. Agitate all bath vigorously, mainly fitst IPA. Final drying should be fast as possible with hot gun or in owen, naturally drying usually produce nothing visible, due to water condensation. At first you should try thin gelatine layer, also I had best results wit thin glass. Have stable fringes!

dmarks
Posts: 2
Joined: Fri Jan 24, 2020 8:44 pm

Re: I'm having problems getting my DCG emulsions to work...

Post by dmarks » Sat Jan 25, 2020 12:22 pm

Thanks all of you for your responses. I have some questions about gelatin hardness.

Is there a good way to test gelatin hardness? One thing I have noticed is that after the fixer (metabisulfite) bath for two minutes or so is that the gelatin is extremely soft. If I touch it with the finger of my glove, it will curl and scrape off of the plate with almost no force whatsoever. I wonder if the gelatin is swelling too much and perhaps is reconstructing at an infrared wavelength? Most of the yellow seems to disappear into blue in about 15 seconds in the metabisulfite solution, so perhaps I am leaving it in the fixer too long. How much does the emulsion shrink in the alcohol baths to compensate for any swelling that might occur in the fixer bath? Also, does all of the metabisulfite need to be rinsed out of the emulsion before the alcohol drying stages? Because the cloudiness/crackling pattern seems to occur if there is K2Cr2O7 or metabisulfite salts left in the gelatin, it does not seem to be strongly dependent (to me) on what salts remain in the gelatin. Also, would it be worth investigating fixing the plate using bright white light or ultraviolet light (365 to 400 nm) exposure? Perhaps baking the plates on the hotplate to harden the emulsion after exposure would help?

Would it make any sense to dip the plates in a solution of potassium alum before exposing to harden them? Perhaps the acidity would also increase the sensitivity as well? One thing I tried was to dip the plates into a deionized water bath before exposing because I thought perhaps the plates were too dry and because the sensitivity of DCG emulsions depends on humidity, the dip in water would increase the humidity and therefore the sensitivity.

Should a desiccator be placed near the placed when the gelatin is hardening on the plates? It is hard to find a place that is both dark and well ventilated. I suppose a dedicated darkroom is needed then for the plates to have a space with enough ventilation and darkness for the gelatin to harden? Perhaps I can use several layers of dark cloth over holes in the box to block the light sufficiently but allow airfow using a fan. What setups do others uses for the hardening step?

I have tried leaning the plate up against rigid objects (bright metallic or white ceramic) that are placed on the optical table, and also taping the coins. The reason I taped the coins was to minimize the coherence length needed to form the hologram, but I will avoid this in the future if this is a problem.

Anyways thanks again for your help. I e-mailed a DCG holography expert at a well-known DCG grating manufacturer and he said that it took him 10 years to perfect his process. I believe it.

Dan
Joe Farina wrote:
Sat Jan 25, 2020 7:11 am
A cloudy layer of DCG or one with dendrites suggests too-soft gelatin. For good-quality DCG holograms, cloudiness, white opacity, and dendrites should be absent, for the most part. It's not always easy to get a handle on gelatin hardness (prior to exposure and during processing) and it's really the main issue with DCG in my opinion. It might be worthwhile to age the plates somewhat (slowly in a refrigerator, or more quickly at room temperature) to provide some bias hardening before exposure.

In my experience, drying the plates (after coating) in a sealed container can cause problems. I prefer to dry my coated plates under a gentle air flow (a fan) overnight.

The main source of the problem may be the taping of the objects to the plate. I would strongly suggest not doing this, as the tape may be moving around during exposure. The relationship between the objects and plate must be rock-solid. Resting the plate directly on the coins might work.

Joe Farina
Posts: 724
Joined: Wed Jan 07, 2015 2:10 pm

Re: I'm having problems getting my DCG emulsions to work...

Post by Joe Farina » Sat Jan 25, 2020 1:06 pm

With regards to leaning a plate against (or near) objects, I think that would only be good if the plate were very carefully supported, on more than one edge. Since this is DCG with long exposures, no movement at all can occur. Supporting the plate in relation to the object is very important. The often-quoted figure for movement is 1/2 of the wavelength of light: if more movement than that occurs (between object and emulsion), no hologram. Thermal variances in the room (small temperature changes) and the fluctuation of humidity can change the size of the objects, components, and table. Thermal expansion in other words. Humidity fluctuations can cause the same issue. If you are using plastic tape to secure the objects, not only is it flimsy, but plastic has a relatively large amount of thermal expansion and contraction.

Regarding gelatin hardness, there is no easy and direct way of determining this, as far as I know. If you are experiencing milkyness, then my first thought would be that, at some point in the process, the gelatin is too soft for whatever is being done to it, at that point. Soft gelatin is OK unless it's being subjected to something which it can't handle, such as higher temperature in one (or more) of the baths.

I think everyone who has done DCG has found a way to get the hardness of the gelatin "in the ballpark" for whatever processing method is being used. Things can be done before and after exposure, it's up to the individual to find the appropriate method. I've tried many, and settled on a few methods which work for me. It wasn't an easy road. By the way, I definitely don't recommend heating a gelatin layer with dichromate/chromate in it. The fumes are horrendous. I would either find another hardening method, or build a special enclosure or shed in your yard outdoors. And hold your breath when approaching the shed, removing plates, and leaving the shed. Then you can take a breath!

DCG is much less sensitive to light compared to silver. When I watched Richard Rallison making DCG holograms at Lake Forest, the room was pretty much flooded with daylight while he was shooting with the helium-cadmium laser.

BobH
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Location: Mesa, AZ

Re: I'm having problems getting my DCG emulsions to work...

Post by BobH » Sun Jan 26, 2020 11:28 am

I'd suggest looking at your dehydration technique. Most use three baths: 50%, 100% and a final 100% IPA, all hot, and all for much longer time than you mentioned. The time in the baths is important, as it takes time for those molecules to do their thing. It takes a few minutes for the water molecules to get out of the emulsion.

The laser is probably OK. For a test object, I'd secure a mirror to the table, glue three washers to it to support the plate kinematically with minimal separation, and make a hologram of that. You'll remove all coherence and stability issues that way, and have a hologram that can eventually be measured for diffraction efficiency.

Finally, I think you need to add a dye to make DCG have enough sensitivity for 532nm, but I may be mistaken. Others here will correct me if that's the case!

Where are you located?

holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Re: I'm having problems getting my DCG emulsions to work...

Post by holomaker » Mon Jan 27, 2020 7:53 am

532nm should work fine for dcg without any additional dyes, you need to put enough amdi to make your plates look like urine in a jar, I think the formula you are using is really for blue light DCG. For 530nm(green) you should be at 4.5-5gams amdi/110 ml h2o.
I found an older picture Of the proper “yellow”your plates should be for 530 nm
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Last edited by holomaker on Tue Jan 28, 2020 2:08 pm, edited 1 time in total.

Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: I'm having problems getting my DCG emulsions to work...

Post by Joe Farina » Mon Jan 27, 2020 10:36 am

That looks more like Daisy Yellow rather than Urine Yellow ;)

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