question about PFG-04

[Joe Farina]

The commercial DCG holographic plates PFG-04 have been available for many years:

https://www.geola.com/product/pfg-04-plates/

So people must be actually using them. But I have no recollection of seeing results being reported, or photos of the finished holograms. The interesting thing is that they have a shelf life of 12 to 18 months. Does anyone know how they do that? My only guess is that the pH of the emulsion is high, which is known to increase the shelf life of DCG. It's also interesting that they make no recommendation to "fix" by normal methods. They only instruct a 100C bake, post-exposure. I've also found that a post-exposure bake is quite helpful for high-pH (color) DCG.

[Joe Farina]

There was also some information on the Integraf site stating that the emulsion was 23-27u. Surprisingly thick.

https://www.integraf.com/shop/pfg-04-di ... ilm-plates

[Martin]

The commercial DCG holographic plates PFG-04 have been available for many years:

https://www.geola.com/product/pfg-04-plates/

So people must be actually using them. But I have no recollection of seeing results being reported, or photos of the finished holograms. The interesting thing is that they have a shelf life of 12 to 18 months. Does anyone know how they do that? My only guess is that the pH of the emulsion is high, which is known to increase the shelf life of DCG. It's also interesting that they make no recommendation to "fix" by normal methods. They only instruct a 100C bake, post-exposure. I've also found that a post-exposure bake is quite helpful for high-pH (color) DCG.

I seem to remember having read (where???) the PFG-04 layers were stabilized thanks to the additon of some noble metal salt...

[Joe Farina]

Thank you Martin. If there's a noble metal salt, it must do an effective job.

[Joe Farina]

By the way, Martin, since you are the best-read person in holography that I know of, do you recall any studies concerning the hardening/dark reaction of gelatin/dichromate at high (~9) pH? I'm drawing a blank. Thanks.

[Din]

By the way, Martin, since you are the best-read person in holography that I know of, do you recall any studies concerning the hardening/dark reaction of gelatin/dichromate at high (~9) pH? I'm drawing a blank. Thanks.

Joe,
It's in Kosar.

dark_reaction.jpg (95.38 KiB) Viewed 5485 times

[Joe Farina]

Thank you Din!

[Martin]

By the way, Martin, since you are the best-read person in holography that I know of, do you recall any studies concerning the hardening/dark reaction of gelatin/dichromate at high (~9) pH? I'm drawing a blank. Thanks.

I'm not aware of any studies about that.

Curiously, in 2008 Jeff and I discussed the potassium chromate side of "DCG" à la Kosar. I wrote:

"...I took another look into Kosar's Light-sensitive systems, 1965. I was thinking of a possibility of improving the shelf life of coated glass plates. Kosar puts emphasis on the low reactivity of (alkaline) potassium chromate. Plates sensitized with gelatin + potassium chromate have very good keeping properties. Dark reaction is said to almost come to a halt at alkaline pH. At the same time their light sensitivity is decreasing considerably.

At lower pH however, at least part of the potassium chromate will turn into potassium dichromate.
I wonder what happens if we were to put a chromate coated plate under an acid vapor atmosphere – say, a chamber with acetic acid. Maybe something stronger like sulfuric acid is required though.
So prior to use the chromate coated plate needs to be kept in that acid chamber for a couple of hours to convert the chromate into dichromate..."

He replied:

"Your basic idea is sound and would work but not with acetic vapour (too weak an acid to acidify chromate to dichromate) and not with sulfuric acid ( plenty strong enough but virtually no vapour) it does work with concentrated hydrochloric acid. [Actually if you had concentrated sulfuric but no hydrochloric but dropped sodium chloride into it would produce HCl vapour to do the job]."


Incidentally - not related to high pH DCG - did you ever read this one?
A review of selected technological applications of DCG holograms
Christo Stojanoff
Proceedings Volume 7957, Practical Holography XXV: Materials and Applications; 79570L (2011) https://doi.org/10.1117/12.874192
Hide Abstract -
The subject matter of this presentation is to demonstrate the engineering applications of holographic optical elements (HOEs) fabricated in dichromated gelatin (DCG) films exhibiting enhanced properties. The composition, structure and the physical, chemical and optical properties of the DCG-film are briefly described in the introduction that is followed by detailed discussion of the developing processes used to achieve the necessary holographic characteristics required by the various applications. Other procedures are used to achieve explicit objectives, e.g., controlling the spectral characteristics of the HOE by inorganic and/or organic additives and using filler material to enhance the UV or IR performance. Stress induced by environmental forces, e.g., wind, or by mechanical strain also changes the performance of the HOE and may be exploited in engineering applications. The optical characteristics of the HOE are also modified by internally induced stress, such as changing the water content of the polymer or using cross-linking agents to modify and harden the gelatin matrix. The formation of thermal or density gradients in the gelatin film during the coating process or at some stage of the hologram development have also an effect on the holographic properties for they determine the conformational state and the mechanical strength of the gelatin film. The ratio between the coiled and the helical structures in the gelatin matrix determines the optical and mechanical properties of the holographic film. Multiple exposures are used to record up to four holograms in single DCG film that are used to reconstruct concurrently several monochrome or RGB beams.

[Joe Farina]

Thank you, Martin, I appreciate the information. I'm not sure if I've seen that particular paper by Stojanoff, but I've read others. Regarding the pH of DCG, thanks for the discussion.

I'm doing some guesswork about practical DCG which is RGB sensitive (488/532/633). Jeff's alkaline (pH 9-9.5) TMG system works to an extent for this purpose. I guess the original goal of the work, which succeeded, was to produce a DCG system with superb red sensitivity.

Color DCG is pretty difficult. I want to get back to basics for a while, with "regular" DCG (gelatin + dichromate). In particular, I'm thinking about the dark reaction on the coated glass plate. Once this reaches a certain age after coating (at a given temp, RH, dichromate concentration, gelatin type, etc.), the hardening is "pushed further down the line" by exposure, and an excellent DCG hologram is the result. The initial dark reaction may be the most important part of the hardening process. The problem with an alkaline DCG formula is that the preceding doesn't hold true anymore. Also, I'm thinking that the preliminary dark reaction should occur on the substrate (the glass) so that the dark-reacted (hardened) "foundation" or "framework" is already in place, in relation to the glass, for the subsequent light-induced hardening. The dark-reacted gelatin layer will then be firmly "locked" to the substrate, and the light-induced hardening will follow this existing framework.

[Din]

A couple of points:
It's true that dark reaction improves the image, up to a point. Beyond that point, performance decreases. Remember, what you're after is modulation, the difference between base hardness and the hardness of the planes - or, rather the ratio of hardness of plane to not-plane. Initially, the gel may be very soft and you need to increase the overall hardness. However, as you push the overall hardness higher, modulation decreases, because modulation is a function of the initial, base hardness. We generally coated on Friday, and left the plates to dark react at least till Monday, usually till Wednesday. In about two weeks, we got optimal performance, but after that images tended to get weaker (this is San Diego). The amount of pre-hardening, in your case with light, must be inversely proportional to the dark reaction times - the shorter the dark reaction, the longer the pre-hardening. Also, the greater the base hardness, the greener the image, and your eyes are not very sensitive in the deep green region ( < 520nm). We were asked to create a deep green skull for a friend; we did it by ageing the plates to four weeks. This is because the harder the plate, the narrower the bandwidth, and the closer is the image to the recording laser wavelength. Bandwidth is caused by differential swelling as the water seeps into the plate, first at the top and later at the bottom. However, if the gel is too hard, water cannot penetrate too much and differential swelling reduces, creating a narrower bandwidth.

If the dark reaction is truly dark - no light at all - then dark reaction is about equal throughout the plate, not specifically at the bottom first. The reason is that dark reaction is a statistical effect, the probability of cross-linking is not based on the position of the cross-linking centre. If you "use up" the cross linking centres due to dark reaction, then there are fewer cross-linking sites and modulation decreases.

Have you thought of pseudo-colour, if the aim is to get a colour dcg hologram? These skulls were both recorded at 488nm.

000-162 color skulls.jpg (13.78 KiB) Viewed 5455 times