Holographyforum.org / holowiki.org

In case anyone is interested, the manuscript version of that paper on scattering is available at:

https://opg.optica.org/directpdfaccess/206aa99e-b995-4b8b-b91f12a0e3c0fdfb_380867/ao-57-3-527.pdf?da=1&id=380867&seq=0&mobile=no

I'm surprised that they're asking $35 for that paper, when the manuscript version is available for free.

Din,

Thanks very much for your help. I will try to get that paper soon. Also, I appreciate your earlier comments on how to reduce scatter. In my recent holograms, I was making a big error with regards to baffling. Not with stray light from my optics (at least I don't think so), but rather with my test-objects, which have been ceramic plate-holder/object support assemblies. The stray light from the ceramic support was going everywhere. I'm in the process of painting every possible surface (except for the object) matt black. By the way, a good velvet-black water based (casein) paint is available at:

https://www.dickblick.com/products/pelikan-plaka-paint/?fromSearch=%2Fsearch%2F%3Fsearchword%3Dplaka%20black

Also, if anyone else in the forum is interested in vacuum de-airing of either epoxy (recommended for Hxtal) or gelatin, I just found and ordered the following:

https://www.sterlitech.com/rocker-410-oil-free-vacuum-pump.html

It's oil-free, relatively inexpensive, and, according to the specs, can pull a vacuum of around 29 inches of mercury. I was impressed with the price/specs, plus the oil-free aspect is very attractive. No oily fumes or backflow to worry about. I'll see how it goes. (The shipping time is incorrect in the above link, I checked with more than one supplier, and the earliest delivery time is in mid-July. Looks like these may be in demand.)

Joe,
Just listened to a webinar on "Materials and Methods for Creating 3D GRIN Diffractive Optics in Photopolymers." In the course of the webinar, the speaker brought up scattering and noise in a holographic volume recording and recommended this paper:
https://opg.optica.org/ao/abstract.cfm?uri=ao-57-3-527

I'm no longer interested, since I'm retired, but, had I been working, this paper would have been quite interesting to me. One of the things I was always fighting was noise and scatter, and we had to fine tune our plate creation and recording methods to minimise these. However, since you're studying scattering and noise, you may be interested in the paper. While this paper talks of scattering in a ploymer, the results can be transferred to recording on DCG.

test

After some more tests this morning, I think the reason for the slightly yellower appearance of the one sample (non-de-aired) is simply because it's volume looks darker, due to the presence of more air/bubbles scattering more light. This would seem to be confirmed by the apparent increase in total internal reflection (the edges of the sample look more "lit up.") The difference is small, but seems noticeable. For samples that are 30mm in depth, a 405nm laser shows 21mW on the power meter transmitted (non-de-aired), and 23mW transmitted (de-aired). Not much difference from a laser power point of view (for a 30mm sample) but it might be more significant from a scattering point of view (noise in a DCG hologram).

Not clearly visible in the photo, but the beam going through the lower (non-de-aired) sample had a blue-green appearance, unlike the upper de-aired sample, where the beam appeared more colorless. [attachment=0]P1010071.JPG[/attachment]

Edit: After more tests later this day, this effect should be probably ignored, because I found out that one sample (the de-aired one) had a flatter "back" (in better contact with the underlying silicone sheet). But the better clarity of the de-aired one in white light (less yellow) was confirmed. Somehow, I get the impression that less total internal reflection is taking place in the de-aired sample.

Thanks Din for your helpful comments.

Today I did another test of de-aired vs. non-de-aired gelatin. This is 280 bloom Gelita photographic gelatin (6 grams gelatin + 47ml water). One test was de-aired at 26 inches mercury, the other one was agitated w/magnetic stirring to introduce bubbles (although most of the bubbles were allowed to rise and pop, so that the test wouldn't be badly skewed in favor of the de-aired sample. I made the mistake of de-airing first, before pouring the non-de-aired sample, so I had to introduce air/bubbles in one of the samples on purpose.).

The results were interesting but not easy to interpret. I shone a spread 405nm laser beam through both of them (side by side). There was no obvious difference in scatter. By that, I mean that there was no sense of cloudiness or "haze" in either one. But, after looking further, I noticed that the laser beam going through the de-aired sample seemed "less visible" from the top. It seemed to show the color of the underlying silicone sheet more clearly (which was pinkish under 405nm). On the non-de-aired sample, the beam going through the cube produced an odd, blue-green color. Perhaps this was fluorescence?

Rather strangely, the de-aired sample (on the right) appeared clearer, and not as yellowish under white light.[attachment=1]P1010072.JPG[/attachment]

Joe,
Just saw your comment about turbitity in the gel while stirring. This may be due to the stirrer whirling at too high a speed. The stirrer should create a circular laminar flow. however, if the stirrer is too rapid, you may start to get turbulence. This could be the cause of the bubbles. Also, the hardening and distorting of your gel cubes could be due to the shrinkage of water at low temperatures. It's not normally noticed because the percentage shrinkage is small and the absolute shrinkage for a large volume is too small to be noticed. But, for the small quantities of water in the gel cube the absolute shrinkage could be high. One reason why you don't put DCG plates in a fridge!

In our case, Joy rigged up a "drying chamber" with a hood from a kitchen cooker above an enclosure. On the face of the enclosure was a plastic sheet. On the base, about 2 feet below the hood, she placed a large sheet of paper on which were placed pennies (NIST approved spacers!) and then placed the plates above the pennies. This ensured that the plates were kept flat while drying - to avoid gel creep across the plates - and were subject to a uniform flow of air. The plastic sheet ensured that no dust settled on the plates while drying.

Just a note about gelatin gelling and drying. After a few days in the fridge, the gelatin (no dichromate) dried, and became like a hard plastic, almost like acrylic. Didn't realize how hard it gets. Also, it shrinks enormously, compared to the gel state. The photo below shows the same gelled cubes I posted earlier.[attachment=0]P1010053.JPG[/attachment]

One property not mentioned is that the helical structure of gelatin makes it slightly birefringent. This sometimes manifests itself in what appear to be odd holographic effects.