beam ratios: silver vs. polymer vs. DCG

[Joe Farina]

Having done my first few photopolymer holograms (Bayfol HX200), it seems that polymer is more dependent on beam ratio equality (ideally ~1:1 for a reflection hologram) compared to DCG. It reminds me of silver halide, but perhaps not as extreme. Maybe this is due to the dyes present in polymer and silver. (My batch of Bayfol is bluish/purplish before exposure, and light pinkish after laser and UV exposure.) On the other hand, DCG (without dyes) is capable of very bright (Denisyuk) results without any additional object light. The two DCG holograms I posted in the gallery were ordinary single-beam Denisyuk. I've read in the past that DCG is very forgiving in terms of object/reference intensity mismatch. I was wondering what might be unusual about DCG in this respect, or if it simply depends on whether or not dyes are present. Thanks.

[jrburns47]

Interesting. I’ve heard from others re the HX200, that very high beam ratios, compared to silver halide, can work as well as lower beam ratios. Someone got good results with the HX200 using a 150:1 beam ratio. If repeatable, that would mean lower power lasers could produce reasonable quality holograms with the HX200. Don’t know anything about DCG.

[Joe Farina]

The Bayfol material is impressive, and it works reasonably well with a very high reference to object ratio. But, compared to (non-dye) DCG, it doesn't seem capable of very high brightness, without a certain quantity of object light. In my limited experience, that is. I did a hologram yesterday with Bayfol, and included some extra object light from a side bounce-mirror. Also, I included a small mirror on the object itself to throw light directly back at the emulsion. As with all types of holograms I've done, these two types of additional lighting resulted in dramatic increases in brightness. But, compared to the best (non-dye) DCG holograms I've done (without any extra object light) the brightness of Bayfol (without any extra object light) doesn't come close, at least in my tests so far.

The really impressive thing about the Bayfol is it's RGB capabilities. On my first two holograms, I did RGB exposures, and all the colors were recognizable in the final hologram. I had mentioned to Martin that Bayer/Covestro/Bayfol must have spent a lot of money developing this material, and he told me that they hold over 100 patents.

[jrburns47]

Yes, I’ve heard that the HX200 requires a “threshold” exposure and that it likes as much total light as possible for complete exposure and brightness. I’ve done a lot of research into it as well as numerous conversations and even have some - but haven’t actually tried it in my own studio - just other people’s studios. Apparently Geola is now selling HX108, 115, and 121 now also.

[Din]

Having done my first few photopolymer holograms (Bayfol HX200), it seems that polymer is more dependent on beam ratio equality (ideally ~1:1 for a reflection hologram) compared to DCG. It reminds me of silver halide, but perhaps not as extreme. Maybe this is due to the dyes present in polymer and silver.

No, not dyes.We (Joy and I) have worked extensively with both polymer and DCG, but we've never recorded any display holograms with polymer. We first worked with polymer when a client requested it and gave us some material in 2012. However, Joy set up the shot, ie set up the ratio, and I just exposed. By 2017, we were working almost exclusively with polymer, but, again, Joy set up the ratios. Now, Joy works as a Senior Research Optical Engineer, and some of her tasks include holograms on polymer. In conclusion, I know quite a bit about DCG, and I know quite a bit about the technical details of the polymer - the physics and chemistry thereof, but Joy knows a great deal about actually working on the stuff. So, this morning, I asked her about this business of ratios.

She firstly said she doesn't work in display, only making lenses, mirrors and other diffractive optical structures for technical applications. Then she said she's more interested in maximising diffraction efficiency (DE), not brightness (it's an uphill struggle to get some people to accept there is a difference between DE and brightness, so I won't bother explaining it here). Bearing that in mind, she said that the material is quite sensitive to ratio - it needs to be 1:1 or as close as possible - for reflection holograms, but not for transmission holograms. When I told her that Jody mentioned using a ratio of 150:1 she said she could believe it for transmission. She told me that, for transmission, she'd never used 150:1, but she had recorded at 20:1 for transmission. She said that the higher ratio for transmission was important for cleanliness, higher ratios apparently make for cleaner holograms. I asked her if the polymer was narrow band, and she said, "Extremely. It has a fairly narrow dynamic range". This would explain the fact that DCG is brighter, since it's got a high dynamic range. She also mentioned that, in a display hologram the ratio depends on which part of the object you're taking the ratio from. Generally, you average the object light, but in a material that has a low dynamic range, the rms of the object light is probably a factor. By the way, silver halide also has a narrow dynamic range. One speculation I have for the difference in ratios for reflection and transmission is that the material works by photopolymerisation. This involves charge transfer - the movement of electrons to the brighter parts of the fringes/planes. In a transmission holograms, the charge transfer is along the film plane, since a transmission hologram has surface fringes. In a reflection hologram, the charge transfer is perpendicular to the film plane, since the mechanism of a reflection hologram are a series of planes perpendicular to the film plane. make of that what you will!

There is a paper by Friedrich-Karl Bruder, Thomas Faecke and Thomas Roelle, the people responsible for the technical side of the polymer, called "The Chemistry and Physics of Bayfol HX Film Holographic Photopolymer" (I used to correspond with Thomas Faecke for technical details of the polymer, and Joy is in touch with him nowadays). Anyway, one section of this paper mentions a PDD model for holographic recording on the polymer. One aspect of this model is the low frequency and high frequency cutoff. In a display hologram, the size of the object, relative to the size of the medium, determines the frequency range in the recording. This may also explain the low brightness.

"Zhao and Mouroulis [20] as well as Noiret et al. [21] for the first time described in 1994 the holographic recording mechanism in dry photopolymers by the model of photo-polymerization driven diffusion (PDD). In this PDD model the local monomer consumption rate was set proportional to the local intensity of the interference field. This means ideal polymerization kinetics is assumed in the PDD model and moreover the recording mechanism is set to be local in time and space. Despite these simplifying assumptions the PDD model is able to predict the fact that the achievable index modulation ∆n1 can drop off significantly if the spatial frequency (SF) of a recorded holographic grating in a dry photopolymer becomes small. This phenomenon is usually addressed as the low spatial frequency cutoff.
However many recording materials also experience a high spatial frequency cutoff, which limits its possible resolution. PDD is not able to predict this high spatial frequency cutoff."

[Joe Farina]

Wow, thank you Din and Joy!

Joy's comments seem to confirm my impressions about Bayfol HX200 photopolymer. When I did the first two, they were just simple Denisyuk with no extra object light. I didn't measure the reference/object ratio, but the reference must have been much, much stronger. White objects were used, not metallic. The resulting brightness was ok, but nothing close to DCG. I also wondered about the 150:1 ratio Jody mentioned earlier. To me, that seems too high to get any kind of reasonable result with Bayfol.

The little 1cm X 1cm mirror included in the third Bayfol hologram (throwing light directly back at the emulsion) produced impressive brightness (although of course there was a small amount of burn-in on the emulsion). But it wasn't as bright as when I've done the same thing with DCG, even with dye-sensitized DCG.

What Joy said about the narrowband nature of polymer also confirms my limited experience so far. (I guess the same thing can be said about silver.) As far as color maintenance capability of the Bayfol is concerned, the material appears absolutely stellar. I associate this with reasonably narrowband holograms, with good maintenance of fringe spacing and spatial integrity. On my first two holograms (RGB) all the colors were recognizable. No wet processing required.

Due to the extreme ease of use of the Bayfol, it's proving very useful for quick checks and tests. The double rubber roller (6-inch "cold laminator" available on eBay for ~$40) works well to almost entirely eliminate bubbles, though I have to be careful when doing the rolling to keep the trailing edge lifted when it goes through the rollers. (The idea being to stop the film from being "plopped down" all at once, which causes a lot of bubbles.)