SILVER HALIDE HOLOGRAPHIC RECORDING MATERIALS 2000
M3 Visual Research Laboratory*, as a service to the holographic community, is proud to present a Consumer's Report on the state of the art of silver halide holographic recording materials. In the wake of the demise of Agfa's Holotest line up there is a real need for a suitable substitute, hopefully one that is brighter, less noisy, and less expensive.
My Artist-Photographer Zone System training compels me to carry out these tests, as understanding the materials results in more control in realizing the final result. As Ruben Nunez has said, we need a Minister of Brightness.
I myself am curious as to what is up with these new products. However, I do not have much faith in literature published by manufacturers, or reports of other holographers who do not test other products side by side, or their technique which may be sloppier than mine. To be really fair, rigorous testing needs to be done, which is usually too boring to be undertaken by the typical holographer.
We purchased samples of five different red-sensitive holographic recording materials: HRT BB-640, two flavors of Slavich plates, PFG-01 and PFG-O3M, plus some Chinese plates from Control Optics. All were of the same 63 by 63 mm or 2 1/2" squares coated on glass, except for the Chinese plates, which were delivered as a sample package of four 4" by 5" glass plates.
Two different sources were used to procure the Russian plates, yet they both bungled the job**. I wanted some PFG-03M plates, the ones that are legendary for ultra-fine grain, in the vicinity of 10 nanometers. From the one I received PFG-03Cplates, which is the true color or panchromatic emulsion. But it nevertheless has the teeny grains, so I thought I was happy.
Until I opened the box! There were only 20, not 30 in it!
From the other source I got another box of PFG-01 instead of PFG-03M. Although I had asked for them over the phone with a verbal description, I guess I had to be more precise with the exact alphanumeric code to avoid confusion.
Russian Plate Scorecard: So I ended up with twice as many PFG-01 plates that I had wanted, no PFG-03M which I wanted most desperately, and something I didn't necessarily want, PFG-03C.
I was introduced to the field of holography over twenty years ago at a photography teachers convention, wherein TJ*** not only showed some Russian holograms but also made a couple of holograms in front of a group of people. A day that changed my life! And he showed off some Russian holograms. It is hard to believe that now I can have my own Russian holo plates.
PACKAGING: The Slavich plates are packed in cute little plastic boxes, not unlike the familiar Empak ones that the Agfa products came in. But they were not all that well-molded, and were difficult to open, which is what it should be, to avoid accidents, but were even harder to close properly, and sometimes gave the feeling that the job was not done well enough to be light-tight.
Cardboard is the word for BB-640 plates' packaging and shame on you. There is a triple box made out of heavy card stock, with a black plastic bag sealing the plates inside from humidity. The plates are stacked inside the light tight baggy, with cardboard interleavings. Tacky, tacky, tacky! I haven't seen any scratches yet, but then I've only shot only a few. I transferred them to a traditional Empak box emptied of its Agfa plates.*
The four Chinese 4" by 5" plates were simply wrapped in black paper and thrown in a box with styrofoam peanuts. To separate them small dots of paper were folded over some of the edges. Admittedly these were a sample package, but it doesn't inspire confidence in the product by the cavalier manner that they were shipped. How could you use a scratched plate in an art piece?
As a standard of comparison, some plates from my secret stash, an unopened box of Agfa Holotest 8E75HD, batch # 591304, expiration date of 02/00, were exposed and processed alongside the rest.
PHASE ONE: ON AN EQUAL FOOTING
To start off with all things being equal, plates from all boxes were exposed with a series of exposures and developed in the developer that has been the household standard for so long, CWC2, with an immersion time of two minutes. (For more details about the process, see the Handout, CWC2.)
This developer bleach combination gets a high brightness out of the Holotest material, signal to noise is directly related developed density, and there is no distortion of the fringes, as the plate can be replaced onto the object and real-time fringes can be observed! The hologram casts a very dark shadow on the object as it steals reference beam light. The CWC2 process and Holotest materials are as peaked as perfectly as possible for this application. It used to be the standard of the industry. Let us see if the new materials can beat it.
For this phase of testing the qualities that the holographic samples are judged on are brightness, signal to noise ratio, and replay wavelength fidelity. The last aspect is important for making copies of reflection holograms, with either contact or separated geometries. Sometimes a different replay color is desired for display purposes, but shrinkage to shorter wavelength replay shows that the material is not replaying the recorded fringe pattern as faithfully as it should.
BLEACHING: I know that this will upset many of you right off, but the bleach that I used for these tests was the dreaded CWPBQ2, for a variety of reasons. First off, I paid $26.10 for a half pound of it 15 years ago, and being a holographic cheapskate I want to use it up. But most importantly, when it is used with the CWC2 developer, it yields a hologram whose fringes are distortion free, as evidenced by brightest replay at the recording angle and the possibility of producing real-time interferometric fringes when the plate is replaced exactly on the object holder. Using the two nested trays method contains the obnoxious odor.
Most of the other rehalogenating bleaches that I have tested (see the Handout, ROUNDUP '90) work pretty much the same, except the Ferric EDTA ones usually exhibit noticeable shrinkage, and the Copper Sulfate prints out like crazy. Just pick your poison. I didnÕt have time to investigate solvent bleaches, to induce shrinkage to shorter replay wavelengths.
EXPOSURE METHODLOGY: The tried and true Standard Object was dusted off for the exposure testing series, which is an antique waffle iron mold sprayed with Krylon #1401 Bright Silver. This paint uses aluminum flakes for its pigment, which in addition to being highly reflective, preserves polarization vectors.
The waffle iron is a not too deep object, with a homogenous background. A series of exposures can be made on one plate with each quadrant having an identical scene. Plus when viewed pseudoscopically it looks like the waffle itself!
Three ball bearings support the plate so it doesnÕt rock. A bar prevents it from sliding down and blocks light from entering the edge of the plate. The iron mold is supported in a kind of goal post arrangement so that it can be tilted to give a decent reference angle, which was about 30 degrees from the normal in this case to prevent the shadows of the waffle texture from getting too long. Figure One shows the rig with a processed plate replaced back into its exposed position generating real-time interferometric fringes, demonstrating the stability of the kinematic device.
The isolation system was my unpatented BIG BEAM as featured in a variety of articles. Photons supplied by a Melles Griot LHP 171 7 mW He-Ne, spatially filtered by Jodon. Incident flux was measured by a borrowed Newport 820 at 40 microWatts per square centimeter.
Figure One: Kinematic Plateholding Object, demonstrating real-time holographic fringes on a well-processed plate.
One quarter of a cardboard square the same size as the plates was cut out, so that ¾ of the plate was blocked while one quadrant was exposed. Exposure doses varied with the material, with the Agfa and PFG-01 receiving 50 microJoules per cm2 minimally and going on up to 3200 microJoules per cm2 for the tiny grainers. The exposure doses are given on a label attached to the center of the plate in most of the following photos. Exposure times varied from 1.25Ó for 50 mJ/cm2 on up to 80Ó for the longest 3.2 millJoules per cm2.
Figure Two shows the preliminary holograms. The standard of comparison was the Holotest, and this batch exhibited behavior like all the prior ones: a threshold of image visibility at about 50 uJ/cm2, increasing brightness until that peaks at around 200 uJ/cm2, and at exposure doses beyond that the plate gets very noisy.
Figure Two: All samples developed in CWC2 and bleached in CWPBQ2. Unpainted backs, holograms laid on a piece of black velvet.
The PFG-01 is claimed to be a drop-in replacement for the above. When exposed and processed alongside the Agfa, it is a close match in brightness but the obvious winner in signal to noise. It also could produce real time fringes when positioned on the object.
I had heard that the HRT BB-640 would need more exposure than the Agfa, so I started with 200 m/cm2 and worked up to 1600 m/cm2 or 1.6 milliJoules/ cm2. The color was orangey, brightness not as good as the others, but this was the lowest noise so far. Hopefully using its own developer will improve the situation, as it is so nice to see almost transparent glass in the darkest areas, as opposed to the veiling haze of scatter with the two above.
I gave the same exposure series to the Control Optics plates, but they shrank into the green! Their instructions say to use a modified CWC2 developer, which is just the usual formulation sans urea.
This makes sense since the urea is a penetrant, a gelatin softener, and it let the layer shrink to the green. We never got around to seeing what its deletion does because the plates were destroyed while cutting the 4Ó by 5Ós down to 2 ½Ó squares. The glass was thinner than the usual 1.5 mm of Agfa. Otherwise the brightness and noise were not unlike the BB-640.
The Slavich PFG-03C also suffered from serious shrinkage. (Not shown.) Its instruction sheet mentions a Hardener step, which would have been implemented in the next round of testing, if it were not noticed that the material had arrived fogged.
There was a gross white haze around the edges of the hologram. When an unexposed plate was plopped into the developer, it almost uniformly darkened. After a few phone calls the situation was straightened out and I received a nice fresh box of Slavich PFG-03M, which was what I had desired all along. The results were not all that bright when done in this manner.
At the end of Round One of Phase One it seems that the PFG-01 is truly a drop-in replacement for Agfa Holotest 8E75HD. Its peak brightness comes at a lower density, with slightly less noise, maybe because as Ilford had always claimed Agfa had quite a few huge (relatively) "rogue grains" floating around which may have been the major contributing factor to noise as well as density. It is surprising that its sensitivity is pretty much exactly the same as Agfa's! The Slavich chemists had certainly done a good job of reverse engineering the Agfa plates.
The other materials do not seem to be peaked with this processing scheme. The shifted replay color and not all that dazzling efficiency (albeit low noise!) points out that we should check out the instruction sheets for these materials to see what the manufacturers' suggest.
PHASE TWO: A PROCESSING SCHEME OF THEIR OWN
What a surprise! Following the manufacturersÕ recommendations improved the quality of the holograms!
I opted to optimize the German material, HRT BB-640 next. Of all the materials it looked the cleanest, and with CWC2 had better sensitivity than either the Chinese plates or Slavich PFG-03C or M.
So I mixed up a batch of
BB AA DEVELOPER
70 g Sodium Carbonate
15 g Sodium Hydroxide
4 g Metol
26 g Ascorbic Acid
One liter of water
The Sodium Carbonate and Hydroxide are there to provide the necessary alkalinity. This bath measures 11.5pH with paper pH strips, which is quite high, and is due mainly to the NaOH. There's a lot more Metol here than in Kodak D-19, and it may keep grain growth down while initiating developer sites.
This formula sets a record for Ascorbic Acid content in a developer. The Vitamin C produces a satisfying fizzing sound when dumped into the Drano/Washing Soda solution. No developing times or temperatures were given, only the guideline of a density of 2.5. You're on your own on how to get there. With all the developing agents and the high pH, you would expect that it be a highly energetic developer. And that may be just what it takes to get this emulsion to perform to its utmost.
For the resulting holograms were extremely bright and low noise! The best tested so far. But the replay color is shifted from the He-Ne red, although it does replay at that wavelength, albeit with a bit of tilt.
Developed in the above stew and bleached in PBQ, I got extremely good efficiency and signal to noise ratio! But since the replay color was not quite the same as the laserÕs, something must be screwy with the gelatin of this emulsion. It is shrinking after processing, both with this developer and CWC2.
A trick that I would use to retain original Bragg plane spacing with the Ilford emulsions having the BIPS* ingredient would be to develop in a pyro-based developer and bleach with PBQ. When pyrogallol develops, it tans the gelatin surrounding the developing grain with its by-products. The gelatin is hardened while it is swollen with water, and it does not collapse as much as it would in a non-tanning developer. Comparing two holographic exposure series plates on Holotest, both developed to the same densities, one in PyroChrome developer and the other in the not-so-tanning CWC2, the same amount of developed silver got removed in the silver solvent 'Chrome bleach, but the higher structural integrity of the gelatin as a result of the tanning results in less shrinkage and not as green a color shift.
The problem with this scheme of color control is that the plumping up of the emulsion is dependent on developed density. The tanning needs to compensate for the natural shrinkage of the gelatin. Usually with the Ilford material one of the exposures produced enough tanning action that the desired effect of real-time fringes could be observed.
So there is a formula included with the BB instructions for a Pyrogallol based developer that I figured would be as energetic as their Vitamin C soup.
BB PYRO DEVELOPER
15 g Pyrogallol
One Liter Water
30 g Sodium Carbonate
7g Sodium Hydroxide
One Liter Water
As is typical with the short-lived pyro concoctions, the two parts are mixed together immediately before use. The pyrogallol concentration is 50% higher than that of the original Pyrochrome recipe, and there is yet another boost to activity with the Metol, which may have a super-additive effect. I followed this developer with rehalogenating CWPBQ2, not with the dichromate based reversal bleach as recommended in the BB instruction sheet, because I wanted to overcome the shrink of the gelatin and not induce any more wavelength shifting to the green, as what would happen using the latter bleach.
Development time with both of these developers was five minutes, with an ambient temperature of Chicago Spring of 72F. Keeping the development time constant and letting the exposure time determine the final density build up yields more easily repeatable results. Five minutes (a couple of good songs) is the maximum I am willing to wait for results.
This developer gave the minimal amount of shrinkage, yielding real-time fringes on the kinematic holder, and a really nice signal to noise ratio.
But I wasnÕt satisfied with the shrinkage of the BB AA developer, so I tried another High Energy developer, Kodak D-8. (I only used 2Õ development time as there was a danger of fogging.) The pH of this one is way up there with the other two, and with its high hydroquinone content it really builds up density! Figure Three shows all three developersÕ results.
Figure Three: BB 640 and 3 Developers, BB Pyro, Kodak D-8, and BB AA, all with CWPBQ2 Bleach. On the left the replay color is laser perfect, the middle an orangey-yellow, and the right a yellowy-green.
When I finally received what I had been looking for I couldn't believe it; here in my hot little hands were samples of the famed Russian holographic technology. High efficiency, high signal to noise and of course the dreaded low sensitivity. But the quality of the hologram should be worth all the sacrifice.
So I started with the classic CWC2, and it was barely OK, as evidenced in Figure Two. Conventional holographic wisdom (or is it lore?) that the Russian emulsions demanded colloidal development because of the super-fine grains. But I had previously tried the typical Western processing scheme on some Bulgarian Holographic Plates in the late Ô80Õs.
In the summer of 1982, Dr. V. C. Sainov brought a bunch of holographic plates that had been manufactured by the Bulgarian Academy of Sciences. They came in two flavors, HP-490 whose sensitivity peaked at 490 but continued on to at least 515 nm Argon green, and HP-650 plates with red sensitivity. He also had his own proprietary developer, FHP-3.
We attempted to record holograms with these materials when experimenting with true color holography although these plates had been sitting in an attic for approximately seven years. The HP-650 was officially pronounced dead on arrival, but the HP-490 still had some life left in them. No fog was found when an unexposed plate was put into the developer, but there was some "development" in the form of shiny silver deposits around the edges of the plates where they had been in contact with some black packing paper. Perhaps some sulfur compounds used to make the paper black were to blame for reducing the silver bromide.
Since we didn't have any of the recommended FHP-3 colloidal developer, we processed the plates in a developer - rehalogenating scheme using the CWC2 developer and CWPBQ2 bleach. The results were quite good, with fairly high efficiency and an incredibly high signal to noise ratio, at all wavelengths we tested, even down to 458 nm.
But at what a price! The exposures were in the 20 to 80 milliJoules/cm2 range, about two orders of magnitude away from the Agfa plates, which were requiring 200 microJoules/cm2 for 8E56HD at 515 nm or 800 microJoules at the same wavelength for 8E75HD. You should expect to see dichromated gelatin performance at these exposure levels, which the material delivers with respect to signal to noise, but not so much in efficiency. The ten nanometer grains keep scatter to a minimum, but are not so efficient in capturing photons. Plus there are not enough of them to provide a very reflective Bragg layer. I had gotten comfortable with developing for only two minutes, but with these super slow plates four might be a better idea. It's only two extra minutes, but it can seem like an eternity.* So it didnÕt seem to unusual to process this way.
But it seems that the GP2 formula is necessary for the best results on PFG-03M as of this writing. The first that I had ever heard of it was on a mimeographed sheet handed out by Nick Phillips at the 1982 ISDH. Even that scholarly tome, Hans BjelkhagenÕs book, gives the formula but does not state its origins.
The rule is not to agitate in this bath. So I just put the plate in a photographic film developing tank (without the reel!) and closed the lid, sitting my trusty Gra-Lab timer (the backwards going clock) for 15 minutes and let it simmer.
The instruction sheet says immersion of 10 to 15 minutes, and figuring that the Helium Neon holographer needs all the speed out of the material, I used 15 minutes. Even with the long development time, I needed 80 second exposures! (HD needs only 5"!)
I used Edwal non-hardening fixer as I had some mixed up for black and white photographic printing. The Slavich fixer formulation is rather simple, just Sodium Thiosulfate with some Potassium Metabisulfite as a preservative. I didn't have any of the latter on hand, and wasn't in a mood to order some. This worked just peachy.
After drying the image looked very bright and clean. The plate itself was not transparent like the others processed in the develop bleach regime, but it was of that reddish-brown color indicative of colloidal development. This added to the signal to noise ratio. And it was capable of real-time fringes, as the developed layer color had only fairly low density at 633 nm.
When flipping over the plate and illuminating the emulsion side to observe the real image, the image color looks a little chirped; it is not a red color, but kind of pink. So it appears that if the final holographic result is to be displayed pseudoscopically, it should be exposed with its emulsion away from the object. (Maybe this is an artifact of not using their fixer?)
Because I am of the old hard core school of develop and bleach, I tried Kodak D-8 as the developer. But this doesn't look so hot, so itÕs not illustrated.
THE PRE-HARDENER PROBLEM: The idea of pre-hardening the plates doesn't seem like such a good idea to me. The only photographic process that comes to mind that requires such a step was the old Ektachrome E-4 color process. The untreated film was not capable of withstanding the high temperatures necessary for short process times, so this pre-developer step was added.
But what really makes me queasy are two of the constituents of the solution; Formaldehyde and potassium bromide. Formalin solution is not easy to come by; even just having the stuff in a darkroom doesn't sit right with me. It belongs in an embalming room*. Perhaps in Eastern Europe this stuff isn't considered that dangerous, but here you will be paying a bundle in Hazardous Material Shipping Charges!
The other component in this bath that raises one of my eyebrows is the potassium bromide. I know from experience that solutions of bromide can erase a latent image! Although it is a minute amount, anything that slows these plates down should be avoided.
But just for the sake of you, dear readers, I got ahold of some Formalin solution and made up a batch of this toxic waste. It appears to be necessary for the -03 emulsions.
Figure Four: The Ultra-Fine Grained Crew. It seems like a tie between the BB 640 developed with Pyro and bleached in PBQ and the PFG-03M and its GP2 colloidal developer.
Figure Five shows the full-plate holograms using the best exposure and developer combination based on the above trial and error exposure tests, 200 mJ/cm2 for the larger grained Holotest and PFG-01, 3.2 mJ/cm2 for the finer grained. They are illuminated on the flip or pseudoscopic side, so they look like bite-sized tasty morsels of that Belgian treat. The chirped aspect of the PFG-03M is visible from this, the emulsion side, but it looks narrow-band red through the glass. Go figure.
We are looking at the emulsion side of this batch in this picture because I painted the glass side with black spray paint to give a better idea of the signal to noise of the emulsion. They were photographed against a Kodak 18% Gray Card to give some idea of relative diffraction efficiency.
PRICING: This is where it gets mind-boggling. One would think that the new kids on the block would try to not repeat the mistake of Agfa and charge an arm and a leg, but they do.
Agfa got caught up in an inflationary spiral; the plates weren't selling, so they had to charge more. And then less people bought, so they charged even more. I remember when the HD series came out around 1980 and 4" by 5"'s were only about $2. At the end, they were $8, which is out of sync with the rest of the cost of living price increases.
So I hope that this Holographic ConsumerÕs Report will help others spend their hard-earned dough wisely!
* aka The Home & Studio of E. Wesly + Sons
**Names withheld to protect the guilty!
*It never failed to amaze me that those slick black boxes were made by Empak, in a suburb of Minneapolis, Minnesota, USA, shipped across the ocean, filled with the Lippmann emulsions in Belgium and back here again. I wonder how much that added to the price of the plates.
*Built-In Pre-Swell, so that the replay color was automatically shifted about 25-35 nanometers even when using a develop-rehalogenating bleach scheme, which should retain the recording fringe pattern spacing. To retain laser color fidelity this bloating agent had to be washed out before exposure. Nobody should be forced to handle things in the dark more than is necessary, especially since the emulsion could be damaged or fogged before shooting the hologram. Let's hope nobody else is brilliant enough to come up with that marketing scheme again! The problems it created! Why put something in that nobody really needed?
.. Tell the tales of woe of the instruction sheets, including how worthless the Agfa ones were. Holographic paper collectors.
.. I still have it in my permanent collection! The earliest publication that I have of the formula in my collection is Ostrovski,...
..My own paper, Recycling of Holographic Plates... Sorry for the sketch footnotes! Maybe when this gets published for real!