Index Matching

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Mounting Holographic Film

When choosing a index matching fluid it is important to have it be safe to handle, it can not evaporate to quickly, it must clean up without destroying the un-developed gelatin and it's Refractive Index should be glose to in-between glass and gelatin.

That may be true for a great many “special applications” but probably not for the recording of display holograms. In the 80s, when we started working with Agfa's 8E75 material (back then coated on Melinex polyester film), we routinely used plain tap water for “index matching”. Though the refractive index of water hugely differs from that of Melinex (that's to say from 1.33 to roughly 1.64 or so), the mismatch did not show up in the final holographic image. Actually, it could only be detected if the film was examined in a transmission mode. It turns out the main issue here is keeping the film stable on a glass plate during exposure. Whereas polyester was very forgiving in that respect, handling nowadays' triacetate has become a lot trickier.

If you are using film instead of glass plates then a method to hold the film stable is very important. The usual method with the AGFA materials is to use an index matching fluid to hold the film emulsion side out to a oversize glass plate. The simplest index matching fluid to use is liquid paraffin. This is sold as lamp oil. Make sure to get the oderless and smokeless variety as it is much more pure. The index of refraction is a little low but it works fairly well. Make sure to wash it off before you develop it. Use a drop or two of dish soap in 1/2 gallon of distilled water as a rinse before development.

I would like to add that other liquids might be equally worth a try. E.g. white spirit and the like: paint thinner, paint cleaners etc. (provided they are low toxic of course). As I have just discovered there is also an odourless white spirit artist's quality, made by Talens. Cedar oil might be another option. Compared to paraffin those liquids have the advantage of being removed quickly from the film – simply by warm air.

It is very important to use the right amount of index matching fluid. Use an eye dropper to measure the fluid onto the plate. A syringe would be useful for larger plates. If you have air bubbles they will show up as blank portions of the holograms because the film will move. If you have too much then you have much to clean up before you can use the sandwich. If you get index matching fluid on the wrong side of the plate it is very difficult to clean it off under the safelight. Before using the sandwich check it in both transmitted safelight and reflected safelight to check for bubbles or smears.

The AGFA materials and now the Fuji materials have a gelatin coating on both sides! This makes using the single plate method very easy. If you are using the Slavich materials they will often curl up on you. This has been attributed to the thinner triacitate base stock but it is likely it is the lack of a second gelatin coating. For the Slavich materials sandwich the film in between two glass plates. This can be done with index matching fluid.

Slight correction regarding Agfa. Yes, they did a couple of film batches with gelatin coated on both sides. But that has been rather an exception. Speculating about the reason for the superiority of Agfa's materials (with respect to ease of handling prior to exposure only!), one might consider the particular triacetate film produced by Bayer or, a special subbing layer, used by Agfa or, the whole thing relates to the topcoat. Personally, I do not think it was just a matter of film thickness. If memory serves, Ilford materials (SP-673, HOTEC) used to have a similar film thickness. And yet, index matching those films on a glass plate became pretty challenging. Slavich (PFG-01 and PFG-03), Red Star, Ultimate and Filmotec all were similarly difficult to handle.

Beside making index matched glass-film-glass sandwiches, glass-film-plastic sheet sandwiches seemed to work reasonably well. E.g. we would pour some index matching liquid on the middle of the glass plate and then apply the holographic film (emulsions side towards the liquid) to the glass. On top of the holographic film (base side) by means of some additional index matching liquid a plastic sheet (PMMA, PC etc. plate or some OHP film etc.) was applied. In those cases, adding a surfactant (Triton-100) to the index matching liquid used make life easier.

Dry Film Mounting

Frank DeFreitas has an interesting mounting method. Here is some information from a post he made to his forum (now retired forum) at holoworld. It also has some very good information about finding the Brewster's angle.

Frank DeFreitas - Mon, May 28, 01 10:51:49 PM When running test after test working with diodes in the now "early" days, I had to come up with a way of not going broke using glass plates -- while also keeping the quality of the holograms intact and consistent -- so that any problems would be from the laser test itself, and not the set-up. Of course, this meant working with film (AGFA-8E75HD at the time).

With both time constraints and the shear number of test shots, I quickly became aware that wet-mounting was just too much to do each time. So here's what I did:

Since the very nature of holography requires that glass plates be manufactured to exacting specs in regards to flatness, I took two old plates and placed them in a standard Clorox bleach solution. Within several minutes, the emulsion turns to a white paste and can be rinsed off with water -- leaving just the glass substrate. This took care of the problem with commercial glass and it's inconsistent "flatness" for sandwiching. NOTE: Since this IS a chemical reaction taking place, when removing the emulsion with bleach, use a fume-hood or do this outdoors.

Once rinsed, clean the glass off with any standard window cleaner and take care to store them so they do not become scratched. Any scratch will show up in the final hologram -- and, if it is deep enough, actually create a shadow -- due to the angled surface of the physical scratch itself.

Then, just take your film and sandwich it between the two pieces of glass. My method was to "squeeze" the glass together with a twisting motion while applying downward pressure. If done properly, you will create your own vacuum and the entire sandwich will stay together as one unit. When you REALLY get the hang of it, you'll find it hard to get it apart!

Now, for the set-up: You will need a polarized laser and make sure that the polarization is properly oriented to your plateholder. You will also need to bring your reference beam (or single beam) in at "exactly" brewsters angle. One way to determine this is to set up your plateholder at brewsters and place a single piece of glass in it. Hit the glass with your spread beam. The glass is going to reflect some of the light hitting it, so place a white card in this reflected light path (in order to view it). If you rotate your laser head, you will notice that this reflected light becomes brighter and dimmer. Find the spot within the rotation where the reflected light is at it's dimmest on your white card, and you've got it. There should be two spots for this with every 360-degree of rotation. With my HeNe, it is at the 3:00 and 9:00 position(s) for side-reference (parallel to table). With the diode, it is at 12:00 and 6:00. With a HeNe, you'll always have a "little" bit of light reflected. With a properly-running diode, the reflection will go completely out on the card (100% -- or VERY close to 100% -- transmission through the glass).

For side referencing, also make sure that your plateholder is not angled toward or away from the incoming laser light, too. It should be straight up and down in relation to the reference beam. This also means making sure that your incoming laser light stays parallel to the surface of the table . . . and is not directed upwards or downwards in any way toward the plateholder during it's travel. If you're using an overhead reference, then it should not be angled in any way from either SIDE. In other words, the ONLY "angle" present should be the reference angle -- no matter what table geometry or set-up you're using.

Place your sandwich into the plateholder and give it time to "settle". It will take much longer with film than with a glass plate. I usually use this time to get chemistry ready, or go upstairs and have a cup of tea and relax, etc.

Do your exposure as you normally would. If you have everything set-up properly as stated above, you will have a film hologram that is every bit as clear, bright and clean as one on glass plates -- without the cost of plates and without any type of index matching fluid or the associated mess and/or extra time.

In closing, I have heard that the .mil thickness of the new film out there is less than it was previously with AGFA . . . so this may require a few "tweaks" here-and-there with settling time, etc.

Regards, Frank


Micheal Harrison has a great article here!