Holographic Film Holders

 

Holding holographic film stable is very important. There are many methods and I hope to show a few. Here are some drawings of some film holders. I designed them to hold plates still for holographic interferometry. I need the abality to hold them at any angle and position over a musical instrument to they need to be very rigid and easily mounted to 1/4" 20 screw.

 

Film Holder 2.5" x 2.5"

Film Holder 4" x 5"

Click on the images to get a CAD file. You will need a plug-in to view .dwf files. I use the auto desk one called Whip. If you right click you can zoom to see the details. If you import this into your CAD software it retains all of the features of the original drawing.

 

Machined aluminum film holders are very expensive and often they are overkill. Since my bench has a metal surface I often use a couple of magnets to grab a piece of glass. This is a quick and dirty method. The main drawback is the magnet is visible in the image.

 

 

 

4" x 5" Film Holder
after Dinish and Joy's

 

This film holder is a copy of one Dinesh and Joy were using when I was there. I made it from 3/4" square aluminum bar. (This is not quite how I made it but I am going to include how I would make it again when I make a second one this week.)

Take a 12" or so section of 3/4" aluminum bar and set a table saw to cut .25" deep. Set the fence .125" from the blade (make sure the fence is parallel to the blade!) Run the piece of aluminum through. (Use two push sticks so you don't get your hands anywhere near the blade. Also a feather board or two would be of use but I did not have any handy.) Then move the fence over about .035 inches or however wide you want the slot. You can make the slot wide enough to accommodate 2 plates and film if you wish.

Then hack saw it to 4.25" or so. (Where the table saw blade exited the part you will notice the slot is wider than the rest of the part. This is because the blade started to ring upon exit and you should make sure to cut off this end.) Set up your cross cut saw on your table saw very square then taking about .030 inches in a pass trim both ends until the ends are clean, square and the piece is exactly 4".

Cut another piece to 6+ inches and clean up the ends on the table saw till it is square and 6" long.

Measure down the 4" pieces 1" from each end and make a line. Measure from the lip .125" and make a line. Where these lines intersect, use a drill press and drill a #36 hole through to the channel. (Marking the hole with a spotting dril is handy to keep the wholde from drifting. At least make sure you have as little of the drill bit sticking out of the chuck as posible and mark the spot with a punch.) Tap to #6 32 tpi. These will be the plate holder screws. (Note: tap from the small side for a reflection plate holder and from the thick side for a transmission holder, or you can make two slots in the same plate holder.)

Mark the two 4" pieces on the ends in the center. (This operation will make the parts "Handed" so make sure to pick opposite ends for this mark.) Drill in a drill press a #F hole about 1/2" deep. Tap to 5/6" 18 tpi.

Measure .375" up and over on the 6" pieces (on the 6" face) and make a mark at each end. Drill through with a 5/16" drill bit. (If you miss this hole or the holes in the ends you can make this hole larger so you can get the parts to align.)

In the center of the bottom of the 4" pieces (away from the slots) drill a #7 hole .425" deep. Tap with a 1/4 20 tpi tap. In the center of the 6" piece, on the same face as the holes, drill a #7 hole .425 or so deep and tap to 1/4" 20 tpi. These will be the mounting holes.

Clean up the corners with a file, and clean up the holes with a countersink tool. Spray paint with Krylon Ultra flat black paint.

Bolt channels to the bas with 2-5/16", 18 tpi, 1" long Allen bolts. Put 4 #6 32 tpi Allen bolts into the channels but first file the ends flat with a 6" mill smooth file. The length should be chosen so it sticks out about .25" when holding a plate. Put 3 1/4", 20 tpi, .75" long Allen set screws into the tapped holes. These now will fit any 1/4" mounting rod.

Spray paint with Krylon Ultra flat black paint. My total time invested was three hours.

A nice touch would be to bevel the front side of the plate holder to 45 deg. You can easily do this with a router and a bearing bit.

What you say? Cut aluminum with wood working tools? The truth is it works very well and I have been doing it for years. It works much better than many woods and it is much faster than milling. I once purchased a 9 HP pin router from Boeing that was used to rout airplane parts. We used it to make electric guitar bodies. If you were going to use your table saw a lot for making aluminum parts I would use a ATB grind 60 tooth blade with large blade stiffeners.

 

Mounting Holographic Film

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. I have tried many different index matching materials and one of the simplest 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. I use a drop or two if dish soap in 1/2 gallon of distilled water as a rinse before I develop.

It is very important to use the right amount of index matching fluid. I 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 I use the sandwich I check it in both transmitted safelight and reflected safelight to check for bubbles or smears.

The AGFA 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 my theory it is the lack of a second gelatin coating. For the Slavich materials I sandwich the film in between two glass plates. This can be done with index matching fluid.

Frank DeFreitas has an interesting mounting method that I have not taken the time to perfect. Here is some information from a post he made to his 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 208.59.249.8

 

 

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Copyright 2002 Colin Kaminski