help- Reflection holograms w/ film sandwiched between glass

MGordon · Post by **MGordon** » Sat Oct 11, 2008 3:58 am

I am a member of a student optics club at UCSD that runs workshops where guests (visitors from various middle schools, high schools, as well as local college students) come to make their own holograms.

video of some of the holograms
Photo of table layout used for PFG03 plates

In the past we've generally used the 2.5" Integraf PFG03 slides, but we've been trying to to switch our setup to make holograms on film instead (we've tried both VRP-M with Argon, and PFG01 with HeNe), as the film is cheaper, and more "user friendly" to give away than breakable glass, especially as we want to start making holograms of larger objects (at least 4"x5" film).

Our table layout uses object beams from each side, and a reference beam (almost) normal to the plate that serves as a third object beam, adjusted with a variable attenuator. Path length differences are 2cm at most. When we tried this out with film, we left the layout the same and expanded the beams with more powerful objectives, then substituted panes of glass held together with binder clips for our plateholder.

So far all our attempts to make holograms by sandwiching film between two pieces of glass have resulted in very poor contrast compared to using the PFG03 plates. All the features are present, it's just the image appears very washed out. The few we tried using VRP with 515nm and our argon laser are even worse (only the very brightest spots on our object show up, regardless of exposure or development times) for some reason, as well.

We also tried simply taping the film to a single pane of glass, with emulsion side facing the object. In that case, the image came out great along the edges, but terribly in the center -- I suppect the reason there was slight movement of the film. (this was also more difficult to do in the dark).

I suspect the biggest problem is that we're using plain glass, without any sort of AR coating to hold the film, so the interference pattern from the laser light reflecting off the glass-air interfaces is washing out the hologram. This seems plausible as the few time we've accidentally loaded our plates with emulsion facing away from our object, the image quality has been dramatically worse than exposures of the same object with the plate loaded correctly.

Has anyone on this forum succesfully been able to make reflection holograms using PFG01 film (not plates)? If so, what method did you use? Was it necessary to use AR coated glass, or some other method of index matching we haven't thought of? Or some other mounting method entirely?

I'm considering buying some AR coated "museum glass" inteded for framing, but I'd like to hear some expert opinions first before working on this further.

Thanks.

MichaelH · Post by **MichaelH** » Sat Oct 11, 2008 2:16 pm

What you've described is most likely caused by movement.

When sandwiching film between glass make sure your film goes all the way to the edge of the glass so that you're clamping the film between the glass at the clamp location.

I'll try some ASCII art...

Imagine the sandwich looks like this:

--------------------------- glass
--------------------- film
--------------------------- glass

and you clamp where the word "film" is. Because there's a gap there, you're going to cause bowing in the middle of the glass. You probably won't see it but even with small squares of glass it's going to happen and where that happens you're going to get movement. Movement in holograms causes either loss of contrast or if there's enough, no image at all.

You can either cut the film so that you have a "perfect" sandwich such as:

--------------------------- glass
--------------------------- film
--------------------------- glass

or you can put tape on the glass as a border to fill in the gap. Blue masking tape has worked well for me for years.

BobH · Post by **BobH** » Sat Oct 11, 2008 3:55 pm

When you cut the film, you may be leaving burrs along the edge that keep your sandwich from squeezing flat. I always burnish all edges I cut before mounting. Also, use thick glass. At least 3mm.

You can also use index matching fluid in all the interfaces to hold the film better. A little messy, but manageable if you set up for it well. Be careful with Cargille oil, though. I found that PFG-01 absorbs it slowly, making it difficult to control the final thickness of the hologram.

MichaelH · Post by **MichaelH** » Sat Oct 11, 2008 5:12 pm

BobH wrote:You can also use index matching fluid in all the interfaces to hold the film better. A little messy, but manageable if you set up for it well. Be careful with Cargille oil, though. I found that PFG-01 absorbs it slowly, making it difficult to control the final thickness of the hologram.

I highly recommend *not* using index fluid until you've gotten past your current stability problem. Once you can get some small sandwiched holograms made move to the more complicated index-matching method. There are good reasons to index match (stability is one of them) but it adds another level of complexity.

BobH · Post by **BobH** » Sat Oct 11, 2008 5:32 pm

But Michael, I syggested index marching oil to get past their current stability problems.

What's complex about putting a few drops of oil in the sandwich, and wiping it off the film before processing? Might get some processing marks at first, but that improves after a while.

MichaelH · Post by **MichaelH** » Sat Oct 11, 2008 8:48 pm

BobH wrote:What's complex about putting a few drops of oil in the sandwich, and wiping it off the film before processing

Because with PFG-01 it never works that simply in practice. Especially for a beginner who's just starting to work with film. PFG-01 has a strong curl that wants to resist the oil before a cover plate can be put on. A beginner invariably uses too little or too much oil, creating uneven coverage on the film or a mess. The glass has also got to be cleaned before another piece of film is put into the sandwich.

If fact, I never use oil for any hologram smaller than 5x7. I've found that a glass sandwich with border tape works perfectly and there's no need to fool with oil that has to be cleaned up. Put the film in a pre-made and hinged (with tape) sandwich. Clamp it up. Put it in the holder. Settle and shoot. Take the film out, put another sheet in and process while the second is settling.

Oil is great when you can't shoot at brewsters or if you're using larger film. Once they get past the "I've never used film before" stage I'd recommend experimenting with oil.

MGordon · Post by **MGordon** » Sat Oct 11, 2008 9:50 pm

Thanks for the advice. Someone in our group brought up the notion of using an index matching fluid but we've been hesitant to use that, especailly for holograms made as part of the workshop since we are trying to make the holograms relatively quickly.

We generally bring in 4-10 people into the room at a time, then set up a new object while explaining our optical table geometry. We try to use one small item from a few different people if possible. We then shoot 3-5 copies of the object in the dark, and develop them all simultaneously in an "assembly line" fashion. In some cases, we actually shoot 2 of one object, chnge the object, then shoot 2 of a second object all in the dark. Once the lights come on, we blowdry the plates, show them off and give them away, then bring in the next group. This entire cycle is done in less than 20 minutes. In a 5 hour workshop we make about 60 holograms on PFG-03 plates.

Adding the steps of placing oil and wiping the hologram and glass down for each exposure would add a lot of extra time to the process when we are making this so quickly (12/hour). Plus it will probably increase our failure rate as it's yet another variable to control (alogn with exposure/development times etc).

We do eventually want to make higher-quality 8x10s just for the members of the group (not to give away) and for those we are willing to take extra time/effort and use matching gel to optimize quality, but not until we get at least decent results using "dry" mounting.

Oil is great when you can't shoot at brewsters or if you're using larger film. Once they get past the "I've never used film before" stage I'd recommend experimenting with oil.

Interesting. I hadn't even though of this as the reason why a lot of the example hologram setups are angled. We haven't been shooting at Brewster's angle. I had assumed angled reference beam was only meant to allow reconstruction of the image without the reflection of the light source reflecting in the viewer's face (or the laser shining in the viewer's eyes in the case of transmission holograms). For aesthetics reasons we decided to have the reference beam strike only a few degrees away from normal (similar to a single-beam layout) so we could use the reference as a third object beam.

What we need to try next is adjusting the angle of the reference to Brewster's, then adjusting the laser's polarization angle so as to maximize transmission. With the plates this wasn't hurting us much as the emulsion is on the surface with no glass between it an the object, but I suspect that's what's been killing us with the sandwich.

Paulos · Post by **Paulos** » Sun Oct 12, 2008 10:23 am

MGordon wrote:.............
In the past we've generally used the 2.5" Integraf PFG03 slides, but we've been trying to to switch our setup to make holograms on film instead (we've tried both VRP-M with Argon, and PFG01 with HeNe).......
......
So far all our attempts to make holograms by sandwiching film between two pieces of glass have resulted in very poor contrast compared to using the PFG03 plates. All the features are present, it's just the image appears very washed out. The few we tried using VRP with 515nm and our argon laser are even worse (only the very brightest spots on our object show up, regardless of exposure or development times) for some reason, as well.
Thanks.

Multiple reflections on film and glass surfaces may cause serious problems,
but I think that your problems mainly comes from the material itself.
Do not compare PFG-03 with PFG-01 or VRP.
PFG-03 is fine grained and can give very bright reflection holograms.
In my opinion, PFG-01 as also VRP - although good for transmissions,
are almost unsuitable for reflection holograms in general, and completely unsuitable for Denisyuks.
Due to their poor resolution, they give noisy and dim reflection holograms.

The reason that the holograms you tried using VRP with 514nm are even worse
is that green holograms demand better resolution, because "green" fringes are closer to each other
(approx. 170nm) than red fringes (approx. 210nm)

I also agree with Michael, "not to use index matching fluid until you've gotten past your current stability problem".

BobH · Post by **BobH** » Sun Oct 12, 2008 2:10 pm

Again, I recommended index matching oil as a diagnostic, to identify the cause of the stability problem. If it's not good for you to use in production, don't use it.

I just don't think it's a technique that's beyond the abilities of your group.

Personally, I'd stick with glass plates. Consider all the time you'll be dicking around with little pieces of curling film in the dark, with a group of newbies watching and waiting, then keeping them all neatly wetted and properly agitated in the processing baths, and finally drying and cleaning the curling little bastards without getting nasty marks all over them. Not fun. Glass is easy to handle and mount, stays on the bottom of the trays, doesn't curl, is easy to dry, and is always flat (giving the best image). It's also way easier to finish, clean and frame for the newbie that proudly takes it away. A raw film based hologram is a pain in the a** to view, and will probably be destroyed before it gets home. That's been my experience.

MGordon · Post by **MGordon** » Sun Oct 12, 2008 7:17 pm

Multiple reflections on film and glass surfaces may cause serious problems,
but I think that your problems mainly comes from the material itself.
Do not compare PFG-03 with PFG-01 or VRP.
PFG-03 is fine grained and can give very bright reflection holograms.
In my opinion, PFG-01 as also VRP - although good for transmissions,
are almost unsuitable for reflection holograms in general, and completely unsuitable for Denisyuks.

On the box it mention those films are "suitable for transmission or reflection", where the PFG-03 claims to be "especially suitable for reflection" due to the higher resolution so we weren't expecting equivalent results. This is why I was asking i anyone on here had actually made these films work as reflection holograms. Your thought about the VRP does make sense.

Again, I recommended index matching oil as a diagnostic, to identify the cause of the stability problem. If it's not good for you to use in production, don't use it. I just don't think it's a technique that's beyond the abilities of your group.

Next time we meet, we will probably try brewster's angle incidence for the holder, then we may try index matching as an additional step to see how significant the difference is, and help us judge whether the issue is stability, multiple reflections, or inherently low resolution of the film.

Personally, I'd stick with glass plates. Consider all the time you'll be dicking around with little pieces of curling film in the dark, with a group of newbies watching and waiting, then keeping them all neatly wetted and properly agitated in the processing baths, and finally drying and cleaning the curling little bastards without getting nasty marks all over them. Not fun. Glass is easy to handle and mount, stays on the bottom of the trays, doesn't curl, is easy to dry, and is always flat (giving the best image). It's also way easier to finish, clean and frame for the newbie that proudly takes it away. A raw film based hologram is a pain in the a** to view, and will probably be destroyed before it gets home. That's been my experience.

For the workshops where we bring in like 50+ people, we may still stick to plates for convenience. Thanks for the advice.