Reflection Holography in Acme Mine #5
Are there any other cave holographers out there? I took a few shots
in West Virginia, Acme Mine #? back in the early 90's. Up til then, I
had worked with holography a good bit, but I still learned from this trip.
I decided that for simplicity I would make a setup to make single step
reflection (Denisyuk) holograms of some helectites I had seen in an
earlier trip. I made a small platform to hold my laser, a battery powered,
5mW HeNe, hacked meter shutter, and a spatial filter sitting on a tripod.
I made a 4x5" plate holder that also sat on a tripod. I also had a 'safe'
green headlamp that I had tested for exposure limits in my lab, and for usability
in Greenville Saltpeter Cave. Caving with a green headlamp was quite interesting.
The setup, settling, and exposures all went well. No dust problems, no
dead batteries, no broken laser. I think I took 5 shots.
Later that day, back at the motel (down from Boston), I started processing.
All the shots came out. They were so good I could see the images while the
plates were still in the final rinse. Uh oh. That's a bad sign. This type of
hologram is very sensitive to the swelling of the gelatin on the plate. I made
a BIG mistake. The RH in most caves is, of course, pretty high. Well, this
swelled the gelatin quite a bit. After processing and drying the gelatin's
swelling pretty much went away. What this did was to move the replay wavelength
(reflection holograms are quite selective) from red to beyond blue thus unviewable.
To view these holograms I actually had to soak the processed plates in water for
a while. I was usually able to get the image to shift back into the visible for
a short time. Between too much viewing, experiments using TEA to permanently swell
a plate, and gravity there are no viewable holograms surviving.
If I were to try some cave holography again, I would make transmission masters
and make reflection copies of those. The setup would be considerably more complicated,
but humidity does not greatly impact transmission holograms.