I assume everyone has read the paper "New Recording Materials for Holography" by Hans I. Bjelkhagen on holographic chemistry and the diffraction efficiency that each can achieve. If you haven't, you should. This experiment is based on that work.
The question is: When one is using VRP-M film from Slavish, which chemistry will result in the brightest hologram:
The JD-2 process is the most common process. CW-C2 and PBU-Amidol bleach is the second most common, but unfortunately needs cupric bromide which is hard to get. It is also the process recommended by Slavich. GP-2 process is the process which the holographic chemistry paper recommended, however they used PFG-3 film.
My holographic setup has been making two beam transmission holograms reasonably reliably. So, I got a couple of The Usual Objects (tm), and laid them out. I then balanced the beam intensities as best as I could, to get a 2:1 ratio, and made three identical holograms. The settling time and exposure were done with a Tandy 102 (this specific computer was manufactured in 1988, oldest box still on active duty. :-) so while they may not be perfectly calibrated, the exposures were identical.
One odd thing about my configuration is that my reference beam hits the film at 45 degrees "above" by tilting the film holder. The reference beam is parallel to the surface of the table.
The film was VRP-M, clamped between two sheets of glass (plain old 1/8" window glass from the local hardware store). The laser was a Coherent Compass 315M (100mw of 532nm). The reference beam was 6.8 microwatts/cm^2, the object was 3.0 microwatts/cm^2. The goal was 50 microjoules/cm^2, so I used an exposure time of 5 seconds. That is actually under-exposed, I had miss-read Slavich's data-sheet. Also note that the light meter I'm using was last calibrated in the 80's, for 632 nm instead of 532 nm. So, the absolute numbers are probably wrong, but the relative measurements should be good.
The three holograms were made one immediately after the other, in hopes of avoiding any long-term changes in the laser and the room, then stored in a dark bag in film can.
The chemistries where then mixed up from the usual stock solutions as close to best practices as possible. Unfortunately there were issues. The GP-2 hologram floated in the developer. One corner was not developed at all. Both the JD-2 and CWC2 and PBU-Amidol holograms looked thin in the developer. They were underexposed, but I did not push process them. After the bleach, they were indescernable. (other than the labels... :-)
The three holograms were then informally pondered with both white light, and a red laser pointer who's collimating lens had been removed. The GP-2 seemed the brightest, with the other two being more of a toss up.
The three holograms were then objectively pondered. The holographic chemistry paper measured the diffraction efficiency every 10 degrees. I do not have a setup to do that easily, so I measured them at a single fixed angle which was approximately perpendicular to the plate.
More specificly, I placed the three holograms back into the original reference beam. I then put the sensor for my light meter on a burette stand about a foot from the plate and at about 0 degrees to the plate. The position is extremely poorly defined, but I did not change it between the holograms so I feel the measurement is still relevant.
|Intensity (up)||Intensity (down)||Delta||Diffraction Efficiency|
All numbers are in micro-watts per cm^2. Raw is the raw reconstruction beam, as measured normal to the wave front (IE: multiply by about 0.7 to compute how much light is actually hitting a square cm of the film.). Nothing, and glass, are the light coming off the film holder, when it contains nothing, and just the glass. The last three rows are then the actual holograms.
Intensity (up) is measured with the hologram in the correct orientation to reconstruct. Intensity (down) is with the hologram flipped so the reconstruction beam is hitting the correct side of the film, but 90 degrees off. Thus there is no reconstruction, and the absolute amount of light that was diffracted (instead of scattered by the emulsion) can be measured. Delta is the difference between Intensity (up) and (down), or the absolute amount of light being diffracted. And finally, Diffraction efficiency is the delta divided by the raw beam intensity.
The results were disappointing. I thought I was doing quite a bit better than that, in terms of absolute diffraction efficiency. They also indicated that the JD-2 process was the best. Which is both logical, why would a poor process become standard, and surprising, why would CWC2 and PBU-amidol have such a good reputation?
However, due to the procedural failures I mentioned above, there is also lots of reason to question these results. Due to the cost of the consumables, I'm not going to repeat this experiment immediately. :-)