Grainy holograms

[rzeheb]

I suspect there is information on this in the old forum archives but......

If someone (me) said that they were getting very grainy transmission holograms, what is the first (or second) most probable cause for this that comes to mind??

I am shooting VRP-M plates using microgreen laser. Developing with D19 which has been diluted about 1:3 with DI water and bleaching with ferric EDTA. My own feeling is that I may be both overexposing my plates and possibly over devolping as well. I noticed that the plates seem the clearest towards the edges and that the grainiest part is right in the center 50% of the plate. Any suggestions would be MOST welcome.

[holomaker]

change light ratios to match the ratios on the clear edges. overmodultion is possible from too bright of object.... too hot in the middle (spread ref. beam)? just thoughts !

[holorefugee]

Also, use long exposures and short development times. The longer the development the larger the silver grains.

[rzeheb]

Thanks for the thoughts/suggestions, that's exactly what I was hoping to get when I posted.... tapping the considerable collective experience represented on this forum. I'm doing a mini experiment to try and sort some of the variables out including using/not using a polarizing filter, shifting the beam ratio towards the reference beam and reducing development time. All changes one at a time of course while holding everything else constant. Hopefully, the net result will give me some hint of what parameters to change to improve the quality of my H1s. Anybody have any additional ideas or suggestions I can try?? Thanks, Ron

[Martin]

Anybody have any additional ideas or suggestions I can try??

If you had sufficient laser power, you could try a different processing schedule. Rather than develop-bleach, develop-fix, will greatly improve the signal-to-noise ratio (though at the expense of DE). For best results you've to develop the emulsion to less density (maybe around 0.6 or so) than in the case of bleaching.

[Johnfp]

What object are you making a holo of?

[rzeheb]

What object are you making a holo of?

I get the same "grainy" effect with multiple different objects. However, what they have in common is that they are all diffusing (rather than metalic) objects, thus resulting in mostly randomly polarized light coming off of the object. In a thread that I started on the old forum it was suggested that I might be struggling with inter-modulation noise, and I tend to agree. However, nothing I do seems to help minimize it very much. I have tried moving my object further from the plane of the plate; maybe that helped a little, maybe not it was hard to tell and fairly subtle if it helped at all. I tried shifting the beam ratio more towards the reference beam; maybe that helped a little but that was also subtle and hard to tell. I also tried putting a polarizing filter in front of the plate to block most of the light coming off the object that was not polarized in the same plane as the reference beam. Since my polarizing film only transmits about 35% of the light that IS in the correct plane, I doubled my exposure time (I probably should have trippled it). The result was a dim hologram with dim noise but probably the noise was still proportionately about the same as it was without the polarizer (though, again, its hard to tell for sure). What the experiment with the polarizer did tell me was that I was not overexposing my plates and I might even have been underexposing a bit. I can mostly get around this problem by reconstructing the H1 about 10 degrees off-axis to the reference beam. The noise largly clears up and the image is still nice and bright. BUT, it still seems to me that I should be able to find a way to, if not eliminate, at least significantly reduce what I assume is intermodulation noise but may be something else entirely.

I am also not convinced that my developing method is not contributing to the problem. My stock D-19 is about a year and a half old. It has been stored under cool conditions and sealed off from the air, and it works great, but I was told by a colleague of mine (who is not a holographer but who did a lot of electron microscopy work) that aging D-19 results in larger grain size in the developed film. If that's true it might aggravate the problem. Maybe fresh developer would be better or maybe a different developing scheme would be better.

If you had sufficient laser power, you could try a different processing schedule. Rather than develop-bleach, develop-fix, will greatly improve the signal-to-noise ratio (though at the expense of DE). For best results you've to develop the emulsion to less density (maybe around 0.6 or so) than in the case of bleaching.

I think I have sufficient laser power, but what do the initials "DE" refer to?

As you can tell, I am a bit at a loss and still very interested in any suggestions. Also, much thanks to everyone that has already offered their ideas.

[Martin]

If you had sufficient laser power, you could try a different processing schedule. Rather than develop-bleach, develop-fix, will greatly improve the signal-to-noise ratio (though at the expense of DE). For best results you've to develop the emulsion to less density (maybe around 0.6 or so) than in the case of bleaching.

I think I have sufficient laser power, but what do the initials "DE" refer to?

Sorry. DE referred to diffraction efficiency. Here it simply translates into "image brightness". With the develop-bleach scheme you clearly get better DE than with the develop-fix method. So in the latter case you'll get much less noise BUT less DE also, since the layer is absorbing more light. Fortunately, when working in the 0.6 - 0.8 density range, you may still reach sufficient DE to make a (H2) hologram.

[Johnfp]

Ok, then I seriously doubt its the object, thanks for the reply. I assume you are making a transmission hologram.

1. As your D19 is a little old and it is really a "large grain" developer used as a happy medium for speed vs grain size, try to make a simpe reflection hologram and see if you have the same issue.
2. D19 is relatively cheap, get some new stuff and see if that helps.
3. I would love to see a photo if you can. If not, does the "graininess" seem to be on the object or the plate? That is, if you move the hologram right to left, do the grains stay in one place on the plate or do they follow a point on the object? How large are the grains in the graininess?
4. How old are the plates? Take one of the plates/film and process it without exposing it. Time it to see if it goes dark at all, hopefully not.

[rzeheb]

Hi John,
Thanks for the list of suggestions. It's very helpful to have specific concrete things to test/try and I will attempt to get to all of them as time permits over the next couple of weeks. My plates are about 2 years old. It did not occur to me that they could possibly be fogged. I don't think they are, but it would be easy enough to test (albeit at the cost of a plate, the thought of which makes me twitchie ). I have not looked at the holos for a couple of days so the following is from memory but in answer to your question #3, I believe the grains are fairly large and seem to be on the plane of the plate rather than in the image itself. It's more or less like driving into the sun with a dirty windshield, the diffused light makes it hard to see anything in front of you but once you get out of the sun it all clears up. Getting "out of the sun" in the case of my H1 holos is simply rotating the plate slightly off axis to the reconstructing reference beam. I'll definitely order more D19 and see if making that up fresh helps as well. Does anybody here have any experience using Microdol-X as a developer? That's supposed to be an ultra fine grain developer. I thought I would order some of that and just try it as well. Tnx, Ron