Image Issues

[Ed Wesly]

One of the tricks of time averaged holographic interferometry is to first find the proper resonant frequency of the vibrating object so that the nodes become apparent. And the best way to do that is by first doing some real-time experiments.

Although I had the advantage of using a Newport Thermo-Plastic Holo-Camera when I worked at Lake Forest College and a Laser Technologies Inc of Norristown PA real time film liquid gate Holo-Camera when I worked at Magnaflux, it is not that difficult to get real-time fringes if you have a good kinematic plate holder. Best example is to be found in Nils Abramson's first book, The Making and Evaluation of Holograms, plus his other papers. The plate rests on three ball bearings and is indexed by two pins on the bottom edge and one on the side. As long as you develop and bleach with non-shrinking formulas you will be surprised how easy it really is to put the plate right back into place and see zero order fringes.

Then the fun begins, as you drive the object illuminated with laser light while you look through the reconstructing hologram. (I envy your Pasco driver! We used to just use Radio Shack speakers!) The fringes will be apparent, but not of the highest contrast. But once you know this resonance frequency, then you can make a high quality single exposure hologram; I have even made a SBR of a vibrating speaker, which just sounds like anathema to the idea of nothing moving during exposing! But before I did that, I analyzed the resonant frequencies of a dud holographic plate painted white placed in the plate holder over the vibrating speaker, using that same speaker to drive the plate. Luckily the two were incompatible in the frequency domain.

Take your time, make sure all your optics are solid, your plate holder especially, and you should be able to get as good results of the image from Tom Rossing's studies of Chinese bells!