help- Reflection holograms w/ film sandwiched between glass

[Colin Kaminski]

The lab looks nice. I am excited you are getting people interested. The set up you are using requires lots of time carding off. That is what would slow me down.

Do you have a good light meter to set the beam ratios? That should allow you to make guesses about beam ratios. However there is nothing to do when somebody brings in a black rock and wants a hologram of it.

[Ed Wesly]

Put it up against a bright background!

[MGordon]

The lab looks nice. I am excited you are getting people interested. The set up you are using requires lots of time carding off. That is what would slow me down.

The setup was more complicated than it needed to be to shoot 4x5's because we wanted to be able to switch the same setup to shoot 8x10s or larger with minimal changes (simply swap out the microscope objectives for more powerful ones to spread the beam more, and/or remove the collimating lenses). We had to use more mirrors to get the lasers up to the higher plane necessary to shoot the larger holograms, and we had to use a lot of table space to have enough room for our beams to spread out and still keep path-length differences reasonable.

Do you have a good light meter to set the beam ratios? That should allow you to make guesses about beam ratios. However there is nothing to do when somebody brings in a black rock and wants a hologram of it.

As for the "black rock" scenario. We actually had a physics postdoc bring in some ferrofluid hoping to get a hologram of it (It looks like heavy crude oil normally, but turns into a solid under presence of a strong magnet). Next time we build a new setup in the spring (our current setup is being torn down as the table is being used by a class), I want to set up a variable beamsplitter and use our more powerful argon laser. Then we should be able to dump a lot more power into the object beams if we want to, and still have reasonable exposure times. Provided the rock is a diffuse black rock (and doesn't have any specular parts on it) we could probably do it.

[MGordon]

Put it up against a bright background!

I tried flipping over the hologram, placing it on a mirror, and then lighting up the hologram to view the phase conjugate image (eg, operate the hologram as a tranmission hologram with the recontruction light bouncing off the mirror and shining back through the hologram) and the result is quite a bit worse -- the image is distorted, and harder to see against the bright specular reflections on the mirror. From what I understand we'd need to record a rainbow hologram, rather than a reflection hologram, to do this.

[Colin Kaminski]

I think Ed was saying to master the dark object against a white background. Then you have a shadow that shows the depth of the difference in distance from the object to the background. I have seen this done and work well. I have also made holograms where an object is set in motion during the exposure in order to blank out the object and leave a shadow against the background. "Shadowgram".

[Ed Wesly]

Yeah, put the dark object against a bright background while making the hologram! White or silver painted board, crumpled up aluminum foil, etc, so that the dark thing is a 3-d shape in front of brightness. Usually something comes out in a dark object that hasn't moved during exposure.

[MGordon]

Interesting idea on the backdrop. That would look pretty cool. I know from shooting a couple 8x10's that we have good depth of field with our setup (long coherence length for the lasers due to the very long cavity). With a backdrop this depth would be obvious even in a smaller 4x5 film.

We will proably set up again sometime in the spring, I'll post another thread with our progress.