Transmission questions

Keef333 · Post by **Keef333** » Wed Apr 04, 2007 6:17 pm

Wow...this place is kinda addictive! Here I am at work posting another question!

I have finally succeeded in making a fairly bright transmission hologram,

but I cant for the life of me get it to project so that I can try making a H1-H2. Whats the deal?

Any help would be great!

Kaveh · Post by **Kaveh** » Wed Apr 04, 2007 7:27 pm

You need to illuminate it with the "conjugate" of the original reference beam, i.e. the "time-reversed" beam. So if your original ref beam was a diverging beam coming from a certain point, you need to illuminate it from the opposite side, and a beam which converges to that point.

This is why it is always best to use a "collimated" beam, i.e. all rays parallel. So to illuminate with the conjugate beam, you only need to flip the hologram through 180°.

Keef333 · Post by **Keef333** » Wed Apr 04, 2007 7:34 pm

So when I make the copy (H2) I wont be using an expanded beam?

Kaveh · Post by **Kaveh** » Wed Apr 04, 2007 7:44 pm

Keef333 wrote:So when I make the copy (H2) I wont be using an expanded beam?

By far the most important is the illumination on H1. On H2 you can get away with much more. But again, if you can, collimate that one too.

The main reason is that the average distance of image points from the H1 is greater than that of H2, and aberrations and distortions are proportional to that distance.

MichaelH · Post by **MichaelH** » Wed Apr 04, 2007 7:47 pm

Keef333 wrote:So when I make the copy (H2) I wont be using an expanded beam?

You can but ideally you want to make the H2 with the reference being the conjugate of the light you'll use to reconstruct the hologram.

in practice though, a "gently" diverging beam works just fine.

JohnFP · Post by **JohnFP** » Thu Apr 05, 2007 10:41 am

If you state you made a bright tranmission hologram then you must be able to see it. Are you looking at it in white light and see a bright rainbow? Or do you mean you are looking at it back in it's original set up (the plateholder) and can see a bright image? If the latter then simply rotate the holographic plate 180 degrees clockwise if you have a side reference beam. Flip the plate bottom to top if you have an overhead reference beam. Stand pretty far back and you should see the image floating in front of the plate on the opposite side of the reference beam (you are also on the opposite side of the reference beam). This is where you should be able to see the image projected onto a white card if placed exactly where the image seems to float in space.

I enforce the statements about the conjugate ref/illum beams but if you can see a nice hologram in the plateholder you should be able to project that image onto a white card, although it will be distored.

MichaelH · Post by **MichaelH** » Thu Apr 05, 2007 11:29 am

JohnFP wrote:I enforce the statements about the conjugate ref/illum beams but if you can see a nice hologram in the plateholder you should be able to project that image onto a white card, although it will be distored.

Not necessarily true.

If the original reference beam was widely diverging, when you flip the hologram and use the same light source, the image will be enlarged. It could be that the image is just too large for the surface he's trying to project onto.

ErichRose · Post by **ErichRose** » Thu Apr 05, 2007 12:46 pm

As mentioned a "gently diverging" reference beam is always best. The lense or spatial filter should be as far back as possible. By throwing a reference beam the length of a 4x8 foot sand table so that it just covered a 4x5 plate, and with a small object, I was able to make a transmission H1 that could be used to make a copy that had minimal/acceptable distortion. Actually many artists have used that distortion to great effect.

Another thing I found was that it can take a little time to "learn to see" the pseudoscopic image. Because it's all inside out and floating in front of the plate it can be hard for the brain to recognize. The eyes want to focus on the most obvious surface and that's the plate and the brain has to learn how to see things all backwards, It's almost like those magic 3D prints. You need to relax your eyes, and even your mind, to really see it. Eventually you get to a point where your brain starts correcting the image and it no longer seems inside out. And that is really weird!

BobH · Post by **BobH** » Thu Apr 05, 2007 2:15 pm

As mentioned a "gently diverging" reference beam is always best.

I can't agree with that! The best reference beam is always that with a divergence matching the reconstruction beam's divergence. Yes, one can make a hologram with unmatched beams, and the resulting distortion might be acceptable to the artist, but suggesting unmatched beams are "always best" is just wrong.

If an artist wants a distorted image, they can create them in many ways and unmatched beams is certainly one. Let's not, however, confuse artistic controls or effects with proper technique.

Keef333 · Post by **Keef333** » Thu Apr 05, 2007 4:50 pm

Ok. So I am trying really hard to keep up. I understand most of what you have all said, but some concepts still confuse me.

Last night I had to take a step back and spend some time reading through the HoloWiki.

Please understand, although I have been successful in creating around 25 holograms thus far, I really have no idea what I am doing as far as the therory involed. I point my little diode (without the colminating lens) at the object (from whatever angle illuminates it best) and position my plate without any plateholders.

Heck, I dont own a single optic and I do my holograms on a bathroom counter in the middle of the night...I dont even have a safelight!