Transmission Rainbow DCG holograms

Dinesh · Post by **Dinesh** » Thu May 20, 2004 1:50 pm

Won't the light from the object focus along an axis perpendicular to the lens axis. True, it won't focus along the lens axis since there's no power there. I think you'll get a focused object perpendicular to the lens axis and smeared along the lens axis.

Kaveh Bazargan · Post by **Kaveh Bazargan** » Thu May 20, 2004 2:19 pm

Whatever it is it won't be an image comfortable to view. I know there have been setups like this for making one-step rainbows, but I think the image will be highly astigmatic, so hard to look at.

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The holographer's home page:
http://www.holographer.org/
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Colin Kaminski · Post by **Colin Kaminski** » Thu May 20, 2004 9:57 pm

"I always reply that I have no idea why one would want to herd cats."

Now you know what I do late at night...

JohnFP · Post by **JohnFP** » Fri May 21, 2004 6:38 am

Come on work with me here guys. This is a Single beam, not a one step focused image rainbow transmission hologram. And being a transmission, I believe the ratio of reference beam to object beam can be such that the object beam can be 10 times less bright at the plate then the reference beam. Right? Also, it could be possible to use slightly concave mirrors in place of the two flat mirrors on the cylindrical lens assembly to get additional light to the object. No I have not worked out the actual radii of the cylindrical lenses yet, just want to see if the theory works first.
What do you think now?

JohnFP · Post by **JohnFP** » Fri May 21, 2004 7:10 am

Althought the drawing does not show it, if the plate is 4" x 5" I would make the cylindrical lens assembly 8" x 10" at a minimum.

Kaveh Bazargan · Post by **Kaveh Bazargan** » Fri May 21, 2004 8:22 am

John, the basic problem is that the image you see the other side will be a complete mess. God knows what it will look like. Have you thought about this?

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The holographer's home page:
http://www.holographer.org/
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JohnFP · Post by **JohnFP** » Fri May 21, 2004 9:20 am

Ahhhh,hahahaha....That is funny. Yes, of course but who know I may be in outer space or something.

Here is what I believed would happen. Take a point on the cube. The light waves that bounce off of it, and you have to think in terms of waves not particles is diverging. As that light from that point hits the first cylindrical lens, lets say on the far side of the lens, that lens focuses that diverging wave back to a point in space, where the plate is, and at the same time the large cylindrical lens focuses the point to the center of the plate. So obviously the object and the image are at the focal point of the first cylindrical lens and the second cylindrical lens.

Then take that same point and pass is though the near side of the first cylindrical lens. The lens focuses the point back to a point in space, where the plate is, and at the same time the large cylindrical lens focuses the point back to the same spot the the first example above mentioned.

Is this so messy there is not way of showing it? Am I that far off. Do I need to get rid of the first cylindrical lens and replace it with just a slit and then get rid of the second cylindrical lens and replace it with a large regular lens?

Kaveh Bazargan · Post by **Kaveh Bazargan** » Fri May 21, 2004 10:08 am

OK. First of all let me make sure the reference beam is coming from above, and not from the side, as the diagram seems to show. You want your dispersion to be up and down, i.e. in a direction perpendicular to the slit used in the imaging.

In principle, you are right about the imaging. One lens is doing the vertical imaging, and one the horizontal. So you would think that this is the equivalent of a conventional lens. But actually, you will see a lot of aberrations, and not a good image at all. This is also why I am not a fan of these double cylindrical lenses for collimators, i.e. the liquid filled ones. So I can't see any advantage of your setup, as opposed to using a conventional lens after the slit.

I stand to be corrected. I remember seeing cylindrical lenses in many systems, but I always avoided them for the inherent aberrations when it comes to imaging.

By the way, why not split the beams conventionally, and illuminate the object that way? You would have more control.

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The holographer's home page:
http://www.holographer.org/
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JohnFP · Post by **JohnFP** » Fri May 21, 2004 10:20 am

Why do cylindrical lenses pose more aberrations then conventional lenses?

Well, I was looking for a novel and unique way to make a single beam, imaged planed, transmission rainbow hologram.

Actually, if someone had an optic grinder the entire cylindrical assembly could be made out of a single lens. Just grind two flats that are tilted to face the focus point of the lens (where the object would be) and glue two mirrors onto those flats. What a cool optic that would be, huh?

Thanks for the dialog as always!

Dinesh · Post by **Dinesh** » Fri May 21, 2004 10:48 am

"Why do cylindrical lenses pose more aberrations then conventional lenses?"
They're not inherently more prone to aberrations, they're just more sensitive to them. Aberrations occur when the lens is off-axis and/or twisted with respect to the object space. If just twisted and/or off-axis, you get coma, if twisted in both axes, ie rotated in both axes, then you get astigmatism as well. In addition, the lens formula is calculated on the basis of what are called 'paraxial' rays, ie the object has to appear small to the lens. Most cylindrical lenses are small so the object size relastive to the lens size is large enough to take it out of paraxial.
Cylindrical lenses are OK for just manipulating light, which is what you use them for in rainbows. If the slit caused by hitting the lens with a raw beam is slightly aberrated, it doesn't matter too much since this is just an object beam. Of course, if it's too aberrated the object beam is also badly aberrated and the object will distort. However when you try to image with them, you need to be extra careful aligning effectively a matchstick relative to an object a couple of inches away.There's also the effects like translation and longitudinal magnification so it ain't usually a good idea to try and image with a cylindrical lens of the type used for rainbows. .