I’m curious what shape the wavefront of a reconstructed real image takes.
Say, for example, when a transmission hologram has been recorded with a collimated reference beam, and is then played back with a collimated beam from the opposite direction – in the style of H1 image transfer – is the reconstructed object wavefront also a planar wave?
- or –
In another example – How about a transmission hologram recorded with a spherical reference beam… When played back for projecting the real image with a collimated beam, is the object wavefront spherical? How about with a spherical reconstruction beam? Does the projected real image take a “really-spherical” shape?
If the answers to these musings are too complex and wordy to peck out on a keyboard, can anyone point me in the direction of a very simple explanation of wavefront shape as relates to holograms and their recording geometry?
Many thanks.
Wavefront shape?
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JohnFP
Wavefront shape?
Hey Justin, just think of a transmission hologram as a lens and apply lens equations for real and virtual image. That's the simplest way to get an understanding.
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Justin W
Wavefront shape?
Howdy John!
Thanks for stopping by and chattin' some.
I'd like to explain a little bit where I'm going with this... I suspect that the answer I seek is based on lens physics - and that's why I'm wondering about this...
Some of (most of) my early holograms that I made were shot with a diverging reference beam (spherical wavefront if I have my terminology correct), and I noticed some cool things about them - the most notable - in my opinion - being that the hologram seemed to be not just an image but a lens as well. As a lens, they would replay differently with different lenses reconstructing them, sometimes shrinking an image, and (the part I'm all a-twitter about) other times magnify the image. I have ideas about doing this on purpose so that I may hologram something tiny then magnify it.
I of course, learned the hard way that doing an H1-H2 transfer with diverging beams leads to an H2 with kooky characteristics (odd proportions and only in focus on the image plane, etc). I would love to be able to do a transfer of a diffractively-magnified image. I bet that would be crazy cool.
I should get down to brass tacks, I guess: I'm trying to picture a transmission HOE that will collect this magnified image from the H1 and return its wavefront to a proper shape for making an H2.
In other words, I'm trying to figure whether I'm interested in collecting a spherical wavefront and collimating it, or uh.... Collecting a spherical wavefront and focusing it? Or something else entirely?
Thanks for stopping by and chattin' some.
I'd like to explain a little bit where I'm going with this... I suspect that the answer I seek is based on lens physics - and that's why I'm wondering about this...
Some of (most of) my early holograms that I made were shot with a diverging reference beam (spherical wavefront if I have my terminology correct), and I noticed some cool things about them - the most notable - in my opinion - being that the hologram seemed to be not just an image but a lens as well. As a lens, they would replay differently with different lenses reconstructing them, sometimes shrinking an image, and (the part I'm all a-twitter about) other times magnify the image. I have ideas about doing this on purpose so that I may hologram something tiny then magnify it.
I of course, learned the hard way that doing an H1-H2 transfer with diverging beams leads to an H2 with kooky characteristics (odd proportions and only in focus on the image plane, etc). I would love to be able to do a transfer of a diffractively-magnified image. I bet that would be crazy cool.
I should get down to brass tacks, I guess: I'm trying to picture a transmission HOE that will collect this magnified image from the H1 and return its wavefront to a proper shape for making an H2.
In other words, I'm trying to figure whether I'm interested in collecting a spherical wavefront and collimating it, or uh.... Collecting a spherical wavefront and focusing it? Or something else entirely?
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BobH
Wavefront shape?
If you're going for an accurate image, you want to match the divergences of all the beams all the way through the process. Holograms not lit in the real world by the sun are lit by a diverging beam. So the reference for the H2 should be converging. If you don't have the spherical mirror to do that, you're going to be stuck with aberrations no matter what else you do. Second best is a collimated beam from a parabolic mirror.
Same is all true for the H1. You're going to be stuck living with the aberrations unless you use a parabolic mirror for a collimated reference beam.
If you want to use HOEs to replace the parabolic and/or spherical mirrors, you have to borrow big mirrors to make them. Or have someone who has them make the HOEs for you. Cheapest way to go is to watch Astromart and/or Ebay for old telescope mirrors from Coulter Optical. They are cheap because of a bad reputation that won't effect their performance as collimating mirrors for holography, and you'll find them at 17.5" f/4.5 (perfect for a 12x16" plate on an 8' long table, or 13". Might need to get it recoated, also pretty cheap considering their value in the holography set up.
You can also just give up on the accuracy thing and make imagery that doesn't require it.
Same is all true for the H1. You're going to be stuck living with the aberrations unless you use a parabolic mirror for a collimated reference beam.
If you want to use HOEs to replace the parabolic and/or spherical mirrors, you have to borrow big mirrors to make them. Or have someone who has them make the HOEs for you. Cheapest way to go is to watch Astromart and/or Ebay for old telescope mirrors from Coulter Optical. They are cheap because of a bad reputation that won't effect their performance as collimating mirrors for holography, and you'll find them at 17.5" f/4.5 (perfect for a 12x16" plate on an 8' long table, or 13". Might need to get it recoated, also pretty cheap considering their value in the holography set up.
You can also just give up on the accuracy thing and make imagery that doesn't require it.
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Justin W
Wavefront shape?
heheheh...
Sound advice, Bob.
I actually have a couple of 8" telescope mirrors. After hearing what you just said, I do wish they were spherical and not parabolic, but sounds like they'll be OK for my pictoral holography...
In fact, I'm hoping they'll be OK for trying my hand at a few HOE's too. I've heard of making holographic collimators (basically a transmission hologram of a collimating mirror? with a diverging beam for a reference?), and can picture laser beam shaping with such a diffractive element, and what I'm trying to imagine is one that will collect this magnified real image (spherical?) and shape it into the same kind of real image wavefront that a transmission hologram made/replayed with a collimated beam will project (planar?).
So it's not so much that I'm trying to find a way around collimating my reference beams. I will be in a position to be exposing and reconstructing with plane waves. I'm mostly just trying to engineer an HOE that will allow me to use one of these really cool magnified images without the aberrations.
This is a hugely hypothetical exercise at this point - I'm still months away from having a working table, and waxing philosophical a lot. And who knows, this may be an endeavor better suited to an experienced holographer, but I'm willing to give it a go.
Sound advice, Bob.
I actually have a couple of 8" telescope mirrors. After hearing what you just said, I do wish they were spherical and not parabolic, but sounds like they'll be OK for my pictoral holography...
In fact, I'm hoping they'll be OK for trying my hand at a few HOE's too. I've heard of making holographic collimators (basically a transmission hologram of a collimating mirror? with a diverging beam for a reference?), and can picture laser beam shaping with such a diffractive element, and what I'm trying to imagine is one that will collect this magnified real image (spherical?) and shape it into the same kind of real image wavefront that a transmission hologram made/replayed with a collimated beam will project (planar?).
So it's not so much that I'm trying to find a way around collimating my reference beams. I will be in a position to be exposing and reconstructing with plane waves. I'm mostly just trying to engineer an HOE that will allow me to use one of these really cool magnified images without the aberrations.
This is a hugely hypothetical exercise at this point - I'm still months away from having a working table, and waxing philosophical a lot. And who knows, this may be an endeavor better suited to an experienced holographer, but I'm willing to give it a go.
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Justin W
Wavefront shape?
EDIT FOR CLARIFICATION: I'd like to rephrase that to read: "I'm mostly just trying to engineer an HOE that will go in between an H1 and an H2 that shall allow me to use one of these really cool magnified images without the aberrations."Justin W wrote:I'm mostly just trying to engineer an HOE that will allow me to use one of these really cool magnified images without the aberrations.
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JohnFP
Wavefront shape?
Take for example: You are looking at a penny with a magnifying glass. The penny and magnifying glass are mounted with the penny in the center of the magnifying glass at some comfortable position to allow a nice enlarged view. When you position your head such that the penny and magnifying glass are all centered the penny is magnified and doen't look too distorted. But as you move your head to the side the penny begins to stretch out of shape.EDIT FOR CLARIFICATION: I'd like to rephrase that to read: "I'm mostly just trying to engineer an HOE that will go in between an H1 and an H2 that shall allow me to use one of these really cool magnified images without the aberrations."
Now what your are asking for is to put another lens between your eyes and the magnifying glass to allow the penny to look enlarged without too much distortion. I think they call this a microscope. The only thing is you got to give up something. And its seems to be depth of field in this case.
A simply way would be to just do some Holographic Microscopy and use an 0bjective to enlarge your image. This is a real ant. No depth of field so when you move your head back and forth, there is hardly any indication of paralax and thus depth, but it was a cool project anyway.
http://www.holograms3d.com/PhotoGal/General/AntHolo.JPG
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Justin W
Wavefront shape?
Howdy John!
Sorry I'm just now getting back to my topic here today - I've been out of the house installing some recessed lighting for family. It among other things came out durn tidy and I'm back home now and a sorts of thankful for your explanation of essential lens functions. There's a lot going on with focusing images without distorting shapes.
Yah. Cool. Back to the drawing board heheh
Sorry I'm just now getting back to my topic here today - I've been out of the house installing some recessed lighting for family. It among other things came out durn tidy and I'm back home now and a sorts of thankful for your explanation of essential lens functions. There's a lot going on with focusing images without distorting shapes.
Yah. Cool. Back to the drawing board heheh