| Making "H2" holograms without a master?
June 17 2003 at 2:39 PM | R. A. (no login)
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| I have another question for all of you. On the following site they briefly talk about using a homemade lens to make H2 holograms without a master.
http://www.techsoft.no/holography/lenses.htm
How does this work? Is it just like putting a large converging lens in front of the object and just making a hologram where the real image is projected? If so, isn't only one plane of the object in focus?
Any info you have to help this confused holographer would be appreciated.
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| | Author | Reply | Bob
(no login)
| | Re: Making "H2" holograms without a master? | June 17 2003, 3:30 PM |
You have it right. The film is placed anywhere within the real image formed by a lens (or "master" hologram), with the aperture of the lens (or hologram) and its distance from the film determining the viewing zone of the final recording.
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| Dinesh
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| | Re: Making "H2" holograms without a master? | June 17 2003, 5:33 PM |
"if so, isn't only one plane of the object in focus?"
No. I've heard many descriptions of photography as 'collapsing the third dimension' and that lenses 'only focus one plane'. Not true. So long as the entire object is beyond the focal length, all points are focused in a one-to-one manner. However, the longitudinal magnification ("stretch") of an image is not the same as the transverse magnification (If you're interested, the longitudinal magnification is the square of the transverse magnification,ie m(l)=m(t)^2 - so long as the object is not too extended). In practice, for small objects, the entire object is focused in much the same manner as an H1 'focuses' an image and `you can place your H2 in any plane through it. As a matter of interest, you can do this with a curved mirror too. The problem is with lighting the object so that the reflected light goes through the lens and is focused without any of the lighting itself going through the lens. You need a bloody large lens for any reasonable sized object.
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| Kaveh
(no login)
| | Re: Making "H2" holograms without a master? | June 18 2003, 12:52 AM |
If your holographic film were used 'photographically', i.e. you had no reference beam, and just recorded the image as in a camera, then correct, only one 'slice' through the object would be focused. But you are doing more. You are recording the intensity and the directions of every ray (i.e. all optical information), using a reference beam. When you reconstruct the image, it is as if you were looking at an image floating in space, with the lens behind it. Your eye is free to 'focus' on any part of the image.
If your reconstruction beam is a laser source, then you won't get any blurring, only distortions of the image points a long way from the image plane designed for the lens.
Now if you use white light (which is one of the advantages of making a 'focused-image' or 'H2' hologram, you will get some blurring for image points out of the plane of the hologram, but this is due to entirely different reasons. One is due to chromatic dispersion, i.e. each wavelength diffracting through a different angle, and the other is due to the finite size of the source.
Let me know if anything here is not clear. I am not sure if I have explained it right.
Oh, if you are familiar with conventional H1-H2 recording, then the same argument applies. You can take a 'photo' of your original object, by putting a photographic film in the projected image of H1, without any reference beam for H2. As the effective aperture of the imaging system will probably be very big, you will have a very shallow depth of focus. You can adjust this by reducing the aperture, i.e. illuminating only a small area of the H1. By illuminating different areas, you can 'photograph' the object from different angles.
Hey, never thought of this. Could be an interesting application!! Have fun.
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| Greg G
(no login)
| | Focused images | June 18 2003, 7:02 AM |
Has anyone tried a hologram using the dual mirror "Mirage" toy?? It's a nice example of a focussed real image.
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| Dinesh
(no login)
| | small area H1 illumination | June 18 2003, 7:56 AM |
In the late eighties I used to get the rough image plane by illuminating the H1 with a raw beam. Since the focus disappears pretty rapidly, as you say, it's easy to get the (rough) plane, and thus the (rough) H1-H2 distance, and the best angle for the H1 by hitting the H1 with a raw beam, bringing the H2 into focus and twisting the H1 without putting in the spatial filter. It made for a quick, easy basic set-up without pinholing. I can't do that today since my Argon would make holes in the H1 emulsion. I have some early Argon H1's with nice black dots on them!
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