sharpness decreases with depth

On: Thu, May 02, 02 04:22:08 AM

Tom Naiser wrote:

I am recording reflection holograms with a single beam setup as described in Franks "shoebox holography". My 35 mW Mitsubishi diode is located about 40 cm in front of the PFG- 03 plate - I am not using any optics for focussing the beam. Do you have an idea why the sharpness of the objects that I record decreases with the depth (i.e. the distance to the holoplate) ? Objects very close to the plate (a few cm) are imaged fairly sharp, but already in a distance of about 10 cm the objects are quite a bit blurred. Thanks in advance! Tom


Tom B. - Fri, May 03, 02 03:27:17 AM

I have noticed the same thing with several emulsions and believe you will also find that: 1) When the hologram is lit by a spread laser beam, deep objects are sharp. They have been recorded clearly, they just aren't reconstructing clearly under white light. 2) With white light, the image is not pure red, but more broadband, sort of orangey red. Sharpness improves considerably with a distant bright source, such as sunlight. I believe these effect are due to unevenly spaced (chirped) fringes in the depth of the emulsion, which could be caused by uneven development and/or uneven shrinkage in the emulsion. Saxby describes chirped fringes as leading to broadband reconstruction at the expense of a progressive loss of sharpness out of the plane of the hologram. I have tried a few things to reduce this such as presoak before exposure and divided development (separate soaks in part A, then part B), but have not yet decided conclusively if they help or not. Drying technique is also probably important. Another thing to try is, for a 45 degree overhead reference beam, tilting the plate 15 degrees so it leans into the object. According to Ward et al, this is the optimum geometry for reducing dispersion blur, but it will not help with chirped fringes. I find it also tends to put the best viewing angle where you would expect it (straight into the plate, rather than a bit below). Not always suitable for some subjects, though.

Thilo K. - Fri, May 03, 02 08:42:18 AM

BTW: Do you get the same maximum depth in a reflection h like in a transmission h with the exception, that you canīt see it in the first one due to blurring ?

Tom B. - Sun, May 05, 02 04:12:10 AM

Good question. Theory says that the depth is limited by the coherence length of the laser, and in a single-beam reflection setup the object beam has to travel twice as far, so half the depth of a SB transmission h. With laser illumination of reflection holograms, I have seen lots of forgotten background objects (wads of Fun-Tak, magnets, basement spiders) out to at least 1 foot (300 cm), but I have not tried comparing with transmission h or tried any distance tests. Another thought on SB reflection blur was that maybe because the object beam is usually so weak compared to the reference that the fringes are poorly defined and so they do a poor job of filtering the reconstruction white light. Must try some split-beam experiments.

Thilo K. - Mon, May 06, 02 07:18:09 AM

Good answer. But there is a question about something: "in a single-beam reflection setup the object beam has to travel twice as far, so half the depth of a SB transmission h" Why do you think so ? I have also noticed that some objects being too far away to be within the coherence length had been recorded anyway. But I have only seen it on transmission hs, which were made using a two beam setup. I think that theory has only been confirmed because no part of a far-away dark object has been seen so far. You can see metallized items due to their enormous brightness when theyīre lit. If you used a combination of metallized spheres being far away from the film, you should be able to see them in the final hologram. As a matter of course, this object is not very interesting esthetically but if it works, it confirms my theory. Maybe you give it a try.

Tom B. - Tue, May 07, 02 01:05:58 AM

Re the single beam refection setup, the object beam has to go through or past the plate, hit the object at distance D, then bounce back to the plate for a total path difference from the reference beam of 2D. In a SB transmission setup, the path difference for the same object distance will typically be less than D, depending on the lighting geometry. If I could get distant objects in reflection holograms to show up unblurred when reconstructed with white light, I would be much more interested in trying some deep image experiments :)

Colin Kaminski - Wed, May 08, 02 01:07:42 AM

Isn't there physical limitation to the depth of a reflection hologram independent of the coherence length of the laser? I have a coherence length of over 7 feet and I still have only inches of usable reflection depth.

Jonathan - Wed, May 08, 02 03:54:03 AM

One of the limitations to visible depth often is simply the inverse square relationship of intensity and distance. I've noticed some surprising things show up if there is sufficient illumination on distant parts of the subject. Another point to consider. Have you ever noticed how sharpness suffers when you move a reflection hologram closer to the reconstruction source, such as a halogen bulb? Points deeper in the image are the first to go, and eventually the entire image is blurry. I think part of the explanation lies in the theory concerning the orientation of the interference fringes. Which basically says that any fringe in the emulsion lies on a plane that bisects the angle made by the incident reference and object beams at that point. This holds for every point on the emulsion plane, and for every point on the object. Reconstruction of a sharp image, throughout the whole depth, therefore depends on matching the original path of the reference beam exactly, and over the whole area of the hologram. This is virtually impossible to do, although if you make a transmission hologram, and then view the virtual image with the same beam you will see very good sharpness through the entire depth. For reflection holograms, usually the replay beam is not the exact match to the conjugate of the reference beam. Even if the reference beam is collimated, the replay beam usually isn't. This means blurring. So for points up close, the difference in the match will cause the point of focus to spread out and be a little blurred, depending on how mismatched the angle of the replay beam is. If the reference beam was collimated or close to it, a halogen bulb far enough back will cause the points up close to appear sharp. But for a point farther away on the object, even with the same difference in match, the spreading out effect will be broader, hence even more blurred. The farther out you go, the broader is the area of focus for that "point" on the original object, simply due to the trigonometry of the wave paths. The problem of a mismatched replay beam would seem to be magnified for points further from the emulsion plane, and interestingly, in either direction as in projection images.

Sergio Oliveira - Sun, May 12, 02 01:31:17 PM

I think the reflection holograms are very correlated to the Kogelnik theory, with the limited thickness of holographic emulsions you will never get a sharp image depth even without chirped or damped fringes. The principal factor here is the limited wavelength selectivity attained with this emulsion, then the sharpness is function of this wavelength selectivity, with will affect first the deep image resolution, remember that all the white spectrum needs to be filtered by the emulsion first... Thick reflection holograms would deliver two things, high efficiency and image depth, but the fringes must be stable, withouth chirping, a very difficult task.

Martin Mueller - Mon, May 13, 02 01:57:16 AM

...actually three things (with very thick reflection holograms), the third one will be greatly reduced image brightness.

Tom B. - Sun, Apr 14, 02 04:03:26 AM

I'm puzzled - why is it that I seem to recall having seen reflection holograms with significant depth (a few feet) and no objectionable blurring. They also didn't seem particularly narrow band. I'm wondering if it could be size-related (these were relatively large), or some optical trickery in the transfer process?

- Sun, Apr 14, 02 11:12:44 AM

Tom: Are you sure that you were dealing with a reflection hologram ? Maybe itīs been a rainbow transfer in which the aperture of the slit was quite large when recorded.

Thilo K. - Sun, Apr 14, 02 11:12:50 AM

Tom: Are you sure that you were dealing with a reflection hologram ? Maybe itīs been a rainbow transfer in which the aperture of the slit was quite large when recorded.

Tom B. - Sun, Apr 14, 02 11:00:44 PM

Definitely reflection holograms as displayed - these were big ones, at least 16 x 20". Just hypothesized that maybe the usable depth scales with the size. If I wasn't so lazy I'd think it through for myself :) These were almost certainly restricted aperture transfers from transmission masters, maybe with some other optics involved, and not stereograms.

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