Lunar Surface hologram

[BobH]

That's one heck of a way to say "no". I think. I apparently need an interpreter to translate from physicist to english.

[Dinesh]

That's one heck of a way to say "no". I think. I apparently need an interpreter to translate from physicist to english.

Bob, it's very difficult to define this without mathematics and without diagrams. If I could show you on a whiteboard and a few hand-waving explanations, I think you'd understand it better. If you are still coming on May 16th, I can explain what I mean much better in person.

However, this is speculation. I don't claim to have invented some new theory, just speculating on something that may not have come to anyone's notice because the situation I'm considering is two beams meeting head on. No one shoots real holograms like that. This all started out from analysing Denisyuk's original paper. There were some puzzling aspects in it, and I'm speculating on an idea that may resolve the puzzling aspects. I don't claim to have re-invented holography or invented a new form of holography! IF you exposed a plate with two head-on beams of unequal strength, then AND ONLY THEN, there may be an effect due to the nature of standing waves. But, it's impossible to express the statement in any other way but handwaving and drawing on a whiteboard, or by use of mathematics. After all, we are talking about a physical effect that can only be understood mathematically. I know there are several metaphors that are commonly used, such as skipping ropes, statements like "light is a wave" and "light spreads out through a small hole", but these are metaphors for interference. Interference is essentially a mathematical concept. Light is a phenomenon consisting of electric field vectors. But, as I've said so often, it's not necessary to know the theory of holography to shoot a hologram, anymore than it's necessary to understand the biochemistry of proteins to fry an egg or thermodynamics to drive a car. You don't need the theory. However, when you're trying to speculate on a theoretical model, you have no option but to use theory. I can use metaphors like clocks, but these are crude metaphors that cannot completely explain the model.

Part of the problem does lie in nomenclature. Both interference and diffraction are the same thing. In one case, you deal with a large number of sources and in the other, you deal with just a few sources. However, the equations are exactly the same. In another situation, Goodman ("Introduction to Fourier Optics") defines diffraction as any propagation of light. He considers a beam of light just crossing a room as diffraction.

By the way, since this is in the "off topic" section, I read in the paper this morning that US students performed 20th in a survey of school students in math amongst the technological countries of the world, yet everyone - teachers, parents, students - are always saying that it's very important to do well in "math and science" (whatever that means). Yet I can't help notice that whenever a scientific topic is explained mathematically, people get disturbed and even angry! If "math and science" are seen to provoke anger amongst those who are seen as technologists, surely this defeats the whole point! If students want to place higher in "math and science", perhaps the students should take an example from people actually working in an apparently technical subject and watch them use math in science, rather than reject the use of "math" in "science"

[Martin]

Thank you for elaborating on the distinction between standing wave and interference, Dinesh.
Thanks also, Joe, for posting the Stroke/Labeyrie paper.

Coming from the display holography area rather, I've to admit I still experience difficulties separating Lippmann from Denisyuk. Looks like a lack of analytical understanding on my part. By the way, Dinesh, I'd greatly appreciate if you happened to make a video of your "handwaveing explanations" at the PORCELAIN CAT GROUP meeting.
Anyway, regarding Lippmann vs. Denisyuk I think one thing not yet mentioned is the bandwidth of the light source(s) involved. Obviously - unless we use some "exotic" non-laser sources (filtered mercury vapor/LEDs) we're dealing with extremely narrow bandwidth light. In contrast to that Lippmann photography may cover light over the whole spectrum. So I wonder how this plays out in respect to the distinction between standing wave and interference.

[BobH]

And the source for Lippman photography is an extended source, not a point source.

[Martin]

And the source for Lippman photography is an extended source, not a point source.

Right - unless it's been what used to be called "photographing the spectrum" - see e.g. figure 4 at http://rsnr.royalsocietypublishing.org/ ... 4/319.full (http://rsnr.royalsocietypublishing.org/ ... l.pdf+html).

[Dinesh]

I've to admit I still experience difficulties separating Lippmann from Denisyuk.

I think a lot of people have this difficulty. It's really a very subtle issue and arises with the difficulty in the popular usage of words like "interference" and "diffraction". Everything you see about the Lippman technique states that interference planes are formed at half lambda intervals; similarly, everything you hear about Denisyuks states that you have planes formed at half lambda intervals. They seem to be the same. But Lippman never claimed that the "reconstruction" method was diffraction.The the essential difference of phase capture is never mentioned. One reason may be that the word "phase" is not well understood by the general public. It's rarely mentioned in popular books on holography. However, because Lippman does not capture phase, Lippman photographs do not capture 3D, as Sergio has pointed out. The difference lies in the mathematics of diffraction, ie the Fresnel-Kirchoff integral.

I know that mathematics and physics is not very popular for the denizens of this forum, and, as I've stated, it's very difficult to "fine tune" some explanations without mathematics and physics. My usual position is: why bother trying to understand it at this level. If you must try and understand it at this level, why the antagonism towards mathematics and physics? (Perhaps it's because people who use physics and mathematics are supposed to be ""smart" and Dinesh ain't "smart". Heck, I couldn't agree with you more!)

However, I am trying to come up with a popular explanation, but it ain't easy! The essence of the explanation lies, I think, in the fact that Lippman describes his technique from an operational view, but the Denisyuk technique is always described from a heuristic point of view. That's one barrier I'm trying to overcome in coming with a popular explanation.

[BobH]

If a point source was used to make a Lippman photograph, would the phase then be recorded? Wouldn't that be a hologram of the reflective surface in contact with the emulsion?

[Dinesh]

If a point source was used to make a Lippman photograph, would the phase then be recorded? Wouldn't that be a hologram of the reflective surface in contact with the emulsion?

The phase of what? The phase of a point source at any distance r is simply kr. If you used the Lippman technique on a point source, you would not get iso-phase contours that go as kr. Also, would it be a Lippman photograph of a point source. What exactly is a "Lippman photograph"? An amplitude recording of a scene with colour added, as Lippman himself described it? Or a phase and amplitude recording as described by Denisyuk? If the former, would you see simply a point, or a dot, on the Lippman surface? As I said earlier, the Lippman technique is operational, it allows the recording of the amplitude(s) of an object(s) by a sequence of operations that rely on the physical mechanism of standing waves. The bandwidth of the illuminant is necessary because the resulting image must elicit a photopic response. If the bandwidth is too narrow, there would be no photopic response.

However, in a Denisyuk (or reflection) hologram, the recording has to be done with a coherent, narrow band source, because you're trying to capture phase information. The retrieval of this phase information can be carried out using broadband sources, but it cannot be recorded with a broadband source (within limits, depending on the depth of the object and other factors). The association of the Denisyuk/reflection holoram with a Lippman photograph is heuristic.

[BobH]

This is why we need to get together for another PCG gathering. This kind of discussion really needs some brewed lube and more than a few napkins.

[Dinesh]

You bring the napkins. The beer is covered. Here's the latest batch with label