Maybe this is the better place to further discuss the text.
I originally thought your new definition was fine, but your remark on how electronic holographic projection would work made me think again. Don't most holographic displays do this already? In addition to that, I also noted the article's section about non-optical holography.
I would now say that Holography is the encoding of a three-dimensional wavefront, for instance of [or "as"?] the light field of a three-dimensional image [or "view" or "object"?].
You seem to doubt the element of encoding is actually there if it would be generated by an electronically controlled array of lasers, but I suppose this would still entice the conversion of physical data into an electronic stream, at least for recorded or programmed variations. Even for a randomly generated light field there would be a specific code/pattern for every instance/form, which could be read from (registered by) the electronic equipment (which probably would involve some digital components, and, by definition, would be encoded).
I'm quite sure the result would be called a hologram, and not just because so many people are confused about the technology. Any reason not to call it a hologram, would most likely be that it isn't done with the traditional techniques that have always been called holography (which I gather have always involved encoding the interference of diffraction as a pattern into a physical structure). Such a reason would basically be similar to still defining holography as a type of 3D photography or a recording.
I think the part for instance, to produce optical elements such as lenses is rather unspecific; surely lenses and optical elements are still widely produced in very different ways, and it's hard for people without the specific knowledge to imagine how it's done. Especially the example of lenses is confusing to me; it makes me think about all the other optical diffraction stuff without interference or lasers that I probably know too little about. The checkout scanners seemed to be an understandable example, at least for me.
Maybe it's best to soon after the definition describe how the technology originated as the photographic recording of the interference pattern of a light field of a physical object and a reference beam of the light source (should diffraction be in here to?), but has also led to computer-generated holography and holographic optical scanning devices (and more?).Joortje1 (talk) 12:17, 16 August 2020 (UTC)[reply]
Epzcaw, Thanks for taking the time to improve this article - it needs it, and I've been trying to avoid getting dragged into it myself precisely because I appreciate it's a mammoth task:( Starting with the first paragraph, I'm afraid I think the CD-audio analogy is really unhelpful in practice. My stab at the opening is along the lines of:
"A hologram is a diffractive record of an interference pattern from which it is possible to reproduce the original light field, resulting in an image which retains the depth and parallax of the original scene. Holography is the science and practice of making holograms."
For me, the rest of the proposed Introduction is generally OK. I'd say a hologram consists of a very fine diffraction grating rather than speckle pattern, and remove "(probably not a live one!)" re. the mouse as both speculative and not scholarly in tone. Since there isn't a History section in the main article, I suggest adding a final paragraph to the introduction: "Holography was first described by Dennis Gabor in 1948, for which he was awarded the Nobel Prize for Physics in 1971".
Joortje1, I find your comments difficult to understand. For example in "You seem to doubt the element of encoding is actually there if it would be generated by an electronically controlled array of lasers," above, what does "it" refer to? The essence of holography is that the phase of the light is recorded, hence the need for a reference beam (even virtually inside a computer). Simplistically: No phase measurement, not holography! Louis Knee (talk) 17:21, 28 August 2020 (UTC)[reply]
Louis Knee, luckily Epzcaw seemed to understand most of what I tried to convey, at least in earlier conversations. My main problem with the curent version and most drafts is the idea that holography can only be recorded from an existing scene, while nowadays an imaginary scene can be programmed. It seems like new holographic display techniques are quite commonly presented with programmed imagery. Whatever your concept of "phase" means, I suppose that this "phase" and its reference beam can be made up to create a hologram. That's why I suggested to replace references to "recording" with "encoding". Similarly, "reconstructing" should probably be replaced with "decoding", since a programmed image previously only existed in the code and in someone's mind. Is there any reason that a programmed holographic image of an imaginary scene should not be called a hologram? If not, how would you record or measure its phase?Joortje1 (talk) 08:42, 21 September 2020 (UTC)[reply]
"Is there any reason that a programmed holographic image of an imaginary scene should not be called a hologram?" No - that's exactly what a computer-generated hologram is. "If not, how would you record or measure its phase?" Um, the phase is calculated by the software. The hologram can then recorded on to a glass plate or whatever, and is then replayed in identical fashion to a hologram of a physical scene. If the replay process is identical, then the word "reconstructed" applies to both forms. Can you provide a reference to this usage of "decoded", please? --Louis Knee (talk) —Preceding undated comment added 21:20, 24 September 2020 (UTC)[reply]
Epzcaw, I've had a quick look at your new version. I thought the second paragraph felt clunky, especially from repetition of "light fields". My attempt at an improved version is below, but still feels awkward - too many "patterns". It's quite hard to squish everything into a single concise paragraph! I'll try to find time for another go at the weekend. (Also some tweaks in para. 4). Not sure of the best way forward - I didn't want to edit your sandbox so I've added my version as a new section below, but that does make it hard to see the changes made. I've no idea why I thought there was no History section in the main article, but I'd leave that final line in the intro anyway: it means the History section can be moved elsewhere in the article if needed, and the Nobel prize reference bolsters (indirectly) the definition of holography. --[[User:Louis
I've deleted my version in the sandbox and moved yours there. My only issue is the first sentence.
Most people will not know what a "diffractive recording" is and I feel that the introduction should be understandable by all, or nearly all readers. How about
Holography is a technique by which a light field can be recorded and subsequently reconstructed in the absence of the original light field. The reconstructed image retains the depth and parallax of the original scene.
I'm afraid I haven't yet been able to find the time to look at this properly. Some comments to give an idea of how I'm approaching this:
hovering the mouse over a wikipedia internal link brings up the start of the article as a pop-up, so the opening sentence should be as complete a definition as possible
according to context "hologram" can refer either to the perceived image or to the artefact storing it. "Recording" shares this ambiguity. (NB we need to add a sentence on this later in the introduction!)
anything that looks like defining a hologram as a "3D image" or "projection" needs to be avoided, as it will be misinterpreted to include cheap stage effects (para. 6)
general readers will not be familiar with wave-fronts, BUT then I'm not sure "light-field" is a well defined term
Having said that, I'm not sure I'd know what a "diffractive recording" is and I came up with the phrase! Looks like something from '80s sci-fi :( You're probably on the right track in changing from "hologram" to "holography" as the core word. This is my slightly pruned version of your opening line:
Holography is a technique by which a light field is recorded and subsequently reconstructed in the absence of the original. The reconstructed image retains the depth and parallax of the original scene.
My quibble is that this is more about what holography can do, rather than how it does it. I'd still prefer a reference to interference or diffraction in there somewhere. Hope this helps. I'll still try to have another look at this in the next few days. Appreciate your efforts! --Louis Knee (talk) 09:39, 20 September 2020 (UTC)[reply]
I think I understand this fear of defining a hologram as a 3D image. Imho, the section Holography#Non-optical provides a much stronger argument to avoid such a definition. Hence my suggestion to use "the light field of a three-dimensional image/view/object" as an example of a holographically created wavefront. However, there is a reason why many encyclopedias and dictionaries refer to holograms as 3D images. I suppose even an acoustic hologram can be regarded as an image containing three-dimensional data (maybe "pattern" would be a better word instead of "image", but still). More importantly, the most common interpretation and use of the concept "holography" involves 3D imagery. That doesn't mean 3D imagery should be called holography and it's a pity people are confused by Pepper's ghost and other techniques that are often misidentified. Holography in fiction often seems to perfectly fit both the scientific and popular ideas, as for instance with The Doctor (Star Trek: Voyager). If you aks me, the current wikipedia definition is way too rigid, a bit too technical (with too little explanation or bluelinking of uncommon terminology) and just not entirely correct.Joortje1 (talk) 08:42, 21 September 2020 (UTC)[reply]
My objection is the same as to Epzcaw's above; that by concentrating on what holography does, you fail to define what it is. "...encodes a three-dimensional wavefront" applies just as much to a Shack-Hartmann device, which isn't holography. In addition, how does a "view" emit a "light field"? --Louis Knee (talk) 22:41, 23 September 2020 (UTC)[reply]
I'm sorry. English is not my native language, so I may sometimes fail to grasp certain nuances in my writings. In this case it wasn't as much about which word to choose, but which of the previous mentioned subjects that apparently could be encoded as a hologram (interference pattern or wavefront) would best define holography in a sentence like the one in my proposed definition above. I understand how this question wasn't very clear.Joortje1 (talk) 06:40, 24 September 2020 (UTC)[reply]
Epzcaw's proposed overhaul seems to have been motivated by my argument that the wikipedia definition excluded computer-generated holography. More recent changes in the actual article even made things worse: holography now apparently is a "real world recording". Most suggestions on this sandbox talk page still seem to subscribe to that idea. Why?Joortje1 (talk) 08:42, 21 September 2020 (UTC)[reply]
I'm not quite sure what you mean by a "real-world device". I believe that a hologram must be a real physical object in order to generate a real physical wavefront. Epzcaw (talk) 19:28, 21 September 2020 (UTC)[reply]
Epzcaw, I agree with this - the outcome of CGH must be a physical artefact in order to reconstruct the image (by diffraction), and digital holography likewise requires a physical device to capture the input pattern. (That doesn't mean we must include the term "physical" in any definition). --Louis Knee (talk) 21:46, 24 September 2020 (UTC)[reply]
The current version of the article (not your sandbox version) calls a hologram a "real world recording" (not my words, not "device"). Such a definition excludes computer-generated holography and in my opinion is worse than the previous "physical recording".
Have you considered that the intention of "real-world" may have been to distinguish our actual world from fiction (e.g Star Wars), rather than the physical from the virtual (computer-generated)? Writing correct, clear, unambiguous text is hard. You've both made a number of points and it will take me a while to chew through them. :( --Louis Knee (talk) 22:41, 23 September 2020 (UTC)[reply]
In the technical meaning of the encoded pattern, a hologram can arguably only be a physical object, although in many a holographic display it's probably an electronically generated pattern that is not as tangible as suggested by "a real physical object". In the very common meaning of the decoded image of a holographic pattern (which the term "hologram" very commonly refers to), a hologram must presumably have a wavefront that is as physical as a real-world holographic recording, but if it's a computer-generated hologram it may better be referred to as an "artificial physical wavefront". I'm sorry if these ideas confuse the definitions and concepts of holography that people were used to, but I thought you agreed when I proposed on the article's talk page that the wikipidia definition should also take cg holography into account.Joortje1 (talk) 05:15, 22 September 2020 (UTC)[reply]
"in many a holographic display it's probably an electronically generated pattern that is not as tangible..." No, the pattern is equally as tangible, to diffract the light to reconstruct the image. Or are you claiming that artefacts created by 3-d printing are somehow not real because they started off as digital files??? --Louis Knee (talk) 21:46, 24 September 2020 (UTC)[reply]
How is a pattern in electro-holography tangible and what does it have to do with 3d printing? Sure, even after the imagery has been designed and computed, there is still physical hardware involved to generate the pattern. What do you believe this hardware does? Print the pattern?Joortje1 (talk) 05:04, 25 September 2020 (UTC)[reply]
"How is a pattern in electro-holography tangible" - by the act of diffracting a physical, actual replay beam of light. "what does it have to do with 3d printing?" 3-d printing is for comparison, to make you think about your proposed words: 3-d printers also "are quite commonly presented with programmed imagery. .... That's why I suggested to replace references to "recording" with "encoding". Similarly, "reconstructing" should probably be replaced with "decoding", since a programmed image previously only existed in the code and in someone's mind." so presumably you would demand that 3-d printers "decode" CAD drawings into plastic? "What do you believe this hardware does? Print the pattern?" Yes - literally for display CGH work. When I saw Martin Richardson's arty-farty CGH stuff in Bracknell a (long) while back, it was definitely on plates on the gallery walls. That's why holographic printers were a thing, e.g. the Zebra Imaging one mentioned on page 8 here. Obviously if you want to modify the hologram at video rate then the technology will be different (page 12), but there still has to be a physical manifestation of the fringe pattern to diffract the light - and this is a specialised sub-topic, probably for holographic display, rather than something that needs thrashing out in the introductory paragraph of the general article. "I'm sorry if these ideas confuse the definitions and concepts of holography that people were used to..." But Wikipedia is not here to invent novel interpretations - "Wikipedia editors must base their contributions on reliable, published sources." Wikipedia:No_original_research --Louis Knee (talk) 23:12, 26 September 2020 (UTC)[reply]
I think we were almost there when you proposed: "Holography is the encoding of a light field as an interference pattern. It is commonly used as a 3D imaging technique, but the principle is also widely applied in different ways." Maybe it should include non-optical holography and be "Holography is the encoding of a wave interference pattern"? And then something like: "It is commonly used as a 3D imaging technique that diffracts light bouncing off an encoded pattern on a 2D surface to produce the light field of an object or scene, but the principle is also widely applied in different ways."Joortje1 (talk) 06:05, 22 September 2020 (UTC)[reply]
I can envisage a device which generates a light field which has been generated by a computer - perhaps an array of very small lasers each of which is driven with the appropriate amplitude and phase so that when viewed by a camera or an eye, an image is formed of the scene (maybe these are there already???). But this would not be a holographic image - no interference is involved - diffraction would, of course, be involved in generating the wavefront as is the case with all wavefronts.
In my view, holography involves a physical interference pattern which diffracts light to create the required wavefront , field , whatever you want to call it.
I think the three of us agree that the current introduction is not satisfactory, but are not in agreement about how this should be done. Please feel free to alter my sandbox. Epzcaw (talk) 18:03, 22 September 2020 (UTC)[reply]
Please check the referenced source about electro-holography. It begins with a decent definition of holography and also seems to confirm what I stated about artifical holography. Much like a photograph doesn't have to be printed, a hologram doesn't have to be an artefact (I can't see how there would be one in electro-holography). Although I think photo-realistic cg imagery shouldn't be called photography (if it is known how it was made; it could easily be misidentified without that information), I don't see any problem in calling a cg hologram a genuine hologram. Maybe that's the influence of sci-fi on the perception of what holography is, but there's also the fact that there are many applications of holography that are quite different from holographic photography (e.g. data storage). I believe laser holophotography was already quite different from the original electron holography. Presumably, holography was perceived more as a technical method from the start (while photography was much more about the resulting image), which clearly has since long been imagined to be capable of producing more than recordings (and applied as such).Joortje1 (talk) 07:05, 25 September 2020 (UTC)[reply]
It may be that we need to rename the page Optical Holography which is what most people mean by holography. There can be the usual disambiguation pages for different sorts of holography. Epzcaw (talk) 09:57, 25 September 2020 (UTC)[reply]
Whilst I see your logic, I don't think it helps much: the electron/neutron/optical holography pages will still need to share a common explanation of holography itself, which would be your Physics of Holography page. Except that the latter will then still need an introductory section for general readers, and that's exactly what we're struggling with here:( --Louis Knee (talk) 23:23, 26 September 2020 (UTC)[reply]
I feel the simple definition that Louis Knee and I more or less agreed on
Holography is a technique by which a light field is recorded and subsequently reconstructed in the absence of the original. The reconstructed image retains the depth and parallax of the original scene. With something additional about using interference and diffraction
is clearer and is something that non-scientists will be able to grasp. Encoding a light field is not something that more people will understand, and is likely to put them off at the beginning. People will be coming to this page to try and get an understanding of how holography works, and it needs to kept as simple as possible. I am proposing to do a new article entitled Physics of Holography (started below), and will suggest removing the current section of that name in the main article to deal with the comment that the article is too technical.
Many years ago when I used to do outreach and lab open days, I used to start by describing a hologram as
a magic window that freezes the light passing through it
but while one can get away with that when there's one set up on the bench behind and you know you can follow up at a tailored level, it isn't suitable for an encyclopedia where users will copy/paste it into their homework. :(
My own attempt at a definition is still in progress; at the moment it looks like:
Holography is the capture and subsequent re-creation of a wave-front, by recording the pattern formed by the interference between it and a second, reproducible wave-front. Illuminating the interference pattern with this second, "reference", wave reconstructs a perfect duplicate of the original.
Note that it makes no assumptions about the nature of the waves, and it precludes neither "forming" the pattern numerically within a computer, nor "recording" it by writing to some dynamic array rather than a static plate.
I like that the first part. I would be happy to go along with that. I suggest replacing the first sentence in the current article with that, and splitting the paragraph.
Holography is the capture and subsequent re-creation of a wave-front, by recording the pattern formed by the interference between it and a second, reproducible wave-front. The recording is referred to as a hologram.
However, I don't think that other forms of holography reconstruct the object beam by illuminating the interference pattern with a second, "reference", wave. I know very little or nothing about them, but I'm fairly sure they involve some sort of digital processing rather than illuminating with a second beam. (talk) 10:36, 27 September 2020 (UTC)[reply]
Don't have time ATM but: second, "reference", wave is exactly what they do. Look at Gabor's original idea: record pattern of electron waves on plate, illuminate with light waves, electron-to-light wavelength shift gives magnification. "digital processing" just means applying the reference wave numerically inside the computer, the physics is still the same. "Holography" just emerges from how interference and diffraction work and these are basic wave phenomena, so true Holography can be done in any sort of wave (as long as there's a way to record the fringe pattern). Not sure if this is clear as it's very brief. Thanks. --Louis Knee (talk) 12:34, 27 September 2020 (UTC)[reply]
Yes, I agree that Gabor's original idea was to record the pattern, and use optical illumination. But that's not what happens in more recent electron holography:
Reconstruction of off-axis holograms is done numerically and it consists of two mathematical transformations.[5] First, a Fourier transform of the hologram is performed.
I bet though that those numerical transformations turn out to be equivalent to those used for digital optical holography, and involve knowing some phase distribution across the image (which is is what the reference/reconstruction beam is for). Unfortunately the digital holography page has little useful info and digital holographic microscopy also omits the reconstruction algorithms, so there's yet another hole to plug... --Louis Knee (talk) 00:06, 28 September 2020 (UTC)[reply]
In acoustic holography, the signals from individual microphones are digitally processed using various beam-forming technique, i.e. summing them with added phase differences.
Don't know much about it either, but again "added phase differences" sounds to me like a synthetic reference beam in disguise. That's my point - the underlying interference and diffraction effects are the same irrespective of medium or of real-vs-simulation. By starting from that core I think it will be easier to explain the physics and let the different applications branch off, into their own articles if needed. (And, anything that doesn't share that physics isn't really holography!) --Louis Knee (talk) 00:06, 28 September 2020 (UTC)[reply]
I don't know what happens with atom or neutron holography.
Maybe
Holography is the capture and subsequent re-creation of a wave-front, by recording the pattern formed by the interference between it and a second, reproducible wave-front. Illuminating the interference pattern with this second, "reference", wave, physically or digitally, reconstructs a perfect duplicate of the original.
I think we are making progress, just slowly. The thing is that working on this eats time, and I only have a couple of hours to spare per week :( Worse, unlike journal articles, it's not as easy to control the scope of the work: changes to Holography end up depending on rewrites of related articles (computer-generated holography, digital holography, digital holographic microscopy, etc., etc.) and it cascades out.
When I suggested a new article on Optical Holography, I should also have said that there should be an article Holography which would describe the general principle of holography (as in your definition) and summarise the different types, including Optical holography), with links to the detailed articles about each method. Most of this is in the existing article.
A new article Optical Holography would then include most of what is in the current Holography article with the general bits stripped away.
My Physics of Optical Holography article will in the main only apply to optical holography.I know very little about how the other forms work. Optical holograms record only the intensity of the interference patterns directly. Acoustic holograms also register the phase directly, but I don't know about any of the others.
Out of curiosity, I retrieved the original Holography article written in 2002.
Holography (from the Greek, holos whole + graphe writing) is the science of producing holograms, an advanced form of photography in which allows the image to be recorded in three dimensions.
The difference between holography and photography is best understood by considering what a photograph actually is: it is a point-to-point recording of the intensity of light rays that make up the image. Each point on the photograph records just one thing, the intensity (the amplitude squared of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.
A separate article for "Optical holography" seems like a good idea. But I'm not sure what the boundaries of that concept are, or whether that's the best term for the photographic recording version of holography. Noting your comment about other forms of holography, the definition in the main "holography" article should be even more generalized. Maybe something can be found in writings about the hypothesis in physics of the holographic principle (or "holographic universe). Wikipedia now calls it: "the description of a volume of space (can be thought of) as encoded on a lower-dimensional boundary to the region(—such as a light-like boundary like a gravitational horizon)." (my parentheis for wording that seems superfluous to a general context). In that concept it's not even a technique, science or art, but probably a natural physical phenomenon.Joortje1 (talk) 15:26, 27 September 2020 (UTC)[reply]
I also liked this definition for the optical variation (presumably including real-time electro-holography): "Holography is a diffraction-based coherent imaging technique in which a complex three-dimensional object can be reproduced from a flat, two-dimensional screen with a complex transparency representing amplitude and phase values" (although I'm not sure about the word "reproduced" in the context of CGH and I miss the technical knowledge to judge te part "complex transparency representing amplitude and phase values". )It's from the article on electro-holography that I previously intended to refer to, but whose url I apparently mixed up with another one. I haven't read the total article yet, but it seems useful.Joortje1 (talk) 15:26, 27 September 2020 (UTC)[reply]
I think the first sentence should be simple, and comprehensible to someone not familiar with the field. Some examples:
A camera is an optical instrument used to record images. At their most basic, cameras are sealed boxes (the camera body) with a small hole (the aperture) that allow light in to capture an image on a light-sensitive surface (usually photographic film or a digital sensor). Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera, the size of the aperture can be widened or narrowed to let more or less light into the camera, and a shutter mechanism determines the amount of time the photo-sensitive surface is exposed to the light.
Photography
Photography is the art, application and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many fields of science, manufacturing (e.g., photolithography), and business, as well as its more direct uses for art, film and video production, recreational purposes, hobby, and mass communication.
Topiary
Topiary is the horticultural practice of training perennial plants by clipping the foliage and twigs of trees, shrubs and subshrubs to develop and maintain clearly defined shapes,[1] whether geometric or fanciful. The term also refers to plants which have been shaped in this way. As an art form it is a type of living sculpture. The word derives from the Latin word for an ornamental landscapegardener, topiarius, a creator of topia or "places", a Greek word that Romans also applied to fictive indoor landscapes executed in fresco.
Higgs Boson
The Higgs boson is an elementary particle in the Standard Model of particle physics, produced by the quantum excitation of the Higgs field,[8][9] one of the fields in particle physics theory.[9] It is named after physicist Peter Higgs, who in 1964, along with five other scientists, proposed the Higgs mechanism to explain why particles have mass. This mechanism implies the existence of the Higgs boson. The Higgs boson was initially discovered as a new particle in 2012 by the ATLAS and CMS collaborations based on collisions in the LHC at CERN, and the new particle was subsequently confirmed to match the expected properties of a Higgs boson over the following years.
I have had access to the correspondence between Max Born and Emil Wolf which relates the story of the trials and tribulations they experienced in writing this book, which has become known as "the bible of optics" - it was started in 1951 ad finally published at the end of 1959.
Gabor was instrumental in recruiting Emil Wolf, who was a lowly research assistant when the project started in 1951; Max Born was a a very eminent physicist who played a vital role in developing Quantum Mechanics, though had not yet won the Nobel Prize.
Gabor was supposed to write some of the book, but didn't have time to do so. Gabor had sent a very long section on holography to them, but Born felt this was much too long so Wolf completely re-wrote it. It took up 8 pages in the book (Section 8.10). The section in the 1999 edition is unchanged apart from a statement about Gabor's Nobel, prize, a mention of Leith and Upatniek's work and a photo from their paper. It was the first mention of holography in a textbook. Wolf later reported that Gabor was very happy with the account given in the book. When
holography became well known, he sent Wolf a reprint of one of his papers with an inscription saying “I consider you my Chief Prophet”
I thought it worth quoting the introduction (Section 8.10 B&W)
In an attempt to improve the resolving power of the electron microscope, Gabor proposed a two-step method of optical imagery. In the first step, an object is illuminated with a coherent electron wave or a coherent light wave. The object is assumed to be such that a considerable part of the wave penetrates undisturbed through it. A diffraction pattern, called a hologram, which is formed by the interference of the secondary wave arising from the presence of the object with the strong background wave, is recorded on a plate. If the plate, suitably processed, is replaced in the original position and is illuminated by the background wave alone, the wave that is transmitted by the plate contains information about the original object. In order to 'reconstruct' the object from this 'substitute' wave, it is only necessary to send it through a suitable image forming system, and an image will appear in the plane conjugate to the plane in which the object was situated.
The object from Gabor's first hologram, made in 1949 in pre-laser days, was a transparency with the names of eminent scientists. The re-construction is clearly legible, though with a lot of noise. It would be good to get permission to put this in Wikipedia.
"...transparency with the names of eminent scientists..." If I remember right, Huygens, Fresnel and Young as the originators of light's wave theory and the Huygens–Fresnel principle etc.
Maths isn't my strong point and B&W scares the heck out of me! My own copy is in a crate in storage at the moment. Note that this excerpt refers to the Gabor or in-line geometry, so 'the strong background wave' is the reference and the object "beam" is paradoxically demoted to 'secondary wave'. Reminds me that "wave optics" is the phrase I was searching for yesterday. I think this excerpt supports my view that the fundamental basis of holography is in the physics of reproducing a wavefront from a stored fringe pattern, with features like 3-d imaging being subsequent consequences of that reproduction process. (Note how the last quoted sentence distinguishes the reconstructed object from the reconstructed wavefront.
There is no "correct" meaning of a word - a word means what people mean by it, and of course, that varies with time, and also with different people and with different groups of people.
So I'm not sure if was can reach agreement about this.
I've found some information about acoustical holography.
Back in 1972, the acoustic wave information was indeed converted to an optical hologram which then acted just like an optically generated hologram to enable an image o fthe original sound source to be made. An introduction to acoustical holography 1972, BP Hildebrand and BB Brenden
The Free dictionary cites the 2012 McGraw Hill dictionary of scientific and technical terms:
Typically, temporal acoustic data are acquired by measurement of the acoustic pressure with a single microphone or hydrophone, which scans an imaginary two-dimensional surface. In some cases, an array of microphones is used and the pressure is measured instantaneously by the array.The measured data are processed in a computer to reconstruct the pressure at the surface of the object as well as the vibration of the surface. The measured time data are Fourier-transformed into the frequency domain, creating a set of holograms, one for each frequency bin in the transform. In the inversion process, each hologram is broken up into a set of waves or modes whose propagation characteristics are known from basic principles. Each wave or mode is then back-propagated to the source surface by multiplication by the known inverse propagator, and the field is then recomposed by addition of all these waves or modes.
A paper from 2014 Inter-noise conference by someone from Brüel & Kjaer Sound & Vibration Measurement:
Near-field Acoustical Holography (NAH) is based on performing 2D spatial Discrete Fourier Transforms (DFT), and therefore the method requires a regular mesh of measurement positions.
Optical holography is effectively a Fourier transform process, and modern electronic and data processing by-passes the interference stage if the frequency of the waves is low enough to enable both phase and amplitude to be measured directly.
B&W is hard work, but I have always found that if you stick at it, it is sufficiently self-contained to enable you to follow the reasoning.
Apologies - I didn't get the expected time at weekend. I did manage to rummage in my box of digital holography papers hoping to transcribe a quote to compare with Epzcaw's electron holography one above, but got hijacked by another project. But, if you have SPIE access you may want to have a look at the following:
T.M. Kreis, M. Adams and W.P.O. Jüptner: "Methods of Digital Holography: A Comparison" in Optical Inspection and Micromeasurements II Proc. SPIE 3098 pp. 224-233 (1997)
J.H. Milgram: "Computational Holographic Image Reconstruction" in Practical Holography XVI and Holographic Materials VIII Proc. of SPIE 4659 pp.12-29 (2002)
In particular, Kreis et al. look at the maths of holography across the different diffraction regimes. I regret not having had the time to check if they use "encoding"/"decoding" - will do next visit.
and also
G. Pedrini, S. Schedin and H.J. Tiziani: "Aberration Compensation in Digital Holographic Reconstruction of Microscopic Objects" Journal of Modern Optics48(6) pp.1035-1041 (2001) DOI:10.1080/09500340010018955
Only Milgram even uses the word "encoding" at all, and again in a specialist sense relating to sampling and information theory issues. Though all the references are more careful than I've been, in that strictly one reconstructs an object or wavefront, rather than "reconstructing a hologram". (Unless you dropped the plate on the floor! :) ) --Louis Knee (talk) 22:23, 17 October 2020 (UTC)[reply]
Regarding
..."modern electronic and data processing by-passes the interference stage if the frequency of the waves is low enough to enable both phase and amplitude to be measured directly"
I'd argue that if you skip the diffraction/interference processes (physical or virtual) that generate the fringe pattern, then that's great but it's not holography any more: see Coherent diffraction imaging or Ptychography, both of which also depend on diffraction patterns but don't make any such claim; or Lippman photography which depends on interference but isn't holography --Louis Knee (talk) 10:21, 6 October 2020 (UTC)[reply]
Louis Knee I'm happy to go along with this. Holography is a process where the object wave and the reference wave are superimposed, the resulting light field is converted to a physical artefact which is then illuminated with the reference wave (or something similar) recreating the object wave which can be used to form an image of the original scene. I'm not suggesting this as the wording for the introduction but a summary of what we are trying to express Epzcaw (talk) 16:45, 7 October 2020 (UTC)[reply]
I've had time to reflect on it, and I can see how B&W's description above (albeit being from an introductory paragraph) could still be confusing. "A diffraction pattern, called a hologram, which is formed by the interference of the secondary wave arising from the presence of the object with the strong background wave, is recorded on a plate." The-fringe-pattern-recorded-on-a-plate - the hologram - allows re-creation of the object beam through diffracting the re-applied reference beam, so in that sense it might be a "diffraction pattern", but note that we've jumped forward to the replay step. The-fringe-pattern-to-be-recorded-on-a-plate isn't the diffraction-pattern-from-the-object itself as in von Laue and X-ray diffraction, but rather the diffraction-pattern-from-the-object-modulated-by-interference-with-the-reference-beam. And this just opens the can of worms of what is the difference between diffraction and interference anyway. Feynman went for interference being interference with a small number of beams, and diffraction being interference of large numbers (channelling Huygens' wavelets). --Louis Knee (talk) 00:12, 7 October 2020 (UTC)[reply]
I think though this illustrates that holography is one of a number of natural effects that emerge from how waves intrinsically behave, and one shouldn't get too hung up on applying particular mathematical formalisms/procedures to them. --Louis Knee (talk) 23:08, 9 October 2020 (UTC)[reply]
Maybe commenting on several of the words that kept popping up in proposed definitions helps to see where some problematic connotations or possible misunderstandings come from and which of the words could actually be most useful. I'll try to write down some conclusions from previous discussions beside my personal opinions. I'll put my signature behind each part so it's easier to comment, but feel free to correct any mistakes in my descriptions if that helps to keep this section more concise.
This seems to be a key word that nobody objected to. I liked how B&W/Gabor defined a hologram as a "diffraction pattern". That seems to be the most basic description and to me it seems focused on the purpose, which seems a bit more to the point than using the word in "interference pattern" (which seems to focus on the encoding process).Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
Seems to be a standard term to describe the way the holographic image gets "projected" when the pattern is exposed to a reference/reconstruction beam. Are we sure that all of the holographic techniques are really based on diffraction? Is Fourier transform (as found in the holographic brain theory) an alternative or is it just another description of what happens in holographic diffraction?Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
A relatively standard term for the data/pattern that is recorded (or created) from the object/scene and to be (re)produced as the result of the diffraction of a reference beam that is bounced off the holographic pattern.Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
Holography is quite commonly confused with several 3D imaging techniques, so it may help to avoid the term in the definition. However, an important characteristic of holography is the conversion of 3D data to a more or less flat surface that will eventually be used to produce the intended 3D representation.Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
I have three separate concerns about references to "3-d" and "3-d imaging":
a) the risk of misunderstanding or misrepresentation that all 3-d images are holograms
b) perceived 3-d images are a side-effect of holography. The holographic plate doesn't "know" that it's supposed to be re-creating one; it simply carries out "wavefront reconstruction" (Gabor) so that the replayed wavefront is an exact replica of the original. If that original scene was of a suitable object then there might be a 3-d image, but otherwise - e.g holographic data storage, HOEs - there won't.
c) The term "3D image" itself is very poorly defined, and some people will take it to include simple stereograms (or almost anything, after 20 years of "3D Graphics Cards" in their computers :( ). --Louis Knee (talk) 00:25, 11 October 2020 (UTC)[reply]
Louis Knee You've put into words a thought I've been trying to formulate since we started this discussion. AS you say, the hologram does not create an image, 3D or otherwise. A camera suitably focussed on the reconstructed wavefront does not form a 3D image (as in the two photos of the mouse - both pictures are 2d images). 3D "imaging" is all in the mind.
I'm afraid it's worse than that: admittedly I spent significant time working on a niche application of a specialist field (holography) but I've spent a lot of time staring at the projected real images and can confirm they are definitely real, visible, 3-d images and projected from the hologram. So, some holograms can create images directly!
This is what I mean by the writing being hard: Wikipedia articles have to end up both accessible to schoolkids doing homework and scientifically/scholarly correct for the experts, all without being incorrect or inconsistent.
I suppose another view might be that referring to "images" is meaningless since in some cases the term is inapplicable, while in others it can apply to multiple items in the process (the subject, the result, possibly even the fringe pattern itself), so it doesn't actually explain anything. --Louis Knee (talk) 22:44, 1 November 2020 (UTC)[reply]
recording
A hologram can be photographically recorded and an CGH pattern can be recorded onto some surface. However, the words "record" and "recording" are commonly associated with truthful registration of an existing phenomenon rather than the computer-generated design, a type of registration that lasts awhile and can be checked later. (Real-time) electro-holography doesn't seem to fit in this picture.Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
encoding
A term all three of us and some existing definitions (including the current Wikipedia version) have used to describe the process of turning the data/image of a wavefront into a holographic pattern. Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
decoding
Since the holographic pattern is encoded from the data/image of a wavefront, I supposed the process of turning the pattern back into a wavefront image could be called "decoding". Louis Knee objected to it for lack of sources using the term in this context. However, an article from Nature seems to use it in more or less the same way: https://www.nature.com/articles/s41598-017-18147-7.Joortje1 (talk) 17:59, 4 October 2020 (UTC)[reply]
"encoding" and "decoding" are a pair - they go together. To me, "encoding" involves a transformation into a significantly different format: we had the example of sound - pressure waves - being encoded as microscopic pits on a CD. Note that replay doesn't happen from the wind rushing past the pits, but from transforming the pit pattern to an optical signal which has to be digitised and processed (to filter out the error detection bits) before one can even think of putting it through a DAC... By contrast, a gramophone record stores the sound by using the pressure waves to push the recording stylus, and plays it back by having the deformed grooves push the stylus around - you can even hear the music directly from the stylus. No-one would ever seriously talk about "decoding" an LP.
Basic holography involves storing the light and dark of the fringe pattern as dark and light areas in a photographic emulsion. I just don't see "encoding" or "decoding" as helpful words here, especially given that there is an established terminology: record, store, transform, reconstruct, replay, etc.
No, the cited Scientific Reports article is obviously using the terms in a different way, similar to encrypt/decrypt as explined in the first few paragraphs: their holograms cannot be replayed simply by illuminating with a reference beam - see lower part of fig. 2 - and instead you need the extra magic crystal they call an SPP to decrypt - or "decode" - the image. --Louis Knee (talk) 23:39, 10 October 2020 (UTC)[reply]
Nowadays, neither acoustic holography nor electron holography (holography with electron waves) involve diffraction, and acoustic holography does not involve interference. Both use digital processing of the recorded signals to calculate details of the source that generated the wavefront that was recorded or encoded. Acoustic holography does not involve any encoding - the phase and amplitude of the signal over an array of points is recorded and then digitally processed. I believe that in their early days, both techniques did generate interference patterns between the object beam and a reference beam - this was then printed as an optical device which would then diffract a light beam to produce an optical wavefront which could be used to create an image of the source of the original wavefront. Developments in digital processing makes the optical part redundant. If they are still called holography, then I would suggest that they are holography. Epzcaw (talk) 09:16, 5 October 2020 (UTC)[reply]
Epzcaw, I sort of agree with the last bit, but my perspective is that "Developments in digital processing can make the optical part redundant - if the digital processing is a proper simulation of the prior optical process, then it is still holography." And, of course, it's still "holography" that makes the fringe pattern to be processed in the first place.
Modern acoustic holography does not involve any interference - the acoustic signal from the "scene" is measured at an array of points, and these signals are digitally processed to form an 'image' of the object which produced the wave. Because you can measure both the phase and the amplitude of the wave, you are missing out the recording of the interference pattern, and the re-illumination of the interference pattern, which are required in optical holography to re-create the object wave. It is effectively a digital lens; it does the same thing to the object wavefront as a lens would do to the original object wavefront. So it is doing what the lens used in holography does; it "images" the original scene by focusing the recorded wavefront.
As far as I can tell (but need to study further and not sure I want to!!), electron wave holography does include fringes (presumably because the frequency of the electron waves is too high to be measured directly, the the processing that is done there is again an imaging one, rather than one that recreates the object wavefront.
Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave
It is a real (physical) process. Not sure I agree fully with the definition but I don't think you can treat it as a digital processins thing.
I'm not sure how relevant all of this is. I'm confident I know what is generally meant (at least in Physics) by optical holography. We can expand holography to all sort of things (like the holographic principle in quantum gravity as Joortje1 suggests) but that, I believe, totally muddies the waters, rather than enlightening the Wikipedia reader.
I believe the holographic principle may more clearly describe the general concept of holography than optical holography. If the theories are correct, it's a very basic principle in physics as well as in human perception and should be considered as more defining as the optical holographic recording technique. I can see how the term differs from "the principle of holography", but it's rather similar. The idea of holography seems to be more about encoding/recording/compressing 3D data onto a 2D surface and decoding/displaying/decompressing it back to its original form (or in case of CGH: generate its intended form), rather than about diffraction and interference of lightfields. That's part of why I'm curious to know how much of those principles exists in all those non-optical and artifical examples of holography.Joortje1 (talk) 22:08, 6 October 2020 (UTC)[reply]
"The idea of holography seems to be more about encoding/recording/compressing 3D data onto a 2D surface..." No - the fringe pattern recorded as a hologram is itself a 3-d entity: see e.g the interaction region in Interference fringes in overlapping plane wavesa plane wave (right) taken from Epzcaw's Draft:Physics_of_Optical_Holography. The reason people assume holograms are thin flat things is because that's what shape photographic plates are!
O.k., so: "3D entity" rather than "3D data"? And it's not necessarily recorded on 2D (flat) surfaces. How can we best describe the conversion to the compressed "bad" picture? Gabor (about the original 1947 idea, in his 1971 Nobel Lecture): "Why not take a bad electron picture, but one that contains the whole information, and correct it by optical means?" He initially called it "wavefront reconstruction".[2]Joortje1 (talk) 18:59, 7 October 2020 (UTC)[reply]
Re. holographic brain, holographic universe, and scratch holography: my personal view is that people are confusing analogy and metaphor with being the same. Just because holography provides an interesting way to think about something doesn't mean that thing is holography --Louis Knee (talk) 00:37, 7 October 2020 (UTC)[reply]
I believe I understand your view and agree that the terms for "holographic principle" and several others are probably coined as an analogy. But, let's compare this with what happened to the term "stroboscopy" / "the stroboscope". It was originally coined by Simon Stampfer for his "stroboscopische Scheiben", basically the first proper animation device (partly inspired by Michael Faraday and just slightly earlier invented by Joseph Plateau, better known by corrupted versions of a product name used for cheap imitations that both Plateau and Stampfer distanced themselves from: the phénakisticope). The term stroboscopy has since much later been used for electronic flash devices to measure mechanical rotation speeds, for the electronic photographic flash, for a video technique to view the movements of vocal chords, for a flickering light in discotheques, etc. Any flickering light can be said to be "strobing" and it's apparently even a makeup highlighting technique.[3] What I don't like is that the principle of animation and film isn't called stroboscopic, but is replaced by incorrect or incomplete theories, such as persistence of vision, flicker fusion, beta movement. The good thing is: most uses of the term more or less refer to the principle that Stampfer described in his patent: quick regular interruptions of the light beams that are bounced off images that are very quickly transferred in front of the eyes so that a motion effect is seen. So, I wouldn't say that all those examples use the term incorrectly: most use flashes (interrupted light) to view a motion effect. I think the article stroboscopic effect defines its subject pretty well (although animation is not included). I don't think it would be good if the term was only used for rotating animation discs, although the word basically means "whirling viewer" and was specifically coined for the discs. Stampfer thought up different applications (including long canvas strips on rollers) and never came up with a name for the principle (neither did Plateau, nor Faraday), but in some incredible roundabout way it basically got labelled with the term that Stampfer coined for its first application. So, back to holography: most uses of the term seem to describe the (lossless) image compression of a wavefront (full 3D data/energy/entity/image) to a lower-dimensional format and back again. That seems to fit pretty well with how Gabor described his original idea (see above), the basic principle of holography, which he happened to apply to electron pictures with optical means, but it has later been applied in non-optical technologies and according to quantum scientists it may correspond to an universal principle. Optical holography is still holography, but I wouldn't say the other examples are not.Joortje1 (talk) 20:50, 7 October 2020 (UTC)[reply]
Passing note: electro-holography is the same as computer-generated holography, but seems to be used when referring to producing time-varying holograms, i.e. holographic TV. The first mention of this I've found was in an MIT paper from 1995 - it doesn't seem to have moved much further on since then in practical realisation. Not to be confused with electron holography.
Electro-holography may usually be CGH, but I don't think it's identical. Electro-holography indeed seems to refer to moving holographic images and is the same as "holovideo". CGH can of course be totally still. MIT reportedly first created electro-holography in 1989. They have worked on electronic capture of object-based holography / real-world input.[4][5]Joortje1 (talk) 22:08, 6 October 2020 (UTC)[reply]
Electro-holography is the same as computer-generated holography in the same way as movies are the same as still photos - movies are made up of a series of still photos taken in succession. The technology is the same for both whether it's done with photographic emulsion or with electronic digital camera. Indeed, our digital cameras act as both still and movie cameras.
Also, both electro-holography and computer-generated holography use a physical device to create the interference pattern which is illuminated by the reference beam to create the wavefront- nowadays they use SLMs (Spatial Light Modulators) Epzcaw (talk) 19:02, 7 October 2020 (UTC)[reply]
To me the "electro-" part implies an electronic device. I'm pretty sure recording of holograms on photographic emulsion (movie film) was already done in the 1960s, long before MIT came up with "electro-holography". In any case, I would prefer to avoid the phrase simply on the rather more mundane grounds of preventing confusion between "electro-holography" and "electron holography"! --Louis Knee (talk) 15:47, 8 October 2020 (UTC)[reply]
Apologies, got waylaid and hijacked by other things. If my laptop battery lasts I hope now to at least grab version 1 as a text file, so I can have a go at it off-line rather than be constrained to brief windows of wi-fi opportunity. --Louis Knee (talk) 22:45, 1 November 2020 (UTC)[reply]
Apologies from me too - I have been away enjoying the delights of Covid infection - picked up in hospital by my husband and passed on to me. We both got away relatively lightly.
I wasn't sure if I wanted to return to this. I would be happy for you to write the introduction based on our discussion here. I will offer minor amendments, Joorte also, and then go for it.
Agree with your comments about writing for Wikipedia - will never satisfy everyone. And the more popular the subject, the more critics you will have. Epzcaw (talk) 09:23, 2 November 2020 (UTC)[reply]
Hope you have recovered OK! Things (in general) really didn't go as I expected - not made it to WP at all since Nov. for reasons I won't share, and very unlikely to get non-trivial time for this for a few months yet :( Louis Knee (talk) 00:39, 8 March 2021 (UTC)[reply]
Fourier transformation is a mathematical technique that can be used to analyse time-varying functions, widely used in a wide range of signal processing applications. For instance, the way a wave is focused by a lens can be mathematically modelled using Fourier transformation. Epzcaw (talk) 09:16, 5 October 2020 (UTC)[reply]
This was a draft article. It was intended to address a comment on the Holography page which said it was too technical. I copied all the "too technical" material from the Holography into the draft article, added more material and made some amendments with a view to removing the echnical material from the main article when the draft article was approved for publication.
Unfortunately, it has been labelled as "Copyright Violation". Most of the article was deleted, though subsequently re-isntated when I showed that there was no such violation. The "Copyright Violation" however remains in place.
The article in question, https://drrajivdesaimd.com/2017/09/17/hologram/, has in fact copied lots of text and two photos (one of of which I contributed to Wikimedia) without attribution so is in copyright violation.
There is currently a 5 month queue to review draft articles, so I see no point in having an articel there with a big "Copyrigth Violation" at the top, since it is likely to stay at the bottom of the queue.
^Coombs, Duncan; Blackburne-Maze, Peter; Cracknell, Martyn; Bentley, Roger (2001), "9", The Complete Book of Pruning (illustrated ed.), Sterling Publishing Company, p. 99, ISBN978-1-84188-143-0
