Talk:Holography/Archive 1

Analog holography
"An alternate method to record holograms is to use a digital device like a CCD camera instead of a conventional photographic film. This approach is often called digital holography. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen."

Has an analog form of electronic holography ever been tried?203.26.37.35


 * The above statment is untrue. A CCD does not have the spatial frequency to record a hologram. The requirements for a transmission hologram are at least 700 lines per mm. —Preceding unsigned comment added by 71.141.30.56 (talk • contribs) 04:13, 26 February 2006


 * Actually, CCDs can and routinely are used to record holograms. It is true that high-resolution single-exposure off-axis holography can require a lot of spatial bandwidth to avoid aliasing.  The spatial bandwidth requirements can be reduced by bandlimiting the object (by spatial filtering the scene, for example), at the price of reduced spatial resolution.  Alternatively, one can record an on-axis hologram and use phase-shifting (instead of the off-axis technique) to suppress the conjugate image, essentially trading spatial bandwidth for temporal bandwidth. -- eyliu 18:39, 14 March 2006 (UTC)

Some recommended corrections
"These versions of the rainbow transmission holograms are formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating which provides the light from 'behind' to reconstruct their imagery. Another kind of common hologram (a Denisyuk hologram)is the true 'white-light reflection hologram' which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer."

Essentially all of the reflective holograms on credit cards, etc., are "embossed" holograms; they're not foil-backed. SPM —Preceding unsigned comment added by 67.185.41.24 (talk • contribs) 17:35, 4 December 2005


 * I disagree with the premise that Denisyuk holograms are "true" white-light holograms and rainbow holograms are not. It is true that Denisyuk holograms have the potential to reproduce color more accurately than film holograms, but this is highly dependent on the experimental methods.  As the thickness of the recording medium tends towards zero, Denisyuk holograms become equivalent to embossed rainbow holograms. -- eyliu 18:39, 14 March 2006 (UTC)

The following statement is not true
"The coherence length of the beam determines the maximum depth the image can have"

It is actually fairly easy to make a hologram containing an image that is far deeper than the coherence length of the laser. The coherence length of the laser determines the maximum path length difference between the reference beam and the object beam in making a hologram, but different parts of a deep scene can be illuminated with different portions of a beam, with each portion delayed by an appropriate amount to ensure that the object beam from that part of the scene has traveled the same distance as the reference beam. SPM —Preceding unsigned comment added by 67.185.41.24 (talk • contribs) 17:43, 4 December 2005


 * While it is true that you can make deeper holograms than the coherence length of the laser the idea of "Multiple Coherence Volumes" is a bit too technical for this discussion. IMHO —Preceding unsigned comment added by 71.141.30.56 (talk • contribs) 04:13, 26 February 2006


 * Probably the simplest correction is to change "coherence length" to "coherence". The counterexample trades temporal coherence (coherence length) for spatial coherence (the counterexample requires that different parts of the beam be correlated, which is the definition of spatial coherence).  Generalizing the coherence requirement closes that loophole. -- eyliu 18:39, 14 March 2006 (UTC)

Zebra holograms
This article is yet to mention Zebra Holograms, which can display "full colour" (RGB and all combination of them) and full parrelex. They are made up of what are called hogels (holographic elements, equivelet to pixels in three dimensions). Each hogel conatins a 1280 line RGB image with 1024 pixels per line. It is my understanding that each hogel is a two dimensinal holographic reprisentation of the entire image. Is my information correct? 203.26.37.35 04:51, 21 January 2006 (UTC)

Are rainbows holographic?
This is perhaps a naive question, or perhaps it has no answer. But I wonder, do rainbows qualify as free-standing holographs? (I suppose the 'hologram' or analog to it would be the rain drop or whatever is bending the light.) oneismany 16:59, 26 February 2006 (UTC)


 * No, the drops in rainbows act like little prismas, please see rainbow. --danh 13:37, 27 February 2006 (UTC)


 * Yes, rainbows are "holographic" to the extent that the entire rainbow image is contained within each spot on the film plane (i.e. within each raindrop.) On the other hand, one could argue that rainbows are not reconstructed via diffraction effects, therefore they are not holograms.  But rainbow holograms do not employ diffraction either.  In rainbow holograms the spacing of the interference fringes can only control the position of the colored bands, and has no effect on the 3D structure of the reconstructed image. --Wjbeaty 02:50, 29 March 2006 (UTC)


 * So is there any isomorphism between prismatic effects and holographic effects? E.g., a description of orthoscopic imagery composed of light that is equally true of holographs and prism effects?
 * On a related note, can a holographic prism refract light? Could holographic raindrops produce a rainbow?  If they could, would the rainbow be a real rainbow or a holographic rainbow?  Would a holographic rainbow be stationary like a holograph, or would it vary according to the position of the viewer like a real rainbow?  oneismany 18:42, 2 March 2006 (UTC)


 * Oneismany, I'm not sure what you mean by a "holographic prism", but if you mean the image of a prism in a hologram (a holograph is a handwritten document), then, no. The image of a prism in a hologram is no more physical than the image of a prism in a mirror, and it doesn't affect the light or matter around it.
 * But if you mean, "can a hologram be made to act like a prism?", then, yes. A hologram is basically a complex diffraction grating, and a diffraction grating will bend different colors of light at different angles, like a prism. -- The Photon 04:43, 3 March 2006 (UTC)


 * I would add one caveat to the comparison of refractive optics (such as a prism) and diffractive optics (such as a grating). The monochromatic behavior may appear similar, but the polychromatic behavior is generally different.  For example, refraction angle generally decreases with increasing wavelength, but diffraction angle increases with increasing wavelength. -- eyliu 18:39, 14 March 2006 (UTC)

Example
This article would benefit from at least one example image. --SparqMan 05:11, 4 March 2006 (UTC)

Real-time holography
I recommend that the section on real-time holography be deleted. The principles of real-time holography are no different than conventional two-step (recording and reconstruction) holography and the concept emerges naturally from the use of a recording mechanism (the photorefractive effect, for example) which does not require photographic developing. -- eyliu 18:39, 14 March 2006 (UTC)


 * May I suggest that this section be retained for the following reasons:
 * Real-time holography (RTH) is a fundamentally different physical process than "conventional" holography, in that film is replaced by a nonlinear optical material, a spatial light modulator, etc.
 * The notion of RTH provides one with a very elegant way to compare/connect nonlinear optical interactions with that of conventional holography, except, that, in the the case of RTH, ALL the beams can interact essentially simultaneously.
 * The notion of ALL beams interacting at the same time has no obvious connection with conventional holography; i.e., why would one (and, for that matter, how could one) have all beams interacting in a given material at the same time.
 * Granted, some of the "formality" of conventional holography has parallels to RTH, but, in fact, it is precisely that parallel connection that provides a heuristic way to relate holography with nonlinear optics.
 * When the connection was made in the early days of phase conjugation, it provided a means by which to enlighten the research community to consider how other classes of NLO interations can be viewed as a RTH picutre: from "conventional" NLO processes (i.e., a third-order nonlinear polarization) to optical coherence processes (e.g., photon echoes, etc.) to simulated scattering processes (SBS, SRS, etc.), to novel classes of materials such as photorefractives, which involve a combination (multi-step) optical processes (involvong space-charge fields that, via the E-O effect, modulate the refractive index spatially, etc.). So, for purely historical reasons alone, this connection is very important in the evolution of the field.  I speak from personal experience, being one of the initial group of researchers that explored this field...
 * LASERMAN; APRIL 20, 2006

pictures!!!
bla bla ,I scrolled all the way down and..NOT A SINGLE IMAGE?? one image is worth a 1000 words. WHATS DOES IT LOOK LIKE ? -- Procrastinating@ talk2me 14:34, 26 March 2006 (UTC)


 * Two-dimensional photographs do a poor job of illustrating the three-dimensionality of holograms. Also, many photographs are copyrighted. -eyliu 31 Mar 2006

Remove technical detail
I think it is better to omit saying that intensity is square of amplitude (though it is true). The reason is, it is quite confusing for many non-technical readers; in fact, many have consulted me in this regard. For them, a physical "device" may be able to record a physical quantity (which in this case is the amplitude). But the doubt they get is, how it could record the "square" of a physical quantity (since "square" for them is just a mathematical / algebraic operation). I thought it is better to delete it and did so - but please be good enough to excuse me if it is wrong. Also note that it need not be explicitly stated for the technical reader (since every student of Physics in his beginning year of Physics Hons., knows what intensity is). —Preceding unsigned comment added by 137.122.30.181 (talk) 20:04, 28 April 2008 (UTC)

question
okay, I read almost all of that page, but I didn't understand any of it, so I don't know if the information I was looking for was there or not. However, I really want an answer to my question, so I'm going to ask it despite the risk of being repetitive. I am reading about Holomovment and the theory is associated with holography. I read that if a piece of holographic film is cut in half, both pieces will still contain all of the original data. How is this possible?

it may work kind of like a mirror, using reflection off the bumpmap to view a 3D image, or like this thing http://www.wisinfo.com.tw/wistek/mirage/mirage_model_22__gigantic_3d_hol.htm, so like a mirror, you would still see the image but it would be smaller, thats my theory i bet its wrong though. but i also have a Q, could you make the hologram larger than the orginal object? 24.3.56.115 15:53, 16 May 2006 (UTC)

HERE'S ONE WAY TO LOOK AT THIS RATHER CONFUSING NOTION (NAMELY, WHY DOES ONE SEE A RECONSTRUCTED HOLOGRAM FROM PIECES OF THE ORIGINAL HOLOGRAM?) If one assumes an object with a diffusely scattering surface (that is, an object whose surface features are not all specular [mirror-like], but, instead, scatters the "reflected light into a large cone of angles), then, the laser light that illuminates the object will scatter back toward the film plane (or, other static or dynamic holographic recording medium) and will, in essence completely illuminate the film.  Since each fearute on the object has microscopic surface imperfections (recall, that it is assume to not be "mirror-like"), then each feature will diffusely scatter the incident laser light into a large range of angles, "filling" the film plane.  By this line of reasoning, all the features on the object's surface will scatter the incident light across the entire film plane. Thus, each piece of the film will have information from the entire surface of the object. Now, the reference laser beam illuminates the film plane also. This combination of beams forms a spatially complex set of "gratings," or, interference patterns, across the film plane. Hence, at each "patch" on the film plane, information from all features of the surface will coherently combine with the reference beam. The "price" one pays for looking at the hologram using only a piece of the original hologram is that the spatial resolution is degraded; that is, the sharpness of the object's detailed features, edges, etc will become blurred, as if it slightly out of focus. The degradation results from diffractive spreading of the reconstruction beam that diffracts from the complex grating formed in the piece of the larger hologram. The diffraction spreads the readout beam (given by the ratio of the optial wavelength divided by the scale size of the piece); the entire hologram has the greatest lateral dimension, hence the least diffractive spread, whereas pieces of the hologram result in greater spreading of the readout beam, thus, degradting the sharpness of the reconstructed image.

By the way, if one were to perfectly image the light scattered from the object onto the film plane, then, there will be a one-to-one "mapping" of each pixel of the object to a single location on the film plane. In this case, every element on the film plane contains only information which has been imaged there. So, if one breaks the developed hologram in this case, the pieces will each contain different aspects of the surface. This is why one typically lets the scattered light "spray" all over the film, and, may employ a simple lens system to help collect the light, but, not to image the light, onto the film plane.

Hope all this helps (and, hopefully, my take on this is correct!!)... GOOD QUESTION!! --- Laserman; June 13, 2006

The assertion that "all the information of the scene is contained in each bit of a hologram" should be completed with "if the scene permits". It is likely that the right side of the plate does not "see" the left side of an object (if this side is occulted by the object itself). A bit of hologram from this region will not allow to see the left side of the object. LPFR 06:58, 18 July 2006 (UTC)

Who really cares about which space television programs use a holography-like tool (end of 1st paragraph)? Maybe that should go.

depth perception from phase or from stereoscopy ?
From the article it seems that the 3D impression in holograms comes from the fact that they capture not only the wavelength (color) but also the phase of light (section 'Technical description'). This is contrasted with so-called "holograms" on identity documents, which achieve a 3D impression by stereoscopy, i.e. different apparent viewing angles of the two eyes (main section 'Holography'). I think this is probably incorrect, for two reasons. First, our eyes are to my knowledge not sensitive to phase, and certainly do not use it to reconstruct depth information. Second, when one changes the viewing angle of a true hologram (contrary to e.g. a photograph), one sees the depicted object at a different angle. This implies that when watching from a constant position, both eye are seeing the object from different angles because they have different vantage points. This would classify as stereoscopy, I'd think. I do not know the details of producing holograms and I'm sure the light's phase information plays a role, but I'd doubt if the perception of depth in the holographic image itself came from anything but stereopsis. 29 Aug 2006


 * Right, the eye cannot not see the phase of the light. However, the fact that the phase of the light is recorded in the hologram means that when re-illuminatated by the reference beam, the original wavefronts from the object are (ideally) completely reconstructed. This means that as the viewpoint is changed, the object's image rotates just as the real object would (up to the limit of the field of view of the hologram, of course). Thus, with two viewpoints (stereoscopic eyes), the object appears in 3D. The light phase isn't responsible for the 3D appearance, but the recording of the phase allows the reconstruction of a 3D appearance. --Bob Mellish 19:47, 29 August 2006 (UTC)


 * Thanks for your response. So we agree on the role of phase in constructing holograms. I think the article on the other hand gives the impression that, rather than being a technical tool that allows one to reconstruct the light coming from the source, phase plays a role in the appearence of the holographic image itself, particularly its depth. Consider for instance: 'both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image.' and, regarding false holograms: 'All depth disappears if you turn the hologram 90° or if you look at it with just an eye. This is not the case with true holograms, which are not based on binocular vision'. The depth in true holograms does disappear when looking with just one eye (unless you compensate by moving it) because their depth illusion is based on binocular vision. Right? Thanks, Jan.
 * A hologram does not record the phase. There is no way to record the phase of light. The retina, photographic emulsion, photodiode, photomultiplier, etc. are just sensible to intensity (power per unit surface). The only way to measure phase is to add the wave to be measured and the reference wave and measure the intensity. But this measure does not even give directly the phase. As the phase cannot be recorded, what a hologram does is to record the place where the phase is "good" as I tried to show in "working principle...".
 * What you get with a stereoscopic pair is very different of what you get wiht a true hologram. Assume that in a scene there is an object F in the foreground that partially hides an object B in the background. In a stereoscopic pair, when you shift your head "to see what is under the object F", the object F will follow the movement of your head and what was hidden will remain hidden and what was visible will remain visible. Simultaneously the 3D deep seems to flatten. Buy a 3D comic: the experience is worth the bucks. This is not the case with true holograms. When you shift your head, hidden zones appear and others are hidden. You can see a series of pictures of a true hologram in this site: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html LPFR 08:45, 1 September 2006 (UTC)

CLARO TV
I am removing the reference to the CLARO TV, since it has nothing to do with holography 84.154.234.43 16:48, 11 July 2006 (UTC)

First holograms
"The first holograms which recorded 3D objects were made by Emmett Leith and Juris Upatnieks in Michigan"

No. Gabor made the first holograms.Restname 23:19, 18 July 2006 (UTC)restname


 * Gabor's holograms were not 3D. Among other things, he lacked a light source with sufficient coherence (e.g. a laser) to record 3D holograms. -eyliu 24 July 2006

Specular reflection.
I have removed the following from the specular reflection article. Perhaps it would be a better fit here. Specular reflection may be important for holography, but holography is not really that relevant to specular reflection.--Srleffler 04:13, 6 September 2006 (UTC)

"Specular reflection is very important for making good scratch holograms, which are optically similar to Benton Rainbow Holograms (AKA: 'White Light Holograms'); see also SPIE article and the FAQ, and the main Wikipedia Holography entry."