"What is a hologram?" For the record:

Al Razutis - Visual Alchemy
FaceBook post April 22, 2015

What is a hologram? post by Al Razutis on FB

I brought up these questions (consistently on FB 'Holography') so I'll add some definitions and comments. Because really, this topic of 'what is a hologram', 'which wave is useful for holography', and who can claim their work to be "a hologram", won't go away into some "classical holography" dustbin, because there's a new flavor now in town, arrays of "zone plates" for displays.

200 years ago? 40 years ago? It is the same science, not a discardable set of ideas. And the ideas (theories, experimental verification) continue today.

Some DEFINITIONS from my favorite sources - listed at end - some forty years ago...

"Holography is an interference method of recording light waves diffracted by a subject illuminated with a coherent light. The diffracted waves are cause to interfere with a phase-related reference wave. If the waves are highly coherent, the relative phase between subject and reference wave remains constant in time producing an observable effect on the intensity distribution of the resulting interference pattern. The photographic record of this pattern, the hologram, contains sufficient information about both the phase and amplitude of the diffracted waves to permit their reconstruction. Wavefront reconstruction takes place, in a second step, when the hologram is illuminated with the reference wave." (Collier pg. 3)

(The photograph, by contrast, contains only the spatial distribution of light intensity in the subject scene)

"The unique characteristic of holography is the idea of recording the complete wave field, that is to say, both the amplitude and phase of the light waves scattered by the object. Since all recording media respond only to the intensity, it is necessary to convert the phase information into intensity. This is done by using coherent illumination...and adding a reference plane or spherical wave to the waves scattered by the object." (P. Harihan pg 1)

--- Is this true for sound waves? For seismic waves? Can one create the above experimental procedure with such and record a "standing wavefront" with the above? Not likely. (If you can, please illustrate.) Is this why we use isolation tables (for long exposures) or short pulses of light (for living subjects)? Yes, and it's been experimentally proven over and over again in the making of what we term "holograms".

More Collier:

"The method of holography applies to all waves: to electron waves, X rays, light waves, microwaves, acoustic waves and seismic waves, providing the waves are COHERENT ENOUGH (*my emphasis') to form the required interference pattern. Holograms have in fact been formed with each of these wavefronts . However, it is in the optical region of the electromagnetic spectrum that hol

Collier continuing:

"Maxwell's theory predicts the presence of two vector orce field in light waves: the electric and magnetic fields....Weiner (1890) was able to demonstrate that a light standing wave pattern (interference) blackned a photographic plate most at the electric field antinodal regions and not at all at the magnetic field antinodal regions. Thus, it is the electric field which is the light force component of major consequences in the forming of holograms." (of any recording medium) ( pg. 5 )

"The object of holography is to usefully record diffraction patterns....holograms can be classified as behaving like plane diffraction gratings or volume diffraction gratings...comparison (between plane and volume diffraction gratings) reveals that the latter (volume diffraction grating) is a more selective criterion for observing maximum diffraction. For volume gratings, once the incident angle has been chosen, the wavelength and diffraction angle are determined. This is not so for plane gratings." (pg. 12)

"Gabor derived the name hologram from the Greek word (holos) meaning whole, noting that a record of phase as well as amplitude information provided a rather complete description of a light wave. The method for preserving phase information comes out ...where we see that relative phase is preserved in a two-beam interference patterns. The basic techique of hologram formation, then, is to divide the coherent light coming from the laser into to two beams" (subject and reference beams). (Collier pg. 14-15)

Not just 'theory', but experimentally verified, and a practice called "holography".

By these definitions we can easily EXCLUDE 'Pepper's Ghost' illusions...
By these definitions we can EXTEND the term "holography" to include those which include "very large high-definition full-colour holograms can now be written as a matrix of tiny elementary holograms" ... "ultra-realistic volume phase holograms may be printed as a matrix of elementary volume holograms using a computer image data." (Ultra-Realistic Imaging, Bjelkhagen, Brotherton-Ratcliffe, 2013)

So what of "scratch-o-grams" where large space diffraction gratings are created mechanically (scratched on plastic)? Are these "holograms"?

That's where we began and the topic I'm revisiting.

Proponent of scratch-o-gram William Beaty contends that scratch-o-grams are 'really holograms' by comparing them to Benton Rainbows ( http://amasci.com/amateur/holo3.html ) in which he asserts: "they employ no diffraction in reconstructing images" and that "a scratch hologram is essentially a Benton hologram greatly enlarged and with fringe count reduced".

-- I disagree. They are not one and same thing!

Beaty further states: "In a conventional 'Benton' or 'Rainbow' hologram, each point on the photographed object generates a particular interference pattern on the film, a pattern called a zoneplate. We can imagine this interference pattern to be the basic '3D pixel' of a hologram."

-- I disagree again.

In my experience, a Rainbow hologram contains (minimal) vertical parallax information. The illuminating slit is not one dimensional, it has height and width, and when the Rainbow hologram is illuminated by coherent light (laser) the 'slit' is clearly visible as a "laser transmission hologram" can show it to be.

What a Rainbow hologram provides is a limited vertical viewing aperture corresponding to the projected 'slit' where the original scene is viewed in 'monochromatic' playback depending on the angle of viewing (i.e. the monochromatic playback is dispersed vertically into viewing zones with separated color playback).

A Rainbow hologram requires a twin beam - object and reference - to record the INTERFERENCE PATTERN of information passing through the slit or H1 with slit. This conforms to the "classical definition" of holography. It is wavelength 'selective' in playback, therefore allowing for spatially coherent (point source) but temporally incoherent light (multiple-color, white light) source to be used.

What is a ZONE PLATE?

"A zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or curved mirrors however, zone plates use diffraction instead of refraction or reflection. Based on analysis by Augustin-Jean Fresnel, they are sometimes called Fresnel zone plates in his honor. The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc."

"A zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there." (Wiki)

"Unlike a standard lens, a binary zone plate produces intensity maxima along the axis of the plate at odd fractions (f/3, f/5, f/7, etc.). Although these contain less energy (counts of the spot) than the principal focus (because it is wider), they have the same maximum intensity (counts/m^2). However, if the zone plate is constructed so that the opacity varies in a gradual, sinusoidal manner, the resulting diffraction causes only a single focal point to be formed. This type of zone plate pattern is the equivalent of a transmission hologram of a converging lens."

"A grating mostly has more than one diffraction order. In a Fresnel zone plate this leads to more than one focus point. The negative diffraction orders even result in diverging beams, in other words: zone plates also behave like a dispersive lens with virtual focus points. When a zone plate is used as focusing lens, normally only the first diffraction order is used and all other orders have to be blocked by a suitable aperture."

Are ZONE PLATES created by mechanical etching holograms? No.

Can one make a hologram (like a Holographic Optical Element) of a point source that acts "as a zone plate?" Of course. I'm not done with Beaty's papers yet, but this will be continued as time permits. Rather than argue every point in the paper, I'm interested in the fundamental question posed by Beaty himself: "Are these holograms?"

1) Optical Holography, Collier, Burckhardt, Lin, 1971
2) Optical Holography, P. Harihan 1984
3) For the classic "Gabor Paper" see: http://www.alchemists.com/fb/gabor-lecture.pdf


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