root@cca.ucsf.edu (Systems Staff) (08/25/89)
I see some quite different things being called "holograms" and
would like to know what's what.
There are what I have understood to be holograms which are formed
by beam splitting and interference pattern generation on a
photographic medium. These are viewed with a matching coherent
light source and form an image by diffraction effects.
OK, that's what I've been told.
Now, I've seen some things which are just multiple image prints
with a surface molded to form a lens array to get some of the
variable perspective image effects that a hologram as described
above yields (but greatly inferior).
I hear people calling these things "holograms". Is there really
something holographic about them? Or is it just the usual case
of ripping off terminology to describe an imitation?
Then I hear about "white light holograms" in contexts that suggest
"genuine" holography but obviously involve a difference in process.
Can someone who really knows what's going on shed a litlle light?
I daresay I'm not the only one who finds this less than clear.
Thos Sumner Internet: thos@cca.ucsf.edu
(The I.G.) UUCP: ...ucbvax!ucsfcgl!cca.ucsf!thos
BITNET: thos@ucsfcca
U.S. Mail: Thos Sumner, Computer Center, Rm U-76, UCSF
San Francisco, CA 94143-0704 USA
I hear nothing in life is certain but death and taxes -- and they're
working on death.
#include <disclaimer.std>halazar@mit-amt.MEDIA.MIT.EDU (Michael Halle) (08/25/89)
Most generally, I guess, a hologram has the ability to record intensity and phase of light. That permits the holographic medium to bend light. Very commonly, this bending is done with a pattern of fringes recorded in an emulsion. In the type of hologram with which most people are familiar, a laser transmission hologram, laser light bouncing off an object interferes with beam coming straight from the source and produces an interference pattern (a bunch of fringes, 300 or so per mm) recorded on a photographic plate. When lit with an illumination beam of laser light coming from the same direction as the reference beam, the developed holographic plate sends out rays of light that exactly duplicate those that came from the original object. So you can think of the hologram as a window frozen at exposure, endlessly replaying light from the object. A laser transmission hologram is just a blur in white light because many vertical perspectives of the object, each perpective a different color, all smear together, the smear proportional to distance from object to hologram. To be able to be seen in white light, the object must be as close as possible to the holographic plate. In fact, a laser transmission hologram is often used as a master hologram, illuminated so as to project an image of the the object out into space. The plate is masked so that only one vertical perspective view is recorded. A second, or transfer, hologram is placed to straddle some plane of the projected object. When this plate is lit with white light, a spectrally colored image of the object is presented to the viewer. This type of hologram is called the rainbow or Benton hologram. Using similar techniques, achromatic holograms or full color holograms can be made. Real image holograms can only be made of objects stable enough to hold still during exposure and small enough to fit on a holographic plate. (The National Geographic globe-o-gram and holographic portraits of people are made with a pulsed laser, so even relatively instable objects can be imaged because they can't move very much during the extremely short exposure.) Holographic stereograms, on the other hand, can be made of anything that can be imaged from different directions. Holographic stereograms are made by replacing the object with a projection screen. A single, 2D perspective view of the object is projected with laser light onto the screen. This view can be made with a movie camera, a scanning electron microscope, or a computer, for example. A vertical slit of the holographic plate is exposed to the projection screen and to a reference beam. The vertical slit is then moved and a new perspective view is projected. This procedure is repeated a hundred times or so, producing a bunch of eye-pupil-width "windows" when illuminated, each one a window onto the view projected when the slit was made. A two eyed viewer sees one view with one eye, another view with the other. The original views are computed so that these two views are a stereo pair. As the viewer moves left or right, different pairs are presented, precisely the views the eyes would see when viewing the original object. When this hologram is made white light viewable, these windows are projected out into space at a convenient viewing distance. Holographic stereograms can also be made in full color. CGHs, or "computer generated holograms" (sort of a misnomer, as computers can do much towards making stereograms), are made by calculating the fringe pattern necessary to bend light to image points of light in space. The pattern is typically written onto a substrate with an electron beam writer. Because the pattern is so fine, computation takes a very long time. As a result, the technology is not really practical for display holographic purposes right now. They are most often used in optical patten matching and other forms of optical computing. Dynamic holography, or holographic video, is based on some aspects of CGH. The state of the art in holographic video (last night, or so) is thirty-two scanlines lines of a static, 3D triangle. The "surface molded" prints that you are talking about are called lenticulars. Lenticulars are much older than holograms, so they aren't really rip offs of holographic technology. These images are recorded photographically, not holographically. The pattern on the surface is a whole bunch of cylindrical lenses. These lenses work similar to a holographic stereogram in that they project discrete images of the object out to different positions in space. The image is recorded on a print behind the lenses. Only a small number of views can be recorded, so the image repeats as you move from side to side. In fact, at some positions, the left and right views may be reversed, producing an inside out, or pseudoscopic, image. Lenticulars are thus cheaper to make but have a lower image quality than holographic stereograms. It's great to see curiosity about 3D, especially multi-perspective technologies. After all, people don't have two eyes just to fill their faces. --Michael Halle Spatial Imaging Group MIT Media Laboratory
andrew@berlioz (Lord Snooty @ The Giant Poisoned Electric Head ) (08/25/89)
The article by Michael was great. I wonder if the slowness of the computer- generated method is limited by compute MIPs or by the electron beam-writing speed? What progress is being made in materials science so as to approach 60Hz frame rates for holograms? Can multiple laser arrays be used in any way in this regard? - recently, it was reported that several thousand lasers have been fabricated on a semiconductor substrate... -- ........................................................................... Andrew Palfreyman There's a good time coming, be it ever so far away, andrew@berlioz.nsc.com That's what I says to myself, says I, time sucks jolly good luck, hooray!
talent@dover.sps.mot.com (Steve Talent) (08/25/89)
In article <2330@ucsfcca.ucsf.edu> root@cca.ucsf.edu (Systems Staff) writes: > >I see some quite different things being called "holograms" and [stuff deleted] >Can someone who really knows what's going on shed a litlle light? holos- from Latin, Greek meaning whole or complete -gram from Latin, Greek meaning record. hologram = complete record A hologram is a device that records a whole image. That is, when you view a hologram you are viewing a reconstruction of the light waves reflected from the object when the hologram was recorded. [disclaimer - I am not an expert in this field but I did a lot of reading and some experimentation as a student (1972-1980)] -- Steve Talent, Motorola Semiconductor Products Sector CAD Mesa, AZ 602-994-6801, ...!{oakhill, sun!sunburn, uunet}!dover!talent
news@blackbird.afit.af.mil (News System Account) (08/26/89)
In article <681@berlioz.nsc.com> andrew@berlioz (Lord Snooty @ The Giant Poisoned Electric Head ) writes: > >The article by Michael was great. I wonder if the slowness of the computer- >generated method is limited by compute MIPs or by the electron beam-writing >speed? I think the problem is with computer MIPs. example: you wish to make a 1" by 1" CGH. you choose to illuminate it with a HeNe laser (wavelength about .5 microns). During the calculation of the intensity values you would use a 2-D array consisting of 50,800 elements on a side. Thats a total of 2,580,640,000 elements in the array. (The array represents the hologram plane) BTW, 50,800 * .5 microns = 1". Now, for each point on the object you must calculate the light intensity from that point to every point on the hologram plane. The calculation involves taking a Sine, Cosine, several multiplies, etc. So, if there are 1000 points describing the object, you've got some 2 trillion very complicated calculations to make. end example. I've calculated it would take a Sun 4 about 280 days to make this calculation. A Cray2 about 6 days. All of these numbers change drastically if you choose to convert the problem such that Fourier analysis is applicable. But, this doesn't seem to work too well for 3-D holography. tom .
hutch@fps.com (Jim Hutchison) (08/29/89)
In article <1311@blackbird.afit.af.mil> tmouser@blackbird.afit.af.mil (Tommy A. Mouser) writes: >I've calculated it would take a Sun 4 about 280 days to make this >calculation. A Cray2 about 6 days. How did you figure this out (A Cray2-?)? Generating 3-D imagery does not always take 280 days on a Sun 4, what is it that makes this problem so unusual. >All of these numbers change drastically if you choose to convert >the problem such that Fourier analysis is applicable. But, this >doesn't seem to work too well for 3-D holography. 3-D holography would seem very suited to Fourier analysis, if for nothing else than to compress the data. On top of that, you could conveniently combine data by merging series. You could build 3-D molecular models without recomputing all the parts, just rotate individual pieces. /* Jim Hutchison {dcdwest,ucbvax}!ucsd!celerity!hutch */ /* Disclaimer: I am not an official spokesman for FPS computing */
ingoldsb@ctycal.COM (Terry Ingoldsby) (08/30/89)
In article <2330@ucsfcca.ucsf.edu>, root@cca.ucsf.edu (Systems Staff) writes: > > I see some quite different things being called "holograms" and > would like to know what's what. > > There are what I have understood to be holograms which are formed > by beam splitting and interference pattern generation on a > photographic medium. These are viewed with a matching coherent > light source and form an image by diffraction effects. ... IMHO a hologram, be it produced by a computer, using lasers with reference beams or an Etch-A-Sketch must reproduce both the amplitude (actually intensity) *AND* the phase (more precisely the relative phase). If either of the above are missing then *I* don't consider it a hologram, though some things are closer than others. > Now, I've seen some things which are just multiple image prints > with a surface molded to form a lens array to get some of the > variable perspective image effects that a hologram as described > above yields (but greatly inferior). ... I doubt this is a hologram. > Then I hear about "white light holograms" in contexts that suggest > "genuine" holography but obviously involve a difference in process. > By my definition, I'm not sure. They sacrifice the 3D aspect in one dimension in order to be able to use white light for reproduction. A professor of mine once said that they used the missing dimension (eg. vertical 3D) to select a part of the white light to do the reprod- uction. White light holograms do appear in a sort of prism of light effect. I have said nothing about whether a hologram has to be 3D, so (IMHO) that does not disqualify a white light hologram. I doubt they preserve phase explicitly, but it must be there implicitly. Hmmm? -- Terry Ingoldsby ctycal!ingoldsb@calgary.UUCP Land Information Systems or The City of Calgary ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb