The first version of this paper was writen in 1990. Subsequent versions were published in New Media Technologies, Ross Harley, editor (New South Wales, Australia: AFTRS, 1993), pp. 123-139; and in Display Holography (Fifth International Symposium - Proc. SPIE 2333), Tung H. Jeong, Editor (Bellingham, WA: SPIE, 1995), pp. 123-137. This paper discusses the aesthetics of holography from the point of view of theories of perception and representation. The paper challenges the notion that holography is exclusively an art of space, revealing it as a time-based medium.

PHOTONIC WEBS IN TIME:
THE ART OF HOLOGRAPHY

Eduardo Kac


1. INTRODUCTION

Every medium has a code, a set of rules or conventions according to which determined elements are organized into a signifying system. The English language is a code as is perspective in painting and photography. In the first case, the elements are phonemes organized into words and sentences according to a social convention: the syntax of English. In the second case, the elements are dots and lines organized into pictures according to a geometric method.

An artist or movement can break the conventions of the medium, as has done C�zanne with painting, Moholy-Nagy with photography and cummings with the English idiom in poetry, and create new elements and rules for combining them. If this is done, the level of predictability (or conventionality) is lowered and unpredictability is increased -- becoming more difficult for the immediate audience to understand it. But once this new rules are learned and the ideas behind them widely understood, the level of unpredictability is lowered and they become new conventions that can be accepted by the audience.

Holographic artists exploring the medium -- as opposed to advertisers using holography, who favor a high level of predictability -- are breaking several visual and cultural conventions. As a matter of fact, holography is so new that many questions are left open about the nature of the medium. Therefore, any attempt to clarify the issues raised by holography on a cultural level has a prospective (and not conclusive) tone, concentrating more thoroughly on general points and on the promise of its potentialities than on the records of its historical achievements so far.


2. THE ETYMOLOGY OF "HOLOGRAM"

The word "photograph" was first suggested by French-born pioneer Hercules Florence in 1833 (1), as a consequence of his attempt to use light to print labels and diplomas on paper with a silver coating. But while photography as we know it today was invented by Ni�pce as a culmination of centuries of research in that direction, holography, as many other inventions in the twentieth century, was the by-product of a search for something else. Namely, a method for improving the quality of images recorded on a electronic microscope.

As opposed to photography, holography did not come as a consequence of centuries of perfectibility. Dennis Gabor, its inventor, needed in 1947 what was to be later called a laser to make three-dimensional holograms , but he invented holography almost fifteen years ahead of the appearance of the first laser (2). Even in the early sixties, when the first three-dimensional holograms were made, the technique was labeled "a solution in search of a problem" by the press. More than thirty years have passed since the invention of holography, but the character traits of the medium as such and its cultural meaning remain incognito.

As observed before, Gabor coined the word " hologram" -- which is widely used to the detriment of the word " holograph" . While the former corresponds to the nomenclature established by its inventor, the latter is used only by those who try to imply that the hologram is an extension of the photograph or by those who are not concerned with their differences. The point to be made here is that the word " hologram" is more precise in naming its referential object.

" Holos" stands in Greek for " total, complete" ; " gramma" means letter and writing. It has the same roots of " graphein" (" to write" ). In ancient Greece, however, the letter was also used as a number, i.e., as a system for the measurement of distinguishable unities (therefore the current use of the suffix in " kilogram" , which does not mean writing with weight but the unity formed by one thousand grams.) So if " gram" designates the unity and " holos" , the total, the word " hologram" means the unity of the whole as well as the wholeness of the unity -- which the word " holograph" could never express. Actually , " holograph" , in any dictionary, designates " a document wholly in the handwriting of its author". So, since in a hologram each part is similar to the whole, a spatial characteristic that will be addressed later, only the word "hologram" should be used and the misleading " holograph" avoided.


3. A HOLOGRAM IS NOT A PICTURE

By avoiding the word "holograph", one is also avoiding the equivocated idea that holography is a kind of photography. After 150 years of developments, photography is a medium the conventions of which are now well established and accepted by the public. Therefore, a comparison between both media might take advantage of the general acceptance of photography not to imply kinship but to serve the purpose of examining a few questions by contrast.

Although Niépce and Florence were also interested in photography as a printing technique, the use of the camera by Daguerre and Talbot gave shape to the medium�s capability of taking accurate pictures of real things. From then on, photography evolved in two basic directions: one, the historical and journalistic approach, has foundations in the recognition of the photograph as a reliable method of documentation; the other, the creative approach, is based upon the invention of non-documental images by means of cropping, solarization, montage, negative manipulation etc. In both cases, though, one wouldn�t be wrong in stating the truism that whether taken with a camera or not, the photograph is ultimately a two-dimensional image and photography is a picture-making technique. It is the art of fixing an image on a plane in such a way (with varying degrees of shade) that the image becomes the plane.

Here a distinction should be made. As opposed to the photograph, the hologram is not a picture and holography is not primarily a picture-making technique. If this is true, all attempts to analyze holography from a straight pictorial viewpoint, or taking simplistic novelty holograms as if they represented the whole world of holography, will prove to be misleading. An examination of an ordinary fact, such as shopping in a supermarket, might make us aware of something that otherwise would be unnoticeable. Every time one goes shopping in a supermarket, one realizes that the checking out was speeded up by the use of a laser scanning process that "reads" the Universal Product Code (UPC) and provides price, an itemized receipt and inventory data to the store computer. The device located inside the scanner that bends the laser light in the direction of the product and back to the microprocessor is a hologram (or a set of holograms) in a glass wheel. This hologram (or set of holograms) doesn�t bear an image at all. What it actually does is just to perform the function of a lens; i.e., it only diffracts light in a particular way. Holograms that do not display images but instead do perform as optical elements are gaining ever more industrial and scientific applications. So if we can not only think of but actually use holograms as optical elements and not like pictures, if they can perform rather than bear an image, they are not extensions of photographs but a new way of recording, storing and retrieving optical information, i.e., information carried by light waves.

The way a hologram stores optically an image can be compared to a certain extent to the way a computer disk stores digitally an image. The digital image has to be transformed into 1s and 0s to be recorded on the disk and to be read by the software on the hard drive. The holographic image has to be codified into an interference pattern to be recorded on the film or plate. This pattern diffracts an incident beam of laser or white light so that the microscopic pattern can be translated into a visual image.

To say that the hologram is distinct from the holographic image means that the first is just the storing medium, while the second is what is stored. All the fluctuations, changes, inconstancies, leaps, turbulences and rhythms perceived in complex holographic art pieces are the result of careful work on the level of the mutable structures by which the visual information is stored, and not on the level of the images themselves. To say that the hologram is not the holographic image means that one cannot "retouch" the image because one can only "touch" the storage medium. The sensorial response to holographic images (and very clearly not to holograms) is ambiguous in the sense that the person who tries to grab the image knows that he/she is not looking at an apple, but nevertheless tries to clutch the luminous image only to have the unusual experience of contrast between vision and touch. The beholder is not looking for deception, he/she is not wanting to be fooled, but wants to have an acute contrasting experience based on the identification of the appearance of a familiar object and its non-correspondent tactile contours. The distinction between a hologram and a holographic image also means that damaging the hologram does not damage the image, because the later is recorded all over the emulsion. In a photograph, as seen from the viewfinder, for every geometrical point on the surface of the object there is one and only one correspondent geometrical point on the surface of the image. In a hologram, as seen from the film holder, for every geometrical point on the surface of the film there is a complete view of all the available information of the object. If one point is missing, the other points will reconstruct the complete image of the object without any problem.

This distinction between the hologram and the holographic image, raised by the example of the supermarket scanner, might ultimately mean that, when looking at holograms that do display images, chiefly at art pieces with complex space-time relations, one should concentrate in appreciating the rhythm orchestrated by his or her own dynamic perception of the informational structure of the piece rather than try to reduce to a monoscopic vision a sensorial experience that demands extensive binocular probing. In other words, the idea of a hologram as "perfected photography", capable of reproducing "better" the appearance of an object (the advertising approach) gives place to the understanding of the hologram as a medium for encoding complex spatiotemporal information (the artistic approach). The complexity of the way information is stored in the hologram might even in certain cases challenge the binocular (or "stereoscopic") perception of images and space by sending absolutely different images to each eye, therefore deterring perception of three-dimensional "objects" to favor a dynamic amalgam of images.

The holographic image, as optical information, is ultimately a spatiotemporal one, and not a volumetric or in-relief one, as distinct from the stereoscopic or anaglyphic drawing or photograph. Anaglyphs and stereographs produce an immutable image of a localized relief without surrounding space, from which the temporal dimension is extracted, while holograms produce images surrounded by actual space the parallax of which has to be perceived in duration.


4. THE QUESTION OF REPRESENTATION IN HOLOGRAPHY

Because holograms are a way for recording, storing and retrieving information, and because they can display an image or replace certain objects in their function (or even perform certain functions that objects cannot) without displaying any image, they pose a complex question regarding the way they represent (or not).

It is very clear from the outset that holograms don�t represent anything, in the same way that a computer disk doesn�t represent anything either. Holograms present things (images) or are things (diffraction gratings in a supermarket scanner). This isn�t so complex a question. But holographic images, yes, they do represent in a way that is peculiar: it is a result of the characteristic traits of the medium as such but is also a consequence of our esthetic code, i.e., the conventions according to which we associate an image to something exterior to itself, as in the case of a holographic portrait. The problem comes from the fact that the syntax and the elements of the vocabulary of holography are not yet widely known, making it very difficult , even for the scholar, to "read" holographic images. The ones that are very easy to "read" are the ones that try to resemble photographs or objects, that try to take advantage of the viewer�s knowledge of another signifying system to convey a straightforward message, usually for advertising or decorative purposes, for commercial rather than for inquiry aims. Those images are of no interest to this essay, because they mislead the viewer by making him/her think that holograms are "optical illusions" rather than a means for recording, storing and retrieving optical information. The emphasis shall be placed in the uniqueness of the holographic image, and not in its reduction to other systems that training and habit have already mastered.

In "On Longing", Susan Stewart demonstrates unwillingly this reductionist approach to holography, or to what she names imprecisely "holographic art", i.e., mass produced stock-images sold as souvenirs or decorative gadgets. Stewart writes:


The complex technology of "holographic art" erases its author and its referent; what matters is that it works, not that it points to something outside itself. Its contents seem strangely unmotivated, strangely out of key with the technical sophistication of its mechanisms: a woman�s face, a parrot in a cage, scenes that resemble those of romantic greeting cards. Content is emptied of interpretability. While modernist art delighted in "making strange" the everyday, this technical art delights in turning the strange into the obvious, in mapping mystery onto cliche. Holographic art is an art like that of commercial television, a mystification of technology accomplished in a gesture which proclaims the innocuousness of all content. In fact, without this univocal and mimetic content, we would not be able to distinguish artistic from scientific uses of the holograph.(3)


Content, in the case of the images that Stewart herself recognizes as resembling greeting cards, are not emptied of interpretability but full of predictability. Those images are designed not to cause strangeness, so that they can be promptly associated to well known images, so that they can convey transparently their content as pictures that resemble other pictures. As says Nelson Goodman, echoing Berkeley, "Pictures in perspective, like any others, have to be read; and the ability to read has to be acquired.(4)" A simple image of an anonymous woman�s face, even if it is in three-dimensions, does not demand the discovery of rules of interpretation. It relies on the experience of looking at actual faces as well as in the experience of looking at pictures of faces. Stewart also mentions, using the word "holograph", that we already described as inappropriate, the "mimetic content" of these particular kind of holograms as if all holograms, including true holographic art pieces, had "mimetic content." Mimetic means imitative, and to imitate is to mirror the appearance, is to resemble. Even if a hologram of an apple can be said to mirror an apple, that doesn�t mean that it represents an apple. We just saw that the woman�s face in Stewart�s "holograph" does not represent a particular woman or women in general; at the most it represents another picture of a woman�s face. So what happens here is not that the referent is being erased; simply, the referent of the image in this case is not an object but another image that poses no interpretation problems. As Goodman explains, some pictures we read by automatic habit; "practice has rendered the symbols so transparent that we are not aware of any effort, of any alternatives, or of making any interpretation at all."(5)

Not all holograms, though, try to minimize their unique characteristics so that they can be grasped as quickly as they would if they were photographs. Baudrillard states that "we are left to know what kind of objects and forms will be hologenic, because the hologram�s destiny is not one of reproducing three-dimensional movies as movie�s destiny was not one of reproducing theater and photography�s destiny was not one of appropriating the concepts of painting.(6)" But to think in terms of "objects and forms" is still to think in terms of volumes and reliefs, and not in terms of empty space or in terms of a four-dimensional space-time continuum. Baudrillard asks: "why will the simulacrum in three-dimensions be closer to the real than the one in two-dimensions?" And he himself provides the answer:


It intends to be, but its paradoxical effect is, inversely, that of making us sensitive to the fourth dimension as concealed truth, secret dimension of all things, that all of a sudden assumes the power of an evidence. (...) In short, there is no real: the third-dimension is the imaginary of a two-dimensional world, the fourth, of a three-dimensional universe... Scaling in the production of a real which becomes even more real by addition of successive dimensions (...): It only is truthful, it only is truly seducing that which plays with one missing dimension.(7)


The "real", like the object of observation in quantum mechanics, depends upon observation, it changes when observed, it is only real when it can be observed. Every observation corresponds to the cultural standard of a certain society at a given time, and the images produced within this framework change together with the model employed to observe, describe or represent any particular notion of the "real". Baudrillard uses the holographic image as a symbol for this cultural relativism:


The meaning, the truth, the real, they can only appear locally, in a restricted horizon, because they are partial objects, partial effects of equivalence and mirroring. Every reduplication, every generalization, every passage to the limit, every holographic extension (whim of explaining exhaustively the universe) makes them emerge in their own derision.(8)


Some holographic images said to be "realistic" will only be so if we state that our notion of the real is changing, because it will be only then that we will be able to establish a symbolic relationship between those images (that challenge our senses and our very notion of what images are and how they work) and the world. In the Renaissance, man placed himself in a vantage point to organize the sensorial (irrational) data of the surrounding world with mathematical (logical) rules, interrupting symbolically its chaotic flux with a schematic picture that expressed the supremacy of his viewpoint; the man of the twentieth-first century is experiencing the satellitization of the gaze, he is looking at the world as a whole from its orbit, not from inside like the Renaissance man did with perspective, but from outside, like we do with holography, to have a totaling view and to be able to scrutinize and control it more firmly, as we try to do with the weather (forecast satellite) or with our neighbor country (military satellite). We now measure the smallest temporal unit to be ever measured: the "femtosecond", or a billionth of a millionth of a second. We now play with atoms at our will in scanning tunneling microscopes linked to immersive virtual reality systems. Even gravity we seem to start to neutralize, with superconductive ceramics. Whether in a microscopic or a macroscopic scale, we are redefining our models. The luminous holographic image, for example, is a clear statement about matter and energy as a continuum rather than distinct entities. Why would an image made of a less dense state of this continuum look more or less realistic than an image made with a denser state? The notion of what is or what looks realistic will follow the ongoing paradigmatic change. "Realism is a matter not of any constant or absolute relationship between a picture and its object but of a relationship between the system of representation employed in the picture and the standard system", as wrote Goodman.(9)

Is a holographic image more "realistic" than a photograph if it is a scientifically more accurate "optical illusion"? Is a hologram an "illusion" in any instance? In order to answer these questions one has to clearly define what one means by illusion. Gombrich explains that in art-historical writing the term illusionism does not connote that illusion or even deception is the main aim of art.


[The term illusionism] was introduced by Franz Wickhoff in 1895 in his famous publications of the Vienna Genesis, an early Christian manuscript, to characterize the deft style of brushwork which had survived from Hellenistic times. The idea that anyone should have confused the illustrations of the manuscript with reality obviously did not enter his mind. What he wanted to convey, quite rightly, was the difference between this style and other, less illusionistic, methods.(10)


The term illusionism therefore never meant that the viewer would be deceived by an image and think that he/she is looking at the object rather than at a representation. But an argument might be raised that the aim of illusion is to create in the viewer the same response that the viewer would have to the object in similar conditions of observation. Descartes knew that "no images have to resemble the objects they represent in all respects (otherwise there would be no distinction between the object and its image)" (11) and, furthermore, as clarifies Goodman, what deceives depends upon what is observed, and what is observed changes with interests and habits. Writes Goodman:


In looking at the most realistic picture, I seldom suppose that I can literally reach into the distance, slice the tomato, or beat the drum. Rather, I recognize the images as signs for the objects and characteristics represented �� signs that work instantly and unequivocally without being confused with what they denote.(12)


In a complementary view, Gestalt psychology explained that apparent motion, like in movies, for example, is not the result of a mistake in the observer�s thinking but a "perceptual fact", i.e., a fact as real as the so-called real movement. Wolfgang Kohler ironically says (13) that it would be hard to convince an audience that the movement perceived in a movie is the result of thousands of mistaken judgements made in a couple of minutes. He contends that the apparent movement is "perceptually real", and that it proves that visual processes resulting from local stimulations occurring in different places under particular temporal conditions do interact one with the other. Likewise, one could say that an apparent tomato is perceptually real � discarding the meaningless idea that holograms are illusions. A hologram resembles another hologram more than it resembles any object, which, of course, doesn�t mean that a hologram necessarily represents another hologram. Representation is a matter of context and illusion a matter of how one understands the relationship between objects and images that represent objects. In informal discourse the word illusion implies absolute pictorial fidelity or unawareness of the distinction between a representation and an object, which ultimately does not occur in any aesthetic setting such as galleries or museums.

The holographic "real image", i.e., the one that stands in thin air between the viewer and the hologram changes our notion of depth, leading the layman to eventually say that there is an illusion, that "it looks as if the thing was really there". Baudrillard explains that holographic "real space" provokes a perceptual inversion, it contradicts our expectations. "Instead of being in a vanishing field for the eye, we find ourselves in a inverted depth, that transforms ourselves in a vanishing point"(14). But he thinks that the holographic image has a spatial presence that eliminates the sense of illusion usually associated with conventional representation.


The hologram is the inverted fascination of the end of the illusion, of the scene, of the secret, by means of materialized projection of all available information of the subject, by means of materialized transparency. (...) The hologram doesn�t have exactly the intelligence of the trompe-l�oeil, which is that of seduction, of always proceeding, according to the rules of appearance, through allusion and ellipse of the presence.(15)


Going further with his argument, he concludes that, to a certain extent, this process represents the end of aesthetics and the triumph of the medium, like in stereophony, "that sophisticated in itself puts an end to the charm and intelligence of music."(16) But does stereophony put an end to the intelligence of music or does it offer another way, with a different "intelligence", of enjoying it? Different ways of producing and recording music will result in different ways of listening, as exemplified by contemporary experimental pieces composed specifically for magnetic tape. While music played live demands a deductive auditory response (for watching the musicians play influences the listener�s perception of music), recorded music suggests a more inductive listening (for stereo-listening brings only a part of the whole spatial-acoustical experience of a live concert). The same is valid for the hologram: while tangible objects and shapes trigger deductive perception, holographic images demand a more inductive perceptual response.

The perceptual and conceptual experience of a hologram will continue to change as new technologies make possible new aesthetic adventures. Writing in 1972, Rudolf Arnheim acknowledged the impact of the then new pulsed portrait:


As holography is perfected, it begins to exhibit the terrifying rigor mortis of all new advances toward illusion. I look at the life-size portrait of the inventor, Dennis Gabor. He stands before me in full volume. As I move from the left to the right, I see a part of his shirt that was hidden before by his jacket, and the reflections on his eye-glasses change. I see his head first from the left, then from the right. So complete is the illusion of the man's three-dimensional presence that his immobility makes him a frightening corpse. The strength of the spell makes me ungratefully aware of what is missing. Instead of an image of a live man, I see a real ghost faking life. It will take a while before this new advance toward realism loses the power of seeming to be reality. It happened before with the motion picture, the stereoscope, the sound film.(17)


In this observation about holography, apparently the only one he ever wrote, Arnheim did not address the problematic notion of "illusionism" and expressed a view of image-making that one might regard as tainted by an evolutionist positivism: holography as progress or "advance toward realism", which he equates to "advance toward illusion". To advance toward realism is a contradiction in terms, because the goal of an absolute realism or illusionism is obviously unattainable, since all forms of representation work against the background of social conventions which change historically. Actually, Arnheim's commentary reveals other interesting contradictions, which may result from his unfamiliarity with the medium or from the attempt to read the holographic image with the critical apparatus one applies to the now familiar code of straight photography. How could holography be an "advance" if the hologram of a live man represents a "frightening corpse"? This definitely sounds more like a technological setback �� an unsuccessful attempt at realism. How can the illusion be "complete" and still make us aware of what is missing? It seems that what is missing is not the living subject of the hologram (Gabor died in 1979), but an appreciation for the unique features of this medium. Arnheim looked at the holographic image and didn't read it as an image, but as a rhetorical "ghost", i.e., the dematerialized appearance of a person. It is as if to the uninitiate a photograph could represent a live man, but a hologram could not. In other words, it is as if it were natural to read photographs and unnatural to read holograms, but we shall not loose sight of the fact that the ease with which we look at photographs today was acquired through a long cultural process that goes back at least to Brunelleschi and Alberti. The rationale behind his commentary breaks down at the end, when he mentions what happened to film and the stereoscope, which no longer seem "to be reality". If these media lost that mimetic "power" it is because their visual conventions have been assimilated by society at large �� a fate that awaits holography and that will be beneficial to it. Holographic art will only earn the consideration it deserves when trained and untrained observers alike can look at a hologram and not be haunted by spirits, specters, apparitions, phantoms, and other supernatural beings of critical discourse.

Holographic images possess several characteristics by means of which they represent things (like in a portrait of a person) or produce self-referential signs (like an abstract composition) and the study of them all would be beyond the scope of this essay. The fact that holographic images demand binocular perception in motion (as opposed to stereographs and anaglyphs) and the fact that they can produce a relief desirably identifiable with the relief of recognizable objects are two prominent aspects and therefore shall be addressed. But we will discuss these features in their reversed perceptual manifestation, the inside out relief and the non-stereoscopic vision (but still binocular), because of their unique holographic qualities.


5. PSEUDOSCOPY AND RETINAL RIVALRY

In the sixties the only kind of existing holograms were laser transmission holograms, i.e., holograms only visible by means of a laser. Those red images displayed in normal conditions a convex relief as such. The optical relief, because it was as convex as the relief of the object, was called "orthoscophic", from the Greek "orthos" (right). But if the light source was kept in place and the hologram flipped around, the relief appeared concave. The inversion also happened if two objects were used as models, one in front of the other. The one that appeared closer in the scene appeared further way in the image. This overall reversion was named "pseudoscopic", from the Greek "pseudo" (false) and is caused by diffracted light rays crisscrossing in front of the film plane.

The "pseudoscopic" phenomenon was discussed in 1868 by Herman von Helmholtz. Studying the image formed in a stereoscope, where two flat photos taken from a distance correspondent to the distance of human eyes are seen simultaneously to give the impression of relief, Helmholtz observed that the perception of relief and solidity was not produced by the movement of the eyes but by binocular perception. He described an experiment that showed that the impression of relief was still produced in a stereoscope despite it being illuminated with an electric spark of very short duration. The spark lasted for less than the four thousandth of a second, during which there could not have been, according to Helmholtz, any recognizable movement of the eyes. He also noted that the image perceived with the right eye is different from the one perceived with the left eye, "otherwise we should not be able to distinguish the true from the inverted or "pseudoscopic" relief, when two stereoscopic pictures are illuminated by the electric spark." (18) Helmholtz describes the pseudoscopic image on the stereoscope saying that "what should be further off seems nearer, what should stand out seems to fall back."(19).

The volumetric image in holography, as opposed to the stereograph, has the quality of reversing itself in all dimensions, the three-dimensions of space and that of time too. It makes no ultimate distinction between left and right, between up or down, between surfaces of curvature positive or negative, between temporal increase or decrease. But instead of traveling through the looking-glass, the holographic image travels the other way: it draws out the image from inside the mirror to our space. The pseudoscopic and the orthoscopic images are symmetric in relation to time; when shaving or making up in front of a mirror, a person moves to the right to see the left side of his or her own image. To see the left side of a pseudoscopic image, the beholder has to move to his or her left side. To see the underside of the pseudoscopic image, the beholder has to move upwards. Perception takes place in time, even if it is a reversible time.

This unusual perceptual experience has a correspondent model in Physics, where "time reflection symmetry" holds that any physical situation should be reversible in time. "According to this principle, if time could be reversed (i.e. run backwards) the time reflection of a particular physical situation would correspond to what one would normally see by reflecting the situation in a space mirror, except that all the particles would be replaced by their anti-particles."(20) In a similar but distinct way, the pseudoscopic image replaces relief by anti-relief, left-right parity by anti-parity and direction by anti-direction. For the holographic artist, pseudoscopy might be more than a stereoscopic "relief inversion"; it might be one possible element for the creation of unexpected rhythms and images with highly unpredictable volumes.

In his investigation of the stereoscopic image, Helmholtz also noticed that binocular vision does not simply coalesce the two distinct images projected on the retinas. He advocated that if one of the pictures in the stereoscopic pair is white and the other black, the resulting picture appears to shine. This phenomenon, called "stereoscopic lustre", according to Helmholtz proved that a "complete combination of the impressions produced upon both retinae" does not take place, because if that would be the case "the union of white and black would give grey."(21)

We know that our normal perception of objects in the world out there depends on the fact that each eye sees a slightly different view of the same object , uniting both views into a coherent whole: what we call, after Julesz, cyclopean vision.(22) The regularity of this principle reflects the constancy of the conditions of observation that we have, which helps us in the elaboration of concepts about the world based on this regularity, like the relationship between what is seen as solid objects and what is felt by touch as solid objects. The action of the senses as an interconnected system also reflects this regularity that grounds our binocular vision. Jean Clair argues that for an artist like Duchamp, "who was repelled by the physicality, the odorous corporeality, of painting, by its excessive grounding in the sensory world, the stereoscopic image showed the way to a purely ideal configuration, the intelligible result of a synthesis certainly closer to the brain � and to the working of a cosa mentale � than to the retinal effect." (23)

Although not having had any further artistic consequence before holography, what Helmholtz described as "retinal rivalry" in the stereoscopic image is a cosa mentale that challenges the regularity of our perception of three-dimensional objects in the world. This is how Helmholtz described it:


There are some very curious and interesting phenomena seen when two pictures are put before the two eyes at the same time which cannot be combined so as to present the appearance of a single object. If, for example, we look with one eye at a page of print, and with the other at an engraving, there follows what is called the "rivalry" of the two fields of vision. The two images are not then seen at the same time, one covering the other: but at some points one prevails, and at others the other. (...) Hence the retinal rivalry is not a trial of strength between two sensations, but depends upon our fixing or failing to fix the attention. (...) If we leave the mind at liberty without a fixed intention to observe a definite object, that alternation between the two pictures ensues which is called retinal rivalry.(24)


This phenomenon can also be observed holographically, but in a somewhat different way. First, in the stereoscope the two flat images are placed side by side, defining very clearly which eye sees what image. In the hologram, all images are recorded simultaneously on the emulsion and it is the diffracted light that sends towards each eye a different image, making it harder to know "from where" the image is coming. Second, while in the stereoscope each image remains flat, in holography each image can be fully three-dimensional, which makes focusing of the eyes more complex. Third, at last, vision in the stereoscope is motionless, while in holography, as in normal vision, the eyes move relative to what they see, scanning the spatial amalgam of the rivaling images.

Some holographic art pieces will display a very complex field, the components of which can only be seen, in discontinuous fashion, when the eyes of the beholder are placed in the direction of the diffracted light. In those cases, the amount of recorded information, and the way this information is presented, neutralizes the action of binocular vision as a system that unifies two different views into a single one. The expectations brought by the viewer are neutralized as well, and he or she cannot perceive individual solid objects but an inconsistent field, where objects and forms shift and coalesce.

Retinal rivalry becomes therefore one element in the vocabulary of holography, establishing a new visual parameter. In holography it is not an exceptional structure or just a "curious experiment", as Helmholtz describes it in the case of the stereoscope. It is a distinctive feature that checkmates our perceptual and symbolic conventions, for our response to visual stimuli depends, as we have seen, upon habits and conditions of observation. If each of our eyes perceives an absolutely different image, vision becomes a somewhat more complex and intense process, and demands with it that the beholder questions the very nature of his previous experiences.


5.PHOTONIC WEBS: TIME-DOMAIN HOLOGRAPHY

We need not examine in detail the technical development of automatic imaging systems, from early nineteenth century until now, to understand that it created the historical, aesthetic, and material conditions for the current digital synthesis of holographic images. In a clear development of painting's aspiration to truth and veracity, photography first attempted to fix images as seen in nature. The camera obscura, used by painters for centuries, became the photographer's essential tool. In the next stage, photographers tried to capture different moments of an action. Muybridge's analysis of motion and Marey's chronophotography paved the way for cinema. As a consequence, Edison and the Lumi�re brothers showed that images representing motion could not only be recorded as stills but set to motion themselves, allowing us to see representations of the recorded events as a temporal flux. Much later, video technology instantiated the recording, eliminating the temporal gap between the action and its playing back and, therefore, reinforcing the congruity beteween the representation and the reference. More recently, personal computers seem to have demolished photography's truth ambition by allowing anyone to manipulate photographic images and to easily recombine them in any desired way. If photography forced painting to redefine its direction in the beginning and middle of the nineteenth century, today computers have a similar impact on photography. How does holography fit in this context? Holograms are already routinely synthesized from secondary sources, including silver photography, video, film, sensing devices, and computer graphics.

Many holograms created today involve electronic image manipulation and digital synthesis, and draw from other artistic fields, such as photography, film, and video. These holograms explore time more intensively than holograms created without the computer and reveal a very important aspect of the medium. A lot of the computer holograms created by scientists or commercial holographers are motionless, or at best have very limited motion, because their images usually aim at reproducing a virtual environment or object with the visual stability typical of laser holography. Since the object in most laser holograms is three-dimensional and stationary, many holographers use the computer to make stationary virtual objects. Holograms thus produced emphasize space instead of time, and volume instead of movement.

Based on a study published elsewhere,(25) I have identified seven domains of holographic temporal manifestation. Not suggesting a hierarchy or the absolute predominance of one cathegory over the other, I propose a critical taxonomy based on the presence or absence of sequencial imagery and the nature of its manifestation in space. While it is obvious that many of these identified time-features will coexhist in certain kinds of holograms, it is also true that in many cases one feature might be predominant in determining the kind of experience the viewer has with the piece. What follows charts holography as an art of time and organizes its structure according to seven principles:

1. SYMULTANEISM. Holograms that present the viewer with one stationary object or construction organize the space so that it can be immediately and simmultaneously perceived by two or more observers in like manner. This allows the viewer to probe a space that remains consistent for the duration of the experience. Most of John Kauffman�s colored rock pieces would fall under this category.

2. TIME SUSPENSION. When a hologram is made that captures light phenomena as its subject matter, the dynamic behavior of light patterns is suspended at the moment of recording. Whenever illuminated, the hologram becomes active again and in its diffractive power reactivates the patterned choreography of the original propagation. Paul Newman�s Light Forms are a good example.

3. FREEZING. A holographic frozen moment implies the use of a pulse so short that it captures a symultaneist scene out of an original dynamic context, in which free motion is not a constraint. Harriet Casdin-Silver�s recent series of hybrid nudes may be included here.

4. LINEARITY. A linear hologram is one in which a serie of images is stored in such a sequence that its frames are meant to be experienced in a particular order. Any other directional readings become distractive in this case. Victor Komar�s holomovies are as linear as traditional movies.

5. TIME-REVERSABILITY. Holographic images can be conceived to be seen in a flux that is bi-directional. No beginning is implied and no conclusive end is suggested. The experience of time-reversible holograms revolves around the possibility of eliminating oppositions such as forward and backward, since these two become equivalent. Dean Randazzo�s Pasqualina, for example, explores tonal transformations that are meant to be reversible in time.

6. DISCONTINUITY. Discontinuos holograms break with the homogenous three-dimensional space reconstructed by a symultaneist hologram, shattering it into discrete viewing zones. These viewing zones can only be seen from restricted points of view. The space created is multifaceted, with controlled zones of visibility, gaps, and visual leaps. Many of Rudy Berkhout�s early holograms explore this principle.

7. REAL-TIME. Real-time holography is computed on-the-fly and projected freely in space in response to the viewer�s command. This "holographic video" can be controlled through the use of dials and other kinds of interface. Still in the early stages of technological development, this technique has not been explored by artists yet. The most prominent example is the apparatus being experimentally developed at the Massachussetts Institute of Technology. One day we will speak of real time transmission of holographic images.

In an article discussing the uniqueness of the digital moving image as distinct from other forms of cinema, Gene Youngblood wrote: "Cinema is the art of organizing a stream of audiovisual events in time. It is an event-stream, like music. There are at least four media through which we can practice cinema �� film, video, holography, and structured digital code ��just as there are many instruments through which we can practice music. Of course each medium has distinct properties and contributes differently to the theory of cinema, each expands our knowledge of what cinema can be and do."(26) This observation is of particular interest because it emphasizes holography as a time-based medium, and not as a three-dimensional imaging technique. The prospect of digital holographic movies of the future notwithstanding, the multimedia nature of the computer compels us to a redefinition or, at least, to an expanded definition of what holography is. Computer holography, as practiced by a small but increasing number of artists around the world, multiplies the expressive possibilities of the medium and asserts time, expressed as changes and transformations, as its main aesthetic feature. Holograms become interactive events that can be perceived in any direction, forward or backward, fast or slow, depending on the relative position and speed of the viewer. Unlike the unidirectional "event-stream" of film and music, as mentioned by Youngblood, four-dimensional holograms are "buoyant events" with no beginning or end. The viewer can start looking at any point. Time is suspended from its extended continuum and can flow forward or backward.

The computer sparks a unique form of visual thinking in which visualization of concepts becomes almost instantaneous and any hypothesis is tested by means of immediate practical experimentation. With holographic video and desktop holographic laser printers being developed today it is clear that in the future holography will merge with digital imaging systems in new and unforeseen ways. In "Design and Immateriality",(27) Abraham Moles observed: "Design by holographic models may still be far in the future, but the concrete activity of design even now partakes substantially of immaterial techniques, or usage of artificial representations, images, and diagrams composed by image-generating machines." This is also true in the fine arts and one can only expect this usage of immaterial techniques to increase. Or, as Frank Popper put it, "the future of holographic art lies no doubt in its combination with video and/or computer art."(28)


6. CONCLUSION

One of the facts that make it delicate to discuss holography from a non-technical point of view is the historical infancy of the medium. By trying to address some of the key issues of holography today we are putting ourselves in the position of the essayist that, around 1869, tried to encompass the cultural meaning of photography. It is clear that by then photography had already been popularized to a certain extent, but no argument should be further developed concerning the transformations that the medium went through in the following one hundred and twenty years. When Nadar photographed Paris from a balloon, he could never imagine that an artificial satellite would in the future photograph Paris from outer space.

We have already seen holographic images produced from satellite orbits. Would these images symbolize that more than one hundred years from now holography will play a major social role, comparable to what photography means socially today? It is hard to say.

Holography is a new medium, and as such is already remapping our perception of the world. As observed linguist Ferdinand de Saussure, recalling physicist Niels Bohr�s dictum, "Far from it being the object that antedates the viewpoint, it would seem that it is the viewpoint that creates the object"(29) , and in holography the viewpoint is an ever changing one.


6. REFERENCES

1 - Cf. Weston Naef, "Hercules Florence, Inventor do (sic) Photographia", Artforum, New York, February, 1976, p. 58; also Boris Kossoy, "Hercules Florence, l�inventeur en exil", Les Multiples Inventions de la Photographie, Minist�re de la Culture, Paris, 1988, p.75, and Naomi Rosenblum, A world history of photography, Cross River Press, New York, 1984, p. 195.

2 - Gabor made his first hologram with a filtered mercury-arc lamp.

3 - Susan Stewart, On Longing, The Johns Hopkins University Press, Baltimore, 1984, p. 12.

4 - Nelson Goodman, "Reality Remade", Languages of Art, Hackett Publishing Company, Indianapolis, 1976, p. 14.

5 - "Reality Remade", p.36.

6 - Jean Baudrillard, "Hologrammes", Simulacres et Simulation, Galil�e, Paris, 1981, p. 158.

7 - "Hologrammes", pp. 160-161.

8 - "Hologrammes", p.162.

9 - "Reality Remade", p. 38.

10 - Ernst Gombrich, "Illusion and Art", Illusion in Nature and Art, Edited by R.L. Gregory and E.H. Gombrich, Gerald Duckworth, London, 1973, pp. 195-196.

11 - Ren� Descartes, Dioptrics, Philosophical Writings, Translated and edited by Elizabeth Anscombe and Peter Thomas Geach, Thomas Nelson and Sons, London, 1971, pp. 243-244.

12 - "Reality Remade", p. 35.

13 - Wolfgang Kohler, The Task of Gestalt Psychology, Princeton University Press, Princeton, 1969, p.39.

14 - "Hologrammes", p.158.

15 - "Hologrammes", pp.158-159.

16 - "Hologrammes", p.159.

17 - Rudolf Arnheim, Parables of Sun Light; Observations on Psychology, The Arts, and the Rest, University of California Press, Berkeley, Los Angeles, London, 1989, p.157.

18 - Hermann von Helmholtz, "The recent progress of the theory of vision", Helmholtz on Perception: Its Physiology and Development, Edited by Richard Warren and Roslyn Warren, John Wiley and Sons, New York, London, Sydney, 1968, p.125.

19 - "The recent progress of the theory of vision", p.122.

20 - E.B. Uvarov, D.R.Chapman, Alan Isaacs, The Penguin Dictionary of Science, Penguin Books, Middlesex, 1983, p.431.

21 - Bela Julesz, Foundations of Cyclopean Perception, University of Chicago Press, Chicago and London, 1971.

22 - "The recent progress of the theory of vision", p. 121.

23 - Jean Clair, "Opticeries", October, MIT Press, Boston, Summer 1978, p.104. 24 - "The recent progress of the theory of vision", pp.122-123.

25 - Eduardo Kac, "Beyond the Spatial Paradigm: Time and Cinematic Form in Holographic Art," BLIMP Film Magazine, Fall 1995, Graz, Austria.

26 - Gene Youngblood, "Cinema and the code", Leonardo, Computer Art in Context Supplemental Issue, 1989, p. 27.

27 - Abraham Moles, "Design and Immateriality: What of It in a Postindustrial Society?", in The Immaterial Society; Design, Culture, and Technology in the Postmodern World, Marco Diani, ed., (Englewood Cliffs, NJ: Prentice Hall, 1992), p. 30.

28 - Frank Popper, "The Place of High-Technology Art in the Contemporary Scene," Leonardo, Vol. 26, No. 1, 1993, p. 66.

29 - Ferdinand de Saussure, Course in General Linguistics, Edited by Cahrles Bally and Albert Sechehaye. Translated by Wade Baskin. Mc Graw-Hill, New York,1959, p.8.

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