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HologramsA Cultural History$

Sean F. Johnston

Print publication date: 2015

Print ISBN-13: 9780198712763

Published to Oxford Scholarship Online: November 2015

DOI: 10.1093/acprof:oso/9780198712763.001.0001

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(p.215) Appendix: A Taxonomy of Holograms

(p.215) Appendix: A Taxonomy of Holograms

Source:
Holograms
Author(s):

Sean F. Johnston

Publisher:
Oxford University Press

I have to start my classes with what holography isn’t . . . you have to unwind it several layers . . . so many of my students assume that, if they’ve seen it in a movie, if they’ve seen it on television, then that’s reality.1

Fred Unterseher, Santa Clara, CA, 2003

In terms of usage of the term ‘hologram’, I have tried to let it go at this point. You can really let it get to you . . . but we kind of lost that battle.2

Gary Zellerbach, 2014

The cultural identity of holograms is closely linked to their mystery, novelty and utility, i.e. to how they behave in particular environments. Their technical properties provide cues for the metaphors that describe them. At the risk of deflating their appeal, this Appendix distinguishes the principal varieties, gives a brief summary of their optical characteristics, and distinguishes them from other technologies.

A.1 Real-World Holograms

Table A1 categorizes the characteristics of the principal varieties of hologram. As argued throughout this book, the gradual evolution of these variants, and the distinct contexts in which they developed, means that texts that describe them are products of their times. Practical books published from the 1980s, as visual properties began to stabilize, are among the better guides explaining the principles, recording and lighting of holograms.3

Table A1 Varieties of hologram

Hologram type

Lighting for recording/viewing

Image position

Image depth

Image properties

Typical subjects

1947–60

Gabor

Filtered monochromatic lamp

Focused centimetres from hologram

Fraction of millimetre

Requires viewing optics (e.g. microscope)

Line drawings and electron-microscope shadowgrams

1961

Denisyuk reflection

Filtered monochromatic lamp/white light

Against hologram plate

Fraction of millimetre

Usually tinted

Reflective objects, curved mirror

1961–

Leith–Upatnieks 2D transmission

Filtered monochromatic lamp

Focused centimetres from hologram

Fraction of millimetre

Requires viewing optics (e.g. microscope)

Line drawings, then photographic transparencies

1964–67

Leith–Upatnieks 3D transmission

Laser

Behind hologram plane

Up to several metres, but usually tens of centimetres

Full depth cues (binocular disparity, parallax, accommodation, convergence); sometimes granular appearance from laser speckle

Laboratory objects, chess pieces, knick-knacks

1965–

Holographic interferogram

Laser

Behind hologram plane

Up to several metres, but usually tens of centimetres

Full depth cues; sometimes granular appearance from laser speckle

Engineering scenes undergoing subtle movement or stress

1965–

Denisyuk reflection

Laser/white light

Behind hologram plate

Usually tens of centimetres

Full depth cues; tinted

Museum objects, still-lifes, aesthetic subjects

1966–72

Image-plane

Laser/white light

Straddling hologram plane

A few centimetres

Full depth cues, but fuzzy and colour-fringed further away from plate

Knick-knacks

1967–

Pulsed laser

Laser

Behind hologram plane

Up to a metre or so, but usually tens of centimetres

Full depth cues; sometimes granular appearance from laser speckle

Rapidly-moving engineering scenes, portraits, figure studies, erotica

c1970–

Multi-channel

Laser/white light

Behind or straddling hologram plane

Usually centimetres to tens of centimetres

Two or more distinct three-dimensional scenes, each visible at particular viewing angles or distinct illumination directions

Product advertising, models

1972–c1985

Benton rainbow transmission

(or ‘White Light Transmission, WLT)

Laser/white light

Behind or straddling hologram plane

Usually centimetres to tens of centimetres

Spectrally tinted along vertical axis; no vertical parallax; fuzzier away from hologram plane

Knick-knacks, aesthetic subjects

1973–c1978

Cross ‘Multiplex’ (integral hologram or holographic stereogram)

White light (for filmed scene); Laser (for hologram recording)/white light

Within a cylindrical hologram

Usually a few centimetres

Spectrally tinted along vertical axis; no vertical parallax; limited horizontal resolution

Filmed outdoor scenes, portraits, erotica

c1974–

Pseudocolour

Laser/white light

Behind or straddling hologram plane

Usually a few centimetres

Three or more overlapping colours

Abstracts, simple compositions

c1978–

Dichromated gelatine (DCG or ‘dichromate’)

and

photopolymer media

Laser/white light

Usually image-plane

Usually a few centimetres

Relatively bright; sometimes low contrast owing to light scatter; fuzzier away from hologram plane

Often pendant- to head-sized models or objects

c1980–

Embossed

Laser/white light

Straddling hologram plane

Usually a few centimetres

Very bright; often distorted by lack of flatness; spectrally tinted

Still-life subjects, security patterns

1981–c1985

McGrew 2D–3D (embossed)

Laser/white light

Straddling hologram plane

A few millimetres

Very bright; limited three-dimensionality

Advertising and children’s stickers

c1967 but esp. post 1985

Computer-generated hologram (CGH)

Laser/white light

Usually straddling hologram plane

Usually centimetres to tens of centimetres

Very bright

Security and decorative patterns; display posters

1990s–

Holographic video

Various

Via computer interface

Usually centimetres to tens of centimetres

Varies

Electronic transmission and reconstruction of holographic image; limited demonstrations

1990s–

Haptic holograms

Various

In front of hologram plane; tactile sensation requires associated equipment

Usually centimetres to tens of centimetres

Varies

Computer-generated touch sensation; limited demonstrations

A.2 Misidentifying Holograms

Because holograms themselves are so varied in their visual effects, other optical technologies can masquerade or be misidentified as holograms. Despite the plethora of claims by companies, promoters and bloggers, most of these pseudo-holographic techniques combine elements available long before holograms were invented, albeit combined in innovative ways in new contexts and for new audiences.

The display of three-dimensionality is one of the most widely recognized characteristics of holograms, although not all holograms do so. To confuse matters further, other technologies can produce some of the optical effects characteristic of three-dimensional images. (p.216) (p.217) (p.218)

(p.219) Visual depth perception relies on four distinct detection mechanisms of the eye and brain:

  1. (a) Binocular disparity: the difference between the images seen by each eye owing to its unique position. Objects near the observer will have distinctly different angles with respect to the two eyes, while distant objects will not. The result is that the image seen by each eye will include objects having different relative positions depending on their respective distances.

  2. (b) Parallax: the sensing of this disparity in the apparent position of viewed objects when the scene is viewed from a different position. It is most obvious when one viewed object blocks another. Vertical parallax is observable only by moving the viewpoint vertically, e.g. by bobbing the head up and down.

  3. (c) Accommodation: the adjusting of the muscles of the eye to focus on a distant object. When viewing a photograph or television screen—even a stereoscopic photograph or 3D television image—all the objects in the recorded scene are focused by the eye at the same distance: they are either sharp or blurry depending on the setting of the lens that recorded the image, and how well the eye focuses on the photographic paper or screen pixels.

  4. (d) Convergence: the rotation of the two eyes in opposite directions to centre the object of interest on the retinas.

A.2.1 Stereoscopic imagery

As discussed in Section 2.3, the oldest form of three-dimensional imaging relies on supplying just two versions of a three-dimensional scene, one to each eye. This will give the perception of three-dimensionality, but only from a single fixed perspective. The most common forms of stereoscopic imaging do not provide parallax (which allows the viewer to ‘look behind’ a foreground object) or visual accommodation (requiring the viewer to focus at different depths of the scene).

The process of visual depth perception by binocular viewing, dubbed stereopsis, was first described by Charles Wheatstone (1802–75) in 1838, and the first practical stereoscope was invented by David Brewster (1781–1868) in 1849. In Brewster’s stereoscope, the perception of depth relies only on binocular disparity, and the viewer’s eyes fuse the two images by rotating (the action of convergence). The other stereoscopic cues of parallax and accommodation are not involved. As a result, the actions of focusing and head movement are decoupled from eye convergence. When the disparity between these cues is too extreme, as in poorly designed 3D movies, the viewer will experience eye fatigue or disorientation.

In their most limited form, stereoscopic displays provide a separate image to each eye for a single observer. The Victorian stereoscope, based on Brewster’s design, allowed the observer to view a card on which two photographs of a scene were positioned side-by-side. The photographs were recorded by camera lenses separated by the same distance as the eyes themselves.4

Later examples of stereoscopic technology are optical schemes that restrict the images visible to each eye in other ways. For motion pictures and television, two techniques became popular. The first employs anaglyphs: overlapping images projected in two colours, one for each eye. By wearing viewing glasses having coloured filters that block the opposite colour, each eye sees only one of the two images, and so the viewer perceives a three-dimensional image from the separate views. The same technique can be used for printed material, too, such as comic book (p.220) illustrations employing two coloured inks. The second popular technique is to employ polarizing filters (typically the Polaroid material patented by Edwin Land in 1929): the two projected images pass through separate polarizers with their polarizing axes opposed, and are reflected from a screen that preserves this polarization. When viewed through the observer’s glasses, which contain similarly oriented polarizers, a different image is seen by each eye, and stereopsis again occurs.5

An extension of this general class of stereoscopic imaging is virtual reality (VR) technologies that employ a viewing helmet with a separate digital display for each eye and a motion-detection system to detect the orientation of the observer and to alter the calculated images accordingly. VR systems have the advantage of being able to recalculate the stereoscopic images on the fly, allowing for both parallax and binocular disparity. The first consumer devices of the 1990s proved unpopular, in part, because of unsophisticated graphics and time lags in the display calculations. The delay between the viewers’ movements and displayed image could cause disorientation and queasiness. The goggles also obscured the local environment, isolating the viewer. Rather than enhancing reality they substituted a noticeably inferior virtual alternative.

Stereoscopic images of this kind are seldom mistaken for holograms because the technology is so clearly linked to the individual viewer. However, as carefully engineered 3D cinema and television programmes have demonstrated, audiences can find the technology compelling and immersive.

A.2.2 Auto-stereoscopic imagery

An alternative to head-mounted apparatus is an auto-stereoscopic display. Such a display allows the observer to perceive depth without glasses. However, a carefully designed optical arrangement is still required to provide distinct views to each eye. The two most common techniques are lenticular surfaces and parallax barriers.

A lenticular surface is one that either incorporates small lenses or other optical elements to restrict the view to a narrow range of angles. In this way, each eye can be limited to a different view. The first American patent for lenticular photography envisaged cylindrical lenses arranged side by side.6 One angle of view through the cylindrical lens channels the left-eye image, and the other channels the right-eye image. Lenticular images can be manufactured as two-dimensional images consisting of interleaved strips of the left- and right-eye images, combined with a moulded plastic overlay that incorporates a fine grating of cylindrical lenses or prisms. Alternatively, such printed images can be mechanically corrugated to achieve the same result. The linear ridges make lenticulars readily identifiable by touch.

Lenticular products were marketed by the New York-based Vari-Vue Company from the early 1940s in the USA, and during the 1960s they were licensed internationally. The products included political campaign buttons, postcards, children’s product prizes, wall hangings and even billboards. Most of their early products featured two distinct images, such as a winking eye or other ‘flicker animation’, rather than three-dimensional views, however, a technique revived in some 2D–3D holograms.

The heyday for such lenticular products was the 1960s, so there was some overlap and cross-fertilization with early holography. Look magazine, for example, included a black and white lenticular picture of Thomas Edison and, a few months later, a colour lenticular advertisement for a (p.221) Division of the Eastman Chemical Company in 1964, the year that three-dimensional laser holograms were announced.7

Variants of this scheme (used, for example, in the Nintendo 3DS hand-held game console, 2010) employ other forms of parallax barrier. These are typically opaque masks manufactured and positioned over the strip image to restrict the viewpoint of each eye, rather like looking through a picket fence at an object close behind it. As with embossed fine prisms and mechanical corrugation, parallax barriers require precise manufacture and are designed for a specific viewing position.

Such lenticular displays are well-suited to digital imaging for hand-held and larger display devices because the required stripe displays can be produced by software, and supplemented by a plastic overlay consisting of the cylindrical grating. In such systems, the observer’s viewing position is usually limited. If the observer is too far off-axis, the viewing geometry will no longer allow each eye to see its intended image and the perception of depth will be distorted or fail. None of the common varieties of lenticular technology allows for vertical parallax: if the viewer’s head is tilted 90 degrees, the perception of three-dimensionality disappears. As a result of these optical and geometrical requirements, lenticular imaging has a perceptibly lower resolution than a regular photograph, cinema image or hologram.

An earlier and more sophisticated form of lenticular imaging employs microscopic lenses rather than prisms or corrugations. The fly’s eye lens is an array of small lenses, often embossed on plastic, which can individually view a scene and record it as an array of miniature photographs. When this image is subsequently viewed or projected through the fly’s eye lens the viewer will be able to change position to gain a sensation of the depth of the scene. The scheme, dubbed integral photography, was introduced by physicist Gabriel Lippmann in 1908.8 The technique allows for both horizontal and vertical parallax as well as accommodation. In practice, the integral photograph acts like a window beyond which the scene is visible, and so resembles a Leith–Upatnieks or Denisyuk hologram. However, unless a very fine microlens array is used (which introduces its own optical and manufacturing problems) the resolution is noticeably poorer than a hologram. The small optical elements reduce the image resolution by diffraction, and make the image appear grainy or pixelated owing to the limited number of lenses that can be used.

Audiences nevertheless link lenticular technologies with holography; the popular understandings and markets overlap so closely that some holography firms, such as Spatial Imaging (London) have shifted the brunt of their marketing and production to lenticulars.

A.2.3 Volumetric imagery

Volumetric displays generate a three-dimensional image in a volume of space that can be viewed from different angles. In this sense, it is a genuinely three-dimensional technique that makes use of the four cues of binocular disparity, parallax, accommodation and convergence. The popular notion of such imagery is close to the depiction of the hologram of Princess Leia in Star Wars, although the film erroneously suggests that this is achieved from a single projector, and that the process is holographic.

This perception of depth can be accomplished either by projecting the image point onto a medium that reflects it, or by generating an image point from a light source. One example of the technique is to project two-dimensional images from multiple projectors onto a column of fine mist or spray. The droplets will reflect the image from two or more projectors back to the eyes of (p.222) the observer. The projected images may be either analogue (e.g. from movie film) or digital (e.g. from LCD projectors). The limitation—as with genuine holograms—is that the viewing volume is relatively small.

For the bright-point method, best suited to digital imagery, one technique is to employ a rotating panel consisting of an array of LEDs, with the LED brightness synchronized to the rotation to sweep out a volumetric image. With sufficiently rapid rotation, a three-dimensional scene will be perceived by the observer. A more elaborate alternative is to cause points in a transparent solid, liquid or gas to emit light, for example by focusing a scanned laser beam in a fluorescent medium. The bright-point method, either using a static-volume or swept-volume technique, is again limited to a relatively small cylindrical space, and image resolution will be determined by the density of LED light sources.

For viewers, the experience is similar to viewing a Cross Multiplex hologram of the 1970s, but with the added appeal of dynamically displaying computer- or video-generated imagery. On the other hand, viewers cannot interact with these mechanically-scanned or laser-scintillated images. Volumetric displays are relatively complex and limited in physical size. Just as with Multiplex holograms, the dedication of the necessary space has discouraged home use.

A.2.4 Video techniques

Contemporary audiences have also been misled by novel video techniques inaccurately identified as holographic. In 2008, CNN touted its introduction of ‘holograms’ in its broadcasts with a visual effect that recalled the Princess Leia scene in Star Wars. Its reporter in a remote studio spoke ‘via hologram’ to the anchor man as the camera panned around the set. Despite the sophisticated synchronization of camera movements on both sets, however, the effect was merely an extension of existing video technology. The reporter was imaged by encircling video cameras, and their positions, movements and optical settings were synchronized with cameras on the main set so that the appropriate remote camera image was selected. The two studio images were overlayed by conventional green-screen video techniques. It should be noted that—as with the Pepper’s Ghost technique described below—television viewers saw the realistic composite image, but the live presenters were not able to see each other except on studio monitors.

More generally, telepresence techniques extend two-way videoconferencing by improving the sense of co-location (e.g. by employing better display technology) or the variety of tasks that can be performed (e.g. to allow collaborative software development or repairing a fault). Rather than being an illusion portrayed for immobile audiences, telepresence provides an enhanced visual experience for participants in two or more locations, but it is neither inherently three-dimensional nor holographic.

A.2.5 Pepper’s Ghost

The visual trick most often misidentified with holograms is Pepper’s Ghost, a spectacular stage effect first seen by Victorian theatre audiences. An off-stage actor, illuminated by a bright lamp, would be visible to the audience via his reflection in an unseen pane of glass on stage. The ghost illusion had been conceived around 1858 by Henry Dircks, a retired civil engineer, possibly with the intention of educating the public about the conjuring tricks of spiritualist charlatans. His scheme was developed and first employed by analytical chemist John Pepper for stage plays from 1862 and patented in 1879.9

(p.223) With a live actor, the reflected image was three-dimensional and startling, and could appear to hang in space if the reflector were suitably positioned (Figure A1). Note that, unlike the artist’s rendition, the actor on stage was not able see the apparition: it was visible only to the audience via the reflection in the glass. For late-Victorian audiences, a magic lantern slide became a simpler and less costly alternative, although in this case the reflected image was, like the slide itself, two-dimensional. Variations of the trick were popular with a generation of theatre and fairground audiences before the introduction of motion pictures.

Appendix: A Taxonomy of Holograms

Fig. A1 Pepper’s Ghost stage illusion (Marion, F., L’Optique (1869), Fig. 73).

This trick has been popular in theme parks and stage spectacles since the 1960s, where the audience members cannot change position enough to detect the two-dimensionality of the projected images. The technique has been revived with images projected from video sources rather than live actors, and with alternatives to the original glass pane reflector. One substitution is to employ a thin reflective film, a water spray mist or fog curtain or even a white mannequin as a screen on which to project the video image. The mannequin is especially well suited to close viewing by audiences who can get a strong impression of three-dimensionality as the projected image ‘wraps around’ the figure. The key to the illusion is to ensure that the audience cannot detect the screen itself, which usually is accomplished by arranging a dark background and ensuring that the reflector or screen is unlit by other sources.

It is worth noting that most implementations of Pepper’s Ghost have none of the features of three dimensional imaging: they require neither binocular disparity, motion parallax, accommodation nor convergence. Observers use these visual cues merely to locate the position of the image in space as they would when viewing a cinema or television screen. But the illusion is more marked when viewers can see other objects nearby—a live performer or furniture—at different distances from the (invisible) screen. Tricked into perceiving the spectre as a living person rather than a projected image, their brains ‘fill in’ the missing dimension of depth.

(p.224) Examples include Disney attractions such as the ‘Haunted Mansion’ (opened in Anaheim, 1969, with large plate glass reflectors)10 and ‘Pirates of the Caribbean’ (updated to include projection onto a curtain of mist in 2006), ‘The Wizarding World of Harry Potter’ (Universal, Orlando, opened 2010), the Hatsuni Miku ‘virtual star’ in Japan (2010) and the Tupac Shakur ‘hologram’ (Coachella festival, 2012), the latter three using a metallized Mylar screen as reflector. Following the Coachella event in particular, popular identification of virtual stage performances as ‘holograms’ dominated web traffic.11

In short: entertainment ‘holograms’ are novel embodiments of a long-standing stage technique. Conventional head-up displays and teleprompters employ the same principle.

Notes:

(1) Unterseher, F. to S. F. Johnston, interview, 22 Jan 2003, Santa Clara, CA, SFJ collection.

(2) Zellerbach, G. A. to S. F. Johnston, telephone interview, 12 Jun 2014, SFJ collection.

(3) See, for example, Unterseher, F., J. Hansen and B. Schlesinger, Holography Handbook: Making Holograms the Easy Way (Berkeley, CA: Ross Books, 1981); Saxby, G., Practical Holography (New York: Prentice-Hall International, 1987); Benton, S. A. and V. M. Bove Jr., Holographic Imaging (New Jersey: John Wiley & Sons, 2008).

(4) Holmes, O. W., ‘The stereoscope and the stereograph’, The Atlantic Monthly III (1859): 738–48. If the distance between lenses is increased, the perception of depth is magnified. This technique became central to stereoscopic photogrammetry, in which photographs from airborne cameras are taken several hundred feet apart as the aircraft moves, to measure the height of terrain and features.

(5) Norling, J. A., ‘The stereoscopic art’, Journal of the Society of Motion Picture and Television Engineers 60 (1953): 268–308.

(6) Coffey, D. F. W., Pat. No. US 2,063,985 A ‘Apparatus for making a composite stereograph’ (1936), assigned to Winnik Stereoscopic Processes Inc.

(7) Look magazine 25 Feb 1964, p. 102; Look magazine 7 Apr 1964, p. 89.

(8) Lippmann, G., ‘Épreuves réversibles. Photographies intégrales’, Comptes Rendus de l’Académie des Sciences 146 (1908): 446–51.

(9) On the disputed origins of the invention, see Pepper, J. H., The True History of the Ghost: And all about Metempsychosis (London: Cassell, 1890). A Renaissance precursor for the illusion, however, is described in Della Porta, G. B., Magiæ naturalis sive de miraculis rerum naturalium (Napoli: 1558), a compendium of magicians’ tricks.

(10) Sweezey, C. O., ‘All that’s 3D is not holography: Disney World’s Haunted House’, Holosphere, 10 (1), Jan 1981: 1.

(11) For a commercial version, see Maass, U., Pat. No. US5865519 (A) 1999–02-02 ‘Device for displaying moving images in the background of a stage’ (1999), assigned to Musion. In more sophisticated implementations, the two-dimensional video images can be generated by compositing (combining image elements from) film or video footage and rotoscoping (tracing a recorded image for substitution of a body double or an inset of computer generated imagery (CGI)).