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Credit: by James D. Trolinger
Most people own at least one hologram, probably without knowing it’s a hologram. Those with relatively little optical background, most of the general public, are most likely to encounter real holograms, face to face, in museums, magazines, art galleries, holographic displays, and security devices (such as those on credit cards). These are optical holograms, produced to create three-dimensional (3D) images of objects, serving various purposes, and so, while many people associate holograms with 3D imaging, others have no clue what holograms and holography are. Most novices do not understand and cannot appreciate the exciting aspects of holograms. After seeing an image in an excellent hologram for the first time, most viewers are surprised, fascinated, or even stunned, and almost always ask, “how was it made?” The discussion immediately gets more difficult, and discretion with the language is needed to keep the further discussion meaningful.
We can help all people to enjoy holography much more if we can provide them an understandable and accurate language and methods to enhance their experience with holograms. They do not need, and, indeed, may be frightened by the much more comprehensive, technical language required by the technical holography community. However, this requires that the technical community maintain accuracy and consistency in its own language and refrain from using confusing or incorrect language. Holography’s evolution across widespread and diverse communities and locations has produced a diverse language, which is often inconsistent even within the technical community itself.
This paper examines the language of holography and its origins, problems, and possible solutions, while recognizing that some of the “errors” in the language are so embedded in society that simple and ideal fixes may be beyond reach. This leaves us with certain questions. For example, “Can the correct language be restored, and should we undertake the task, or are we stuck with every 3D image being called a hologram?”
Breakthroughs sixty years ago, 1960’s revolution, (the anniversary we celebrate in this issue) created an explosive revolution in holography and cleared the way for many applications in science, industry, and art, including display, advertising, security, inspection, diagnostics, computing, data storage, new optical components, spectroscopy, microscopy. Holographers in many different fields now employ a wide range of recording and reconstruction processes, and widely varying, which are largely unknown to the public.
The early pioneers created a useful and accurate language, much of which has been almost lost in history. Gabor’s original paper was titled “Microscopy by Reconstructed Wave-fronts,” which probably led his early followers and others to appropriately call the process “wavefront reconstruction”. The inventions and language of early pioneers provide us with three separate categories for optical holograms: the in-line hologram of Gabor, the off-axis hologram of Leith and Upatnieks, and the white light reflection hologram of Denisyuk, now known as Gabor holography, Leith Upatnieks holography, and Denisyuk holography.
How did these pioneers explain holography? Adolph Lohman, a highly respected early researcher, described holography in the following way: “The propagation of the light wave from the object is interrupted somewhere by the photographic plate on which the hologram is recorded. The light wave is frozen in the hologram. When properly illuminating the hologram later in the second step, the wavefront reconstruction, the light wave ‘frozen’ on the hologram is revived. It continues its travel to form an image as if the light wave had not been interrupted. Unlike classical methods of image formation, the hologram does not necessarily bear any resemblance to the image which will be formed when it is properly illuminated. The hologram appears as a window through which a viewer can observe a 3D image of an object in the space behind. The image can also extend into the space in front. ”
Denisyuk described a hologram as the “optical equivalent” of an object itself. Illuminating a hologram of an object produces the same light wave for viewing that would be produced by the object itself. Using these concepts, a hologram can be described to a layman as a window on which information is recorded that enables it to transform light coming from a point source of light into a different light wave, i.e., one that would have come from a 3D object sitting behind the hologram. A viewer looking through the hologram sees a 3D image of the object sitting behind the hologram. Transmission holograms transform light passing through the hologram, and reflection holograms transform reflected light.
Figures 1–2 can be helpful for answering the inevitable question “How is a hologram made?” A common manifestation of an optical hologram is a photograph of the interference pattern resulting from overlaying two laser light beams originating from the same laser, known as the object and reference beams, as shown in Figure 1. Lasers are used because proper interference requires coherent light; this can be provided by a laser. The object beam comprises light reflected from an illuminated object, and the reference beam arrives unperturbed from the laser.
When such a transparency of the photograph is later illuminated with a similar reference beam of light (Figure 2), a visible 3D image of the object can be seen in the space originally occupied by that object. The viewer can move around and continue viewing through the hologram, and the image will continue to be visible from different angles. The hologram acts like a window with the object sitting behind it.
A hologram is not an image or a person, as incorrectly defined in respectable dictionaries and Wikipedia; it is a recording. What one can see when looking through a hologram is a 3D image, which is either behind or in front of the hologram, or both.
More recently, holograms have been described as windows that allow a viewer to look into a different place and time. With holographic movies and with real time holography, the viewer can see action in true 3D, taking place in a different place and time.
A completely general definition covering all holograms is unlikely to be understandable or even useful to many people who will see and enjoy optical holograms; however, broad definitions and language are needed to cover the newest forms and applications of holograms, especially non-optical holograms, such as acoustical, digital, and real time holograms, as well as the many types and uses of holography. We conclude that an optimal approach is to adopt at least two consistent definitions: one that is comprehensible to the general public and a second to be understood and used more by the technical community.
In technical language, a hologram is a recording of any type of complex wave, such as a light, sound, x-ray, radio, or synthetic wave, that employs the principles of diffraction and interference phenomena to store information sufficient to enable reconstruction of a nearly perfect replica of the original wave. Holography is the process of recording and using holograms, usually a two-step process, in which the first step records or creates the hologram, and the second step employs it to reconstruct and manipulate wavefronts and images. This technical definition is consistent with the layman’s definition, and it also rules out other forms of 3D image recording.
Stereo photography and other imaging methods that provide an illusion of 3D are often confused with holography. Experienced and knowledgeable hologram viewers spend much more time looking at images in holograms than images in stereograms, because there is much more to see. When experienced viewers observe holograms on display they are often in constant motion to assist with the viewing, a process that is sometimes referred to as the “holography dance.” We can teach the general public about the “holography dance” to improve hologram appreciation. Because pure hologram images retain all of their 3D properties, distance perceptions, vertical and horizontal parallax, and focus, they are almost always more interesting in appearance than stereoscopic images (and can even be exciting). Nevertheless, owing to our fascination with 3D imagery, many techniques have emerged and continue to emerge for providing 3D illusions. Of these, many, like stereoscopy, are very effective entertainment devices that are not based on holography; nevertheless, they are often mistaken for holography and even passed off deliberately as holograms for publicity.
Holography cannot yet and may never improve all 3D imaging techniques, which are quite effective in providing the desired 3D information/illusion for various applications. Virtual reality (VR) headsets are an example where video stereo photography is being pushed to its very limits in recording and computation, so as to achieve effective illusions of large, dynamic scenes for games and training purposes. Holography may eventually compete with existing VR headsets, but the necessary hardware and software does not yet exist. The same is true for video recordings of rock stars in action, none of which are holograms. Holograms have been made of stationary rock stars, but not as a live video depicting singing and dancing on a stage.
Digital holography (which became viable only after the more recent advent of high-resolution digital technology) is possibly the fastest growing technical holography topic, opening the way for many exciting new applications to come. Techniques developed by this industry have enabled faster and lower-cost hologram production than anyone had imagined. In science and medicine, digital holography is revolutionizing microscopy and imaging. Unlike analogue holography, which relies mostly fixed recording materials, digital holography employs electronic recording, computers, and software. Everyone will be seeing more of digital holography in the near future. With digital holography, a scientist, in principle, can peer through a window located on earth and see what is taking place behind a partner window (digital hologram) that is located on Mars.
Journal
Light: Advanced Manufacturing