It is an honour to be invited to be Guest Editor of the ISJ for this Special Holographic edition particularly when approaching the end of my career primarily in optical metrology. I first became involved at AWRE Aldermaston using high speed/ultra high speed photography. Later, when with CEGB research laboratories, I collaborated with the University of St Andrews on early argon laser development with a view to applying it to several imaging situations.
My interest in holography was aroused during the mid 60’s when I attended a lecture by Professor Gabor. One of my then current problems was recording water droplets forming in steam turbines. I saw holography’s potential as a tool of measurement for such applications.
Around 1967 I acquired a small pulsed ruby laser produced by Barr and Stroud; it proved an effective, if limited, tool for holography. However, I realised that pulsed holography was a possible solution for “in reactor” inspection in biologically hostile areas. At that time it was not possible to obtain a powerful portable laser for applications under the arduous conditions imposed by in-reactor applications. In association with J. K. Lasers we funded and set about developing such a system from a prototype which J K Lasers had constructed for Nick Phillips. This laser finally proved commercially a very popular.
Through EU sponsorship I enjoyed a Chair at the University of Rome for three years applying holographic techniques to the restoration of ancient artefacts. I also had the opportunity to co-operate with several artists using pulsed lasers; the cover picture is one example of this work.
The 90’s were spent in the USA where I was funded for the development of NDT using both holographic interferometry and acoustics/laser Doppler imaging techniques. The team I lead took the concept of a portable holographic ruby laser/camera to what must be its ultimate. More recently I was appointed to an Honorary Chair at the University of Aberdeen.
Over the years much of my work has been published in this journal. My full publications list can be seen: https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B_Lensk3qxZTZTNiYWE5YzgtNDk0OS00MzlmLThiNmItYzFmNGZjYjVkMjQ1&hl=en
John M Webster PhD, MinstP, C.Phys, C.sci. [email protected]
Introduction
G Saxby
3 Honor Avenue, Wolverhampton WV4 5HF, UK
Historians of science and technology have frequently suggested that scientific breakthroughs tend to occur when the time and the circumstances are right, instancing such events as the introduction of printing, calculus, anaesthetics, semiconductor technology and the laser. But not every invention fits this thesis: for example, the invention of the laser seems to have been so far ahead of its time that it was at first stigmatised as ‘a solution in search of a problem’. Holography seems to have had an equally messy birth and upbringing.
The much older technology involved in photographic imaging depends on Newtonian principles, with some help from optical insights by Gauss and others. This model for the behaviour of light, the ray model, is still the basis for optical designs, though the precise nature of the image formed by a lens does need a more sophisticated model. Holography makes little or no use of the ray model: light is represented by transverse waves. But whereas in photography, the light waves are incoherent, i.e. in a broad spectral band and random phase distribution, in holography they are required to be of a single frequency and in phase. What this means in practical terms is that holography can record phase relationships in light beams, whereas photography cannot.
The intensity of a light beam is half the time average of the square of its amplitude (A2), and when two incoherent wavefronts occupy the same space their combined intensity is the sum of their average intensities, namely ½ (A12 + A22). However, if the wavefronts are coherent, the amplitudes of two such wavefronts occupying the same space add algebraically, so that the average intensity is not ½(A12 + A22) but ½(A1 + A2)2, i.e. ½ (A12 + A22 + 2A1A2), and the ‘2A1A2’ part contains a term that is independent of time, showing up as a stationary energy distribution varying in space. A screen shows this as an interference pattern.
The mathematics underlying the holographic principle is well established: you can find it laid out in full in HariharanCitation1 as well as in other optics textbooks; those unfamiliar with the handling of complex numbers can find a simpler (if less rigorous) treatment in Saxby.Citation2 A photographic record of the interference pattern can be made by positioning a photographic emulsion in the space; and if this record, after development, is repositioned in the same space and illuminated by just one of the original beams, a second beam will also appear, and the wavefronts that constitute this beam form a precise replica of the other original beam. This is the holographic principle. In practice the first beam (the reference beam) will be a simple plane wave, but the second beam (object beam) will have been modified by having been reflected from (or transmitted by) an object. Plainly, if the eyes of a viewer intercept this second ‘reconstruction’ or ‘replay’ beam, the visual experience will be identical with that of viewing the object itself, from a viewpoint corresponding to that from which the original object would have been viewed. Thus the two eyes of the viewer will perceive the image from appropriate viewpoints, giving stereoscopic perception. Indeed, the image will have full horizontal and vertical parallax, a true three-dimensional image.
So when, and how, did holography emerge? The making of a hologram clearly depends on the possession of a coherent light source, which the invention of the laser eventually supplied. However, the interference of two light beams had already been demonstrated as early as 1802 by Thomas Young, and clothed with the garment of mathematical respectability by Heinrich Helmholtz some years later, so that all the principles that underlay holography were known by the 1850s, roughly the same time as photography became of age. However, it was a further 50 years before Gabriel Lippmann employed the principles of interference in image making. He placed a mirror behind an ultrafine-grain panchromatic emulsion and recorded the interference planes produced by the various colours in the subject matter. When illuminated by white light these colours were recreated, giving a photographic image in natural colour. This achievement earned him the Nobel Prize for Physics for 1908. A further near approach to the holographic principle was made in 1930 by Frits Zernike, who discovered the phase contrast principle for rendering small transparent objects visible: this led to the phase contrast microscope in the early 1940s and eventually to the Nobel Prize for Physics in 1953. However, it was 1947–1948 before the principles underlying holography were finally set down by Denis Gabor,CitationCitation3,4 who at the time was occupied in attempting to improve the image quality of electron microscopes. His filtered mercury light source had a coherence length of only about a millimetre, and this limited his images to pinhead size. His in-line set-up also produced a spurious real image that was in the way of the genuine one.
The apathy that greeted Gabor’s papers is perhaps understandable: the rigorous treatment of the mathematics did not make for light reading, and the illustrations were unconvincing. Little was done in the way of further research until the 1960s, when Emmett Leith and Juris Upatnieks, working on radar research in the USA, rediscovered Gabor’s papers, found a way of avoiding his experimental difficulties and, with the help of the newly-invented laser, produced the first successful three-dimensional holographic image. At about the same time Yuri Denisyuk, working on his own in the Vavilov Institute in St Petersburg, invented the reflection hologram, employing a system based on Lippmann’s geometry, using coherent light and a reflective object behind the photographic emulsion rather than a mirror.
The eventual recognition of holography as an important advance in optical technology led to Gabor’s being awarded the Nobel Prize for physics in 1971. The omission of any recognition of the contribution to its development by Leith and by Denisyuk did cause some unease in scientific circles; the full story of this is told in Johnson.Citation5
The relevance of holography in the parallel worlds of technology and art was slowly recognised, first in the Soviet Union as Denisyuk’s superb reflection holograms began to be used to record museum artefacts and then to display these in travelling exhibitions; secondly, as research engineers began to use holography’s ability to measure microscopic distortions in stressed components by using secondary interference fringes; and thirdly, by creative artists exploiting the ability of holography to produce semi-abstract patterns in light, in three dimensions. This ability was greatly extended by the discovery that reversing a hologram in the replay beam would generate a pseudoscopic (reversed perspective) real image in front of the hologram, and this image could be used as ‘object’ to make a second, or transfer, hologram, allowing a degree of manipulation of the image. Further research by Stephen Benton at MIT led to the ‘rainbow’ white-light transmission hologram, and Steve McGrew evolved a method of producing surface-relief rainbow holograms by an embossing process. The way was now open to mass-produce holographic imagery, which rapidly developed into the big business it is today.
At this time holography was still confined to the optical table and to full-size images, but in 1970 Lloyd Cross began to develop the principle of the holographic stereogram. This hybrid of holography and photography codes a series of still photographs into a contiguous array of holograms in a single recording material, which when transferred gives a convincing three-dimensional image with full horizontal parallax. Many artists have taken up this technique to produce three-dimensional (often animated) images, freeing holography from the optics lab, at least in the production of the initial imagery. Advances in laser technology have also led to the development of holography of moving subjects and to full-colour imagery using a combination of three or more lasers of different wavelengths.
By 1990 it seemed that there was little left to learn about holographic principles and that any further advances would be confined to materials and techniques. This has largely proved to be true; and although holography has found an important niche in security (is there any important document nowadays that does not bear a hologram?) and in advertising — not forgetting the explosion of decorative wrapping paper that occurs at festive times — holography no longer carries the magical reputation it once had with the public. However, it is still an important research tool, and is itself still the subject of much research, as the papers in this issue of Imaging Science Journal confirm.
REFERENCES
- Hariharan P. Optical Holography, 1996, 2nd edition, pp. 13–17 (Cambridge University Press, Cambridge).
- Saxby G. Practical Holography, 2004, 3rd edition, pp. 417–421 (CRC Press, Boca Raton, FL).
- Gabor D. A new microscopic principle. Nature, 1948, 161, 777–778.
- Gabor D. Microscopy by reconstructed wavefronts. Phys. Rev., 1949, 85, 763.
- Johnson SF. Holographic Visions, 2006, pp. 142–144 (Oxford University Press, Oxford).