Seeing the whole picture: Dennis Gabor and the invention of holography
Popular culture’s depiction of holography has long overshadowed the actual science. As an optics feat first developed in the late 1940s—a proposed workaround to resolution issues in electron microscopy—Hungarian-born researcher Dennis Gabor won the 1971 Nobel Prize in Physics for its invention.
The Nobel committee saw the profound in Gabor’s achievement. His concept of holography—to see more by illuminating interference patterns in waves—has become a strong thread woven through many areas of science, particularly photonics. As just one example: the recent, stunning insights about gravity across the vastness of space derive from painstaking analysis of interference patterns in light as recorded in the LIGO Observatory’s gigantic interferometers.
But starting in the mid-1960s, and with the invention of the laser, holography has also fueled numerous, mostly unrealized visions for lifelike, holographic 3D representations of people and objects. From optical benches to Hollywood studios, this idea of holography has left a long trail of disappointment.
Though he, too, would come to embrace holography’s broad potential, including for arts and entertainment, Gabor avoided any hype about its humble beginnings.
“I wanted to improve the electron microscope which, tantalizingly, had stopped short of resolving atomic lattices,” Gabor wrote about his invention in a 1971 SPIE conference presentation. “My idea was to take a bad picture, even a quite hopeless looking picture, but one which contained the whole information, amplitude and phase of the electron waves and to reconstruct from this bad picture, called a ‘hologram,’ the real one by means of light waves.”
The word hologram is from the Greek word “holos,” meaning “whole.” Gabor’s experiments with holography—he also called it wavefront reconstruction—began in 1948. It was a two-step process that began with recording on a photographic plate the interference pattern between a coherent electron beam (the object wave) and a coherent background (the reference wave). In the second step of reconstruction the hologram is illuminated with visible light and the original wavefront is reconstructed so that the aberrations of the electron optics could be corrected by optical methods.
First holographic reconstruction (1948). Photo credit: SPIE Proceedings Volume 12151
Gabor was born in 1900 in Budapest. His life of prolific invention—he held more than 60 patents—began at age 10 with a patent application for a merry-go-round inspired by aviation innovators like the Wright Brothers in Dayton, Ohio.
In 1927, Gabor received a doctoral degree in engineering at the Technical Hochschule Berlin-Charlottenburg and went to work for Siemens & Halske AG in Berlin. He fled Nazi Germany in 1934 with an invitation to work at the British Thomson-Houston Company (BTH) in the UK.
In his Nobel lecture, Gabor recounted his early efforts with holography at BTH: “It was a lucky thing that the idea of holography came to me via electron microscopy,” he said, “because if I had thought of optical holography only [the director of research] could have objected that the BTH Company was an electrical engineering firm, and not in the optical field.” As luck would have it, a BTH sister company made electron microscopes and so Gabor got a green light to carry out some optical experiments.
They were not easy.
At a time before the laser was invented, the best Gabor could do to achieve the right balance between light coherence and intensity for wavefront reconstruction was to use a high-pressure mercury lamp. To achieve spatial coherence, he had to illuminate with one mercury line a 3 µm diameter pinhole. That left just enough light “to make holograms of about 1 cm diameter of objects, which were microphotographs of about 1 mm diameter, with exposures of a few minutes on the most sensitive emulsions then available,” he told the Nobel audience.
Because of the small coherence length, Gabor was forced to arrange everything on one axis, and this early method became known as in-line holography. It also gave rise to another problem: “There is not one image, but two,” he explained of the optical reconstruction. “Each point of the object emits a spherical secondary wave, which interferes with the background and produces a system of circular Fresnel zones.”
Gabor persisted. He found that if an electron hologram is taken with a lens with a terrible spherical aberration, one of the images can be corrected with suitable optics, and the effect is to wash out the unwanted second image. “Such a very bad lens was obtained by using a microscope objective the wrong way round and using it again in the reconstruction.”
Still, holographic images were never satisfactory. What’s more, Gabor’s work was never in sync with other developments in electron microscopy that were advancing its capabilities by other methods, and by 1955 he had abandoned the research.
Holography research did not go into hibernation, however, as Gabor would later recount. It went underground.
Unbeknownst to Gabor or anyone else in the West, Soviet researcher Yuri Denisyuk had independently developed the reflection hologram, a purely photographic method of 3D image reconstruction by way of light interference patterns using white light and monochrome light of another color. His work for the Soviet Navy at the Vavilov Institute in Leningrad (now St. Petersburg) was a clandestine secret of the Cold War.
Meanwhile, at a classified laboratory at the University of Michigan, physicist Emmet Leith, working on the development of an optical data processor for the US military, recorded interferential radar waves—another reinvention of holography between 1955-56 that he would develop into a complete theory of holography in the microwave region.
Gabor took up a professorship at Imperial College London, where his prolific scientific career included work on magnetic recording devices, thermionic energy converters, 3D picture projection, and more. He maintained an interest in science and society issues and wrote popular books like Inventing the Future (1963). He may have been disappointed with holography in its early stage, but it was far from his only research interest.
In the course of his work, Leith discovered Gabor’s early efforts, and he eventually teamed up with another Michigan professor, Juris Upatniek, to tackle many of the problems Gabor had set aside. When Leith and Upatniek introduced laser light to holography in the early 1960s, producing some of the first lifelike 3D images, research in the field exploded along with high hopes for applications including art, photography, cinematography, and display.
Gabor, though approaching retirement, re- entered the field. “He actively pursued holography with a vigor that was truly astonishing,” Leith wrote in a 1980 SPIE proceedings paper. “He conceived a workable holographic movie system, made worthwhile contributions to the rather difficult theory of holography in 3D recording materials, advanced ideas for translating machines based on recording of associative pairs of information beams…Gabor was a doer and a reader, but he was foremost a doer.”
The development of the laser recharged holography, Leith says, and the 3D reflection of objects—its most popular and widely recognized impact—would not be possible sans laser light. “However, the rise of modern holography was definitely underway prior to the laser era.”
Gabor died in 1979. Leith suggests that proper evaluation of the impact of Gabor’s invention of holography should not be compared to the fevered dreams of the entertainment industry. Rather, holography’s impacts on spatial filtering theory, interferometry, integrated optics, optical scanners, and adaptive optics, to name just a few, all build on Gabor’s original idea that shining light on the interference patterns of waves can reveal the whole picture.
William G. Schulz is Managing Editor of Photonics Focus.