History of Holography

“I want to talk about the Universe as if it were a hologram. As you probably know, a hologram is the representation of a three-dimensional object on a two-dimensional surface, similar to a photographic plate. Supposedly I was represented by a hologram in one of the first episodes of Star Trek. The Next Generation. I say supposedly because, despite appearing as three-dimensional in the Enterprise space ship, the TV screens in those years could not, and still cannot, display three-dimensional holographic images. This will be the next technological revolution. In that episode I played poker with Isaac Newton, Albert Einstein and Commander Data”.

This is how the scientist Stephen Hawking started his presentation in Tenerife in 2014, where he defined holography in a more attractive way than any dictionary. The Spanish Royal Academy says that it is "a photographic technique based on the use of coherent light produced by laser. The interferences caused by the reflected light of an object with indirect lighting are exposed. After being developed, the photographic plate is lit by laser light, forming the three-dimensional image of the original object".

Ordinary photography is capable of reproducing a two-dimensional image, which is obtained by focusing the light reflected by an object on a photographic plate that records the intensity of the light it receives. In this way, the two-dimensional map, once developed, reconstructs the image corresponding to the focused plane. However, a hologram is formed by a confusing scheme of bright and dark points where all of the object's optical information is enfolded, with the fundamental particular feature that each part of the hologram contains in turn all the information captured in the full hologram. When the plate is lit with laser light, the holographic image appears, which reproduces the object three-dimensionally, displaying the unfolded optical information.

Dennis Gabor, father of holography

The Hungarian Dennis Gabor, who invented the hologram, explained his discovery in simple terms in this article published in 1948: "The purpose of this work is a new method for forming optical images in two stages. In a first stage, the object is lit using a coherent monochrome wave, and the diffraction pattern resulting from the interference of the secondary coherent wave coming from the object with the coherent background is recorded on the photographic plate. If the properly processed photographic plate is placed after its original position and only the coherent background is lit, an image of the object will appear behind it, in the original position."

Holography, according to the Open Mind article Dennis Gabor, Father of Holography published by the Applied Physics professor of the University of Alicante and member of the Augusto Beléndez Spanish Royal Society of Physics, took its first steps in 1947 in a laboratory of an electrical engineering company where Gabor worked on the improvement of the electronic microscope. This instrument increased by one hundred times the resolution power of the best optical microscopes and was very close to resolving atomic structures, but the systems were not perfect enough. The limitation was related to the spherical aberration of the microscope's magnetic lenses. To solve this problem, Gabor asked himself: "Why not take a poor electronic image, but which contains its ‘total’ information, rebuild it and correct it using optical methods?".

He came up with the answer to this question while waiting to play a tennis match on Easter Sunday in 1947, and it involved considering a two-stage process. In the first stage, the recording, he would produce the interference diagram between the object electron beam (object wave) and a "coherent background" (reference wave) that would be recorded on a photographic plate. Gabor called this interferogram hologram, from the Greek ‘holos’, which means ‘the whole’, as it contains the total information (amplitude and phase) of the object wave. In the second stage, the reconstruction, he would lit the hologram with visible light, rebuild the front of the original wave and would be able to correct it using optical methods to obtain a good image. Thus, the physical principles of holography are based on the wavelike nature of light and interference (in the recording stage) and diffraction (in the reconstruction stage). Gabor spent the rest of the year working on his ‘new microscopy principle’.

Giving up

Around 50 articles on Gabor's technique were published in the 1950s. However, only small and blurry images were achieved. Researchers lost interest for two reasons. First, the impossibility of obtaining optimal results when they applied the electronic microscope method, and second, the hologram reconstruction stage, which was imperfect. Gabor's method generates a hologram on an axis whose quality is poor due to the overlapping of the virtual image and the real, or combined image. When gazing at the virtual or real image, another unfocused image always appears as a background. In 1955, after researching several optical montages with the aim of minimizing the effect of the combined image, Gabor gave up his research into holography.

But, as the Spanish scientist says, everything changed in the 1960s. The holographic explosion, which originated in the United States in the early 1970s following the invention of the laser in 1960, and thanks particularly to the contributions by Emmett Leith -who recorded the first hologram of a three-dimensional object in 1964- rehabilitated Gabor, who from being virtually unknown received the Nobel Prize for Physics in 1971 “for his invention and development of the holographic method”.