http://nobelprize.org/nobel_prizes/physics/articles/biedermann/index.html Prize-Awarded Methods Among the Nobel Prizes in Physics, two scientists have been honored for their remarkable methods to record and present images: Gabriel Lippmann, awarded in 1908 "for his method of reproducing colours photographically based on the phenomenon of interference," and Dennis Gabor, awarded in 1971, "for his invention and development of the holographic method." Both methods had the same goal of carrying image reproduction further in a way that was quite different from other earlier attempts made for the same purpose. To achieve this, Lippmann and Gabor chose a revolutionary approach to fundamental physics instead of following an evolutionary progress in engineering. In 1886, when the art and technology of photography was still struggling to transfer the colors of nature to adequate tonal values in black and white, Gabriel Lippmann conceived a two-step method to record and reproduce color images directly through the wavelengths in the object and the subsequent photograph. While Lippmann improved photography from black and white to color, Gabor's holography extended photography from flat pictures to a three-dimensional image space. Procedures to offer to each eye of the viewer its own parallax – stereoscopy – are as historical as photography itself. But Gabor's idea of a "hologram" was to store all the information in all image space and not just in one slightly different second photograph. Ideas Behind the Methods Interestingly, the physics behind both inventions can be understood on the same principle, namely using the wave nature of light, which involves encoding the image field by interference, recording the structure in a photographic plate, and then reading out the image field again by sending light and getting it modulated in this structure. Lippmann's Color Photography How could Gabriel Lippmann make use of interference effects to achieve color photography? The primer on wave optics and interference told us that light of different wavelengths will generate standing wave patterns at corresponding period lengths. Lippmann started out with a pattern of standing waves, where a wavefield meets itself again after it is reflected in a mirror. He projected an optical image as usual onto a photographic plate, but through its glass plate with the almost transparent emulsion of extremely fine grains on the backside. Then he added the interference effect by placing a mercury mirror in contact with the emulsion. The image went through the emulsion, hit the mirror, and then returned the light back into the emulsion. A suitable thickness of this photographic layer corresponds to around ten or more wavelengths. The image projected onto the plate did not plainly expose the emulsion according to the local distribution of irradiance. Rather, the exposure was encoded when the wave field returned within the emulsion and created standing waves, whose nodes gave little exposure, whereas the bulges gave maximum effect. Hence, after development, the photographic layer contained some twenty or more lamellae of silver grains with different periods for different colors in the image. When, after development, white light is shone on the plate in reflection, it will be scattered at these silver grains in all directions. Into the direction from which the standing wave pattern had been generated, the scattered light fields having the same wavelength as the period of the lamellae will be in phase, interfere constructively, and together create a strong color image. Certain elegant insects and butterflies have created such periodic lamellae without having been taught the optics of scattering or diffraction. We see that in essence, this form of imaging builds on a symmetric two-step process of interference and diffraction: first by encoding the image into an interference pattern, and then reconstructing the image by diffraction at this pattern. Gabor's Hologram The same two-step principle holds for Gabor's idea of wave front reconstruction. From Lippmann, we learned how to record and retrieve color information on a flat picture in contact with a photographic plate. If Gabor wants to reconstruct wavefronts in three-dimensional space, he needs a field of view, and we imagine that he instead has to abandon wavelength range. The process has to be done in monochromatic light. The reference for interference is no longer the reflection of the image field itself (in holography usually called object field), rather it has to be provided by a separate reference field. The angle between the reference field and any point from the object field determines periodicity and orientation of the resulting, much more complicated interference structure, which he called a "hologram." This also means that in order to obtain decent interference, the coherence length has to be larger than the path difference between any point at the object field and the reference field. When Gabor conceived the process of wavefront reconstruction in 1948, then intended to correct aberrations in electron microscopes, the available mercury arc lamp restricted his optical feasibility experiment to an object size of a few millimeters. A breakthrough came first in 1963 when Leith and Upatnieks at the University of Michigan demonstrated three-dimensional images from holograms made by laser. A highlight of the art became the portrait hologram of Gabor made with a pulse laser in the spring of 1971; the volume of the object space is several cubic meters. --

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