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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