It was a small start, but one that pointed toward three-dimensional imaging. In a microscope, Gabor saw “one plane
after another of extended objects...just as if the object were
really in position.” His report in the May 15, 1948, Nature
gathered considerable attention and helped him land a professorship at Imperial College in London, where he continued his
research. Others soon followed his lead.
However, progress was slow. Low coherence of available light sources limited imaging to small transparencies.
Moreover, the coherent light and the light scattered from the
transparency followed the same paths in Gabor’s design, so
illuminating the hologram reconstructed a pair of images in
the line of sight, one in focus and one out of focus. The twin
image problem degraded holographic images and discouraged
developers. By the mid-1950s, Gabor and most others had
largely abandoned holography.
Optical signal processing
About that time, Emmett Leith was planting the seeds of a
holographic revival at the University of Michigan’s Willow
Run Laboratory. A native of Detroit, Leith had taken optics
courses as a physics major at Wayne State University and
joined the lab staff in 1952 after receiving a master’s degree.
Established in 1946 as a military research arm of the university, Willow Run in 1953 began a major project in the hot,
new—and highly classified—field of synthetic aperture radar.
The idea was to scan a small antenna over a large area,
building up an image with the high resolution possible only
with a much larger antenna. Flying the antenna pointing
The two sidebands in a communication signal or
in-line hologram.
Adapted from E. Leith and J. Upatnieks, J. Opt. Soc. Am. 52( 10), 1127 (1962).
The object is placed in the lower part of plane P1; the hologram is recorded at plane P2.
Adapted from E. Leith and J. Upatnieks, J. Opt. Soc. Am. 53( 12), 1377 (1963).
Prism
P1 P2
Incident beam
Object
Hologram
recording
plane
z
out the side of a plane could collect the data in a series of
strips, but the trick was converting the raw data into a three-dimensional map.
In 1954, Leith was assigned as a junior engineer to the project
headed by Lou Cutrona and Wes Vivian. Some months later,
Russell Varian, one of the brothers who founded Varian Associates, suggested processing the radar data optically. The appeal
was that focusing an image through a lens produces a Fourier
transform of the image in the focal plane of the lens. Fourier
transforms were vital for analyzing the huge volumes of radar
data, and the electronics of the time weren’t up to the job.
Leith and Wendell Blikken first designed a complex
incoherent optical system for the task. Initially it didn’t seem
promising, but they soon improved it. “The biggest advance
came when we began looking into coherent light, and many
of the problems just melted away,” Leith recalled later. At
the time, in 1955, mercury lamps were still the best available
coherent sources, but their requirements were modest—only a
line source of monochromatic light because they were processing signals in one dimension. Eventually they realized a point
source of white light could give essentially the same benefits,
if the optics focused all the light from the point source onto
another point.
While developing the theory of their optical processing system in September 1955, Leith realized that the diffracted light
waves produced by illuminating the data record were optical
replicas of the original radar, but with optical rather than radar
wavelengths. That led to a theory that mirrored Gabor’s wavefront reconstruction approach, but shrank the long radio waves
to shorter optical wavelengths rather than stretching the short
de Broglie wavelength of electrons to the optical domain.
Yet Leith knew nothing about other work on holography
until October 1956, when he discovered a paper by Paul
Kirkpatrick and Hussein M.A. El-Sum in the Journal of the
Optical Society of America. He was stunned that others had
already developed many of his concepts, although not the
radar application.
It took time to make optical processing a success. The first
eight test flights failed to produce usable data. But Leith and
the Willow Run group perfected the technique, and by 1960 it
was the standard way to handle synthetic aperture radar. It was
so successful that the military classified it, keeping the details
under wraps for years.
Off-axis holography
Holography intrigued Leith, but optical processing kept him
too busy to do experiments until 1960, when Willow Run
hired Juris Upatnieks as a new research assistant in the optics
group. Born in Latvia in 1936, Upatnieks fled with his family
when Soviet troops occupied Latvia in 1944. He spent years as
a refugee in Germany before the family immigrated to the U.S.
in 1951. He had a fresh degree in electrical engineering from
the University of Akron (Ohio), but lacked a security clearance.
While he waited for the paperwork, Leith put him to work on
