Color film

The invention:Aphotographic medium used to take full-color pictures.
The people behind the invention:
Rudolf Fischer (1881-1957), a German chemist
H. Siegrist (1885-1959), a German chemist and Fischer’s
collaborator
Benno Homolka (1877-1949), a German chemist
The Process Begins
Around the turn of the twentieth century, Arthur-Louis Ducos du
Hauron, a French chemist and physicist, proposed a tripack (threelayer)
process of film development in which three color negatives
would be taken by means of superimposed films. This was a subtractive
process. (In the “additive method” of making color pictures,
the three colors are added in projection—that is, the colors are formed
by the mixture of colored light of the three primary hues. In the
“subtractive method,” the colors are produced by the superposition
of prints.) In Ducos du Hauron’s process, the blue-light negative
would be taken on the top film of the pack; a yellow filter below it
would transmit the yellow light, which would reach a green-sensitive
film and then fall upon the bottom of the pack, which would be sensitive
to red light. Tripacks of this type were unsatisfactory, however,
because the light became diffused in passing through the emulsion
layers, so the green and red negatives were not sharp.
To obtain the real advantage of a tripack, the three layers must
be coated one over the other so that the distance between the bluesensitive
and red-sensitive layers is a small fraction of a thousandth
of an inch. Tripacks of this type were suggested by the early pioneers
of color photography, who had the idea that the packs would
be separated into three layers for development and printing. The
manipulation of such systems proved to be very difficult in practice.
It was also suggested, however, that it might be possible to develop
such tripacks as a unit and then, by chemical treatment, convert the
silver images into dye images.Fischer’s Theory
One of the earliest subtractive tripack methods that seemed to
hold great promise was that suggested by Rudolf Fischer in 1912. He
proposed a tripack that would be made by coating three emulsions
on top of one another; the lowest one would be red-sensitive, the
middle one would be green-sensitive, and the top one would be bluesensitive.
Chemical substances called “couplers,” which would produce
dyes in the development process, would be incorporated into
the layers. In this method, the molecules of the developing agent, after
becoming oxidized by developing the silver image, would react
with the unoxidized form (the coupler) to produce the dye image.
The two types of developing agents described by Fischer are
paraminophenol and paraphenylenediamine (or their derivatives).
The five types of dye that Fischer discovered are formed when silver
images are developed by these two developing agents in the presence
of suitable couplers. The five classes of dye he used (indophenols,
indoanilines, indamines, indothiophenols, and azomethines)
were already known when Fischer did his work, but it was he who
discovered that the photographic latent image could be used to promote
their formulation from “coupler” and “developing agent.”
The indoaniline and azomethine types have been found to possess
the necessary properties, but the other three suffer from serious defects.
Because only p-phenylenediamine and its derivatives can be
used to form the indoaniline and azomethine dyes, it has become
the most widely used color developing agent.Impact
In the early 1920’s, Leopold Mannes and Leopold Godowsky
made a great advance beyond the Fischer process. Working on a
new process of color photography, they adopted coupler development,
but instead of putting couplers into the emulsion as Fischer
had, they introduced them during processing. Finally, in 1935, the
film was placed on the market under the name “Kodachrome,” a
name that had been used for an early two-color process.
The first use of the new Kodachrome process in 1935 was for 16-
millimeter film. Color motion pictures could be made by the Kodachrome process as easily as black-and-white pictures, because the
complex work involved (the color development of the film) was
done under precise technical control. The definition (quality of the
image) given by the process was soon sufficient to make it practical
for 8-millimeter pictures, and in 1936, Kodachrome film was introduced
in a 35-millimeter size for use in popular miniature cameras.
Soon thereafter, color processes were developed on a larger scale
and new color materials were rapidly introduced. In 1940, the Kodak
Research Laboratories worked out a modification of the Fischer
process in which the couplers were put into the emulsion layers.
These couplers are not dissolved in the gelatin layer itself, as the
Fischer couplers are, but are carried in small particles of an oily material
that dissolves the couplers, protects them from the gelatin,
and protects the silver bromide from any interaction with the couplers.
When development takes place, the oxidation product of the
developing agent penetrates into the organic particles and reacts
with the couplers so that the dyes are formed in small particles that
are dispersed throughout the layers. In one form of this material,
Ektachrome (originally intended for use in aerial photography), the
film is reversed to produce a color positive. It is first developed with
a black-and-white developer, then reexposed and developed with a
color developer that recombines with the couplers in each layer to
produce the appropriate dyes, all three of which are produced simultaneously
in one development.
In summary, although Fischer did not succeed in putting his theory
into practice, his work still forms the basis of most modern color
photographic systems. Not only did he demonstrate the general
principle of dye-coupling development, but the art is still mainly
confined to one of the two types of developing agent, and two of the
five types of dye, described by him.