[rollei_list] Re: Old film
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 31 May 2005 00:05:28 -0700
----- Original Message -----
From: "John A. Lind" <jalind@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 30, 2005 9:09 PM
Subject: [rollei_list] Re: Old film
> At 12:00 PM 5/30/2005, Richard K. wrote (in part):
>
>>I don't know why Kodak has been trying to kill off
>>Kodachrome but they
>>have for a long time. Probably supporting the special
>>processing is the
>>problem. Kodachrome evidently has excellent dark storage
>>properties while
>>Ektachrome type films have better resistance to fading
>>under projection.
>>I've seen quite a lot of Kodachrome that still looks
>>pristine after 60 years.
The stability of incorporated coupler color films has been
very substantially improved over the years. Modern color
films probably have a lifetime of 50 or more years before
dye fading becomes too great to get good prints or reproduce
them. The early films had lifetimes of no more than 15
years.
All color films are B&W to start with. The color is
generated by a complex reaction during development.
Incorporated coupler films, often called chromogenic films,
have dye generating chemicals called couplers in them. These
couplers are anchored so that they can not diffuse either
from layer to layer or within a particular layer. Agfa
invented the method of attaching the coupler molecule to
very large long chain molecules that are too big to move
around in the gelatin. This method is the one currently used
in all color films that have couplers. Kodak came up with a
method of encapsulating the coupler molecule in a resin. The
resin allowed penetration of processing chemicals but did
not allow the coupler or the dye that resulted from it to
escape. The resin itself could not move around in the
emulsion.
The couplers produce dye in combination with the reaction
products of certain kinds of developers, thus there is a
complex reaction: the developer reacts with the exposed
halide to produce silver and also a reaction product. That
reaction product further reacts with the coupler to produce
the dye. In reversal films the first developer is of a type
which does not generate the necessary kind of reaction
product so no dye is generated. The second developer is of
the right kind so a positive dye image is generated by it.
In color films the silver image produced by the developers
is unwanted and is removed by a bleach that does not affect
the dyes. Thus, although the original image is a silver
image just as in B&W film, none of the silver is left after
processing. Large labs recover the silver. Nearly all the
silver used in the manufacture of the film can be recovered
after processing.
The "amateur" photographers to whom Kodachrome is
attributed were Leopold Mannes and Leopold Godowsky, both
well known classical musicians. Mees, in his farewell book
on retiring says that the joke at Kodak was that Kodachrome
was invented by God and Man. In fact, in Joseph Fiedman's
book on the history of color processes, he says that
probably Mannes and Godowsky were responsible only for the
differential bleach penetration process. I don't have the
book in front of me and am not sure I remember names
properly but the original idea for the generic process
probably dates from work done in the 'teens. The name of the
original investigator is right on the tip of my brain, how
frustrating.
The differential bleach process was extremely complex and
required astonishing control over all sorts of variables.
The film had to be dried between each step to insure the
rate of diffusion would be predictable. Evidently, the dry
film was floated on the surface of the bleach until it had
penetrated the necessary layers. The bleach removed the dye
and also converted the silver image back to a halide. It was
probably a combination of ferricyanide and something else.
The differential re-exposure process is the one currently
used. Kodak changed from the original process to the current
one perhaps a year and a half after releasing Kodachrome to
the market. Initially, Kodachrome was available only as 16mm
motion picture film. Later, probably after the conversion to
the new process, it was made available in 35mm still film,
various sizes of roll film, and sheet film up to 16x20, all
processed in Rochester by Kodak.
Because the couplers are in the second developers rather
than the film Kodachrome must go through four developments.
Kodachrome consists of three layers of B&W emulsion with a
yellow filter layer made of colloidal silver under the top
sensitive layer. It probably also has separator layers
between the emulsion layers. The top layer is sensitized
only to blue light. This is the blue recording layer. The
yellow filter under it removes blue light from the rest of
the film. the second layer is orthochromatic, sensitive to
blue and green, but, because of the yellow filter it records
only green light. The bottom layer is a specially sensitized
emulsion that is sensitive to blue and red, but not to
green. There are a number of sensitizing dyes that will
produce this effect. In fact, if you look at the spectral
response of most panchromatic films you will see a dip in
the green region. In any case, the bottom layer is the red
recording layer. In order to have a positive color image the
three layers must be reversed and the proper dye generated
in each layer. The top layer is yellow, the center layer
magenta, the bottom layer cyan.
In the current process the film is first developed in a
plain developer so that there is a negative silver image in
all three layers. This developer is such that it does not
destroy the red sensitization of the bottom layer. The first
reversal step is to flash the film with red light through
the base. this exposes only the bottom layer because the
other layers are not sensitive to red light. The film is
then developed in a developer containing a cyan coupler. The
developer is of the sort that generates reaction products of
development which will further react with the coupler to
make the dye. Since the only areas of the emulsion which can
be developed are those which were not developed during the
first development, a positive silver image and a coincident
positive dye image result. The film is rinsed and then goes
is flashed with blue light from the front. Since the yellow
filter layer is still intact the blue light exposes only the
top layer. This is then developed in a developer containing
the yellow coupler. Now the process changes. the center
layer can not be flashed because the top and bottom layers
have now had all the silver in them developed so are quite
opaque. So, the third developer contains a chemical fogging
agent along with the dye coupler. Since the only developable
halide left is in the center layer the fogging agent can not
have an effect on the other layers. The center layer is
developed with a coupler for magenta. At this point a
complete positive dye image in complementary colors exists
in on the film and all the halide has been developed to
metallic silver. The next step is to remove the silver using
a bleach which does not affect the dye image. This bleach
also removes the yellow filter layer, which remember is also
made of silver. After this step the film contains a full
color positive image. The film is then washed and probably
stabilized and is finished.
The original process was much like this except that the
three color developers could work in all three layers. After
initial development the negative silver image and yellow
filter layer was removed leaving the remaining halide. Then
the entire film was fogged with light. Then the film was
developed in the cyan coupler developer. This produced a
cyan image in _all three layers_. The film was washed and
dried. Then it was floated on a bath of a bleach which
penetrated the two top layers, removing the cyan dye and
converting the metallic silver image back to halide. Note
that the bleach was so closely controlled that it did not
penetrate to the bottom layer in which the cyan dye and
silver image remained. The film was then developed in the
second developer resulting in a magenta image in the two top
layers. Again the film was washed and dried. Then it was
bleached again, this time in a bleach which penetrated only
the top layer. It was then developed in the final bath, the
yellow coupler developer. After this a full color dye image
was on the film along with the positive silver image. the
silver was removed by a bleach which did not affect the dye
and the film finished. Whew! While both of these processes
are fussy the original one worked mainly because some very
dedicated people made it work.
For a time Ilford made a reversal color film similar to
Kodachrome but it used very thin layers of pre-fogged
emulsion to separate the layers. During the first
development these layers became opaque. The top and bottom
layers could then be selectively fogged with white light and
reversed in the color developers. The center layer, as in
Kodachrome, was reversed by a developer containing a fogging
agent or by the use of a separate fogging bath. The opaque
silver separator layers were removed along with the image
silver and filter layer. I don't think Ilford made this
material for very long and I have no idea of what it looked
like.
There have been an enormous number of attempts at full
color photography beginning almost as early as photography
itself Hand colored Daguerreotypes are not uncommon and
various color separation and chemical processes were tried
well back in the 19th century. Very few of these processes
had any success or even the possibility of success. Some
were eventually successful but could not be made to work at
the time of their invention because the technology and
science if photography was not sufficiently developed. Now,
of course, color is routine but I think we should remember
the perfectly tremendous amount of effort by a great many
men and women it took to get us here.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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