[rollei_list] Re: The end of Kodachrome?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 31 Dec 2010 18:36:52 -0800
----- Original Message -----
From: "Edward Meyers" <aghalidebw@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, December 31, 2010 5:42 PM
Subject: [rollei_list] Re: The end of Kodachrome?
Kodachrome is basicallly a B&W film, which is developed in
special color
developers.
Ed Meyers
True but also true of all color films. The difference is
that the "couplers" which are the chemicals that become the
dyes, are incorporated in the emulsion layers of modern
films but were not in Kodachrome where they are in the
reversal developers. In both cases the color from a reaction
between the coupler and reaction products of the developer
as it converts the silver halide to metallic silver. In
reversal films all three layers are first developed in a
non-reacting developer to a black and white silver image. In
a color negative film the first developer has is such as to
produce the necessary reaction products to further react
with the couplers to make the dyes. The silver image is then
bleached out. For reversal the color development takes place
in the second or reversal developer. In modern films which
have the coupler in the emulsion all three layers are
developed at once as in the negative film. For Kodachrome
three separate reversal developments are necessary, one for
each color. In the later Kodachrome process the layers are
developed separatlely by separate exposure of two and a
fogging developer for the third. The process works because
the panchromatic sensitivity of the layer closest to the
support is preserved by the type of developer used to
develop the layers to negatives. Since the remaining halide
is still light sensitive it is made developable by flashing
through the base with red light. The center layer is
orthochromatic so is blind to the red light and the top
layer is blue sensitive so is also insensitive to the red
light. The base layer is then developed into a positive with
a developer which also produces the cyan dye. The top layer
is then re-exposed using blue light. Since the yellow filter
layer between the top and second layers is intact the blue
light can not reach the center layer. The film is then
re-developed in a developer with the yellow dye coupler in
it. The center layer can not be light flashed because both
top and bottom layers are now opaque, all halide in them
having been converted to silver. So, it is developed in a
reversal developer containing a fogging agent plus the
coupler for magenta dye. The film now has a positive magenta
image in the center layer, a yellow positive image in the
top layer and a positive cyan image in the bottom layer, all
imbedded in the silver. The silver, including the yellow
filter, is now removed by a bleaching bath leaving only the
positive subtractive dye images in the film. There are other
steps such as rinses between developments and washing but
this is the essential process for Kodachrome. Note that all
the images of both variations of film color film start out
as silver images. All color films can be developed to B&W
silver although there may be residual color due to masking
or filter material which is often difficult to remove.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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