[rollei_list] Re: The end of Kodachrome?
- From: John Jensen <jwjensen356@xxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sat, 1 Jan 2011 14:15:36 -0800 (PST)
Richard, I understand the processes (and had read similar material many years
ago). But what always struck me is the permanence of Kodachrome over many
years. So many of my reversal slides from Anscochrome (worst), Ektachrome
(better), and Agfachrome (best) have color shift. Do you have an opinion as to
why Kodachrome is best of all in this regards? After all, they all have color
dyes.
John
--- On Fri, 12/31/10, Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:
From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
Subject: [rollei_list] Re: The end of Kodachrome?
To: rollei_list@xxxxxxxxxxxxx
Date: Friday, December 31, 2010, 6:36 PM
----- 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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