Marjolein Katsma said:
Bart van der Wolf (
[email protected]) wrote in
[email protected]: SNIP
Then how can the orange/reddish color be there on a film that's
been developed as B&W (i.e., no pigments supplied during
development)? I'm sure I'm missing something here.
With the single exception of Kodachrome film (where colors are
introduced during subsequent processing steps), all color (and
Chromogenic B&W) films already have dyes (actually dye-sets per film
family) in their emulsion. Some of those dyes are masking layer dyes
in color negative films, but those masking layer dyes are not present
in slide films because those are intended for projection. This will
require a specific color (slightly greenish/blue) of light to
trans-illuminate the color negative films and 'remove' the mask color,
or one can use different exposure times per channel as a temporal
color filter.
When Chromogenic Black&White film will be processed and printed with
the same equipment and on the same paper as color negative film, it
also requires a masking layer to be able and use a similar exposure on
that paper. That paper by the way is not neutral in response, but it
is made to compensate for non-linearities in the color negative film.
That's why Chromogenic B&W film has similar characteristics as Color
negative film, except for the CMY color layers and mask color.
I also don't understand what you're saying about "polluted" color
layers.
Polluted with what, caused by what?
If e.g. Green light strikes a color negative emulsion, then the
Magenta dye layer will increase in density with processing, but the
Yellow layer will also increase a bit in density. This will pollute
whatever the Yellow layer's density should be due to Blue light
exposure, thus after inversion Blue will be lighter (desaturated) than
it should and that's caused by Green light exposure instead of
exclusively Blue. So one could say that Blue in the resulting image is
polluted by green exposure.
Similarly, Blue light exposure causes the yellow dye build-up but also
Green and Red light will cause some Yellow dye formation.
Depending on the emulsion, the Yellow layer (Blue in the inverted
image) could be formed by 60% Blue, 30% Green, and 10% Red exposure.
In order to remove that 'pollution', the masks lose density in an
inverse rate to exposure and effectively removes the impurities. The
impurities result from impurities in the dyes (they are not strict
band-pass filters but they overlap in transmission), and cross dye
layer chemical pollution (chemical reaction affects multiple layers).
I hope this short (believe me, there's a lot more to it) course in
Sensitometry helped to clarify a bit. I guess my 3.5 years
Professional Photographer's "Fotovakschool/Apeldoorn" education
provided 'some' foundation.
Bart