filmscanners: Re**n: Rescans and archiving

["Alan Tyson" <AlanTyson@bknowl.freeserve.co.uk>]

I refer the honourable gentleman to my answer of 26th
June.....

AIUI, emulsions such as C41, E6, and ordinary colour print
papers have the  dyes synthesised not in a chemical factory,
but in situ in the emulsion during processing. 'Colour
coupler' molecules in the emulsion and the oxidation product
of the
colour developer link together to give a long molecule with
the desired colour. This is a remarkably clever bit of
chemistry.

In contrast, in Kodachrome and Cibachrome processes the dye
molecules are presynthesised in one piece. In Kodachrome
they are diffused into the emulsion during processing, and
in Cibachrome, the unwanted dyes are selectively bleached
out during processing.

It is thus inherently easier in the latter processes to use
permanent dyes; the colour chemists have fewer constraints
because they don't have to meet the requirements of
developer chemistry as well as everything else.

Regards,

Alan Tyson

----- Original Message -----
From: Rob Geraghty <harper@wordweb.com>
To: <filmscanners@halftone.co.uk>
Sent: Friday, December 14, 2001 12:05 AM

> I've heard that too, although I don't understand what
difference it makes
> when the dyes are added!
> (but I can see that it does)
>
> Rob