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From: hitchner@hydra.riacs.edu (Lew Hitchner)
Newsgroups: comp.graphics
Subject: Re: Color scanning from B&W scanner problem
Message-ID: <1941@hydra.riacs.edu>
Date: 5 Jan 90 20:34:25 GMT
References: <720004@hpvcfs1.HP.COM>
Sender: uucp@riacs.edu
Reply-To: hitchner@hydra.riacs.edu (Lew Hitchner)
Organization: RIACS, NASA Ames Res. Center, Moffett Field, CA
Lines: 27

You are right about the color filters being the problem.  Colored cellophane
is a horrible choice for color separation filters.  You must use "real"
filters,
i.e., something (glass or plastic) that has a clearly defined stop band and
pass band for the spectral range needed for each of the red, green, and blue.
The filters you should buy are called wratten filters.  These are not available
at most retail photo shops, but should be available from photo supply
companies,
direct from Kodak or another photo industry co., or possibly at larger printing
or copying businesses.  Wratten filters are named by a number that seems
to have
nothing to do with the wavelengths of the filter.  The filters numbers
are integers
in the range 0 to 100 (or therabouts), some with an "a" appended, such as 23a.
I don't know the optimal filter numbers for r,g,b separation.  You'll
have to ask
for a catalog or ask an expert.  However, I do know that filter numbers
22 thru 29
pass red (approx. 560-700 nanometers) and block the rest, and that
filter numbers
57a to 59 pass green (approx. 500-550 nm.) and stop the rest.  Sorry, I
don't know
the filter numbers for blue filters.  But, I hope this helps get you
started in the
right direction.

          Lew Hitchner