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From: mcdonald@aries.scs.uiuc.edu (Doug McDonald)
Newsgroups: comp.graphics
Subject: Re: Color scanning from B&W scanner problem
Keywords: don't do colour with bw scanners
Message-ID: <1990Jan7.155917.29080@ux1.cso.uiuc.edu>
Date: 7 Jan 90 15:59:17 GMT
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In article <5443@udccvax1.acs.udel.EDU> cygnus@vax1.acs.udel.EDU (Marc Cygnus) writes:
>
>I just noticed the subject line :-)
>
>I don't recall if this has been said before, but it's probably a good
>idea if you not waste your time trying to force a BW scanner to do
>colour. Besides the fact that the lightsource is probably not a good
>wideband source, and alignment is _ridiculous_ (unless you tape your
>source image to the underside of the scanner's bedcover... yuck), you
>should keep in mind that the light will have to pass through the filter
>_twice_.
>
>When doing camera work with colour separation filters, it's suggested
>that 2 f-stops of compensation be used with the filter (and that's when
>the light only passes through once).
>
I actually tried scanning my MacBeth Color Checker on an Apple
scanner.
Results:
   The color response of the scanner is apparently designed to
give "correct" gray scale values of color material (i.e. blue
darker than red darker than green), because it does do so, quite well.
It has enough response in the blue and red to be useful. But
of course it is only a 16  level (4 bit) scanner, so would
be useful mostly for scanning colored line art. You probably
could get some sort of tri-grayscale image by varying the contrast
and brighness, but it wouldn't look very pretty.

The Wratten filters mentioned previously are certainly NOT right
for using in such a scanner. (In fact, they are probably not right
for use in front of a camera, for photographing general subjects.
They are TOO narrow. They are for PRINTING use, or for doing color
separations from three or four color printing or color photos.)
For that you need bigger filters, and ones with higher transmission,
due to the double pass nature. The theatrical gels are indeed
available in suitable colors, and are available in big sizes.

With the proper filters you should be able to get excellent
scans of colored line art.

Of course, with a Mac you are going to have fun figuring out
what to DO with color separations!

Doug McDonald