Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!cs.dal.ca!ug.cs.dal.ca!legrady
From: legrady@ug.cs.dal.ca (Tom Legrady)
Newsgroups: comp.graphics
Subject: Re: Color scanning from B&W scanner problem
Summary: Try Roscolux filters
Message-ID: <1990Jan5.190700.28712@ug.cs.dal.ca>
Date: 5 Jan 90 19:07:00 GMT
References: <720004@hpvcfs1.HP.COM>
Reply-To: legrady@ug.cs.dal.ca.UUCP (Tom Legrady)
Organization: Math, Stats & CS, Dalhousie University, Halifax, NS, Canada
Lines: 37

In article <720004@hpvcfs1.HP.COM> stevem@hpvcfs1.HP.COM (Steve Miller) writes:
I recently purchased the color scanning package Chroma-32 from Discovery
Software.  This package supposably lets you get color scans using your    
greyscale scanner by scanning the image three times with three different
color filters (plastic 8.5X11 sheets).  This package runs on a Macintosh
on which I have an HP ScanJet+ 256 gray level scanner hooked to.

The Problem:

The color scans don't have good color and I think I know why:  The color
filters suck!  The red filter doesn't block all of the blue or green.  The
green filter doesn't block all of the red or blue light etc..  The result
is muddy looking (under saturated) colors.  An easy test demonstrating that
the filters aren't very good is taking a picture with lots of color and
placing one of the filters over it (say the red filter).  In theory you
should only see red color coming through:  whites and reds should be a 
bright red and all other colors should be darker shades of red or even black.
However, this is not what you see.  The red filter allows fairly large
amounts of green and blue to come through.  Likewise the green and blue
filters allow other colors to come through.

----------------------------------------------

Roscolux produces colour filters for theatre, video and cinematic use.  They
make a large assortment of colours, and the sample books include a graph
of the absorption spectrum ( very approximate! ). Check "Theatrical Equip-
ment and Supplies" in your Yellow Pages (tm).

Note that some of these may be overkill. Their darkest blue passes only
about 1%. 

It may be better to use some filter that passes a wider range of colours
and do some processing on the resulting data.


Tom Legrady
legrady@ug.cs.dal.ca