VueScan Color Management

[Alan]

OK, so I just read Fraser et al.'s "Real World Color Management" and am
trying to apply it to my workflow. I know, a little knowledge is a
dangerous thing, so what I say here may be dangerous.

Setup:
Macintosh 10.3.5
Cinema Display
Epson Perfection 4870 Scanner
Monaco EZ Color calibration
VueScan 8.0.17 Professional

First I calibrated my screen, which seemed to go well.

Then I calibrated by scanner. I have calibrated it both under VueScan
and EZ Color with both transmissive and reflective targets.

A) Creating the color spaces/correction tables.

1) I can't figure out how to use VueScan to create the "uncorrected"
tiff files that EZ Color needs to build its color correction tables.
Epson Scan has an option to create "uncorrected" scans, VueScan does
not. So I scanned the targets in Epson Scan, fed them to EZ Color, and
came up with profiles for my scanner.

2) I had already used VueScan to create profiles (I think in VueScan
7.8.x?) and I am using these.

3) All these profiles were saved in /Library/ColorSync/Profiles

B) Looking at these tables I found that

1) The VueScan color correction files are very small: 4-8 K. I suspect
that they must just be applying a simple curve to each channel. The EZ
Color tables weigh in at > 200K, and presumably include look-up tables,
and could cope with interactions and non-linearities better.

C) Applying the tables I found that the behavior of VueScan seems at
times illogical and incorrect, however I can find settings that give me
good scans.

In the color tab of VueScan are the following three fields:

* Scanner ICC profile
* Output color space
* Monitor color space

I tried setting Scanner Color Space to the VueScan icc and the EZ Color
icc file.

I tried setting Output color space to Adobe RGB and ProPhoto RGB. Adobe
is a relatively large gamut, and ProPhoto is a very large gamut.

I set Monitor Color Space to Adobe RGB and ProPhoto RGB.

D) Some answers:

1) ALWAYS KEEP THE "MONITOR COLOR SPACE" AND THE "OUTPUT COLOR SPACE"
THE SAME. When they are different, Vuescan shows you the RGB numbers
appropriate for one color space as they would be interpreted in the
other space. While interesting, there are few reasons why you would
want to do this. I thought for a long time that "monitor color space"
should be set to the icc for my monitor--this is NOT correct. This
difference can easily be seen by changing these menus.

2) THE MONITOR COLOR SPACE SETTING HAS NO EFFECT ON THE SCAN. This is
as it should be, however, if you ignore the advice above, you will
wonder why the "what you see is not what you get."

3) THE OUTPUT COLOR SPACE AFFECTS THE SCAN, EVEN WHEN IT SHOULDN'T. The
idea of a color space is that it maps RGB values to colors. When saving
the file, VueScan is supposed to map the color values (from the scan)
back to RGB values in the chosen color space. This mapping + the
inverse mapping (in, say, Photoshop) should be invariant.

It shouldn't matter which space you use as long as you don't have
out-of-gamut colors, the colors should be the same. You can test this
by opening an image in Photoshop and changing its color space from
Adobe RGB to ProPhoto RGB--the on-screen colors (usually) don't change.
This is not true for VueScan. Scanning with "Output Color Space" equal
to ProPhoto RGB gives more saturated colors and different hues than
when it is Adobe RGB. This seems to me to be a (big) bug in VueScan.

What is most interesting about this is that if I set the "Monitor Color
Space" and "Output Color Space" to the same values, the image shown in
the "Scan" tab of VueScan appears to be the same. However, when saved,
differences can be seen.

4) I got the best results when I used the VueScan profile for my
scanner, and saved the results as Adobe RGB. There were pronounced
color casts, and variations in saturation, when I either used the
Monaco scanner tables or when I used the VueScan tables and outputted
to Pro Photo RGB.

5) You're milage may vary.

 

[Jeff Randall]

Alan:

The scanner profiles generated by VueScan are simple -- they only
specify the RGB primaries, RGB gammas, and the white point the same as
a monitor profile. Some folks refer to these profiles as matrix
profiles. The advantage of matrix profiles is that they are exposure
independant as well as small and fast. VueScan's built-in profile is
just one Ed created with the scanner he tested while the custom
profile you create is "tuned" to your specific scanner. My experience
has been that unless you require absolute color fidelity and won't be
post scan tweaking b/w points, curves, saturation, etc., then
VueScan's matrix approach works very very well. I basically use it
100 percent of the time.

EZ Color scanner profiles and the like, create color lookup tables and
thus are much larger. These CLUT type profiles require that you scan
each image with exactly the same settings, including exposure. To
create these profiles with VueScan, you can scan the target with color
balance-None and output color space-device or you can just output a
raw scan (raw ignors color tab settings) and create the profile. Raw
48-bit has a gamma of 1, raw 24-bit has a gamma of 2.2. Some
profiling packages do not well with a gamma of 1 so you will need to
experiment.

Good luck.

Jeff Randall