Q-60 question

  • Thread starter Thread starter Bauie
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Bauie

Are the grey patches on a Q-60 supposed to be pure grey? ie, no hint of
color at all? The one I just purchased has quite a lot of cyan in the
lighter greys when viewed with the naked eye. How can you calibrate a
scanner when a grey target is not grey at all? Am I missing something
here?

Before assigning my scanner profile patch #5 reads: R151, G181, B165
From my fine art prining days I have a Kodak "photographic step tablet
Click to expand...
no. 2" are these supposed to be pure grey?

At the end of the day what I want to do is arrive at settings that I
can apply to every color scan that will remove any color bias
introduced by the scanner (Nikon super coolscan 4000)
Thanks for any help.

Mark
 
Bauie said:
Are the grey patches on a Q-60 supposed to be pure grey? ie, no
hint of color at all?
Click to expand...

They are not necessarily exactly neutral but usually close (when
viewed at a 5000 Kelvin color temperature lightsource), and since the
actual values are supplied with the target, close to neutral gray is
good enough. What *is* important is the accuracy of the accompanying
data file.
The one I just purchased has quite a lot of cyan in the lighter
greys when viewed with the naked eye. How can you
calibrate a scanner when a grey target is not grey at all? Am
I missing something here?
Click to expand...

The calibration procedure compares the actual (as given in the
target's data file) with the perceived (by the scanner) colors. The
actual colors on the target (if close enough to the intended colors)
will provide a reasonable (for the purpose of interpolation) spread
throughout the possible gamut for the material it was produced on.
Before assigning my scanner profile patch #5 reads: R151, G181, B165

step tablet no. 2" are these supposed to be pure grey?
Click to expand...

Yes, reduced silver densities have a good/uniform absorption
throughout the visible spectrum.

If you want to compare the actual IT8/Q60 colors with the RGB readings
in a given colorspace, you could use
<http://www.brucelindbloom.com/ColorCalculator.html> for conversion.

Bart
 
Bauie said:
Are the grey patches on a Q-60 supposed to be pure grey?
Click to expand...

The IT8 specification does define them as absolute neutral grey under
D50 illumination (this is the same as direct sunlight) but it is very
difficult to manufacture them so accurately, therefore you get the
reference file with your target that has measured values for the
target that you have.
ie, no hint of color at all? The one I just purchased has quite a
lot of cyan in the lighter greys when viewed with the naked eye.
Click to expand...

If this is so when viewed under D50 light then it is not a very good
target.

The whole horizontal scale at the bottom of the target is defined to
be neutral gray.
How can you calibrate a scanner when a grey target is not
grey at all?
Click to expand...

ICC profilers can take this into account, however they do need to
"guess" more in this situation and not all of them are very good in
that.
Before assigning my scanner profile patch #5 reads: R151, G181, B165
Click to expand...

The RGB values do read the same both before and after the profile is
Assigned.

What the raw data from the scanner is not relevant. What the RGB
values are after:

1) Assign Scanner profile
2) ConvertTo RGB working-space

depends on

a) what RGB working-space you use (here the whitepoint setting is of
particular interest and
b) what conversion options you use in the step 2 above.

Usually the most accurate conversion settings from an acquire device
profile in Photoshop are:

Engine = MicrosoftICM (or ColorSync on Mac)
Intent = Relative Colorimetry
Blackpoint Compensation = OFF

The target image on screen should now (after the above two steps)
appear very similar to you vision compared to the target itself when
it is illuminated with D50 light at such illumination level that is
comparable to the output of the screen (adjust the light so that the
Dmin patch of the real target has about the same luminance as it has
on the screen).

If it does not appear similar then the profile is not good.
From my fine art prining days I have a Kodak "photographic
step tablet no. 2" are these supposed to be pure grey?
Click to expand...

I have not seen specs for that. But if it is very old it is probably
not in very good condition anymore, what comes to the hue of it.

If you have Photoshop and Excel you may want to try my XLProfiler
from: http://www.aim-dtp.net/aim/calibration/xlprofiler/index.htm you
can use it also to create an accurate simulation image for your
target.

Timo Autiokari
 
Timo Autiokari said:
The IT8 specification does define them as absolute neutral grey under
D50 illumination (this is the same as direct sunlight) but it is very
difficult to manufacture them so accurately, therefore you get the
reference file with your target that has measured values for the
target that you have.
Click to expand...

When I purchased my IT8 targets (for Ektachrome and Kodachrome), all I
got was the slides -- no reference file. I did find, however,
reference files on the Kodak web/ftp site and downloaded these to use
with the calibration process. Did I do the right thing?

I've been using Vuescan since the IT8 process is available. Is there
a method to use IT8 with older versions of Nikonscan (i.e., v3.x that
supports LS2000/SCSI under MacOS9)?

David
 
My understanding is that you get the numbers for the particular Q60,
(mine is 1999:10) then through some complicated math you read the
output from the scanner and somehow compare it to the numbers.

For example, my A1 patch reads: L 8, a 2, b 0, Kodak says it is L 11, a
9.64, b 2.35. What exactly I'm supposed to do with the difference
between these numbers is what I'm trying to find out.

On Kadak's data I have no idea what X, Y, Z, S_X, S_Y etc mean, I
understand the Lab color model numbers though

Mark
 
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