Datacolor User to User Support Group.

>>Bob Petruska <petruska@...>: I will be running the tests over the weekend and report back to the forum my findings.

I will be most interested in your report...


>>I'm sitting here thinking that even though I have a calibrated NEC 
monitor how do I know that the printer and the monitor think alike 
with the same file data?  What I'm getting at is that if I have two 
calibrated mid range quality monitors like a NEC and Samsung sitting 
next to each other I would bet that the same image file would look 
slightly different between them.

If you are looking at the same image on different monitors using a color management aware software (let's say Photoshop with the same Working Space on all the monitors), then the images should display identically because that's the whole point of HARDWARE monitor calibration.

Many years ago, Bruce Fraser (sadly missed) discussed color management, monitor calibration and printer profiles (on Epson printers but it should apply to Canon printers also) in various posts on the old Compuserve Publishing Production forum and I saved those posts.

I hope you will find Bruce's explanations useful:

"At the heart of all color management there's a thing called the
profile connection space, or PCS. In current implementations it's
always either CIE Lab or CIE XYZ, and the math for getting from one
to the other is fairly trivial.

The unique property of these spaces, as opposed to RGB or CMYK, is
that they unambiguously represent a perceived color.

Profiles are essentially lookup tables that say what device values
(RGB or CMYK) will produce specific perceived colors.

Color-managed applications like Photoshop use working spaces so that
they can make the image look right on different monitors.

The Epson profile doesn't try to match your monitor, it tries to
match the working space, via Lab. Photoshop also does an on-the-fly
transform from the working space to your monitor's space, so that the
image appears correct on your monitor, whatever it is. It'll do a
different transform for someone else's monitor, but as long as the
monitor profile is accurate, the image will appear identical.

You profile your printer so that, when it gets fed known color
values, it produces those colors. You profile your monitor so that
when it gets fed known color values, it produces those colors.

Working spaces contain known color values, because they're referenced
to Lab. So when you feed a file in a working space to
correctly-profiled devices, the color comes out right. The printer
and the monitor are entirely separate from each other. They only talk
to each other through the color management system.

So it really doesn't matter what standard you choose to calibrate
your monitor to. All that matters is that you have an accurate
description of that standard (a good monitor profile), and the color
management system will ensure that the signals that get sent to your
monitor will make it display the image correctly. It'll do the same
thing on my monitor, even if it's calibrated to completely different
settings from yours."

AND:

"What color management can do, and do well, is to translate the monitor image into the color gamut and dynamic range of your printer, as faithfully as possible, and at the same time show you how the print will appear, allowing you to make any necessary optimizations before you print.

In applications like Photoshop, images live in an abstract RGB working space that isn't tied to the quirks of a specific device, such as Adobe RGB (1998).

To display the file accurately on your monitor, Photoshop looks at your monitor profile, and adjusts the values being sent to the video card so that the color displays correctly on your monitor. It will do the same when the same image is displayed on my monitor, or on Tom's or Dick's or Harry's. Other than that, the monitor is out of the loop.

When you print using color management, the CMS looks at the source color space (e.g. Adobe RGB) and at the output space (e.g. your printer profile). It derives the Lab values that are represented by Adobe RGB values and by printer RGB values, then builds a big lookup table to go from the one to the other. Your monitor is out of this loop.

To see how the print will appear before you make it, you need to use the CMS to simulate the output, which it does by converting the data on the fly from Adobe RGB to printer RGB, then to your monitor RGB. That's what Photoshop's Proof Setup features are for. In this case, the monitor is in the loop, but your printer profile doesn't need any knowledge of it.

Profiles are like dual-language dictionaries. They're essentially lookup tables that have device signal values (RGB or CMYK) on one side, and device-independent CIE values (LAB or XYZ) on the other. No profile needs any knowledge of any other device's behavior: all the profiles use CIE LAB or CIE XYZ as the interchange medium (they're called the Profile Connection Space, because that's their role)."

AND:

"If you calibrate the monitor, save the profile, load the profile, and
let Photoshop update the profile, you'll get an accurate display of
your image at that calibration. If you fail to take any of these
steps, Photoshop won't be displaying the image properly. When it does
display the image properly, it doesn't matter how your monitor is
calibrated. Right now I'm looking at the same image on a 5000K gamma
1.8 monitor, a 6500K gamma 1.8 monitor, and a 6500K gamma 2.2
monitor. The image looks identical on all three.

Once more, the file-to-print transform is entirely separate from the
file-to-monitor transform. You can whack your monitor out into all
weird states under the sun, and it will still have zero influence on
the print. Forget about the idea of calibrating the monitor to the
print. That isn't how it works. You calibrate the monitor to some
known CIE values, and you profile the printer to some known CIE
values.

Lets say you have two pieces of wood, each of which is some length.
You could say, 'piece o' wood #2 is 1.8 times the length of piece 'o
wood one.' That's like trying to calibrate your monitor to your
printer, and it isn't all that useful.

But if you measure them, you can say 'piece o' wood 1 is 10 inches
long. Piece o' wood 2 is 18 inches long.' 

That's how color management works, only the inches are CIE units instead."


Terrie
http://tlbtlb.com/
tlbtlb@...