Using Lab to Determine Proper CMKY Values

[billw@mlode.com]

Greetings,

I have tried, with limited success, to use Lab values in Photoshop to determine a matching set of CMYK values for my press.

The press is an Indigo on which I have printed a target and made a few profiles with different attributes. I also made a proofing queue for the press on our Oris proofing rip.

We have a customer / artist that wants us to match an existing label that was printed offset. The samples from the offset printer are not consistent in color and the artist has supplied me with two different labels that have the gold and a beige color they want to match. The artist also gave us some suggestions for color from an old copy of the AFGA Process Guide. We used to have that guide, but no more, so I have no way of knowing how close the CMYK inks in the guide are to the Indigo's CMYK.

I measured the Lab values from the two target colors; created a Lab document in Photoshop; created the two colors and then converted the document to one of the profiles to obtain the CMYK values. While it worked pretty well for the beige color, it did not work so well for the gold color. For the beige the dE was under 1, but for the gold the dE was near 3.

I tried conversions using the different profiles I made (GCR3, start 0, TAC 320; GCR1 start 0, TAC 320; GCR3, start 40, TAC 320, and a few more), with all giving me close to the same CMYK values for both the gold and beige. By the was the TAC is a guess, as I did not include a CMYK progressive value ramp on the target - I know, shame on me.

Has anyone done this technique to find CMYK values in Photoshop? If so, a little guidance would be appreciated. I tried both absolute and relative, black on, off, etc.

Thanks.

-Bill-

[Bret Hesler]

I have done this, and it gives you a pretty good starting point, but sometimes you need to tweak the values a little. I usually use a CMYK document, and in color settings set the destination profile as my working space and Absolute Colorimetric. Then I plug the Lab numbers into the color picker and it gives me the CMYK build for that profile.

If you are making profiles with Profilemaker, the ColorPicker application is actually built for just this purpose, and it lets you find the lowest Delta E combination.

Bret

[gordo]

You could also use good old steam power technology by printing a color atlas on your Indigo, something like one of these: Quality In Print: The Color Atlas - helping designers to specify color
or print out do a ring-around of CMYK screen tint builds.
Then make a direct visual (or measured) comparison between his sample and your actual press color.

best, gordon p