With my printer, I need to be able to print colors in CMY, not CMYK. Is there any way to convert Pantone to CMY values without the K?
The digital values of Pantone inks are given by Pantone. The only useful digital values representing a color of an ink without specifying the printer and the paper can be defined only in the Lab color space because it represents the color appearance based on human perception. From there it can be converted to any color space representing a printer and paper used.
One way to convert from the given Lab color to a CMYK color space is to use color management which happens automatically in CS6 when you switch to the CMYK slider in a CMYK document when you have a pantone color. In the previous versions you have to choose the Lab method for the Spot color option from the Swatches' panel menu. If the Lab color gets converted with adding a value to the K, there is nothing you can do but to put it to 0 which will alter the color.
The other way is manually to try playing with the CMY values without using K until you get a color that matches as much as possible the pantone color displayed with the Lab method on your screen.
The usual way with color management is if you really have a CMY printer that doesn't use black ink, then you get a spectrophotometer with a calibration software that will print and measure color samples and will create a color profile of your printer. Then you assign this printer profile as the color space to your document from Edit > Assign Profile and when you convert a Pantone Lab color values to CMYK it will give you only CMY color, matching as much as possible the pantone color. Usually printer manufacturers provide with the printers color profiles for the printer and the different papers
purchase a printed Pantone swatch book, coated or uncoated,
which one is nearer to your printer paper (or buy both).
A comparison of printed ink and monitor preview is quite use-
less (different viewing conditions; many spots inks are out of
gamut for monitors).
Set in program: color management OFF
Set in printer (driver): color management OFF
Print page 2 of this doc:
It contains all CMY0 colors (K=0), 0 step 10 to 100
Then check by a loupe whether pure colors #00, 0#0, 00#
are printed by pure inks C,M or Y.
The result is TRUE or FALSE.
TRUE: your printer accepts really CMY.
Choose for your Pantone sample (Swatch book) the nearest
printed swatch, interpolate visually and read values CMY.
FALSE: most likely your printer accepts RGB values.
Print page 19 of the same doc.
It contains all RGB colors, 0 step 25 to 250
Continue as above.
If the swatches #00, 0#0, 00# are not printed by pure inks,
then your color management is not OFF.
(1) All this makes sense for vector graphics (stroke, fill, text).
For images one needs a complete ICC profile.
(2) Most likely your printer prints with CMY inks and expects RGB
numbers. In my experience only PostScript printers are able to print
CMYK directly - then always including K.
Printer test pages
Best regards --Gernot Hoffmann
that´s a good idea, how to convert an already available CMYK profile
into a CMY profile.
Unfortunately it's not helpful for the OP - he doesn't have a CMYK
profile for his printer and inkjet or toner printer gamuts a very different
to offset process gamuts (graphic below).
Even if the OP's printer should really work with CMY number inputs
(Device CMY), it would be difficult to create a profile.
I read, it had been possible with ProfileMaker 4, but the feature has
been removed in newer versions.
Best regards --Gernot Hoffmann
Gamut volumes in CIELab. Offset versus Inkjet. Not CMY but CMYK.