Converting CMYK images from coated to uncoated colorspace

Hi,

I have a CMYK image assigned with a Fogra 39L profile for coated substrate but the image needs to be reproduced on uncoated substrate based on Fogra 29L profile.

What will be the correct way of converting an image that's in Fogra 39L (coated) to Fogra 29L (uncoated) colorspace?

Will these 2 methods of conversion result is exactly same result:

a) Convert the CMYK (39L) to RGB (Adobe 1998) and then convert it back to Fogra 29L (uncoated) colorspace

OR

Converting the Image directly from Fogra 39L to Fogra 29L

What should be the correct way of handling these kinds of conversions, where input data is already in CMYK colorspace?

Thanks

The best way to convert an already-separated CMYK file to another CMYK output space is via a device link profile (in this case, one that is specifically built to effect a conversion between a source of coated Fogra 39L and a destination of uncoated Fogra 29L). With a properly-built device link, besides achieving a usually smoother-looking result, you can also control a number of parameters such as the purity of the primaries, black-only elements, etc.

Photoshop CS4 will finally be supporting device links, by the way -- a cheer to our gracious hosts at Adobe for finally doing something many of us had long wished for!

As for the more "traditional" way of converting between the two spaces (i.e., a straight ICC profile-to-profile conversion), the extra step of doing an intermediate conversion to AdobeRGB (or any other RGB working-space) is quite redundant, and brings no detectable advantages.

HI MARCO

>Photoshop CS4 will finally be supporting device links...<<




what you meen in "device links" i dont understand this paragraph?

thank

Koki,

a standard profile for CMYK contains all informations
for a mutual conversion CIELab <--> CMYK.
A device link profile for two profiles CMYK1 and CMYK2
replaces the standard workflow CMYK1 <--> CIELab <--> CMYK2
by one profile CMYK1 --> CMYK2 (for only one direction).
According to Marco, transforms by device link profiles
are smoother than traditional transforms (which goes there
without any proof, but it's indeed advertized by companies ...).

I'm using device link profiles for proof printing:
The input profile is for instance ISOCoated, the output
profile that of the proofing inkjet. The device link profile
is calculated once and applied very often during the calculation
of device pixels. Therefore it's fast, but concerning the
quality I don't see a difference, compared to proofing with
traditional calculations via CIELab.

Best regards --Gernot Hoffmann

thank!!!!!!!!!

Koki,

A devicelink profile is a special kind of profile. It is not 'inside the profile'. They are very handy but not common, and software like Photoshop cannot use it yet (may be CS4 can?). Some RIPs and high level printers do use them. Refer to the documentation of your machines if you don't know wether they can take advantage of them or not.

So with a devicelink profile, you can convert, for example, a document from an offset coated CMYK with 340% ink to an uncoated newsprint with 280% total ink preserving the black as pure black and some other details.

If you want to know a bit more, you may look for instance an on-line manual of Link-o-Lator <http://www.colorwiki.com/wiki/Link-o-lator_Interface>

THANK YOU GUSTAVO!