'Safest' way to convert PMS - CMYK

Thanks for the Help link, Didier. Good info.

That helps, but the original problem still persists, since different color libraries (from the same vendor) specify different Lab values for a given spot color. I guess that is to be expected to some extent, especially since so many spot colors are out of CMYK gamut. But, you would think Pantone would keep at least Lab values consistent for a given color on a given paper grade over the course of many years. I'd expect Pantone 300C to look the same on #3 coated stock from one year to the next, since it uses the same ink formulation on the same paper grade (as long as it is printed carefully and consistently with industry standard inks, papers and process control). That should result in Lab values that are a very close match from one year to the next. So, the designer is still faced with the problem of which set of Lab values to select.

Of course, Pantone 300M, printed on an uncoated #5 sheet, may well have different Lab values than 300C printed on gloss stock. It does make it confusing for the designer, and also for very discriminating clients (luckily, they are not always as picky as we designers are).

My current solution is to look at the Pantone Solid Coated library, and note the Lab and CMYK equivalents in my given CMYK working space, using Relative Colorimentric. If the color is out of gamut, I click the OOG warning and see what colors I get. Then I look at the Color Bridge PC library to see what colors I get. They're usually different. I might even look at the Solid to Process Library. The one that gives me the best "eyeball" match on my carefully calibrated monitor is the one I usually use, but I also exercise some judgement based on past experience.

For example, 2935C is one of the corporate logo colors of my previous employer (out of CMYK gamut, of course).  Here's info from three different Pantone libraries for coated stock (USA inks and standards):

Color Bridge PC

100c, 52m, 0, 0   43L, -13a, -50b This color is printable and is a decent match to the swatch book

Solid Coated

100c, 65m, 2y, 0    36L, -4a, -63b (this color is OOG so you have to bring it in gamut to evaluate)

100c, 65m, 2y, 0    39L, -6a, -49b (this is the Lab equivalent when brought into gamut—prints way too purple from lots of experience)

Solid to Process Coated

100c, 46m, 0, 0    45L, -17a, -50b (this one is too light and errs a bit toward cyan, but helps avoid purple shifts on press)

So, what did I use? From experience in printing this color over and over on corporate brochures, fliers, post cards, posters, etc, on lots of different presses, I settled on 100C, 50M, 0Y, 0K (between Color Bridge and Solid to Process). It errs slightly on the cyan side, but I find that much more acceptable that a purple shift if the magenta is laid down too heavy.

That illustrates the mess they have made out of specifying spot colors. Sort ot takes the science out of it, doesn't it? Well, nobody said it was perfect, and it is a good guide to get us closer to where we want to be, faster. But, judgement, knowledge and experience still come into play. I did a job for a customer recently, and their corporate logo uses 300C, very close to my old corporate color, 2935C. So, I purposely erred on the side of less magenta based on years of struggling with my own corporate color. They were happy with the printed job, fortunately. They selected the printer themselves after the job was designed and submitted. I submitted it as a PDF/X1-a file, stripped out all profiles to prevent conversions, and specified the original design intent as SWOP_Coated #3v2. I also submitted proofs and cautions about color matching, to let myself off the hook.

We have a ways to go.

Lou

jethrodesign wrote: (The numbers are very different, for instance our dark blue PMS 539C is either 100C, 49M, 0Y, 70k using the Pantone conversion, or 100C, 77M, 47Y, 51K using LAB conversion. Those are pretty radically different numbers and kind of make me nervous).

The first build is Solid to Process.

One good feature about Solid to Process, you will notice that almost all of the builds are comprised of only 2 or 3 primaries. Hardly ever 4.

The colors are fully saturated, then darkened with black when necessary. (Black also desaturates because CMYK is not a cube like Lab)

The Lab conversion technique introduces the yellow gray component to the 539 color. In theory no problem with that, and if the press does its job it should be OK. That is the color management solution.

But with this Lab conversion you have four ink variables for the critical color swatch instead of only 3. In any production system, an additional variable is more chance for error.

I would use both color management and a Pantone build library. Create a blank CMYK in PS and assign the appropriate profile. For example Lou's suggestion of SWOP2006 grade 3.

Go to the color picker. Pick the solid 539 and hit enter.

Go back to the picker again. Change to the Solid to Process library. This identifies the closest STP match to the solid Lab in your current CMYK color space. It happens to be 5395 (FWIW) using relative colorimetric.

Going to process coated you end up with DS 327-1. This is 35C 0M 0Y 100K. So not a great result using that library.

There are other solutions you could try too. Some people use Saturation intent for solid colors, which may yield a more pleasing result. Others create a specialized max GCR version of the CMYK space and convert the Lab to that, ensuring that black is used as the gray component.

Bridge may be OK to use too. It is the new Pantone build system after all. However like the Lab conversions, many Bridge builds use 4 colors. Another problem with Bridge, it is not the Adobe default, so it's not widely used.

Something else too when using Lab source for solids. If the job is on a dull coated sheet you want to use Matte. The Solid Coated is for a gloss sheet. There is not a huge difference as with uncoated, but there is a difference nonetheless.

Jethro,

In general, I never use spot colors in a design unless my ultimate intent is to actually print with premixed spot inks. I recently completed a job where my customer's logo is specified as a bright blue (Pantone 300C), but they told me they did NOT want to print the brochure as a 5/C job (ie, CMYK + PMS 300C).Pantone 300C lies outside of the normal CMYK gamut. I told them this and they just told me to get as close a match as I could with CMYK.

I used Pantone Color Bridge PC (Process coated) to select the best CMYK equivalent to 300C based on their current recommendation. They recommended 100C/42M/0Y/0K. On my calibrated monitor it looked like a reasonable match, so that's what I went with. I also have two swatch books and looked at CMYK equivalents to 300C in both, just as a check. (By the way, the Pantone solid to process guide I have suggests 100C/44M, but I chose the color bridge numbers, since I'd rather err on the side of less magenta). I set up InDesign with color management turned on, and set the CMYK profile to SWOP2006_Coated3v2 (IDEAlliance profile, which conforms to the latest G7 criteria).

As in your case, the final printer had not been selected, and was not selected until after the job was completed. So, I converted the job to PDF using PDF/X1-a, but specified SWOP2006_Coated3v2 (Grade #3 paper, with a 300 TIL). I felt this was fairly safe. The job printed fine, but that's a crap shoot when you don't know who the printer will be. I was cut out of the loop after the job was submitted to my customer.

To cover my butt, I sent high quality printed proofs on a nice, bright, coated paper from my calibrated inkjet (custom profiles), which I set up to simulate the above SWOP profile. I also put in writing that the printer's final job should come very close to my proofs if they did everything right and met industry standards. I also told them my PDF had stripped out all profiles, converted everything to the SWOP profile (preserving CMYK numbers, so I didn't convert black plate data into CMYK), etc. I also supplied the original InDesign file with tagged files, fonts, images, etc. I didn't want to get hung if the job ended up looking like crap. As it turns out, the customer selected a decent printer, provided my proofs as a guide, and the job printed fine (this time).

I sometimes wonder if Pantone looks at their sales figures, decided they need a boost, and releases new swatch books with new numbers just to pump up sales! Your color (539C) is a deep, dark blue. Here are the some of the numbers I came up with:

1.  100/77/47/51 gives 15L, -7a, -19b in the PS color picker as the Lab equivalent.

2.  100/49/0/70 gives 15L, -9a, -32b (same density, much bluer)

3.  Pantone Color Bridge PC gives 100/58/23/77, with a Lab equivalent of 8L, -11a, -24b, darker tone, but color in between the two above.

4.  Pantone solid coated guide gives a Lab equivalent of 15L, -9a, -21b.

The good news is that this color is so darn dark, that a bit of color shift one way or the other may not be all that critical. Pick your poison.

Good luck.

Lou