The Print Geeks: Color Control 101
On May 4, WhatTheyThink.com launched "Print Geeks" with a webinar to review several common causes of color management failure. Resident geeks Jim Raffel and Shelby Sapusek of ColorCasters were joined by contributor Dan Gillespie of Alder Color Solutions for this session. Richard Romano had the unenviable job of keeping three print geeks on topic and on the clock.
The panelists set out to run through five common areas that color management fails in the world of inkjet printing. Along with presenting the necessary amount of color theory and color management basic knowledge, widely varied experiences from the field were shared by Raffel, Sapusek and Gillespie.
This article only touches upon the conversation that took place during the webinar. Be sure to watch the recording to catch the lively back and forth between three of the industry’s leading color management print geeks.
RGB and CMYK are different than L*a*b*.
A significant amount of design work occurs in the RGB color space, which is an additive color space where colors are created by adding light to an otherwise dark surface. Think of your computer monitor which starts out black until you turn it on and display an image.
Inkjet printing occurs in the CMYK color space, which is a subtractive color model. While this seems counterintuitive, since we are adding ink to a white sheet of substrate, what’s actually happening is that ink is allowing less and less light to reflect off the substrate and return to our eye.
Both the RGB and CMYK color spaces are what we call device dependent. That means that a set of RGB values that displays the correct color on your monitor will likely look different on someone else’s monitor. The same is true of a set of CMYK values that produce just the right color on your printer, but an entirely different color on another printer.
The solution to this problem is the device independent L*a*b* color space. With just three coordinates, we can define any color in a 3D model, and those values are portable between RGB and CMYK color spaces when combined with ICC profiles and the color management Profile Connection Space (PCS). RGB is converted to L*a*b* with an appropriate ICC profile attached, and then the L*a*b* can be converted to the output CMYK space of your printer with an appropriate ICC profile attached.
Leaving source profiles defaulted to SWOP
In your design apps and RIP software, there is an area to define your color management settings. One important setting to take a look at is the source CMYK and RGB profiles. The defaults many applications choose (U.S. Web Coated (SWOP) v2 and sRGB IEC61966-2.1) are relatively small color gamuts that have other issues as well.
The SWOP profile is more than 25 years old and pre-dates CTP plates and the G7 Gray Balance method. This makes printing with this profile in a digital space less than ideal. By changing this default to the Coated GRACoL 2006 (ISO 12647-2:2004) CMYK profile, not only is a larger color gamut achieved, but also one which is gray balanced. Gray balanced profiles create images that have a common visual appearance across medias, inks and printers.
Choosing the wrong rendering intent
Basically, rendering intents determine how colors that don’t fit perfectly in one color gamut, like Adobe RGB (1998), get mapped into another, like GRACoL2006.
The tradeoff is typically between color accuracy and pleasing appearance. The most accurate choice would be the absolute rendering intent - with the tradeoff being lack of pleasing color in some cases.
The relative colorimetric rendering intent is a nice tradeoff between color accuracy and is visually appealing.
Finally, the perceptual rendering intent is focused on mapping colors in such a way that is most visually pleasing but at the expense of color accuracy.
There are other rendering intent, but they are variations on these three. Finally, when available, it makes sense to apply the Black Point Compensation option to the Relative Colorimetric rendering intent.
Skipping a daily nozzle check of your printer
When it comes to monitoring overall print quality, there is no simpler or quicker tool at your disposal than a nozzle check. Doing this check daily is a great way to make sure your printer is ready to go and doesn’t need a cleaning. Also, by keeping all your past nozzle checks, you can refer back to past tests to see if today’s is better or worse than the past.
Lack of verification and process control
Color management will fail quickly without some form of color verification and process control program in place.
Color management professionals use software tools to evaluate color quality as they are dialing in systems with profiles and other settings tweaks. These verification results can be used as your stake in the ground against which future prints are compared.
Anytime a verification fails, it’s time to figure out why. Results over time can also be reviewed to look for trends in color quality degradation. In this way color can be restored to the baseline before a verification failure and prevent bad product from getting out the door.
The session wrapped up with some great questions from the attendees; but you’ll have to watch the video to hear the Q&A. The video can be found on WhatTheyThink.com under the Webinars tab. You can also download the presentation slides.
