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May 1, 2004 12:00 AM
Every imaging device, from the lowliest desktop inkjet printer to the mightiest pressroom iron, has its own personality. Color imaging will only produce accurate results if we consider the characteristics of each device, which requires a global framework for color control.
In this article, you'll learn how color can be controlled using the International Color Consortium (ICC) philosophy as well as by using reference printing conditions, such as Specifications for Web Offset Publications (SWOP) and General Requirements for Applications in Commercial Offset Lithography (GRACoL).
Every color-imaging device behaves differently — hence the need for color management. Individual desktop printers have different inks, paper and printing technologies. And of course the differences between a desktop printer and an offset press are even greater. If you ignore these differences, inaccurate and inconsistent printing are the obvious results.
Closed- and open-loop color systems can help bridge the gaps between devices. Since a closed-loop system performs a compensation for each device-to-device translation, it works best with a small number of devices (see Fig. 1a). A closed-loop workflow is fixed — the user knows where the images came from and how they will be viewed and printed. In a typical scenario, images are always acquired from the same drum or other scanner, displayed on the same monitor and destined for a single proofing/printing process, such as ColorArt or Chromalin. In this closely controlled situation, it's relatively easy for users to achieve the desired results. Skilled operators learn device characteristics and compensate accordingly.
An experienced operator, for example, would know a particular scanner has a color cast, responds weakly to blue, or always produces unsaturated images and needs a tone-curve correction. Typical corrections might vary depending on whether the image is low or high key, has dominant flesh tones and how it will be printed. Color corrections could involve a certain amount of gut feeling as well as adjustments made using the RGB/CMYK values shown by an onscreen dropper tool.
Closed-loop color has worked well for years. Operators learned device characteristics and compensated for them accordingly. But times are changing — old-time prepress specialists have given way to all-purpose employees. Today, the same person who creates graphics in Photoshop and pages in QuarkXPress may also generate separations. And as digital cameras, inkjet proofers/printers and monitor proofing have mushroomed into integral components of graphic-arts workflows, it can be impractical to compensate for the behavior of each individual device.
How can you achieve color that's consistent and accurate — even if you don't have skilled personnel or a fixed workflow? Many printers are exploring open-loop options. Rather than connecting all devices together, open-loop systems (or color-management systems, as we'll call them in this article) use a hub-and-spoke configuration to communicate color between devices (see Fig. 1b). The hub-and-spoke model provides greater flexibility, an important consideration, since images frequently originate from many sources and are viewed and output on different monitors and devices.
A color-management system is similar to an airline's system of hub cities. The hub system dramatically reduces the number of flights needed to cover a large geographical area. To add a new city, the airline simply schedules a flight into its hub city, and that destination is instantly connected into its network.
But just as an airline passenger can't make any flight connections without flying into the central hub, every device must have a profile before it can enter the system. You need a scanner profile for a scanner, a monitor profile for a monitor, a printer profile for a printer and so on.
A color-management system connects all devices in and out of a central profile connection space. The central connection space takes a device's internal color scale (either RGB or CMYK, depending on the device) and converts it to CIELAB, a three-dimensional device-independent scale used for measuring color. (See “CIELAB: measuring color on many different media,” November 2003.) A single profile that connects the device (and its images) to the central space is all that's needed to add a new device. When a device and its profile (which describes device characteristics) are available to the network, the characteristics are immediately known to all of the system's devices. Users can immediately transfer images between the new device and any other device in the network.
Color-managed workflows let users process images in simulation mode. In other words, images can be processed without changing the underlying pixel values while still providing a preview of a job on a particular stock. The preview is obtained by “pretending” to apply a printer profile to the image. Since no colors are discarded, it's easy to reuse/repurpose images — you don't have to scan them again.
In a color-managed workflow, you can process images to the printer space. The image can then be returned and viewed on the monitor to provide a preview (soft proof) of the expected printed result.
If you're proofing, you can also process an image using a press profile and then print locally to provide a proof on an inkjet or other printer. The press profile is used first, followed by the local proofer profile.
The ICC is a regulatory body that supervises color-management protocols between software vendors, equipment manufacturers and users. It has worked diligently to create a standardized file format and protocols for use in color communication, but in general, the industry has been slow to adopt ICC color management. It may be too complex for some commercial environments. Nonetheless, it is good for scanner characterization, previewing images to get an idea of printed colors, and proofing a press on an inkjet or dedicated proofer.
ICC profiles can be used as the basis for other products. Many color-management products, including some for monitor proofing, feature internal ICC profiles. (See “Coming to a screen near you” on p. 22.) Rather than dealing with the intricacies of profiles and their contents, users can concentrate on processing and viewing images and getting the job done.
Reference printing conditions, for example, are becoming an increasingly popular way of working, and the main mode of implementation for these is via an ICC profile.
Working together, committees of printers, print buyers and suppliers have developed several standard or reference printing conditions. Since the specifications are based on experimental press runs, they represent results that the average printer can achieve.
Reference printing conditions include:
A press is considered to meet the relevant specifications if it can hit specific measurable print parameters. If the pressroom can hit SWOP guidelines, for example, we assume the press is operating as a SWOP press. If you operate a press to the SWOP specification, you don't have to make a custom press profile; you can use a SWOP profile. The same holds true in the prepress department. Since the job will be printed on a SWOP press, everyone involved in the prepress process can use a widely available SWOP profile. (Photoshop already includes a SWOP profile in its list of output profiles.) And last but not least, SWOP gives press operators a straightforward goal.
In 1995, the SWOP committee printed and measured a 928-patch IT8.7/3 target on a SWOP press. The group measured six selected press sheets using Gretag SPM100 and the X-Rite 938 spectrophotometers. They averaged the measurements from these sheets and published a set of IT8.7/3 CMYK values and their measurements in LAB/XYZ. The data is published in a document called “Graphic Technology — Color Characterization Data for Type 1 Printing, ANSI CGATS TR 001-1995.”
Since “TR” stands for Technical Report, the data is referred to as TR 001 data. This data was the first publicly available and widely accepted colorimetric characterization of a sheetfed offset printing press and is exactly what users need to make an ICC output profile.
Using TR 001 data, you can make a SWOP printer profile, and, provided the press is operating according to SWOP specifications, this profile can be considered representative of that press. At early stages of the production process, a generic SWOP profile can help clients and designers visualize the impact the printing process will have on specific colors.
In an exciting development for sheetfed commercial printers, GRACoL-certified press sheets were released this past February (see Fig. 2 on p. 18). These sheets are printed according to the GRACoL standard for Grade 1 and Grade 2 gloss-coated paper. Using these sheets, graphic-arts professionals can determine if their proofing systems are reasonable predictors of the final printed product.
Measuring the targets on the press sheet provides data that can be used to create an ICC profile. Note that the GRACoL press sheet contains both IT8.7/3 and ECI 2002 printer targets that can be used to make an output profile. Therefore, the GRACoL specification can be “expressed” or represented by an ICC profile. The measurement data for the GRACoL press sheet is also posted online (www.npes.org/standards/toolsDTR004.html). Provided its presses produce sheets that are in accordance with the GRACoL specification, both a printing company and its clients can use a GRACoL profile.
The GRACoL target and test images are called DTR 004 and are used in the same way as TR 001 data for the SWOP process. The “D” stands for draft (the data is in a three-year trial). Early feedback indicates GRACoL could potentially do for commercial sheetfed printing what SWOP has done for publication printing.
You can buy GRACoL press sheets online from IPA, the Assn. for Graphic Solution Providers (www.ipa.org). Sheets range in price from $85 to $950, depending on whether the press sheet is batch certified or individually measured.
European printers also are attempting to use a standardized output profile for printing. The European Color Initiative (ECI) has created profiles that can be used as the default setting for the CMYK working space in Photoshop and elsewhere.
The group also offers a profile that is valid for offset printing on glossy or matte coated paper, and other profiles for Process Standard Gravure. Anyone printing to a European standard such as FOGRA or System Brunner Eurostandard can thus obtain reference ICC profiles that are based on median production values for broad categories of papers and printing processes.
Few users want multiple profiles for different press conditions. Since five or six reference printing conditions can cover the full range of printing processes from newsprint to heavyweight catalog papers, it is convenient for printers to print to these specifications and for their customers to use corresponding ICC profiles in their workflows.
Abhay Sharma is a professor of digital color imaging at Western Michigan University and member of the ICC. Contact him at firstname.lastname@example.org.
Today's color management is basically International Color Consortium (ICC) management. The ICC is a regulatory body that supervises vendors, equipment manufacturers and users. It specifies the framework for the profile connection space as well as profile format.
ICC got started about 10 years ago. In the early 1990s, several leading color technology companies were developing systems for color desktop publishing. Since there wasn't a common color-management framework, each application had to align itself with a specific hardware vendor.
When new systems were introduced, many key workflow features were missing. Since different vendors' systems were largely incompatible with each other, users couldn't mix and match components.
At the same time, Apple Computer had been working on ColorSync, an operating-system- level technology that facilitates the communication of color information among multiple hardware and software components. It is an open-platform system that all vendors can link into. Many vendors were interested in adopting or interfacing with ColorSync, but were secretive about their intentions.
What eventually became the ICC started in 1992 at a FOGRA meeting. After the regular agenda concluded, vendors discussed interest in a unified color-management system. Although the ICC used Apple's ColorSync model as the basis and framework for a modern color-management system, it is completely independent organization.
From its eight founding companies (Adobe, Agfa, Apple, Kodak, Taligent, Microsoft, Sun and Silicon Graphics), ICC has grown to about 70 companies.
The consortium meets three times a year. The main work of the ICC is done in groups, each dedicated to a specific issue. Currently there are Architecture, Chromatic Adaption Transform, Communications, Graphic Arts Workflow and Profile Assessment groups. For more information, see www.color.org.
Abhay Sharma's book, “Understanding Color Management,” explains the basics of color science and reviews a wide range of profiling hardware and software options. The book also details International Color Consortium (ICC) color management — what it is, how it works and how to use it. Cost is $54.95; see amazon.com/colormanagement to order.
Sharma will also help coordinate IPA's proofing roundup at its Technical Seminar June 8-10 in Chicago. Visit www.ipa.org.