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Color management for digital printing

May 1st, 2010 / By: / Feature, Graphics

Ensure precise color and efficient workflow by following these techniques.

The modern worlds of prepress systems are designed for fully digital, modular and open production workflows. Input systems for image capture, hardware and software for color processing, and output devices play key roles in prepress workflows. This involves a variety of device-specific color systems.

No two scanner RGB systems, monitor RGB systems or output CMYK systems are identical. Therefore, the modular nature of these systems requires new methods of ensuring the accurate communication of color information. At the same time, an increasing volume of data is repurposed for a variety of different media, which requires a workflow that is media independent for color data processing.

Color management should allow for flexible proofing of the color data. First, the color data should be displayed on the monitor precisely as it will be produced on press. This applies to CMYK data and to other color data, and is referred to as a soft proof. Second, the color data should be output on a digital color printing system in such a way as to simulate the print run hard proof. To this end, a digital color printing system should be sufficiently flexible to simulate any type of print production from one output to the next.

Offering a technical solution for these reproduction targets is what color management is all about. Basically, a workflow involves four device-specific color spaces or color gamut: scanner/camera RGB, monitor RGB, the CMYK space of the printer process, and, frequently, the CMYK space of a proofing process. The main function of a color management system is to convert the RGB space of the digital file to subsequent color systems. In doing so, the scanner space must be adjusted to the color space of the output device. The colors that cannot be reproduced must be replaced as inconspicuously as possible by colors of the target space. In many applications, this involves what is referred to as color space compression, or gamut mapping.

Color-managed workflow

The main function of a color management system is to ensure the unequivocal colorimetric transition between the various color systems involved in the reproduction workflow. To achieve this objective, the following tools are required:

  • Application software for generating input and output profiles (e.g. X-Rite ProfileMaker 5 Platinum) and suitable color measurement technology
  • Color profiles (e.g. ICC color profiles) colorimetrically describing the respective color reproduction characteristics of the input and output devices
  • A color computer (Color Management Module or CMM) that performs the actual conversion of the color data from one color space to another with the aid of color profiles
  • An application (e.g. Adobe Photoshop) that is capable of utilizing the various functions of the CMM to initiate the actual color conversion process

Color space conversion

In practice, an original image (source color) must be converted for a specific output device (destination color) as there is a difference between the gamuts and the colorimetric characterizations of the color systems. The color space conversion is defined as the connection between a device-specific color space and a reference color. During the process, each RGB value of a scanner color space is assigned to the corresponding color value in the reference system. In ICC terminology, the device independent reference or connection space is referred to as the Profile Connection Space (PCS). The ICC standard generally uses the CIELAB system as the PCS. The conversion of a scanner RGB color space to an output CMYK color space uses two separate conversion processes: RGB to LAB and LAB to CMYK.

A scanner RGB color space (the source) generally has a much broader spectrum than the CMYK color space of a printing system (the destination). The color management system has to specify the parameters that will determine how the source color space should be represented, such as the black point, the white point and the dynamic range of an image. This frequently involves color space compression. The system also has to decide how colors from the source that cannot be reproduced in the destination can be replaced by other colors as inconspicuously as possible.

For Mac users, any ICC compatible application can call on ColorSync (Apple’s CMM) and use it for color conversion. To perform a color conversion requires at least two profiles: one source profile and one destination profile. There are no limits regarding the source and destination color spaces. Any conceivable combination is possible, provided the corresponding profiles are available. ColorSync allows two or more profiles to be linked. The most common case is the combination of two profiles (source and destination profile) or three profiles (source, destination and simulation profiles). This function is vital for proofing functions. A color adjustment method can be selected for each color conversion step.

For Windows users, a fully ICC compatible CMM is available. Image Color Management (ICM) offers similar functions to ColorSync’s, and provides the current monitor profile at the operating system level for use by all applications.

The graphic design of the CMM dialog boxes varies from one application to the next. Depending on the function of the program within an ICC workflow, the CMM functions are utilized to a greater or lesser extent.

Application setup

Frequently, the entire color conversion process, from scanner RGB space to the output CMYK space, is performed in an ICC-savvy application, such as Adobe Photoshop. This enables completed CMYK files to be adjusted via ICC profiles when the job is sent to the printer. In addition to implementing a number of ICC functions, Photoshop includes an internal CMM, Adobe Color Engine. Adobe replaces unspecified RGB color space with an RGB working space. This is a digitally generated RGB color space, normally based on different TV, video and phosphor standards.

The user has a choice between standard RGB systems. The RGB working color space can be stored as ICC compatible ICC profiles and embedded in image data. Photoshop uses the ICC system profile to display real color data. In Photoshop’s color conversion model, all input color data (RGB, CMYK, LAB) must pass through the RGB working space to achieve a real color monitor display. Consequently, this may change the way CMYK images are displayed on the monitor if the RGB working space is changed.

The RGB working space must be defined in the Edit/Color Settings menu. There are several different standards to choose from. The best model depends on the individual applications. For printing, a relatively large color space should be selected to minimize data loss during real color monitor display and during the separation process. Adobe RGB is a good recommendation with roughly the same gamut as most CMYK color spaces used in printing technology. The RGB color space Wide Gamut, which is artificially enhanced, is not recommended. It may cause colors to be displayed on the monitor that cannot be reproduced. A good color space for creating RGB images intended for the internet is the sRGB color space.

The CMYK working space specifies the internal conversion between CMYK and LAB and therefore the separation of RGB and LAB data. This means that when a mode change from RGB/LAB to CMYK is performed in Photoshop, the separated CMYK is generated according to the Color Settings dialog box.

Edit/color settings menu

Adobe’s color settings menu window in Photoshop allows you to choose RGB and CMYK working spaces. It is best to use industry standards to start (Adobe RGB and SWOP for CMYK). If you are working with a particular printer, find out what they prefer and adjust your settings accordingly.

Color management policies include Convert, Preserve Profiles, Preserve Numbers (ignore embedded), and Off (Preserve is usually best; you can always convert later).

“Ask when opening” will help you troubleshoot workflow problems and keep you aware of any embedded profiles or of any profile mismatch when you open new files. Save the settings so your choices can be easily retrieved or shared with other Creative Suite applications.

The CMYK working space is also used for soft proofing, a real color display that simulates the final print result (View/Preview/Working CMYK).

Take advantage of Adobe Bridge to keep all of your Create Suite color settings synchronized.

InDesign and Illustrator color settings dialog box

Preserve Numbers was introduced in CS2. In earlier versions it was not possible to have color management off and still see accurate previews and proofs. It is still recommended to convert all RGB files to your standard working space, because not all RIPs can handle multiple RGB color spaces.

PDF and color management

The Portable Document Format from Adobe is quickly becoming a universal document exchange format in the publishing industry. Since it has been optimized based on the requirements of a modern media world, PDF is suitable for designing interactive documents for the internet and for prepress applications. PDF has become a medium independent exchange format between data suppliers and data recipients. Like other application files, PDF can be incorporated into an ICC workflow.

PDF supports all relevant color spaces in reproduction (RGB, CMYK, Gray, LAB and XYZ). If the basic settings for a PDF file in Adobe Acrobat Distiller are carefully selected, the color data in the master document remains unchanged. Acrobat Distiller supports the ICC standard. This program distinguishes three different ways to process color data:

  1. Any source color data or computer-generated colors can be embedded in the PDF file and output should remain unchanged.
  2. Prior to generating a PDF file, the PS data can be processed in accordance with individual profile settings via the ICC color adjustments function of the computer. In addition, ICC profiles can be embedded.
  3. Internal distiller color processing options are available. For example, CMYK data embedded in a document can be converted to a standard RGB during the distilling process. This is useful if the PDF data is to be used solely for display on a monitor.

These three options make PDF completely open and allow it to be incorporated at any point in an ICC workflow. The user can choose whether they wish to use the PDF purely as a transfer interface for neutral color information and perform the color conversion process at a later point and time, or directly color convert the PDF data.

Functions and formats

To implement the concept of a standard, platform independent, reproducible color space conversion, each application should offer access to all of the ICC color adjustment functions incorporated in the operating system. User interfaces should be clearly defined, with understandable options and commands complying with ICC terminology. The following functions should be supported:

  • Use of the system profile within the application (the monitor profile)
  • Optional use of the default CMM for an ICC color conversion
  • Ability to select the Rendering Intents when performing a color conversion
  • No limitations with respect to what source and destination color spaces can be converted and combined
  • Linking of three color spaces (source, destination and simulation) when performing a color conversion
  • Optional incorporation or deletion of ICC profiles in color data formats

Apart from the applications, the data formats used in the publishing industry must adhere to a number of conventions to ensure ideal ICC workflow. One important feature is the ability to incorporate ICC Source Profiles in a color data file. It is important for both application programs and data formats to be able to store all color models required in modern reproduction technology.

Brian Ashe is senior technical consultant at X-Rite/Pantone. He brings 25 years of experience to the field of color management, and is a regular speaker at a variety of color conferences, such as Clemson University, FFTA, GATF, and a guest lecturer at RIT and Parsons School of Design.

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