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Color Management for Proofing

In this part we look at profiling for proofing, meaning you want your large-format inkjet printer to look like another printer or printing process, such as an offset press or a production printer.

By Rich Adams, GIA

In Part 1 of this series, we looked at monitor calibration, which helps you standardize your monitor to a known contrast and color balance values so that WYSIWIF ("what you see is what's in the file"). In Part 2, we looked at standard working spaces, which are places to store color data until you know what you want to do with it (archive it, display it on-screen, print it). In Part 3, we looked at printer profiling, so that output you print will match what you saw on the screen.

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  • So What's a Proof?
    A proof is one (or a few) copies that show how a print job will look. Proofs originated in commercial printing, where the lithographic, gravure, or flexographic presses take a lot of work to set up. The idea was to print a fast, economical single copy of a job to show the customer, before spending the time and materials to make printing plates, ink up the press, and adjust ink and water balance-only to find that the customer didn't like the color.

    As low-cost color printers were developed, designers and photographers also began making their own proofs. Today, proofs can be divided into two types, depending upon how they're used:

    • Contract proofs show the appearance of color, or how photos will look when printed. The customer may be asked to sign off on the proof, indicating his or her acceptance of the job, hence the term "contract proof." Contract proofs need to be highly accurate in color.
    • Preproofs, design proofs, or "comps" show early design stages of a job. Depending upon the stage at which they're made, preproofs may or may not need accurate color. Preliminary comps are economical prints mostly for internal use, usually from a desktop laser or inkjet printer, that show early design stages of a job, such as layout, position, and general color appearance. This type of preproof may not need accurate color. Final comps, used to present finalized designs to customers, may need to provide higher quality and more accurate color matching.

    Quick History of Proofing
    The first proofs were made on special "proof presses" that were separate from the production press and a little easier to set up. In the 1970s press proofs were replaced by laminated photographic systems like DuPont Cromalin and 3M Matchprint. In the early '90s continuous-flow inkjet proofs like IRIS and DuPont Digital Cromalin began to replace analog proofs. Customers who wanted halftone dots and press-like color began using digital dot proofs like the Kodak Approval and Creo Spectrum. In the late 1990s, large-format inkjet proofers, which had been considered too low in quality to use as contract proofs, made major quality advances and, with color management, have made large-format inkjet proofs the most common type of contract proof today.

    Inkjet Technologies
    Today's large-format inkjet printers use an ink-saving technology known as drop-on-demand, in which microscopic ink droplets are ejected from the print heads onto the substrate. Piezo inkjet print heads use pulses of electricity to eject the ink, while thermal print heads use heat.

    Today's inkjet proofers are used to image a wide variety of media, including photographic, signage, fine-art, and textile materials. In this article we discuss inkjet printing for making comps and contract proofs.

    Inkjet Proofs
    Printers. The kind of inkjet printer you use depends on the kind of proof you want. Low-cost desktop printers like the Canon i960, Epson 1280, and HP Deskjet 9650 are good for making preliminary comps. Higher-end printers like the Canon i9100/9900, Epson 2200/4000/7600, or Lexmark 5150, are good for higher quality final comps. Top-of-the line printers like the Epson 7600/9600/10600 or HP Designjet 30, 130, or 5500, when used with a RIP, can be used for contract proofs.

    Media. Most printer manufacturers offer media that's compatible with their equipment. For proofing jobs that will be printed by lithography, a photo media may not be the most appropriate, as the brightness and gloss will be much higher than the paper used for the actual job. Many proofing system manufacturers, including Agfa, DuPont, and Kodak Polychrome Graphics, now sell large-format media for proofing.

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    Color profiles. Inkjet inks have color characteristics much different from those of lithographic printing inks. The color match you get with an inkjet printer is highly dependent upon the proper use of accurate ICC color management profiles. For the color accuracy you need for final comps or contract proofs, you're going to need three profiles:

    • proofer profile-This is an ICC profile for the inkjet printer, made with the same ink, media, and RIP or driver used to print the job. Some media manufacturers make profiles available on their web sites, while others may come with the printer or RIP. For the most accurate profiles, you may want to get a color management system, which could entail an investment of $2,700 or more.
    • press profile-This is an ICC profile for the printing press, the color of which you want to match. This could include a lithographic sheetfed or web press for commercial or publication printing, a flexographic press for label or package printing, or a gravure press for long-run publication or package printing
    • monitor profile-If you're a content creator, you'll undoubtedly want to proof your work on-screen. For this you'll need a monitor profile, along with the press profile. You can make a "poor man's" monitor profile with a visual calibration application like Adobe Gamma or Apple ColorSync, but for the most accurate monitor profile you need a monitor colorimeter and profiling program such as GretagMacbeth's i1 Display or Monaco's Optix.
    • standard working space profile-A standard working space is a place to store data until you know what you want to do with it (display, proof, or print it). (See the article "Standard Color Working Spaces" for additional information.) Adobe Photoshop and other applications support several standard RGB working spaces, including (from largest to smallest) Adobe RGB, ColorMatch RGB, and sRGB. Most content creators today use the largest, Adobe RGB. Users with CMYK workflows can use one of Adobe's standard CMYK working spaces, such as U.S. Sheetfed Coated or U.S. Web Coated (SWOP). Of these, Sheetfed Coated has the larger gamut, but SWOP is more common, because SWOP (Specifications for Web Offset Publications) was the first color specification developed.

    Proofing Workflow
    Profiles. To use your color profiles for accurate final comps and contract proofs, you'll need to establish a color-managed workflow that uses the four profiles described above-proofer, press, monitor, and standard working space. Three terms used to describe ICC profiles used in proofing workflows are:

    • source profile-This is the ICC profile describing the color space that the image is currently in. The source profile is generally a standard working space profile, but could also be a scanner or digital camera profile.
    • destination profile-This is an ICC profile that describes the device to which the profile will be output. The destination could be a color monitor (for display), an inkjet printer (for proofing), or a printing press (for production output), for example.
    • proofing profile or reference profile-This is an ICC profile that describes what device the image should look like when proofed. When proofed on a large-format printer, for example, the proofer profile could be that of a lithographic press.

    RIPs used for proofing will generally have a place to specify the three types of profiles.

    RGB or CMYK workflow. The first decision you should make is whether to use an RGB or CMYK workflow. RGB standard working spaces typically have a bigger color gamut than CMYK working spaces, so are good for getting the most color saturation and for repurposing content, e.g., from print to web. Your company or client may want a CMYK workflow, which will also work but is not going to keep all of the color saturation of the original images.

    Use Photoshop's Proof Setup dialog box to set up a proofing workflow (RGB or CMYK) by specifying the proof profile (in this case, for an Indigo digital press). When printing the image to an inkjet proof, set up Photoshop's Print with Preview dialog box to select the proof profile (here, Indigo Glossy) and the destination profile (here, HP 5500). The same procedure applies to both RGB images (here, the document is in ColorMatch RGB) and CMYK images (document in U.S. Web Coated).

    Soft proofing. To view accurate proofs on the monitor, your viewing program needs to convert color from the standard working space to the monitor profile. Whether you're working in Adobe RGB or U.S. Web Coated (SWOP) CMYK, for example, and use Photoshop's Proof setup (Figure 2), Photoshop will convert from the standard working space profile (source profile) to your monitor RGB (destination profile) with reference to the press profile (proofing profile)

    Printing the Proof. You can print proofs from applications like Photoshop or using a third-party RIP. Most large-format printer RIPs have proofing capability. When printing a proof in the RGB workflow, the application or RIP needs to be set for the standard working space (source profile), the proof printer (destination profile), and the press profile of the device to be matched.

    Paper tint. Using absolute colorimetric rendering to make a proof causes the background to be tinted to match the press (or final output) paper, if this paper is darker than the proof paper. Relative colorimetric rendering avoids the paper tint. Some customers like the paper tint and feel it makes proofs look more accurate, while others find it distracting.

    Rendering intents. One feature of inkjet proofs is that they can be set up to match the color of paper being proofed. If you're proofing a job for newsprint or buff-colored paper, you can make an inkjet proof on white paper in which the background has been tinted with yellow ink to match the job's paper stock. Some customers like background tinting, while others find it distracting. Either way, background tinting is controlled by the rendering intent.

    Rendering intent describes the way out-of-gamut colors are moved into gamut with color management. If you use the large Adobe RGB standard working space and want to print on a press using uncoated paper, which has a much smaller gamut, to be represented in the print, all of the colors outside the uncoated paper's gamut must be either lost or moved inside the gamut.

    Rendering intent. Diagram illustrates colorimetric rendering, which maps colors from a larger gamut to a smaller gamut "by the numbers," sometimes mapping multiple colors to the same point but is useful for spot colors. Perceptual rendering, though less numerically accurate, maintains a distinction between mapped colors and is used for photos.

    For final output, use perceptual rendering, which moves colors relative to each other to maintain their distinction (Figure 4). For proofing, however, where you're generally moving from a small gamut to a larger one, use colorimetric intent, which matches colors "by the numbers." The ICC recognizes two types of colorimetric intent: absolute and relative. Relative colorimetric intent readjusts the white point of the file to that of the substrate, while absolute colorimetric does not. The bottom line is, if you use absolute colorimetric rendering, the proof will be tinted to match the paper of the press, while if you use relative colorimetric, it won't be tinted.

    Proofing PANTONE colors. To check for PANTONE colors that can't be reproduced accurately, open a PDF or EPS document with named PANTONE colors in Photoshop, set the Proof Setup to the printer's ICC profile, and turn on the Gamut Warning. Out-of-gamut colors will be grayed out. This example shows a set of PANTONE colors without the Gamut Warning off (left) and on (right).

    Proofing PANTONE® Colors. Some inkjet printer RIPs have automatic PANTONE calibration for printing spot colors and can accurately reproduce 70-90% of PANTONE colors. To use this feature, define PANTONE colors as named spot colors (not as CMYK or RGB process colors) and save in EPS or PDF format. The RIP's lookup table converts named colors to the closest-matching CMYKcm (or other ink set) values based on the color profile selected.

    To soft-proof PANTONE colors, open the document in Photoshop in LAB mode, set the Proof View to the printer profile, and turn on the Gamut Warning. Out-of-gamut PANTONE colors will be grayed out.

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