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Predictability of Spot Color Overprints

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Predictability of Spot Color Overprints Predictability of Spot Color Overprints Robert Chung, Michael Riordan, and Sri Prakhya Rochester Institute of Technology School of Print Media 69 Lomb Memorial Drive, Rochester, NY 14623, USA emails: [email protected], [email protected], [email protected] Keywords spot color, overprint, color management, portability, predictability Abstract Pre-media software packages, e.g., Adobe Illustrator, do amazing things. They give designers endless choices of how line, area, color, and transparency can interact with one another while providing the display that simulates printed results. Most prepress practitioners are thrilled with pre-media software when working with process colors. This research encountered a color management gap in pre-media software’s ability to predict spot color overprint accurately between display and print. In order to understand the problem, this paper (1) describes the concepts of color portability and color predictability in the context of color management, (2) describes an experimental set-up whereby display and print are viewed under bright viewing surround, (3) conducts display-to-print comparison of process color patches, (4) conducts display-to-print comparison of spot color solids, and, finally, (5) conducts display-to-print comparison of spot color overprints. In doing so, this research points out why the display-to-print match works for process colors, and fails for spot color overprints. Like Genie out of the bottle, there is no turning back nor quick fix to reconcile the problem with predictability of spot color overprints in pre-media software for some time to come. 1. Introduction Color portability is a key concept in ICC color management. Color portability between colors captured and colors displayed (or printed) is accomplished in an ICC-based pre-media software in two color conversions. As depicted in Figure 1, the first step is to translate color data captured into the RGB working space via the input device profile. The second step is to translate color data from the RGB working space into the monitor space (or CMYK printer space) via a monitor (or output) profile. Figure 1. Color portability in color management workflow There are two profiles used in a color conversion. The source profile provides the input-to-PCS (A-to-B) conversion, and the destination profile provides the PCS-to-output (B-to-A) conversion. A color conversion is also described as the A-to-B-to-A conversion. In a late device-binding workflow, pictorial color image data are converted and saved in the working space, such as Adobe RGB (1998). Thus, the digital file can be rendered to any number of output devices with the appearance of the monitor display. Color rendering intent of pictorial color images is either perceptual or relative colorimetric. In the context of pictorial color image reproduction, color portability means WYSIWYG or good color agreement between display and print as the result of correct pre-media settings. Why not an exact match between display and print? The reason is that color device is gamut-limited. In this case, monitor gamut is smaller than the working space, and printer gamut is smaller than monitor gamut. 1a. Predictability of pictorial color images from display to print Color predictability emphasizes the use of A-to-B-to-A color conversion to simulate printed color closely with a display. As shown in Figure 2, the source or simulation profile is the printer, and not the working space. Thus, color predictability is realized when (a) the display gamut is larger than that of the printer, and (b) the color rendering intent, used in the pre-media settings, is absolute colorimetric. Submitted to the 35th Int’l Research Conference, Sept. 7-10, 2008, Valencia, Spain 2 Figure 2. Color predictability in color management workflow 1b. Spot color portability Spot colors are colors that designers choose from a color swatch library, e.g., Pantone solid coated library, when creating artwork using pre-media software. There are many custom color libraries and each library contains more than 1,000 spot colors. The fact that in-gamut spot colors can be accurately displayed on the monitor is because there is a spot color-to-PCS conversion available and the pre-media software acts as the API to complete the color conversion for monitor display (Figure 3). Figure 3. Spot color management workflow There are two ways to reproduce spot colors in hardcopy. The first method is to print spot color alone as specially formulated ink on a printing press (Figure 3). In this case, these are high chromatic colors, e.g., Coca-Cola red, IBM blue, etc. The second method is to print spot colors as CMYK composites on a CMYK digital printer. There is a RIP-based look-up table that performs spot color -to-CMYK conversion prior to hard copy output. Due to gamut limitation of the digital printer, spot color accuracy is compromised (Pantone, 2008). In this research, hard copies from spot color printing using specially formulated ink serve as the reference. 1c. Research questions on spot color predictability When a spot color is specified in pre-media software by default, colors underneath the spot color are removed or knocked out from the digital file as if the ink is opaque. Due to the advent of pre-media software development, the rule of "printing spot color alone as an opaque ink" has been changed. Today, pre-media software, such as Adobe Creative Suite, allows spot colors to overprint on top of other colors with different blending and transparency effects. As a result, spot color overprint features offer many pre-media color choices to designers. There are two research questions raised in this research. The first research question is, “What is the predictability of spot color solid between display and print?” The second research question is, “What is the predictability of spot color overprint between display and print?” If we can answer both questions with confidence, we will extend the understanding of color management from process color to spot color in the graphic arts industry. 2. Methodology This research takes the following four experimental steps: (a) building a viewing platform to enable display- to-print comparison; (b) validating the predictability of process color between display and print; (c) testing the predictability of spot color solid between display and print; and (d) testing the predictability of spot color overprint between display and print. To elaborate, the first step is to set up an experimental condition for display and print comparison. The second step is to validate the experimental condition by means of psychometric analysis. The last two steps are to answer the research questions regarding spot color predictability. 2a. Building a viewing platform to enable display-to-print comparison It is customary to view a monitor display under dim surround and a hard copy image under bright surround. ISO 12646 (2006) specifies that the ambient illumination of the monitor shall be low, i.e., less than 32 lux. Submitted to the 35th Int’l Research Conference, Sept. 7-10, 2008, Valencia, Spain 3 ISO 3664 (2006) specifies that the viewing illumination of the hard copy shall conform to ISO viewing condition P2 or 500 ±125 lux. In a pilot study, many observers found it difficult to evaluate display-to-print color match with the display under dim surround and print under bright surround. The inability of human visual system to fully adapt to both viewing conditions is a major cause of the difficulty. The work of Katoh (1995) elaborates on the phenomenon, known as partial adaptation. To overcome the difficulty of adaptation, this research modified a viewing booth to accommodate the display- to-print comparison in a common bright surround. As shown in Figure 4, a GraphicLite viewing station, manufactured by GTI Graphic Technology, Inc., was modified so that a hard copy (A) is placed next to a 20” Apple Cinema Display (B) surrounded by a gray mask (C). A second monitor, located outside the viewing booth, is used for logistic purposes. The 20” Apple Cinema Display is referred to as the soft proofing monitor in this research. Figure 4. Display-to-print comparison under bright surround To configure the viewing condition and pre-media settings between display and print under bright surround, the following steps were carried out in this research: 1. A digital printer was profiled using the IT8.7/4 profiling target and X-Rite ProfileMaker 5. A grayscale print, as shown in Figure 4, was made as a reference. 2. The soft proofing monitor was calibrated and profiled under a number of white point and gamma combinations using the X-Rite Eye-One Pro. In a preliminary test, the grayscale print was compared to a number of displays of the grayscale PDF file using different monitor profiles. The monitor profile with D50 and 2.0 gamma was found to yield the best display-to-print match. 3. Both the monitor profile and the printer profile were placed in the ColorSync folder. 4. In the pre-media color settings dialog box, (a) the soft proofing monitor ICC profile acts as the RGB working space; (b) the digital printer ICC profile acts as the CMYK working space; (c) the color rendering intent was set to absolute colorimetric. This color setting was synchronized in all Adobe CS components using Adobe Bridge. 5. The grayscale PDF file was opened in Adobe Acrobat Pro and displayed on the soft proofing monitor. Go to Advanced / Print Production / Output preview and verified that (a) the correct printer ICC profile was chosen under “Simulation Profile,” and (b) the “Simulate Paper Color” option was checked. 6. The observer was asked to adjust the intensity of the ambient light in the viewing booth so that the white point and intensity of the hard copy matched that of the monitor.
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