By YU-JU WU
February 2006
Summary
Spot color is widely used in gravure printing to obtain a colorful appearance. With new developments in printers, inks, and media, an inkjet printer can be treated as a digital proofer for gravure printing, providing significant time and cost savings compared to conventional procedures for job approvals. The implementation of digital proofing is integrally related to color management. In the digital proofing process, a third-party RIP can be employed to interpret raster and vector data files for a specific printer, distributing the right amount of ink on
the substrate, therefore, providing better control for accurate digital color reproduction. The objective of this work was to establish a digital proofing system for gravure spotcolor printing. An Epson Stylus Pro 4000 digital printer combined with two commercially available RIPs (CGS ORIS RIP and GMG ColorProof RIP) were tested and compared. ICC profiles were generated for the Epson Stylus Pro 4000 using the actual production printing substrate. The quality of spot-color reproduction was evaluated in terms of the ∆E in L*a*b* color space for selected spot colors. The overall results suggested that the shadow area of each spot-color test chart has relatively high ∆E values due to some spot colors in the test charts being out of the color gamut of the test printer. Overall, the printer with RIP software offers better spot-color reproduction resulting in lower ∆E values than printing without RIP. The Epson 4000 printer with the CGS RIP delivered better color reproduction in Yellow spot colors, while the Epson 4000 printer with the GMG RIP had better color reproduction in Blue-347, Black-392, and Red-314 spot colors. Among the seven selected spot colors, Yellow-385 is the hardest one to reproduce.Introduction
Gravure printing is the printing process that is usually employed to meet long print-run requirements. Long print runs are accompanied by relatively long leadtimes, high initiation costs, moreover, coupled with relatively high press setup times and makeready (Suchy, Fleming, & Sharma, 2005).
Recently, the trend in the printing industry includes shorter run lengths and work with fast turnaround time. New developments in digital printing have made it possible to produce small quantities of high-quality color products at affordable prices. With its unique properties, digital printing can be used for proofing, providing significant time and cost savings compared to conventional procedures (preparing cylinders and printing proof samples) for potential product verification.
The implementation of digital proofing is integrally related to color management (Sharma, 2004). The key to achieving the best quality color reproduction is to combine the right equipment, software, and media. The use of a raster imaging processor (RIP), for example, has provided better control for accurate digital color reproduction in inkjet printing. The ink can be adjusted and limited in the RIP, achieving the right amount of ink distribution correctly on the media. Because ink is distributed correctly on the media, a larger color gamut can be obtained (Anonymous, 2005; Rich, 2004).
However, the way to control and improve spot-color reproduction quality for inkjet printing is still a challenge to printers. Some commercial RIP software manufacturers represent the latest developments in spot-color matching that incorporate spot-color edit functions in their systems, allowing the user to print spot color more accurately. With the right combination of equipment, software, and media, accurate spot-color reproduction can help inkjet printers to meet clients’ demands and stay competitive in terms of quality and process efficiency.
The overall objective of this study was to establish a digital proofing system for product gravure printing. The selected spot colors were printed on the substrate by a drum cylinder gravure proofing press in order to obtain original L*a*b* values and to generate a digital test form. Spot-color test charts were printed via commercially available RIPs on the actual production printing substrate. Color gamuts and L*a*b* values of different output combinations were compared.
Experimental
The objective of this investigation was to establish a digital proofing system for gravure spot-color printing. Color management with ICC profiles was used to investigate the reproduction of specific spot colors. The Epson Stylus Pro 4000 digital printer was tested for actual printing on a production printing substrate. Two commercially available RIPs—CGS ORIS RIP and GMG ColorProof RIP—were tested and compared.
Seven spot colors were selected for evaluation: blue B-347, black B-392, red R-314 and R-349, and yellow Y-355, Y-357, and Y-385. Each chart consisted of 66 patches of different gray levels, generating a chart with a variety of shades for the color (as shown in Figure 1). The colors were printed on the substrate using a drum-cylinder gravure proofing press. Each specific measured area on the individual chart was measured five times for L*a*b* values—to reduce measuring errors—and the average value was computed as original data. According to these original data, the spotcolor test charts in digital form were generated using Photoshop CS, so that these charts could be used for actual digital printing reproduction.
Figure 1: Spot-Color Test Chart
ICC profiles were generated for the Epson Stylus Pro 4000 using the actual production printing substrate. The devices were profiled as CMYK devices. For the CGS ORIS RIP, the calibrated linearization of the printer was used to output an ECI2002V CMYK chart. For the GMG ColorProof RIP, an ECI2002V CMYK chart was printed without any ink limitation because a specific full-gamut color profile was used to reproduce spot color.
Those printed charts were then measured with a GretagMacbeth SpectroScanT, operated by GretagMacbeth Measure Tool 5.0.4 software. The measurement files were used to generate profiles using GretagMacbeth ProfileMaker Pro 5.0.4. ICC profiles were loaded in the RIPs’ spot-color functions. Seven spot-color test charts were then printed via the CGS ORIS RIP and the GMG ColorProof RIP, and the L*a*b* values for each color patch of the chart were measured using a GretagMacbeth SpectroScanT.
The quality of spot-color reproduction was evaluated in terms of the ∆E in L*a*b* color space. The Epson 4000 printer with the CGS ORIS RIP and Epson 4000 printer with the GMG Colorproof RIP were employed to print spot-color charts, and the color gamuts of the two output combinations were compared using ColorThink 2.1.2 software, while X-Rite Monaco GamutWorks 1.1.2 software was used for gamut volume comparison.
Results and Discussion
In this study, the Epson Stylus Pro 4000 printer was profiled using the CGS ORIS RIP and GMG Color Proof RIP on the commercial gravure printing substrate. Selected spot-color test charts were printed via the CGS ORIS RIP and GMG ColorProof RIP. The color gamuts of two kinds of output combinations were tested and compared using ColorThink 2.1.2 and X-Rite Monaco GamutWorks 1.1.2 software. The quality of spot-color reproduction was evaluated in terms of the ∆E in L*a*b* color space and color.
Gamut comparison
Figure 2 illustrates the color gamut comparison for the GMG RIP and ORIS RIP (with L*a*b* values of original data for reference). The GMG Color Proof RIP demonstrated a larger gamut for the Epson Stylus Pro 4000 Printer on the actual gravure printing substrate, especially in the yellow, red, and magenta areas. The gamut volumes for the GMG Color Proof RIP and CGS ORIS RIP are 446,586 and 382,043, respectively. It is important to note that some shadow tints in the spotcolor test charts are out of the color gamut of the printer.
Figure 2: Epson 4000 gamut comparison – GMG (true color) vs. ORIS (wireframe)
Original & digitally printed L*a*b* values comparison for GMG ColorProof RIP
The ∆E values calculated for original and actual printed L*a*b* values for each color (Epson 4000 with GMG RIP) are shown in Table 1 and Figure 3. Table 1 indicates that the average ∆Es for B-347, B-392, R-314, R-349, Y-355, and Y-357 are lower than 2. The Y-385 spot color has the highest ∆E value of 4.5. Figure 3 represents ∆E comparisons of original and printed L*a*b* values for the GMG RIP. The comparisons clearly demonstrate that the ∆E of the Y-385 test chart is significantly larger than those of others. For the Y-385 test chart, the ∆E values are over 4 from 50 percent to solid tints. In contrast, the ∆E values of the B-347, B-392, R-314, R-349, Y-355, and Y-357 test charts increase significantly at shadow areas. Those spot-color trajectories traversing out of the color gamut of the test printer, as shown in Figure 2, contribute to higher ∆E values.
Table 1: ∆E comparison of original & printed L*a*b* values for GMG RIP
Figure 3: ∆E comparison of original & printed L*a*b* values for GMG ColorProof RIP
Original & digitally printed L*a*b* values comparison for CGS ORIS RIP
The ∆E values calculated for original and actual printed L*a*b* values for each color (Epson 4000 with ORIS RIP) are shown in Table 2 and Figure 4. Table 2 indicates that the Y-385 spot color has the highest ∆E value of 4.5, followed by B-392 (2.6), B-347 (2.4), R-314 (2.3), R-349 (1.4), Y-355 (1.1), and Y-357 (1.0). As shown in Table 2, the CGS ORIS RIP has better reproduction capability in Y-355 and Y-357 spot colors. Figure 4 presents a line chart of the ∆E comparison of original and printed L*a*b* values for the ORIS RIP. The comparisons clearly demonstrate that the ∆E values of the Y-385 test chart are significantly larger than those of others. For the Y-385 test chart, the ∆E values are over 4 from midtones to solid tints. The ∆E values of B-347, B-392, and R-314 test charts increase significantly at shadow areas. Those spot-colors trajectories traversing out of the color gamut of the test printer, as shown in Figure 2, contribute to higher ∆E values. For the R-349, Y-355, and Y-357 test charts, the ∆E value of each color patch is lower than 4.
Table 2: ∆E comparison of original & printed L*a*b* values for ORIS RIP
Figure 4: ∆E comparison of original & printed L*a*b* values for CGS ORIS RIP
∆E Comparison of original & printed L*a*b* values for spot-color test charts
In this part, a line chart was employed to compare ∆E values for different output combinations (Epson 4000 printer with CGS ORIS RIP and Epson 4000 printer with GMG Color Proof RIP) for an actual gravure printing substrate. Figures 5 to 11 present line charts of ∆E comparisons for the Blue-347, Black-392, Red-314, Red-349, Yellow-355, Yellow-357, and Yellow-385 charts respectively. As shown in Figure 5, Figure 6, and Figure 7, the ∆E values of the CGS ORIS RIP are larger than the GMG Color Proof
RIP (especially in shadow areas). For the B-347, B-392, and R-314 charts, the ∆E values of two RIPs are lower than 4 below 80 percent tint, while the ∆E values increase significantly from the tint of 80 percent for either Epson 4000 with ORIS RIP or Epson 4000 with GMG RIP. pared to those of other test charts (∆E values range from 0 to 2.5). It shows that the CGS ORIS RIP and GMG Color- Proof RIP have better reproduction capabilities in Red-349, Y-355, and Y-357 spot color in terms of lower ∆E values. The Y-
385 test chart, compared to other spotcolor charts, has relatively high ∆E values. As shown in Figure 11, the ∆E values of the two RIPs increase significantly from the tint of 80 percent. With the exception of shadow area, the ∆E values of the two RIPs were controlled between 0-5 at highlights and midtones.
Figure 5: ∆E comparison of original & printed L*a*b* values for B-347 Chart
Figure 6: ∆E comparison of original & printed L*a*b* values for B-392 Chart
Figure 7: ∆E comparison of original & printed L*a*b* values for R-314 Chart
Figure 8: ∆E comparison of original & printed L*a*b* values for R-349 Chart
Figure 9: ∆E comparison of original & printed L*a*b* values for Y-355 Chart
Figure 10: ∆E comparison of original & printed L*a*b* values for Y-357 Chart
Figure 11: ∆E comparison of original & printed L*a*b* values for Y-385 Chart
Conclusions
For color gamut comparison, the GMG Color Proof RIP demonstrated a larger gamut on an actual production printing substrate than the CGS ORIS RIP did, especially in yellow, red, and magenta areas.
The shadow area of each spot-color test chart has relatively high ∆E values, due to some spot colors in the test charts that are out of the color gamut of the test printer. For the Blue-347 and Black-392 test charts, the actual production printing substrate cannot accept such large amounts of ink at solid areas and causes ink smearing. Although color gamut is still the key point to achieving the best quality spot-color reproduction, the absorbency of the substrate should be taken into account in the spot-color reproduction process.
Among the seven selected spot colors, Yellow-385 is the hardest one to reproduce. Spot color Y-357, in contrast, has relatively low ∆E values compared to those of other test charts.
Overall, a printer with a third-party raster imaging processor offers better spot-color reproduction resulting in lower ∆E values. The Epson 4000 printer with the CGS RIP delivered better color reproduction in Yellow spot colors (Yellow-355, Yellow-357, and Yellow-385), while the Epson 4000 printer with the GMG RIP has better color reproduction in Blue-347, Black-392, and Red-314 spot colors. Table 3 shows the summary of ∆E comparisons for different output combinations.
Acknowledgement
The author would like to thank Omnova Solutions, Inc., for partial financial support for this project, and to GMG America and CGS Publishing Technologies for software donation. The author would like to express gratitude to Dr. Alexandra Pekarovicova and Dr. Paul D. Fleming from the department of Paper Engineering, Chemical Engineering, and Imaging at Western Michigan University for their helpful advice and assistance with the work.
References
Anonymous. (2005). “GMG arrives downunder.” Australian Printer magazine, p. 46.
Kapel, D. (2005). “Proof Positive: New inkjet proofs look good and cost less.” American Printer, 122 (11). pp. 12-16.
Rich, J. (2004). The RIP Report – Using and Choosing ICC-Based RIPs that Drive Inkjet Color Printers.Gaithersburg, MD: Rich & Association LLC.
Sharma, A. (2004). Understanding Color Management. NY: Thomson Delmar Learning.
Suchy M., Fleming, P. D. III., & Sharma A. (2005). “Spot Color Reproduction with Digital Printing,” Proceedings of the IS&T NIP21: International Conference on Digital Printing Technologies, Maryland, September 18-23, 2005.