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FLEXO Magazine : July 2013
tion for all things related to lighting. In 1931, the Vision and Colour Technical Committee of the CIE set its first standards, which have been a leading force in colorimetry. To accurately describe color, the CIE defined a “standard observer” as a way to control an important variable in color measurement: the way human’s view color3. The CIE 1931 standard observer is a composite made from a group of individuals and is representative of normal human color vision. In order to obtain the results, the observ- ers viewed a pure white screen. On one side a test lamp with a pure spectral color was emitted, while on the other, three lamps emitting various amounts of red, green and blue light attempted to match the color of the test lamp. The results from the match were obtained and recalculated to all positive values, in order to create the first CIE color model called XYZ. Later on, other models were derived from this first model in response to varying concerns, one of them being the CIE LAB color model3. Another critical component of accurately measuring color is the SPD of the illuminant. When measuring print samples, the ISO 3664:2009 standard sets the spectral power distribu- tion of the illuminant to D504. Though D50 is the standard for comparing print samples, many illuminants in the real world have varying SPDs that are much less defined. In commercial settings, the SPD of illuminants is often not taken into consid- eration, due to economic or environmental reasons1. Figure 1 shows the difference in SPD of D50 and other illuminants that include Illuminant D65, which represents average daylight and Illuminant A, which represents typical incandescent lighting. Differences in the non-visible spectrum of an illuminant can also have a large impact on color. Illuminants that contain an ultraviolet (UV) component can attribute to fluorescence, a phenomenon where non-visible UV light is absorbed and reemitted as visible light. Fluorescence occurs in substrates that contain re-emitted Optical Brightening Agents (OBAs). OPTICAL BRIGHTNING AGENTS OBAs are special dyes added to substrates to enhance their brightness. OBAs work through the process of fluores- cence by absorbing invisible UV radiation at wavelengths bellow 400 nanometers (nm) and re-emitting the light at the visible blue end of the spectrum at around 400-450 nm5. OBAs are typically added to most substrates that have a high bright- ness because it is more economically and environmentally preferable than additional bleaching processes1. The fluorescing effect causes substrates to emit more blue wavelengths, which cause the appearance to be whiter. Due to the transparency of inks, OBAs also can have an affect on printed colors. This is especially true for areas with low percentage halftone dots due to larger areas of the substrate being exposed2. It is important to consider the impact of OBAs when accurately measuring color, due to the large percent- age of substrates that contain them for printing and proofing. MEASURING COLOR Color values on a printed sample are measured using a spectrophotometer. The vast majority of spectrophotometers in the graphic arts rely on a single illuminant, Illuminant A5. This differs from visual inspection that is performed under D50 according to ISO 3664. Because Illuminant A emits very mini- mal UV light compared to D50, it can cause a visual mismatch between two printed pieces, even when instruments using Illuminant A indicate there is none. According to ISO 13655, it is possible to test for florescence by measuring the CIELAB difference of a sample under both Illuminant A and D65. “ T he resultant spectral data is then used to compute the tristimulus values, relative to D50, for both sources. The CIELAB difference between them is a mea- sure of the fluorescence6.” This research seeks to answer the following question: How significant is the effect of metamerism on a spot color’s range of tints when printed on substrates that contain OBAs? The first step is to confirm florescence in a substrate based on ISO 13655 standards. Then, the second step is to confirm florescence between M1 (D50) and M2 (UV-Cut) measurement conditions. Finally, the third step, after printing test patches, is to determine the DE 2000 of M1 (D50) and M0 (Illuminant A) measurement conditions. This is done to understand issues that might arise from trying to measure color with an instru- ment and by visual comparison. It is expected that the DE between the two illuminants will grow larger as the tint percentage becomes smaller, due to a larger unprinted area where the substrate’s fluorescence will affect color. Two substrates with the same brightness and known to contain OBAs were used to determine the impact of the OBAs on color. Tint patches were printed on MeadWestvaco’s The first step is to confirm florescence in a substrate based on ISO 13655 standards. The second step is to confirm florescence between M1 (D50) and a M2 (UV-Cut) measurement conditions. The third step after printing test patches is to determine the DE 2000 of M1 (D50) and M0 (Illuminant A) measurement conditions to understand issues that might arise from trying to measure color with an instrument and by visual comparison. It is expected that the DE between the two illuminants will grow larger as the tint per- centage becomes smaller due to a larger unprinted area where the substrate’s fluorescence will affect color. 2.2 Procedure Two substrates with the same brightness and known to contain Optical Brightening agents were used to determine the impact of the OBAs on color. Tint patches were printed on MeadWestvaco’s Crescendo C1S paperboard and Tango Coated Cover C1S. For this study Pantone 281, Pantone 299 and Pantone 375 were chosen as shown in Figure 2. Each color was applied to the substrate using a HarperScientific Phantom QD Flexographic proofing system with dot coverage percentages of 10, 35, 50, 75 and 100. All instrument measurements were completed using a Konica Minolta FD-7 spectrodensitometer with a UV LED light source (spectral reflectance measurement: 380 to 730nm, spectral irradiance 360 to 730nm). To start, both substrates were measured and confirmed to have florescence. The substrate was mea- sured using M0 (Illuminant A) and D65 and the DE00 was calculated to confirm the presence of OBAs as per ISO 13655. The substrate and tints were then measured using M0 (Illuminant A), M1 (D50) and M2 (UV-Cut). From here it was possible to calculate the DE00 between the illuminants for each tint patch. Figure 2: Above are the approximate representations of the chosen colors. 3. Results Measurement of the unprinted Crescendo C1S under M0 (Illuminant A) and D65 yielded a DE00 of 3.31. Furthermore, a measurement under M0 (Illuminant A) and M1 (D50) resulted in a DE00 of 1.36, and a measurement under M1 (D50) and M2 (UV-Cut) resulted in a measurement of 6.43. Measurement of the unprinted Tango Coated Cover C1S under M0 (Illuminant A) and D65 yielded a DE00 of 3.29. Further- more, a measurement under M0 (Illuminant A) and M1 (D50) resulted in a DE00 of 1.35, and a measure- ment under M1 (D50) and M2 (UV-Cut) resulted in a measurement of 6.41. Table 1 on the next page show the measurement results from both of the substrates. In both substrates it is clear that the 10% tint patches had the largest DE. The DE diminishes as the halftone dot percentage becomes larger. This signifies that the unprinted substrate plays a large role in color difference. Figure 2: Above are the approximate representations of the chosen colors. www.flexography.org July 2013 FLEXO 31