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FLEXO Magazine : November 2009
40 FLEXO NOVEMBER 2009 www.flexography.org circles stand for the unpolarized and polarized readings respectively. It can be seen that the polarized filter results in lower reflectance (and consequently higher density) for both IFTs. Additionally, at the lower end of the spectrum, where the blue density filter is used, the difference between the low and high density for the polarized readings is larger than from the unpolarized density difference, meaning that polarized filter readings are more sensitive to changes in IFT. An additional observation is that the polarized filters have flat responses for wavelengths higher than 500nm. This is caused by the exclusion of the specular reflections from the metallic surface that would have provided information about the color appearance of the sample. In comparison, the un- polarized readings do not have such a flat curve. The spectral measure- ments were then converted to density readings. Figure 2 displays another bronze metallic sample. It includes the responses of the Status I and Status T densities for the blue filter. It can be seen that Status I filter samples the spectral re- flectance over a narrower range. It also has a higher peak, which translates into an amplified and thus higher and more sensitive density reading. On the other hand, Status T den- sity has a lower peak that spreads over a broader range of the spectrum. Figure 3 analyzes the two different status density responses of the same bronze color sample. The left column displays Status T density and the right shows Status I. It can be seen that Status I densities have higher readings. Also, the polarized filters have higher readings for both status densities (circles). More analytically, the density difference between the polarized and unpolarized readings is slightly higher for Status I (1.815-1.110=0.705 for Status T and 1.95-1.23=0.72 for Status I). This is observed in all the FIGURE 3. Status T and Status I density responses for a bronze metallic color sample. FIGURE 4. a*b* plot of bronze metallic color. FIGURES 5, 6 & 7. L* and C* analysis of a bronze metallic color sample. TABLE 1. Density difference of the blue filter for low and high density bronze metallic color samples. TECHNOLOGIES & TECHNIQUES
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