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FLEXO Magazine : November 2009
FLEXO NOVEMBER 2009 www.flexography.org same both for Status T and Status I densities. It can be seen that unpolarized readings have a flat response and are not able to distinguish between the different IFTs. On the other hand, polarized readings range from a density of approxi- mately 0.60 to a density of 1.10, being sensitive enough to distinguish between the different IFTs. Next, the colorimetric coordinate of lightness is examined. It is seen that polarized readings provide a sensitive response for monitoring IFT variations for process control. The differ- ence in lightness for polarized samples ranges of 25.0 Delta L, whereas for unpolarized the difference is only 10.0 Delta L. Finally, Figure 12 displays the chromatic difference for silver metallic colors. It is observed that there is not an important chroma difference between polarized and unpolarized read- ings, and moreover the reading is low, about 2.00 C. This indi- cates that there is no hue difference as well between unpolar- ized and polarized readings, as that would be insignificant due to the strong achromatic nature of the samples. CONCLUSION For process control of metallic color samples using 0/45 geometry, polarized readings have better sensitivity than unpolarized readings in monitoring changes in IFT. This holds true for both metallic color samples. Additionally, for bronze metallic color samples, Status I densities are more sensitive than Status T densities. Likewise, polarized readings have a more sensitive lightness response than unpolarized readings both for bronze and silver metallic colors. For the bronze metallic color samples, there was a differ- ence between the Status T and Status I density readings, with Status I being more sensitive to IFT changes. For the silver metallic colors, however, since the visual filter is being used, there is no difference between the different status densities. Analyzing the chromatic dimension for bronze metallic colors, which can be expressed either in chroma or a* and b*, it was concluded that an unpolarized filter provides higher sensitivity. For the silver metallic colors, there was no differ- ence between the two filters, due to the strong achromatic component of the silver color. FURTHER RESEARCH This analysis used only silver and bronze metallic colors. In order to be able to generalize the findings, it is suggested to conduct the same experiment with various metallic colors, of different formulations. It would be also important to extend the metrological speci- fication by including the integrating sphere and gonio-spec- trophotometry. Additionally, the extent to which these findings would be identical using a D50/2-degree observer would be interesting to determine. A final suggestion for further research would be to corre- late IFT changes to the perceived color changes, involving an analysis mostly of the appearance attributes of metallic colors. ACKNOWLEDGEMENTS First, I would like to thank Gravure Research Professor Robert Chung for his involvement in setting the objectives and discussing the result of this experiment. The help of Research Associate Franz Sigg was important in providing the Excel template that allowed the conversion of the spectral reflectance readings to the different status densities. Also, the R.I.T. School of Print Media and Dr. Patricia Sorce for providing the funds that made this research and publication possible. Special thanks also to the whole Test Target group, FIGURE 12. The chroma response of polarized and unpolarized readings of silver metallic color samples of different IFTs. FIGURE 10. Density reading of the visual filter for polarized and unpolarized metallic samples of varying IFT. FIGURE 11. The lightness response of polarized and unpolarized readings of silver metallic color samples of different IFTs.
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