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FLEXO Magazine : November 2009
38 FLEXO NOVEMBER 2009 www.flexography.org There is a need for consistent metallic color printing at the longest possible ink mileage while achieving the demand for visual appeal. This paper describes an initial exploration of process control of metallic color using commonly available metrology in the graphic arts. In this methodology, spectral reflectance data across the visible region are measured using 0/45 geometry with and without a polarized filter. Spectral reflectance values are, then, used to derive densitometric (Status T and Status I) and colorimetric (CIE L*a*b* and CIE LCh) values. They represent potential process control parameters for metallic color printing. Two sets of metallic color samples were measured. One set represents six different formulations of bronze metallic colors at two different ink film thicknesses (IFT). The other set repre- sents a single silver sample with six IFT variations. In order to determine which one of the color measurement parameters provides the most sensitive response to ink film thickness (IFT) variation among metallic samples, correlations between IFT, representing the independent variable, and densitometric and colorimetric parameters, representing dependent vari- ables, were investigated. It was concluded that polarized readings are more sensitive to IFT changes than unpolarized readings for both metallic col- ors, both for Status T and Status I densities, as well as lightness (L). Status I density readings of the blue filter are more sensitive than Status T density readings for bronze metallic colors. For the silver metallic colors, there is no difference between the two status densities, since the visual filter is the same for both. The chromatic dimension for bronze metallic colors, expressed ei- ther in chroma (C) or a* and b*, has a more sensitive response to IFT changes without a polarized filter. LITERATURE REVIEW The appearance of metallic colors is largely a function of their spectral reflectance in combination with a wide range of surface effects. This combination is difficult to characterize and measure (CGATS/SC3 N 447, 2001). Surface effects have primarily a specular reflection, and the spectral reflectance involves mostly the scattered light that is diffusely reflected from the ink film. The diffuse reflection is caused by the light that is scattered in the ink film and is diffused at the edges of the flakes. Thus, the smaller the diameter of the flakes, the higher the diffuse reflection and the higher the density read- ing at 0/45 geometry. However, the thickness of the ink film that achieves the desired appearance cannot be measured precisely from commonly available instruments that utilize 0/45 or 45/0 measuring geometry, due to the strong specular reflection of the light. Previous studies (Rosenberg, 2001; Mannig and Verderber, 2002) explain that the reflection of light from the metallic flakes is stronger than the "ideal white diffuser" that is used as the reference for color assessment. Densitometry compares the light that is reflected from the unprinted surface (Io) to the light that is reflected from the printed surface (I1), by means of D = log10 Io/I1. Since the reflection of light from the metallic flakes is higher than the reflectance of the unprinted surface, the density reading of the white point might be drasti- cally reduced. Furthermore, CGATS/SC3 N447 (2001) notes that the metallic appearance is not measured precisely by 0/45 or 45/0 geometry because the particular geometry does not correspond to the exact angle of specular reflection. Metallic appearance, however, is not the focus of this paper, and as such Rosenberg's study (2001), which discusses this topic, is of secondary importance. Likewise, Matthew Adby Process Control for Metallic Color Printing A Study of Commonly Available Metrology in the Graphic Arts By Dimitri Ploumidis • Two sets of metallic color samples were measured. • Correlations between IFT, as well as densitometric and colorimetric parameters, were investigated. • Polarized readings are more sensitive to IFT changes than unpolarized readings for both metallic colors, both for Status T and Status I densities, as well as lightness. TECHNOLOGIES & TECHNIQUES
Sustainable Fall 2009