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FLEXO Magazine : November 2009
www.flexography.org NOVEMBER 2009 FLEXO (2003) describes metallic luster, which is measured by means of a sphere spectrophotometer. Metallic luster is defined by Gary Field (1998) as the ratio of specularly reflected light to the diffusely reflected light from the same surface. The study of Mannig and Verderber discusses the reduction of the effect of the specular reflectance by utilization of polarized filters, having more relevance to process control applications. The present paper provides additional insight toward this direction. Polarized filters block the diffuse reflection of light that is reflected from the metallic flakes. In this manner, polarized filters significantly reduce the amount of reflection due to the metallic mirror-like surface and increase the density reading, allowing a more accurate measurement of IFT (Sigg, 2005). This study further proposes the use of narrowband Status I density (20 nm bandwidth), instead of the wideband Status T density (100 nm bandwidth) that is standard in the U.S. Status T densities were defined to match as closely as possible the spectral products historically used in evaluating original artwork meant to be color separated (ISO/CD 5-3, 2006). In order to achieve this visual match it was important to sample a wider range of wavelengths. However, this decreased the sensitivity to the response at the peak wavelength of the filters. Status I spectral density is applicable to the evalua- tion of graphic arts materials, such as process ink on paper. Status I spectral densities are derived from narrowband filters that amplify the spectral reflectance at the peak wavelength of each filter, and as such provide higher densities and higher sensitivity to IFT variations (Kiphan, 2001). Metallic colors, however, do not peak on the wavelength where each filter is meant to measure. Bronze metallic colors, having a primarily yellow hue, shall be measured by the blue filter (dB). Silver metallic colors have a grey hue, and they shall be measured by the visual filter (dV). However, there is no difference in the visual filter between Status T and Status I densities. The visual filter is calculated for both status densi- ties by formula dV= -log10 (Y/100). A final remark is that the calculation of lightness (L) is also based on Y. This would mean that the response of both dV and L would be identical, if not for the logarithmic nature of dV, that reduces the sensitivity for spectral products of high reflectivity. OBJECTIVE The objective of this paper is to evaluate the commonly available metrology used for process control of metallic inks. The focus is on metrology for process control of metallic inks using 0/45 geometry. The spectral reflectance of a number of different formulations of metallic colors will be measured both with and without a polarized filter. The spectral reflectance data will be converted to colorimetric values (CIE L*a*b* and CIE LCh) and densitometric values, using both Status T and Status I densities. The limitations of this paper is that the specification of metallic inks will not be addressed, as it more closely relates to their appearance attributes. Moreover, the integrating sphere geometry and gonio-spectrophotometry will not be addressed, as they are not commonly available in printing plants. METHODOLOGY The measuring instrument used in this experiment will be the Spectrolino Spectroscan spectrophotometer, measuring the spectrum from 380nm to 730nm at 10nm intervals, and using a 0/45 geometry. The measurements will be done both with and without a polarized filter. The spectral data will be converted to Status T and Status I densities, utilizing an excel spreadsheet that was provided by Franz Sigg, Research Associate at RIT. The colorimetric coordinates will be specified in CIE L*a*b* and CIELCh, which will be exported from GMB's MeasureTool 5.0.1. The illuminant used will be D50, and the standard observer will be 10-degrees, because it is more appropriate for uniform colored areas larger than a 4-degree field of view. The metallic color samples were created by Eckart using an IGT printability tester. Six bronze metallic colors with differ- ent formulations were provided and measured. Each bronze sample came at two IFTs, one low and one high. Additionally, Eckart created one silver metallic color at six different IFTs. After the measurements and the computations, the data were analyzed and evaluated by means of graphical analy- sis, with the focus to relate the measurement responses with relation to IFT changes. RESULTS Bronze metallic colors. The spectral reflectance of the low- and high-density bronze metallic color samples was measured both with polarized and unpolarized filter. Figure 1 displays the spectral reflectance curve of one of the samples. The response of the rest of the samples was identical. The orange lines represent the low-density readings, and the green lines the high-density readings. The squares and FIGURE 1. The spectral power distribution of one bronze metallic sample. FIGURE 2. Status T and Status I density spectral reflectance responses for a bronze metallic color sample.
Sustainable Fall 2009