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FLEXO Magazine : September 2014
pared to color values measured on the LED UV cured sheets. GRAPHICAL REPRESENTATION OF COLOR DIFFERENCE Averages of the L*a*b* values were graphed using CHROMiX ColorThink software. In the two dimensional mode, the axes repre- sent a and b values, where the a values mea- sure the color value from red to green and b values measure the color value from blue to yellow. In the three dimensional graphical display, L is represented on the vertical axis. Conventional UV values are represented by a square on the graph and LED UV values are represented by a circle on the graph. The color gamut of sRGB, well known in the RGB display industry, is displayed as a reference. Graph 3 through Graph 8 represent the differ- ence in the color values for cyan, magenta, yellow and black, achieved using convention- al UV and LED UV curing systems. By comparing the placement of the circles (LED UV values) and squares (conventional UV values), there are many color values that show very little difference, especially in the two dimensional model. However, some color values show a trend in the squares extending further away from the center of the graph. This represents the wider color gamut achieved with conventional UV curing systems. COLOR DIFFERENCE, SHOWN NUMERICALLY Color differences between the conventional UV and LED UV systems were determined using the delta E value of the L*a*b* averages. The industry standard CIE76 (Internation- al Commission on Illumination) L*a*b* formula was chosen. The delta E value was calculated using the following formula: Delta E values were calculated for each pair of pressruns with the same substrate type and press speed. The resulting delta E values are shown in Table 1: SEPTEMBER 2014 | FLEXO 69 Graph 4: Conventional vs. LED on paper at low speed (circle = LED value, square = conventional value) Graph 3: Conventional vs. LED on paper at high speed (circle = LED value, square = conventional value)