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FLEXO Magazine : February 2012
to base all color decisions throughout the print job. Use a spectrodensitom- eter to take multiple measurements in different areas of the designated color swatch in the color guide, and average the measured results into one value. This becomes the color reference when choosing or formulating ink and checking color throughout the printing process. Why take multiple measurements in different locations on one color swatch? Color guides not only have color vari- ance between different guides, but also can substantially vary within the color swatch itself. By averaging the measure- ments you are minimizing the impact of color swatches that have inconsistent color. That specific color guide should accompany the job throughout the print production process. Lastly, a customer may provide L*a*b* color values accompanied by a proof. While the L*a*b* color value offers an empirical method for check- ing color, it does not guarantee that what is coming off the press will visually match the proof or meet the customer 's expectation. Remember, customers cannot "see" an L*a*b* value, only a printed representation in the form of a proof. There are many factors that influ- ence the visual match of proof to press. One of the main influencing factors is paper, and is covered in the section that follows. COLOR OF PAPER The color of paper influences how we perceive color. It is commonly understood in our printing industry that paper is the fifth color. The "color" of paper heavily influences how our eyes visually perceive color. Often, inkjet proofs will contain optical brighteners to increase the whiteness of the paper to make the paper look "less yellow." Adding chemicals to the paper, causing invisible ultraviolet light to re-emit more toward the blue end of the spectrum so that the human eye perceives the paper to be whiter, even though in actuality it is bluer, does this. In addition, optical brighteners show large color shifts when viewed under dif- ferent lighting conditions, such as Tung- sten, fluorescent, D5000 (normal day light). So the press sheet will appear more yellow than the proof, adversely affecting the visual match seen by the human eye. The surface texture and coating differences between the proof and press papers will also contribute to visual color differences. If the proof is from a previous job, ex- posed to both open air and light, it will lose a certain amount of color fidelity and no longer accurately represent the customer-defined L*a*b* color values. The "blueing" effect of optical bright- eners will fade over time causing the paper to yellow, and not only lose the visual accuracy, but also the empirical measureable accuracy as well. In order to determine whether the customer-supplied proof contains any optical brighteners, use a spectroden- sitometer to take an L*a*b* measure- ment of the non-printed area of the proofing paper. Remember, in general, if the b* value is greater than -3, there are optical brighteners in the paper. www.flexography.org FEBRUARY 2012 FLEXO 31