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FLEXO Magazine : June 2014
Controlling Print Using Colorimetry Pros & Cons, Formulas & Charts Danny C. Rich, Ph.D., Sun Chemical Corporation; Ray Cheydleur, X-Rite Corporation; Greg Imhoff [Editor’s Note: While the specific example cited in the following article is offset, the concepts discussed and standards detailed apply to flexography as well.] T his article shows how modern colorimetry can supplement or replace historical densi- tometry in setting aims and tolerances for the reproduction of graphic images. ISO Status Density was developed to provide information on the amount of dye or ink utilized in a reproduc- tion process. Thus, if one puts the correct amount of ink down on the substrate, the image will be correct. Densitometry functions by measuring the amount of light absorbed and not reflected back to the observer. The appearance of a print is controlled not by light absorbed, but by light reflected into a read- er’s eyes. Colorimetry captures the appearance of a print in terms of attributes like: • Hue • Chroma • Lightness • Darkness • Grayness On press or near press inspection systems will track and report on these perceptual variables using metrics that are much more sensitive to the impact on the reader. These tracking systems will compare current appearance to the desired appearance and produce numerical trend charts and conformance re- ports. New algorithms must be used to relate the perceptual variables to the settings of the ink keys on the press to maintain the overall appear- ance of the reproduction. Modern publication printing involves setting up one or more web fed presses to run similar images targeted to different geographic regions. In most cases, the number of impressions can be large and the first impressions should be visually similar to the last impressions. Historically, the setup of the press and the control of the quality of the printing throughout the run were documented using an ISO Status Density reading. Usually, these readings came from small patches along the lead or tail edge of the print that would normally not be observed by the reader. Industry specifications, such as SWOP, would establish aims for these readings along with a narrow tolerance on either side and the pressman would setup his press and order inks that would provide prints whose densi- ty readings were within those tolerance windows. More recently, it has been observed that prints produced to very tight density specifications may still have visual differences, especially in the “Densitometry measures what you do not see and colorimetry measures what your customer wants to buy. ” 38 FLEXO | JUNE 2014 TECHNOLOGY & TECHNIQUES Anilox Roll, Doctor Blade, Ink Selection Guide