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FLEXO Magazine : March 2014
cent—or 25 percent, 50 percent and 75 percent—depending upon the printing method and real estate available on the printed substrate. In any case, the press operator must know in advance what value to measure and the expected result. A reflectance densitometer is an instrument that measures density and, in most cases, dot gain over a printed substrate. A spectrophotometer is an instrument that measures light and creates a spectral light wave normally translated in L*a*b* values, which refers to a specific color. In most cases they are also capable of calculating Delta E or Delta E 2000. “Light” and “observer” are two variables to define before measuring. A PROBLEM TO SOLVE During my years as a printer consultant, the most common mistake I have found in our industry, throughout the world, is the lack of properly set tolerances. In many cases, companies select a standard to follow and assume that the tolerance will be acceptable, only by definition. In my experience, this is the cause of color management failure. The consequence is a lot of time and money spent on the press to correct this initially simple problem. When we talk about color, there are two tolerance defini- tions in use today: one in terms of Delta E and the other in terms of density. In most cases, print buyers, brand owners, sales people and graphic designers talk about Delta E. There is often a lack of understanding of how Delta E is defined. Some people consider Delta E “classic,” while others will consider Delta E CMC or 2000. In addition, only a few people will consider the two most important attributes when measuring with a spectrophotometer: illumination and standard observer. This means that it is very common to find that the selected metric doesn’t match because it is simply not used properly. The second way to define the tolerance, density, is normally used internally in production and in the pressroom in particular. Printers who try to control color normally place a control strip on one side of the sheet or web. This control strip usually includes solid patches of process colors and one or more tonal values. In some cases they also include solid patches for spot colors. Press operators measure density and compare the number to a pre defined table and accept a tolerance of ±0.05 of the reference density value. This comes from the ISO standard, although it is also common industry practice. I have found some companies that use different values—such as ±0.03 , for example. Ink and prepress departments usually use both, depending on what they are controlling. The reality is that for the customer (print buyer), the densi- ty is of little use. Customers care about getting the right color with the desired quality in a consistent way. Density is an easy way to print by numbers on a press and keep it stable and on time. Image 2 40 FLEXO MARCH 2014 www.flexography.org