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FLEXO Magazine : June 2014
lamp), a sample aperture and imaging optics, a filter set and a basic light detector. Absorbance was read directly from the amount of voltage emitted by the photoelectric detector. From Lambert’s law, the density will be proportional to the amount of ink present on the paper. Image 1 shows the spectral products of the four ISO Status T density functions used in controlling print in North America. As we’ve de- scribed, the density will be the logarithm of the total amount of light that is passed through or transmitted by the filters when the ink print is placed under the instrument aperture. In modern instruments, we can simulate this effect by reading the spectral reflectance of the printed ink and then multiplying the measured reflectance at each wavelength by the ISO spectral product at that wavelength and then summing the results in a single number that represents the ISO status reflectance and then apply the logarithmic transformation to the reflectance to obtain the ISO status density. Notice that the peak of the ISO Status spectral product occurs at or near to the lowest reflectance or maximum absorption region of the printed inks. Also note that for the SWOP standard process yellow, the ISO density curve captures some of the non absorbing region of the ink. ISO Status E, used primarily in Europe, has a narrower response and will report a higher density value, to note smaller shifts mainly in Yellow, for the same ink print. Similarly, if one contaminates the yellow with a small amount of magenta or warm red, the measured density will be higher. CIE COLORIMETRY The CIE adopted a method for colorimetry in 1931 that utilized a con- cept known as color matching functions. These are tabulated curves indicating the amounts of a primary color that must be additively mixed together in order to match the color of a single wavelength of light. Like the density spectral products, the color matching functions can be overlaid on a reflectance curve to show the spectral regions for which they apply (See Image 2). Both the CIE color matching functions and the ISO spectral products are reduced to single numbers by multiplying the reflectance values by the weights (CIE or ISO) for each wavelength and summing the val- ues across all wavelengths. Thus even the mathematics of the methods are equivalent to this point. DENSITY IN CONTROLLING PRINTING In daily use, the pressman would bring the press up to speed, select some test prints and read the reflectance of the solid patches of the inks. The instrument would convert the reflectance using the logarith- mic amplifier and when the pressman liked the print, he could read the density and then use that value to maintain the correct amount of each ink being transferred to the substrate. In doing so, the film thickness or ink film weight is the primary application for a density reading. The pressman has been taught that if the ink is consistent and the substrate is consistent, then the press needs only to lay down the same amount of ink on every impression for the print to be consistent. Eventually, print workers began to note that just knowing the thick- ness of the ink layer was not enough information to control the two color overprints, the tone scale or the neutral colors in the print. So many special metrics based on density were developed. Concepts like “ink trap” for predicting the density of the overprints and dot gain were developed. Dot gain was based on the work of Neugebauer who had first reported a model for predicting the color of a halftone by computing the fractional area of a dot that was covered with ink and the colors of the ink and substrate. The human eye is spatially additive—light reflected from the ink and from the substrate are added together in the visual Image 1: Plots showing the TR001 Process Solids and the ISO Status T Spectral Product Image 2: Plots showing the TR001 Process Solids and the CIE D50/2 degree color matching functions 40 FLEXO | JUNE 2014