If practical colorants could be found to meet theoretical requirements then there would be no need for color correction in multi-color printing. Color correction is necessary because the available yellow, magenta, and cyan inks absorb and transmit in all parts of the visible spectrum. It is possible to measure how much these colorants deviate from the ideal with the use of a reflection densitometer. The red, green, and blue filter positions on a densitometer will read the cyan, magenta, and yellow densities respectively. From these densities the amount of color correction necessary to compensate for the inks deficiencies is expressed in terms of percent masking. This is usually computed by the formula: percent mask = highest unwanted density/wanted density. The percent mask is often computed from the densities found when measuring a solid area of a particular colorant. The ratio of unwanted densities to wanted densities is often assumed to remain constant for solids and tints. This is the proportionality rule and is assumed to hold true in what are known as the masking equations (which are the basis for photographic masking for color correction.) The problem is that when a graduated scale of a single color is printed from a light tint to a solid, the proportions of the wanted and unwanted densities do not remain the same for varying tint values and solids. This is known as proportionality failure. This paper deals with the statistical evaluation of the three factors (and the interactions between them) which have been suspected of being the primary causes of this phenomenon. The three factors being tested are solid ink density (ink film thickness), type of paper, and half tone screen ruling. The results indicated that screen ruling was the most important factor for all colors with solid ink density being the next in importance. But the type of paper used was not found to influence proportionality failure, except with magenta ink. This, along with the fact that there was very little proportionality failure in yellow ink, may indicate a certain degree of confusion between proportionality failure and the influence of various factors (including paper) on the 'purity' of a color. It was shown in this experiment that the optimum printing levels which produce the least amount of proportionality failure were at the lowest solid ink density practical, the finest screen ruling, and on uncoated paper (yellow notwithstanding since it displayed comparatively no proportionality failure.) The implications of these results indicate that certain modifications in color correction methods may be necessary if printing conditions deviate widely from these optimum levels.
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Department, Program, or Center
School of Print Media (CIAS)
Levine, Daniel, "An evaluation of the primary factors which influence proportionality failure" (1976). Thesis. Rochester Institute of Technology. Accessed from
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