Quantitative color correction with a single gray mask requires that the two unwanted color densities of each ink must be equal. The unwanted densities can be equalized by appropriate choice of the separation filters. By use of DIN inks with daylight illumination, it is possible to fulfill the Luther condition as well as achieve equality of the unwanted color densities. The required mask must be exposed with blue, green, and red light, the proportions of these exposures being calculated from the unwanted densities of the three inks. To calculate the superimposition of three halftone images, additivity of color density behind each separation filter and purely additive color mixture of the halftone images were assumed. The result of the calculations shows that any mixture of inks produces the same unwanted color density behind each of the three separation filters. This means that any ink mixture can be masked by the same gray mask for all three separations. The unwanted color densities of some ink mixtures have been calculated and compared with a realizable mask density, with the result that, theoretically, any ink mixture can be masked with a single mask to within ±0.01 density units. This accuracy is better than that required by a practical process, but it depends on the following three conditions: equality of unwanted color densities for the pure inks, additivity of color densities in the superimposition of the pure inks, and purely additive color mixture. Thus the system is a very simple and theoretically correct first approximation to a practical masking process. The above assumptions are generally not fulfilled, but the deviations arising are of second order and do not seem to spoil markedly the color quality of a print.
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