The effect of screen ruling on the saturation of pastel gamut colors

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Title: The effect of screen ruling on the saturation of pastel gamut colors
Author: Chittick, John T.
Abstract: Reproduction of pastel colors by offset lithography produces images that have less color saturation than their continuous tone counterparts. This problem is primarily attributable to the halftone structure which approximates continuous tone pastels with large amounts of white paper and small amounts of chromatic ink. This thesis investigates the effect of screen frequency upon pastel color saturation. Single and two color overprint tints of nominal five, 10, and 15 percent printing dot areas were printed in 85, 100, 120, 133, 150, and 200 lines screen rulings. The reflectances of these colors were measured with a colorimeter and converted to CIEL*a*b* color notation to provide metric lightness and the chromatic co-ordinates from which chroma was calculated. Physical dot area was calculated by the Yule- Nielson equation for each single color patch. A more complicated graphical technique was used to find this data for the overprint colors of the same physical dot areas. For both single and overprint colors, as screen ruling becomes progressively finer, pastel saturation increased with physical dot area held constant. For this experiment, the average chroma difference at the 10 percent physical dot area between the coarsest and finest line screen ruling is 2 chroma units. This difference could also be achieved by changing physical dot area by one percent. The amount of saturation increase is dot area and hue dependent. The yellow printer showed the greatest saturation increase. The saturation increase increased with dot area for a given screen frequency. 2 Lightness decreased whenever saturation increased. When the saturation associated with the different screen rulings were compared at equal darknesses, an increase in saturation could not be statistically determined. The saturation increase from increased screen ruling at equal physical dot areas occurs because of the increased ink/light/paper interaction. Optical dot area is calculated by the Murray·Davies equation which does not compensate for this change in optical interaction. The experiment shows that at equal optical dot areas, chroma will be the same regardless of screen ruling.
Record URI: http://hdl.handle.net/1850/8845
Date: 1980-11

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