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Dot gain
Dot gain, or tonal value increase, is a phenomenon in physical printing processes including offset lithography, flexography and gravure that causes printed material to look darker than intended. It is caused by halftone dots growing in area between the original printing film and the final printed result. In practice, this means that an image that has not been adjusted to account for dot gain will appear too dark when it is printed. Dot gain calculations are often an important part of a CMYK color model.
Dot gain is defined as the increase in the area fraction (of the inked or colored region) of a halftone dot during the prepress and printing processes. Total dot gain is the difference between the dot size on the film negative and the corresponding printed dot size. For example, a dot pattern that covers 30% of the image area on film, but covers 50% when printed, is said to show a total dot gain of 20%.
However, with today's computer-to-plate imaging systems, which eliminates film completely, the measure of "film" is the original digital source "dot". Therefore, dot gain is now measured as the original digital dot versus the actual measured ink dot on paper.
Mathematically, dot gain is defined as:
where aprint is the ink area fraction of the print, and aform is the prepress area fraction to be inked. The latter may be the fraction of opaque material on a film positive (or transparent material on a film negative), or the relative command value in a digital prepress system.
Dot gain is caused by ink spreading around halftone dots. Several factors can contribute to the increase in halftone dot area. Different paper types have different ink absorption rates; uncoated papers can absorb more ink than coated ones, and thus can show more gain. As printing pressure can squeeze the ink out of its dot shape causing gain, ink viscosity is a contributing factor with coated papers; higher viscosity inks can resist the pressure better. Halftone dots can also be surrounded by a small circumference of ink, in an effect called "rimming". Each halftone dot has a microscopic relief, and ink will fall off the edge before being eliminated entirely by the fountain solution (in the case of offset printing). Finally, halation of the printing film during exposure can contribute to dot gain.
The Yule–Nielsen effect, sometimes known as optical dot gain, is a phenomenon caused by absorption and scattering of light by the substrate. Light becomes diffused around dots, darkening the apparent tone. As a result, dots absorb more light than their size would suggest.
The Yule–Nielsen effect is not strictly speaking a type of dot gain, because the size of the dot does not change, just its relative absorbance. Some densitometers automatically compute the absorption of a halftone relative to the absorption of a solid print using the Murray–Davies formula.
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Dot gain
Dot gain, or tonal value increase, is a phenomenon in physical printing processes including offset lithography, flexography and gravure that causes printed material to look darker than intended. It is caused by halftone dots growing in area between the original printing film and the final printed result. In practice, this means that an image that has not been adjusted to account for dot gain will appear too dark when it is printed. Dot gain calculations are often an important part of a CMYK color model.
Dot gain is defined as the increase in the area fraction (of the inked or colored region) of a halftone dot during the prepress and printing processes. Total dot gain is the difference between the dot size on the film negative and the corresponding printed dot size. For example, a dot pattern that covers 30% of the image area on film, but covers 50% when printed, is said to show a total dot gain of 20%.
However, with today's computer-to-plate imaging systems, which eliminates film completely, the measure of "film" is the original digital source "dot". Therefore, dot gain is now measured as the original digital dot versus the actual measured ink dot on paper.
Mathematically, dot gain is defined as:
where aprint is the ink area fraction of the print, and aform is the prepress area fraction to be inked. The latter may be the fraction of opaque material on a film positive (or transparent material on a film negative), or the relative command value in a digital prepress system.
Dot gain is caused by ink spreading around halftone dots. Several factors can contribute to the increase in halftone dot area. Different paper types have different ink absorption rates; uncoated papers can absorb more ink than coated ones, and thus can show more gain. As printing pressure can squeeze the ink out of its dot shape causing gain, ink viscosity is a contributing factor with coated papers; higher viscosity inks can resist the pressure better. Halftone dots can also be surrounded by a small circumference of ink, in an effect called "rimming". Each halftone dot has a microscopic relief, and ink will fall off the edge before being eliminated entirely by the fountain solution (in the case of offset printing). Finally, halation of the printing film during exposure can contribute to dot gain.
The Yule–Nielsen effect, sometimes known as optical dot gain, is a phenomenon caused by absorption and scattering of light by the substrate. Light becomes diffused around dots, darkening the apparent tone. As a result, dots absorb more light than their size would suggest.
The Yule–Nielsen effect is not strictly speaking a type of dot gain, because the size of the dot does not change, just its relative absorbance. Some densitometers automatically compute the absorption of a halftone relative to the absorption of a solid print using the Murray–Davies formula.