Dot gain

Dot gain, or tonal value increase, is a phenomenon in offset lithography and some other forms of printing which 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.

It 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 ${\displaystyle a_{\text{print}}}$ is the ink area fraction of the print, and ${\displaystyle a_{\text{form}}}$ is the pre-press 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.

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