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.
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.
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).
The Yule–Nielsen effect, sometimes known as optical dot gain, is a phenomenon caused by absorption and scattering of light by the substrate.
A common value for dot gain is around 23% in the 40% tone for a 150 lines per inch screen and coated paper.
However, it will tend to be larger when the halftone pattern in finer and when the substrate has a wider point spread function.
In instances in which both critical printing fractions are non-trivial, Viggiano recommended that a cascade of two (or possibly more) applications of the dot gain model be applied.
To a certain extent, the models described above are empirical, as their parameters cannot be accurately determined from physical aspects of image microstructure and first principles.
Such models were used by Pearson and Pobboravsky, for example, in their program to compute dot area fractions needed to produce a particular color in lithography.