Dot gain (also known as 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 diameter 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
Types of Dot Gain
There are basically two types of dot gain, optical and mechanical. Mechanical comes from the presses, and there are all kinds of differences that play a role. Each machine, and each printing method has its own rules and quirks when it comes to dot gain.The second type of dot gain is known as optical dot gain. This is dot gain caused by how the dot appears to the eye on the page. Optical dot gain is effected by the printing method, paper, inks, lighting and no doubt the shape of your eyeball. So there is a more subjective aspect to optical dot gain, simply because it may be more noticeable to some then others.
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 increase of screen dot sizes in offset printing has two basic reasons: One is a characteristic of the lithographic printing technique and based on the competition of two liquids to wet the plate surface. The other one is caused by mechanical action between cylinders. In addition to that in screen areas we find higher color intensity than we would expect only from area coverage. This part of dot gain is due to light gathering below the dots.
CONTROLING DOT GAIN
Dot gain can be measured using a densitometer and color bars in absolute percentages. Dot gain is usually measured with 40% and 80% tones as reference values. A common value for dot gain is around 23% in the 40% tone for a 150 lpi screen and coated paper. Thus a dot gain of 19% means that a tint area of 40% will result in a 59% tone in the actual print. Modern prepress software usually includes utility to achieve the desired dot gain values using special compensation curves for each machine. Dot tone can be controlled best during the preparation of files. If you or your printer anticipate an issue, than leave sensitive files in their most editable formats such as native photoshop or similar files. A pre-press staff person will be able to adjust the halftones, quartertones, mid tones and even three-quarter-tones (that’s a lot of tones!) in order to adjust your files for the best printing possible.