What You Need To Know About Dotgain

By Author: Dean G
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Dot gain (the amount a dot grows when printed on press) is caused by ink spreading as it is absorbed by the paper. To determine the amount of dot gain occurring, you need to know the difference between the tonal values as defined in the file/image, and the corresponding halftone dot size measured on the press sheet.

Measuring Dot Gain
Using a densitometer, dot gain is measured from the 50% patch of each process color on the press sheet's color bar. Dot gain is the net percent increase in a 50% screen. For example, a dot gain of 20% would be due to a 50% screen in the file being reproduced at 70% on the press sheet.
More dot gain occurs in midtones than in quarter and three-quarter-tones. Quater-tone dot gain results in a 25% screen printing at 40% or so; three-quarter-tone dot gain results in a 75% screen printing around 88%. The is good to keep in mind when evaluating tonal values in a 4-color image or graphic.

ICC Profiles
Photoshop automatically compensates for dot gain when an image is converted from RGB to CMYK for print production. How an image is converted is determined by the ICC profile ...
... that is selected. An ICC press profile is a file that characterizes the color capabilities (including the amount of dot gain) of a particular press/paper combination (i.e. web press/coated paper).

Coated Papers: It is recommended using the SWOP2006_Coated3v2.icc profile for converting RGB images into CMYK for those images printing on coated paper. The SWOP2006_Coated3v2.icc profile can be found on the official SWOP website (www.swop.org), in the Resource Center section (www.swop.org/resources/).

Uncoated Papers: For uncoated paper, we recommend using the U.S. Web Uncoated v2.icc profile, which ships with Photoshop.
Rather than using profiles, Photoshop's Custom CMYK Setup can be used to determine how your RGB images are converted into CMYK; however, doing so will not result in as good of a conversion as using an ICC profile. If your image has already been converted to CMYK using Photoshop's default color settings, it will still print okay since Photoshop will have used its default profile (U.S. Web Coated (SWOP) v2.icc) for the conversion.

Total Area Coverage (TAC)
In 4-color printing, if you adjusted the darkest area of your image to be 100% of each color (CMYK), you would have a TAC of 400%. In theory, this would result in a very deep shadow area; however, in reality, it would be a mess on press. Having too high of an ink coverage leads to problems such as ink contamination and drying problems. To avoid these problems the TAC for uncoated paper should not exceed 280% and 300% for coated paper. Each of the profiles recommended above use appropriate TAC limits as part of their characterization data set.