Types of dot gain
There are different types of dot gain in the prepress and printing process.
Imaging devices & media
The optical system in computer to plate systems or imagesetters is not always perfectly linear. In order to make sure that the media are exposed sufficiently, the laser beam is a bit wider than needed so that the lines that are exposed slightly overlap each other. Depending on the process (positive/negative), this may cause either a slight dot gain or a dot loss.
Media such as plates or film also can be non-linear: some are but polymer plates, for instance, can have a dot gain of 5 percent of so.
Mechanical dot gain can be summed up as any physical growth or loss the dot experiences by things like gear play or over-exposure. Mechanical dot gain in turn is divided into two sub- groups: directional and non-directional.
Directional dot gain would be dot gain incurred through doubling or slurring. Slur is the deformation of a dot due to surface speed difference of two cylinders. The difference in speed causes the dot to elongate in the printing direction. Although often present, slur has no significant contribution to overall dot gain. It typically adds only 1% to 2% in the worst cases. Doubling is basically a registration problem on multi-colour presses
Essentially Ink is transferred from the printing plate to the blanket and from the blanket to the paper. Each time pressure is applied, increasing the physical diameter of the printed dot. The ink that is used, the fountain solution, the blanket, the pressure (over/under packing) and the speed at which the press runs all influence this area.
When ink is absorbed in the paper is spreads in all direction increasing the dot diameter. This effect is more pronounced 0n newsprint, plastics, foil and uncoated paper than it is on coated stock.
Optical dot gain refers to the appearance of a given halftone to the human eye, or densitometer, when viewed at normal viewing conditions. When light hits the printed surface, it becomes slightly diffused around the dots. The human eye (as well as measuring devices) perceive this as a darkening. Dots appear to be larger than they really are.
Applications like Adobe Photoshop will automatically compensate when images are converted from RGB to CMYK. This is done based on the selected preferences set in Photoshop or Adobe Bridge. All designers need to be aware of this (see above) and make sure that their software is correctly configured for the printing process that will be used to print their job. They also need to be aware that vector based applications don’t compensate for dot gain. If you draw infographics for a newspaper, you need to make sure that flat tints don’t get too dark in print and allow for this.
Prepress operators are expected to make sure that plates delivered to the press are linear, with a typical tolerance of around 2%. Today’s highly automative systems come with calibration tools to achieve this. If a system has 5% gain, instructing the RIP to image a 50% tint as a 45% tint assures that the end result is once again 50%. This process is called linearization.
Given the fact that so many people supply files that are optimized for sheetfed offset printing with a dot gain of 12 to 20%, operators may tweak other devices such as digital presses to mimic the dot gain behavior of offset presses.