The phenomenon of dot gain is prevalent in the printing world. It can wreak havoc on all types of printed materials, causing them to look darker in colour than they were intended to be. Sometimes expressed as a percentage, dot gain is the difference between the size of dots on the source file and the size of the corresponding dots on the printed piece. So for instance, if the source file is at 50% but is at 65% after printing, the dot gain is 15%.
Furthermore, dot gain differs depending on the colour used, and is slightly different between the 4 CMYK colours of cyan, magenta, yellow and black.
Dot Gain Causes
There is more than one cause of dot gain. It can occur as the result of machinery, paper and even light.
Dot Gain via Printing Press
During offset printing, ink gets transferred from a printing plate to a blanket before being transferred from the blanket to paper. Each transfer alters the dot more in terms of increased diameter. But the type of blanket used, as well as the ink type and fountain solution as well as the pressure applied all contribute to the percentage of the dot gain on the printed final product.
The absorption of printed ink into paper also causes dot gain. During absorption, ink travels vertically as well as horizontally, which can increase dot diameter. This effect is more easily seen on newsprint and uncoated inkjet labels than on coated paper.
Dot Gain due to Light
Dot gain can also be perceived although it may not actually exist. This is called optical dot gain, and occurs when light makes contact with a printed surface and causes its dots to take on a diffused appearance. The human eye perceives the dot to look darker and larger in size than it actually is.
Computers and other Media
Dot gain can also be caused by imaging media and devices. For example, a computer’s optical system cannot be considered to be linear. This is why, during printing that any laser beams used are wider than necessary; this accounts for any errors in perception of the optical system of the computer. But even these will result in dot gain or loss, however slightly. As far as media is concerned, dot gain can occur via the film or plates used. This is because different media will handle ink differently.
Minimising Dot Gain
Although it is possible to minimise some types of dot gain at the both the prepress and press stages, the phenomenon is typically identifiable at the design stage, Therefore, it is the designer’s responsibility to be aware of it as well as anticipate how it will affect a finished product. The designer can also control dot gain in their image editing program, although this will not necessarily prevent it from occurring later on.
Many photo editing programs compensate for dot gain automatically. This automatic conversion usually occurs during image conversion from RGB to CMYK colour. However, in order for proper conversion to take place, the software must be configured according to the printing process which will be used.
During pre-press, the operator must ensure that any plates are as linear as possible. A tolerance of 2% is typical, and the process of linearisation should be utilised. Another thing to consider at the pre-press stage is that many times, a customer will supply files which have been optimised for a particular printing process, such as sheet-fed offset. This will cause the amount of dot gain to change. However, one print process can mimic the dot gain amount of another with some tweaking.
Although the dot gain phenomenon does have the potential to cause many headaches during the printing process, the fact that it can be addressed during several stages and from early on is a relief, both to customer and operator.