What is Dot Gain?
“Dot gain” refers to the apparent growth in dot percentage that occurs from when a dot is measured on film (or in the application file) to when it is measured on the printed sheet with a densitometer. Dot gain is created by the pressure between the plate and blanket, which causes the ink film covering the dots on plate to spread outward. The term tone value increase (TVI) also refers to the same phenomenon. In reality, TVI is the more appropriate term of the two, because not all modern printing technologies (for example, some inkjets or proofers) actually use dots. In addition, the term TVI puts more emphasis on the aspect of this process that we are most concerned with – the change in tone from original to reproduction.
What Causes Dot Gain/TVI?
There are actually two primary components of dot gain. The first component, which we have already touched on, is mechanical dot gain. It is caused by the application of pressure to an ink film during printing. The second component is what is known as optical dot gain. Optical dot gain occurs when light is trapped by the substrate and scattered around the perimeter of the dot. It can be thought of as the shadow of the dot within the substrate.
How is Dot Gain Calculated?
The optical component of dot gain makes it difficult to directly measure. As a result, the method most commonly used to calculate dot gain requires density readings of a solid patch of ink and and a corresponding tone patch. One of the many formulas created to calculate dot gain can then be used to determine the change in total area of the dot. Due to the complexity of measuring optical dot gain, there is no universally accepted formula for calculating total dot gain. The one most commonly used, and which is built into most densitometers, is the Murray-Davies formula. The values obtained by these calculations can be compared against the target values in your own in-house specifications or against a published set of standard values such as GRACoL. Keep in mind that some standards will specify allowable tolerances for the midtones only, as this is where dot gain is greatest.
Some form of dot gain is inherent in any printing process, and as such it should not necessarily be seen as positive or negative. We can, however, use dot gain as a process control metric by comparing expected dot gain values with the actual results we achieve on-press. For example, if we have a 50% tone, and expect a total dot gain of 24% during printing, then a measurement of the corresponding tone patch on our press sheet should have a value of 74%. If our reading yields a value significantly different than this expected size of 74%, then our processes should be examined to determine the cause for failure.
In terms of colour balance, controlling the consistency and balance of dot gain for each ink colour is more important than its actual value. If any colour produces a significantly different dot gain value than its counterparts, severe colour shifts will occur. As a rule then, dot gain values should vary as little as possible between colours. Understanding this will help you ensure efficient, repeatable, and high quality results.