What is Dot Gain?
Dot gain is often also referred to as TVI (Tonal Value Increase). It is a printing phenomenon, which occurs in offset lithography that causes the small halftone dots that make up an image to grow in size during the printing process.
Dot gain is caused by halftone dots growing in size between the original printing plate and the final printed sheet. The image above illustrates this— the original halftone dot (dark red) expands as the ink is transferred to the paper. The result is an enlarged dot size (inclusive of the dark red and red areas). If adjustments are not made to the plate during the CTP (Computer to Plate) process, images will appear too dark when printed.
Several factors can contribute to dot gain such as the type of substrate, the type of ink, ink viscosity, squeeze, water and ink balance, etc. Paper and other substrates play a major role in the effect of d
ot gain because they all have different ink absorption rates; uncoated papers can absorb more ink than coated ones, and thus can show more gain. To illustrate this, take an eye dropper filled with coffee and place one drop on the Sunday newspaper and another on some nice glossy paper—the drop on the newspaper will really expand; the other not as much.
While dot gain in offset printing is inevitable, it is not entirely bad because it is measurable, controllable and predictable. At Impressions Incorporated, we monitor the effects of dot gain from several press runs. The information gathered is used to create plate curves. In simple terms, a plate curve is used to cut back a tonal value, so that when the dot gains on press, it gains back to what it was originally. This is one of the fundamental ways a printer ensures that the press sheet matches the approved proof.