In prepress and printing dot loss refers to halftone dots either disappearing or reducing in size. This is the opposite of dot gain, the process in which halftone dots increase in size.
Dot loss can happen in prepress as well as during the printing process:
- In prepress tiny dots such as a 1 or 2% dot may not appear because of limitations in the output device (e.g. a computer to plate device) and/or the substrate (the printing plate or film). Incorrect exposure or focus settings of the output device are other potential causes of dot loss. In the development process of plates, an over-aggressive developer or excess pressure of clean-out rolls can cause it as well.
- Some printing processes such as screen printing or high-speed web printing can have difficulties reproducing small halftone dots. The threshold value that can be maintained differs from press to press with the printing substrate and speed of the press also playing a role. Whereas some newspaper presses manage to maintain a 3% dot, there are screen printing presses in which any dot smaller than 15% simply disappears. There are other factors that can cause dot loss in the mid-tones and shadows. With flatbed presses, for example, dot loss in certain areas of the printed sheet may be caused by a weak/delaminated print table, an undersized frame, or an oversized squeegee.