Solid Ink Density (SID) is the fundamental measurement of Optical Density in a 100% ink patch, quantifying the film thickness of the ink laid down on the substrate. Measured by a Densitometer, SID only tells the operator how much ink is on the paper, regardless of the color's hue or chroma. This value is key to establishing consistent Process Control at the start of any print job.

The difference lies in what they measure: SID is an absolute, non-tonal measurement (100% solid area), while Dot Gain is a tonal measurement (halftone area) that quantifies how much the halftone dots spread beyond their intended size. High SID (too much ink) is often a contributing factor to excessive Dot Gain, making the control of one essential to the accurate management of the other.