Physical dot gain of offset: Understanding and determination

This report consists of two parts. In the first, a methodology for estimating physical dot gain from scanned images of test patches is proposed. Mathematical relationships bridging the image intensities with the physical dot coverage have been worked out, taking into consideration of light scattering inside the paper substrate and the light reflection at air-paper and air-ink interfaces. In the second part, the possible causes for the physical dot gain in the light of ink-transfer and ink-setting are discussed. The physical dot gain obtained reveals significant differences in the light tones compared to the middle and dark tones. This indicates differences in ink-transfer and ink-setting between the prints consisting of well isolated dots in the light tones and those of overlapping dots in the middle and dark tones. A tentative explanation based on process-competition between press, ink spreading, and free ink-splitting, is given.