Spectral analysis and minimization of moire patterns in color separation

Undesired moiré patterns may appear in colour printing for various reasons. One of the most important reasons is interference between the superposed halftone screens of the different primary colours, due to an improper alignment of their frequencies or orientations. In this article we explain the superposition moiré phenomenon using a spectral model which is based on Fourier analysis. After examining the basic case of cosinusoidal grating superpositions we advance, step by step, through the cases of binary gratings, square grids and dot screens, and discuss the implications on moirés between halftone screens in colour separation. Then, based on these results, we focus on the moiré phenomenon from a different angle, the dynamic point of view: we introduce the moiré parameter space, and show how changes in the parameters of the superposed layers vary the moiré patterns in the superposition. This leads us to an algorithm for moiré minimization which provides stable moiré-free screen combinations for colour separation.