The Color Gamut of Halftone Reproduction

Colour mixing by a halftoning process, as used for colour reproduction in graphic arts and many forms of digital hardcopy, is neither additive nor subtractive. Halftone colour reproduction with a given set of primary colours is heavily influenced not only by the colourimetric properties of the full-tone primaries, but also by effects such as optical and physical dot gain and the halftone geometry. By computer simulations based on basic spectrophotometric measurements on actual prints, we demonstrate that such effects not only distort the transfer characteristics of the process, but also have an impact on the size of the colour gamut. In particular, a large dot gain, which is commonly regarded as an unwanted distortion, expands the colour gamut quite considerably. We also present an image processing model that can describe and quantify the effects of physical and optical dot gain on different media and with different halftoning methods.