Deducing ink thickness variations by a spectral prediction model

Most existing techniques for regulating the ink flow in offset presses rely on density measurements carried out on specially printed patches. In the present contribution, we develop a methodology to deduce ink thickness variations from spectral measurements of multi-chromatic halftone patches located within the printed page. For this purpose, we extend the Clapper-Yule spectral reflectance prediction model by expressing the transmittance of the colorants composed of superposed inks as a function of the ink transmittances and of fitted ink layer thicknesses. We associate to each ink an ink thickness variation factor. At print time, this ink thickness variation factor can be fitted in order to minimize a difference metric between predicted reflection spectrum and measured reflection spectrum. The ink thickness variations deduced from multi-chromatic halftones allow to clearly distinguish between normal ink volume, reduced ink volume or increased ink volume. This information can then be used for performing control operations on the printing press.