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AssigneeEcole Polytechnique Federale De Lausanne (Epfl)
InventorsRoger D. Hersch, Peter Amrhyn, Matthias Riepenhoff
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The present invention proposes a method and a computing system for deducing ink thickness variations from spectral reflectance measurements performed on a printing press or on a printer. The computed ink thickness variations enable controlling the ink deposition and therefore the color accuracy, both in the case of high-speed printing presses and of network printers. Ink thickness variations are expressed as ink thickness variation factors incorporated into a spectral prediction model. The method for computing ink thickness variations comprises both calibration and ink thickness variation computation steps. The calibration steps comprise the calculation of ink transmittances from measured reflectances and the computation of possibly wavelength-dependent ink thicknesses of solid superposed inks. Wavelength-dependent ink thicknesses account for the scattering behavior of non-transparent inks or of inks partly entering into the paper bulk. The ink thickness variation factors are fitted by minimizing a distance metric between the reflection spectrum predicted according to the thickness variation enhanced spectral prediction model and the measured reflection spectrum. The ink thickness variation enhanced spectral prediction model can be applied both in the visible wavelength range and in the near-infrared wavelength range. This enables computing unambiguously the thickness variations of the cyan, magenta, yellow and black inks. Furthermore, a spectral reflection may be measured over a stripe of a printed page and used to predict the ink thickness variations occurring within that stripe. This enables the real-time control of the ink deposition process on a printing press.
A method for computing ink thickness variations for the control of printers or printing presses, the method being based on a thickness variation enhanced spectral prediction model, said method comprising calibration steps and ink thickness variation computation steps, where the calibration steps comprise the calculation of ink transmittances from measured reflectances and the computation of ink thicknesses of solid superposed inks and where the ink thickness variation computation steps comprise fitting of ink thickness variations by minimizing a distance metric between a predicted reflection spectrum and a measured reflection spectrum, where said predicted reflection spectrum is predicted according to the thickness variation enhanced spectral prediction model, where said thickness variation enhanced spectral prediction model comprises for each ink a single ink thickness variation factor, said single ink thickness variation factor being independent of ink superposition conditions.
An ink thickness variation computing system for the control of printers or printing presses comprising a reflection spectrum acquisition device, a module for computing and storing calibration data and an ink thickness variation computing module, where the module for computing and storing calibration data is operable for deducing ink transmittances from spectral reflectance measurements and operable for computing initial ink thicknesses, where the ink thickness variation computing module is operable for computing ink thickness variations by minimizing a distance metric between a reflection spectrum predicted according to a thickness variation enhanced spectral prediction model and a measured reflection spectrum, and where said thickness variation enhanced spectral prediction model comprises for each ink a single ink thickness variation factor, said single ink thickness variation factor being independent of ink superposition conditions.
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