Deducing ink thickness variations by a spectral prediction model

Abstract

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 multichromatic 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 to minimize a difference metric between predicted reflection spectrum and measured reflection spectrum. The ink thickness variations deduced from multichromatic 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.

Review of spectral reflectance models for halftone prints: Principles, Calibration, and prediction accuracy

“…The Yule-Nielsen model, the Clapper-Yule model and the Low scattering Clapper-Yule model are less accurate but need less measured samples for their calibration. Different criteria determine the optimal model for a given application: the number of calibration samples that can be printed, the types of halftones used in the reproduction process, the necessity of considering the transmittance of the inks [23] and the ease of using the model in inverse mode. If illumination and viewing conditions vary, the Clapper-Yule and low scattering Clapper-Yule models should be preferred since they incorporate explicitly the illumination and observation geometry.…”

Recovering neugebauer colorant reflectances and Ink‐spreading curves from printed color images

“…(12) to multiple inks is not appropriate. Hersch et al have shown that the relative thickness of a printed solid ink layer depends on the presence of other superposed inks [18]. Beer's law enables expressing these ink thickness variations.…”

“…This is possible because the ink spreading curves are calibrated in the first calibration step performed in Eq. (18). Moreover, a weight  i = 1 indicates that within the measured reflectances of the calibration set, there is also the measured colorant reflectance R i .…”

“…(12)- (16) enable the first calibration step in Eq. (18) to recover colorant reflectances similar to real reflectances even when there is not enough information in the image calibration set. However, because of the PCA, the recovered colorant 10/33 reflectances are restricted to a limited set of reflection spectra.…”

“…In this case, the colorant reflectance is set to , old i R the spectrum recovered in Eq. (18). A weight equal to 1 indicates that the colorant reflectance can be freely calibrated between the minimal value allowed for a reflectance and the paper reflectance.…”