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Summary:
The quality control to avoid waste of printed products is essential in the printing houses when fast printing presses are used. The tone rendering of four-color images is generally judged through a visual evaluation of print darkness produced by the distinct inks. The measurement of the optical density at specific test spots on a printed sheet is most urgently required for modern web-offset presses which are rather unstable in their inking characteristics when compared with the established technology of large gravure presses. New types of optical sensors, i.e. densitometers, have been designed and implemented for the on-press quality control of fast web-fed presses at the Technical Research Centre in Finland. The operation of the optical transducers at their installation is governed by a microprocessor. The densitometer assembly is capable of measuring the optical density of a small test spot on a paper web moving at a speed up to 10 m/s. The opacity of pure paper left unprinted by the offset plate gap is used a reference level during each press revolution. The chromaticity of overlapping ink layers is determined using a proper selection of gelatine filters. To overcome the slow drift of illumination intensity due to the aging properties of halogen incandescent lamps, the measurement procedure includes the dark level determination which is carried out at present intervals. The optical measurement devices can be applied to print darkness evaluation on paper and on aluminum foil. On paper the transducers are used in a diffuse mode whereas the print density on glossy metal surfaces is to be measured in a specular mode. The specular measurement method employs a polarization filter which cancels the first-surface reflection from a wet print. The density measurements are used as a basis for the closed loop inking control; typically eight sensors each with four color filters are used for one web. To overcome the slow drift of illumination intensity due to the aging properties of halogen incandescent lamps, the measurement procedure includes the dark level determination which is carried out at present intervals. The optical measurement devices can be applied to print darkness evaluation on paper and on aluminum foil. On paper the transducers are used in a diffuse mode whereas the print density on glossy metal surfaces is to be measured in a specular mode. The specular measurement method employs a polarization filter which cancels the first-surface reflection from a wet print. The density measurements are used as a basis for the closed loop inking control; typically eight sensors each with four color filters are used for one web.