Artificial neural networks for optimising camera‐based colour measurements of prints enhanced with pearlescent pigments

Pearlescent pigments are widely used in printing due to their optical, chemical and physical properties. To analyse the effects of goniochromism they produce, the colorimetric characterisation of materials printed with pearlescent pigments requires multi‐angular measurements. In this study, the colours of prints enhanced with pearlescent pigments were measured by means of a digital camera, relying on the empirical camera characterisation method. Since this method is time‐consuming, it was altered to enable estimates of colorimetric values for different geometries to be measured on the basis of images captured at one viewing angle. This approach was based on the use of artificial neural networks which were shown to provide sufficient flexibility for the given task. The results indicate that the images obtained at the viewing angle of 45° aspecular (measuring geometry 45°/asp 45°) accurately estimate CIELab values for all of the tested measuring geometries. The proposed method is therefore not only time‐efficient but also reduces the associated errors due to the camera's movement, and enables the estimation of colorimetric values for those viewing angles inaccessible by camera.     

Comparative study of different methods for the assessment of print nonuniformity and their correlation with the human visual system

Print nonuniformity is the variation of optical density (reflection) on the print and can occur in several different types that can be classified into two basic groups: random and systematic variations. This article examines two types from each group of variations that are most commonly found in digital printing systems: blotches and streaks, where amplitude and size were varied. Three most commonly used methods for measuring the print nonuniformity were used: Grey Level Co-occurrence Matrix (GLCM), ISO 13660 and Integration (also Improved Integration) method. The results obtained by measuring were compared with a visual assessment to find the link between objective and subjective analysis and to define and determine which method/parameter is the best for the measurement of which type of print (non)uniformity. The obtained results indicate the possibility of selecting the measurement parameter, but that the choice of the parameter depends on the type of nonuniformity. It is concluded that in the case of the GLCM method, the entropy and energy parameters are strongly related to the visual assessment for all three sets of samples. These parameters can be used regardless of the type of nonuniformity. The ISO G parameter can be used to measure both graininess and mottling, while the ISO M parameter is only suitable for measuring mottling. Integration method could be used to measure systematic error. On the basis of conclusions of the research, it is possible to propose the development of an industrially applicable solution for measuring print nonuniformity of digitally printed samples.     

Customized daltonization: adaptation of different image types for observers with different severities of color vision deficiencies

Universal Access in the Information Society - The color vision deficiency, popularly called daltonism or color-blindness, manifests with limited color discrimination ranging from slightly reduced...