Ole Norberg

Paper Whiteness and its Effect on The Reproduction of Colors

The whiteness level of a printing paper is considered as an important quality measure. High paper whiteness improves the contrast to printed areas providing a more distinct appearance of printed text and colors and increases the number of reproducible colors. Its influence on perceived color rendering quality is however not completely explained. The intuitive interpretation of paper whiteness is a material with high light reflection for all wavelengths in the visual part of the color spectrum. However, a slightly bluish shade is perceived as being whiter than a neutral white. Accordingly, papers with high whiteness values incline toward bluish-white. In paper production, a high whiteness level is achieved by the use of highly bleached pulp together with high light scattering filler pigment. To further increase whiteness levels expensive additives such as Fluorescent Whitening Agents (FWA) and shading dyes are needed. During the last years, the CIE whiteness level of some commercial available office paper has exceeded 170 CIE units, a level that can only be reached by the addition of significant amounts of FWA. Although paper whiteness is considered as an important paper quality criterion, its influence on printed color images is complicated. The dynamic mechanisms of the human visual system strive to optimize the visual response to each particular viewing condition. One of these mechanisms is chromatic adaptation, where colored objects get the same appearance under different light sources, i.e. a white paper appears white under tungsten, fluorescent and day light. In the process of judging printed color images, paper whiteness will be part of the chromatic adaptation. This implies that variations in paper whiteness would be discounted by the human visual system. On the other hand, high paper whiteness improves the contrast as well as the color gamut, both important parameters for the perceived color reproduction quality. In order to quantify the influence of paper whiteness pilot papers with different amount of FWA but in all other respects similar were produced on a small scale experimental paper machine. The fact that only the FWA content changes reduces the influences of other properties separated from the paper whiteness in the evaluation process. A set of images, all having characteristics with the potential to reveal the influence of the varied whiteness level on color reproduction quality, were printed on the pilot papers in two different printers. Prior to printing the test images in the experiment, ICC-profiles were calculated for all the used printer-substrate combinations. A visual assessment study of the printed samples was carried out in order to relate the influence of the paper whiteness level to perceived color reproduction quality. The results show an improved color rendering quality with increased CIE whiteness value up to a certain level. Any further increase in paper whiteness does not contribute to an improved color reproduction quality. Furthermore, the fact that some printing inks are UV blocking while others are not will introduce a non uniform color shift in the printed image when the FWA activation changes. This non uniform color shift has been quantified both for variations in illuminant as well as variations of FWA content in the paper.

Color measurements on prints containing fluorescent whitening agents

Papers with a slightly blue shade are, at least among a majority of observers being perceived as whiter than papers having a more neutral color1. Therefore, practically all commercially available printing papers contain bluish dyes and fluorescent whitening agents (FWA) to give the paper a whiter appearance. Furthermore, in the paper industry, the most frequently used measure for paper whiteness is the CIE-whiteness. The CIE Whiteness formula, does in turn, also favor slightly bluish papers. Excessive examples of high CIE-whiteness values can be observed in the office-paper segment where a high CIE-whiteness value is an important sales argument. As an effect of the FWA, spectrophotometer measurements of optical properties such as paper whiteness are sensitive to the ultraviolet (UV) content of the light source used in the instrument. To address this, the standard spectrophotometers used in the paper industry are equipped with an adjustable filter for calibrating the UV-content of the illumination. In the paper industry, spectrophotometers with d/0 measurement geometry and a light source of type C are used. The graphical arts industry on the other hand, typically measures with spectrophotometers having 45/0 geometry and a light source of type A. Moreover, these instruments have only limited possibilities to adjust the UV-content by the use of different weighting filters. The standard for color measurements in the paper industry governs that measurements should be carried out using D65 standard illumination and the 10o standard observer. The corresponding standard for the graphic arts industry specify D50 standard illumination and the 2o standard observer. In both cases, the standard illuminants are simulated from the original light source by spectral weighting functions. However, the activation of FWA, which will impact the measured spectral reflectance, depends on the actual UV-content of the illumination used. Therefore, comparisons between measurements on substrates containing FWA from two instruments having light sources with different UV-content are complicated. In this study, the effect of FWA content in paper on color reproduction has been quantified for an officetype paper. Furthermore, examples are given on how color measurement instruments give different readings when FWA is present. For the purpose of this study and in order to ensure that only the effect of FWA was observed, a set of papers with varying additions of FWA otherwise identical, were produced on a small-scale experimental paper machine. The pilot papers were printed in three different printers. Two spectrophotometers representative to the instruments used in the Graphical Art Industry and the Paper Industry respectively where used to measure the printed papers. The results demonstrate how the use of spectral weighting functions for simulating standard illuminants works properly on nonfluorescent material. However, when FWA is present, disparities in UV content between the light source and the simulated illuminant will result in color differences. Finally, in many printing processes, some of the used inks are UVblocking, this further complicates the effect of FWA in printed material. An example is shown on how different color differences are obtained for different process ink combinations when the amount of FWA added to the paper is varied.

Retinal spectral image analysis methods using spectral reflectance pattern recognition

Conventional 3-channel color images have limited information and quality dependency on parametric conditions. Hence, spectral imaging and reproduction is desired in many color applications to record and reproduce the reflectance of objects. Likewise RGB images lack sufficient information to successfully analyze diabetic retinopathy. In this case, spectral imaging may be the alternative solution. In this article, we propose a new supervised technique to detect and classify the abnormal lesions in retinal spectral reflectance images affected by diabetes. The technique employs both stochastic and deterministic spectral similarity measures to match the desired reflectance pattern. At first, it classifies a pixel as normal or abnormal depending on the probabilistic behavior of training spectra. The final decision is made evaluating the geometric similarity. We assessed several multispectral object detection methods developed for other applications. They could not proof to be the solution. The results were interpreted using receiver operating characteristics (ROC) curves analysis.

Impact of illumination spectral power distribution on radiance factor of fluorescing materials

The spectral radiance factor and thereby the appearance of fluorescing material is known to depend strongly on the spectral power distribution (SPD) of the illumination in the fluorophores excitation wavelength band. The present work demonstrates the impact of the SPD in the fluorescence emission band on the total radiance factor. The total radiance factor of a fluorescing paper is measured in three different illuminations. The presence of peaks in the SPD of fluorescent light tubes dramatically decreases the luminescent radiance factor. This effect will impact the appearance of fluorescing media under illuminations with large variation in SPD, which includes recent LED illuminations.

Extension of Murray-Davies tone reproduction model by adding edge effect of halftone dots

We propose expanding the Murray-Davies formula by adding the effect of edges of solid inks in a halftoned image. The expanded formula takes into account the spectral reflectance of paper white, full tone ink and mixed area scaled by the fractional area coverages. Here, mixed area mainly refers to the edge of an inked dot where the density is very low, and lateral exchange of photons can occur. Also, in such area the paper micro components may have higher scattering power than ink, especially, in uncoated paper. Our methodology uses cyan, magenta and yellow separation ramps printed on different papers by impact and non-impact based printing technologies. The samples include both frequency and amplitude modulation halftoning methods of various print resolutions. Based on pixel values, the captured microscale halftoned image is divided into three categories: solid ink, mixed area, and unprinted paper between the dots. The segmented images are then used to measure the fractional area coverage that the model receives as parameters. We have derived the characteristic reflectance spectrum of mixed area by rearranging the expanded formula and replacing the predicted term with the measured value using half of the maximum colorant coverage. Performance has clearly improved over the Murray-Davies model with and without dot gain compensation, more importantly, preserving the linear additivity of reflectance of the classical physics-based model.

Improved spectral vector error diffusion by dot gain compensation

Spectral Vector Error Diffusion, sVED, is an interesting approach to achieve spectral color reproduction, i.e. reproducing the spectral reflectance of an original, creating a reproduction that will match under any illumination. For each pixel in the spectral image, the colorant combination producing the spectrum closest to the target spectrum is selected, and the spectral error is diffused to surrounding pixels using an error distribution filter. However, since the colorant separation and halftoning is performed in a single step in sVED, compensation for dot gain cannot be made for each color channel independently, as in a conventional workflow where the colorant separation and halftoning is performed sequentially. In this study, we modify the sVED routine to compensate for the dot gain, applying the Yule-Nielsen n-factor to modify the target spectra, i.e. performing the computations in (1/n)-space. A global n-factor, optimal for each print resolution, reduces the spectral reproduction errors by approximately a factor of 4, while an n-factor that is individually optimized for each target spectrum reduces the spectral reproduction error to 7% of that for the unmodified prints. However, the improvements when using global n-values are still not sufficient for the method to be of any real use in practice, and to individually optimize the n-values for each target is not feasible in a real workflow. The results illustrate the necessity to properly account for the dot gain in the printing process, and that further developments is needed in order to make Spectral Vector Error Diffusion a realistic alternative for spectral color reproduction.

Experimental Analysis for Modeling Color of Halftone Images

Reflectance models such as the monochrome Murray–Davies (MD) and the Neugebauer color equations make inaccurate predictions owing to changes in reflectance or tristimulus values (TSVs) of halftone dots and the paper between the dots. In this paper, we characterize the change of micro-TSVs as a function of printed area in spectral halftone image by a power function and compare its prediction efficiency using theoretically and experimentally measured limiting TSVs assuming dots of uniform thickness. We found that experimentally accounting for dot thickness variations as solid and mixed areas more precisely explained the single-model parameter that captured the observed lateral light scattering effect. The results showed that incorporating empirically modeled TSVs of the dots and the paper between dots, as well as introducing a new term addressing mixed area in the MD equation, produced CIE ∆\( {\text{E}}_{\text{ab}}^{*} \) in the range 1.22–1.76, and the overall gain was more than 1 ∆\( {\text{E}}_{\text{ab}}^{*} \).

Microscale halftone color image analysis: perspective of spectral color prediction modeling

A method has been proposed, whereby k-means clustering technique is applied to segment microscale single color halftone image into three components—solid ink, ink/paper mixed area and unprinted paper. The method has been evaluated using impact (offset) and non-impact (electro-photography) based single color prints halftoned by amplitude modulation (AM) and frequency modulation (FM) technique. The print samples have also included a range of variations in paper substrates. The colors of segmented regions have been analyzed in CIELAB color space to reveal the variations, in particular those present in mixed regions. The statistics of intensity distribution in the segmented areas have been utilized to derive expressions that can be used to calculate simple thresholds. However, the segmented results have been employed to study dot gain in comparison with traditional estimation technique using Murray-Davies formula. The performance of halftone reflectance prediction by spectral Murray-Davies model has been reported using estimated and measured parameters. Finally, a general idea has been proposed to expand the classical Murray-Davies model based on experimetal observations. Hence, the present study primarily presents the outcome of experimental efforts to characterize halftone print media interactions in respect to the color prediction models. Currently, most regression-based color prediction models rely on mathematical optimization to estimate the parameters using measured average reflectance of a large area compared to the dot size. While this general approach has been accepted as a useful tool, experimental investigations can enhance understanding of the physical processes and facilitate exploration of new modeling strategies. Furthermore, reported findings may help reduce the required number of samples that are printed and measured in the process of multichannel printer characterization and calibration.

Extending color primary set in spectral vector error diffusion by multilevel halftoning

Ever since its origin in the late 19th century, a color reproduction technology has relied on a trichromatic color reproduction approach. This has been a very successful method and also fundamental for the development of color reproduction devices. Trichromatic color reproduction is sufficient to approximate the range of colors perceived by the human visual system. However, tricromatic systems only have the ability to match colors when the viewing illumination for the reproduction matches that of the original. Furthermore, the advancement of digital printing technology has introduced printing systems with additional color channels. These additional color channels are used to extend the tonal range capabilities in light and dark regions and to increase color gamut. By an alternative approach the addition color channels can also be used to reproduce the spectral information of the original color. A reproduced spectral match will always correspond to original independent of lighting situation. On the other hand, spectral color reproductions also introduce a more complex color processing by spectral color transfer functions and spectral gamut mapping algorithms. In that perspective, spectral vector error diffusion (sVED) look like a tempting approach with a simple workflow where the inverse color transfer function and halftoning is performed simultaneously in one single operation. Essential for the sVED method are the available color primaries, created by mixing process colors. Increased numbers of as well as optimal spectral characteristics of color primaries are expected to significantly improve the color accuracy of the spectral reproduction. In this study, sVED in combination with multilevel halftoning has been applied on a ten channel inkjet system. The print resolution has been reduced and the underlying physical high resolution of the printer has been used to mix additional primaries. With ten ink channels and halfton cells built-up by 2x2 micro dots where each micro dot can be a combination of all ten inks the number of possible ink combinations gets huge. Therefore, the initial study has been focused on including lighter colors to the intrinsic primary set. Results from this study shows that by this approach the color reproduction accuracy increases significantly. The RMS spectral difference to target color for multilevel halftoning is less than 1/6 of the difference achieved by binary halftoning.