Whiteness and Fluorescence in Paper: Perception and Optical Modelling

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Whiteness and Fluorescence in Paper

Perception and Optical Modelling

Ludovic Gustafsson Coppel

Supervisor: Prof. Per Edstr ¨om, Mid Sweden University Examiner: Prof. H˚akan Olin, Mid Sweden University

Assistant supervisors: Ass. Prof. Caisa Johansson, Karlstad University and Dr. Siv Lindberg, Innventia

Department of Natural Sciences, Engineering and Mathematics Mid Sweden University

Licentiate Thesis No. 47 Sundsvall, Sweden

2010

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Mittuniversitetet Department of Natural Sciences, Engineering and Mathematics

ISBN 978-91-86073-97-8 SE-851 70 Sundsvall

ISSN 1652-8948 SWEDEN

Akademisk avhandling som med tillst˚and av Mittuniversitetet framlägges till of- fentlig granskning f ör avläggande av teknologie licentiatexamen tisdagen den 9 november 2010 i sal O111, Mittuniversitetet, Holmgatan 10, Sundsvall. Seminariet kommer att h˚allas p˚a engelska.

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°Ludovic Gustafsson Coppel, 2010

Printed by Kopieringen Mittuniversitetet, Sundsvall, Sweden, 2010

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To Eliott, Elina, and Rebecka

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Abstract

This thesis is about modelling and predicting the perceived whiteness of plain paper from the paper composition, including fluorescent whitening agents. This includes psycho-physical modelling of perceived whiteness from measurable light reflectance properties, and physical modelling of light scattering and fluorescence from the paper composition.

Existing models are first tested and improvements are suggested and evaluated.

The standardised and widely used CIE whiteness equation is first tested on commercial office papers with visual evaluations by different panels of observers, and improved models are validated. Simultaneous contrast effects, known to affect the appearance of coloured surfaces depending on the surrounding colour, are shown to significantly affect the perceived whiteness. A colour appearance model including simultaneous contrast effects (CIECAM02-m2), earlier tested on coloured surfaces, is successfully applied to perceived whiteness. A recently proposed extension of the Kubelka-Munk light scattering model including fluorescence for turbid media of finite thickness is successfully tested for the first time on real papers.

It is shown that the linear CIE whiteness equation fails to predict the perceived whiteness of highly white papers with distinct bluish tint. This equation is applicable only in a defined region of the colour space, a condition that is shown to be not fulfilled by many commercial office papers, although they appear white to most observers. The proposed non-linear whiteness equations give to these papers a whiteness value that correlates with their perceived whiteness, while application of the CIE whiteness equation outside its region of validity overestimates perceived whiteness.

It is shown that the quantum efficiency of two different fluorescent whitening agents (FWA) in plain paper is rather constant with FWA type, FWA concentration, filler content, and fibre type. Hence, the fluorescence efficiency is essentially dependent only on the ability of the FWA to absorb light in its absorption band.

Increased FWA concentration leads accordingly to increased whiteness. However, since FWA absorbs light in the violet-blue region of the electromagnetic spectrum, the reflectance factor decreases in that region with increasing FWA amount. This violet-blue absorption tends to give a greener shade to the paper and explains most of the observed greening and whiteness saturation at larger FWA concentrations. A red-ward shift of the quantum efficiency is observed with increasing FWA concentration, but this is shown to have a negligible effect on the whiteness value.

The results are directly applicable to industrial applications for better instrumental measurement of whiteness and thereby optimising the use of FWA with the goal to improve the perceived whiteness. In addition, a modular Monte Carlo simulation tool, Open PaperOpt, is developed to allow future spatial- and angle-resolved particle level light scattering simulation.

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Acknowledgements

This thesis is the result of three intensive years when I have had the opportunity to meet and work with many people in projects involving several companies, university and institutes. I want to thank all representatives in the reference groups of the Human Product Interaction (HPI) research cluster at Innventia, and of the ongoing PaperOpt project, for valuable inputs, comments, and encouragements.

I want to thank my supervisor, Per Edstr öm, for his confidence in me and support throughout this work. Together with my manager, Marie-Claude Béland, Torbj örn Widmark, and my earlier Master’s Thesis supervisor, Nils Pauler, Per was also very active in setting up this licentiate project, which later became a PhD project thanks to the PaperOpt project Per initiated.

My assistant supervisor from Karlstad University, Caisa Johansson, is thanked for valuable comments to the manuscripts. Thank you also Caisa and Erik Bohlin for our fruitful collaboration in the PaperOpt project.

I am grateful to Markku Hauta-Kasari for the good time and help I got during my stays at his group at the University of Eastern Finland in Joensuu. Special thanks go to Jussi Kinnunen for help with the bispectrophotometer.

My colleagues in Stockholm at Innventia are thanked, not only for enjoyable cof- fee breaks and lunches. Annika Lindstr ¨om, Annika Kihlstedt, Caroline Cederstr ¨om and Maggan for help with lengthy evaluations in the perception lab and measurements. My co-authors, Siv Lindberg and Staffan Rydefalk for their expertise and support. Hjalmar Granberg for numerous discussions about light scattering.

My colleagues in Örnsk öldsvik at the Digital Printing Center are thanked for the very good atmosphere in the Paper Optics group. My co-authors, Mattias Ander- sson and Ole Norberg for all the good time in Ö-vik and at conferences. Magnus Neuman with whom I have more and more discussions about light scattering and fluorescence.

This work was financially supported by the Swedish Governmental Agency for Innovation Systems (VINNOVA), the Kempe foundations, and the Knowledge Foun- dation (KK-stiftelsen), which is gratefully acknowledged.

Last but not least, I want to thank my family in France who always believed in me, and my wife Anne and my children for all the good things that happened during these three years...

Thank you all!

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List of Papers

This thesis is mainly based on the following papers, herein referred by their Roman numerals:

Paper I Coppel L.G., Lindberg, S., and Rydefalk, S, Whiteness assessment of paper samples at the vicinity of the upper CIE whiteness limit, in Proc. 26th Session of the CIE, Beijing, China, pp D1–10, (2007).

Paper II Coppel, L. G. and Lindberg, S., Modelling the effect of simultaneous contrast on perceived whiteness, in Proc. 4th European Conference on Colour in Graphics, imaging, and Vi- sion, Terassa, Spain, pp 183-188, (2008).

Paper III Coppel L.G., Andersson, M., and Edstr ¨om, P., Quantum efficiency of fluorescent dyes in different furnishes, manuscript to be submitted to Applied Optics, (2010).

Paper IV Coppel L.G., Andersson, M., Edstr ¨om, P., and Kinnunen, J., Limitations of the efficiency of fluorescent whitening agents in uncoated paper, manuscript to be submitted to Nordic Pulp and Paper Research Journal, (2010).

Paper V Coppel, L. G. , Edstr ¨om, P. and Lindquister, M., Open source Monte Carlo simulation platform for particle level simulation of light scattering from generated structures, in Proc.

Papermaking research symposium, Kuopio, Finland, (2009).

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Related conference papers not included in the thesis:

Coppel, L. G., Perception and measurement of the whiteness of papers with different gloss and FWA amount, in Advances in Printing and Media Technology, Vol. XXXVI, Proc. 36th iarigai, Stockholm, Sweden, p. 83-89, (2009).

Coppel, L. G., Norberg O., and Lindberg, S., Paper whiteness and its effect on perceived image quality, to appear in Proceedings of the 18th Color Imaging Conference, San Antonio, Texas, (November 2010).

Bohlin, E., Coppel, L.G., Andersson, C., Edstr ¨om, P., Characterization and modelling of the effect of calendering on coated polyester film, in Advances in Printing and Media Technology, Vol.

XXXVI, Proc. 36th iarigai, Stockholm, Sweden, p. 301-308 (2009).

Bohlin, E., Coppel, L.G., Johansson, C., Edstr ¨om, P., Modelling of brightness decrease of coated cartonboard as an effect of calendering - microroughness and effective refractive index aspects, to appear in Proceedings of the TAPPI 11th Advanced Coating Fundamentals Symposium (2010).

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1 Introduction

1.1 Background and Problem Motivation

Whiteness of paper is a commercially important property, and a marketing goal for product segments such as office papers although the perfectly corresponding instrumental measurement remains elusive [1]. According to Ganz [2], the assessment of whiteness depends on individual preference, the level and spectral power distribution of the sample irradiation, the colour of the surround, and the acquired precon- ceptions in various trades. Despite the problems presented by the colorimetry of fluorescent samples, most observers are able to arrange white samples of different luminous reflectance, hue and saturation in a one-dimensional order according to whiteness, although little general agreement on whiteness can be reached. Regard- less of this disparity, attempts have been made to elaborate a standardised whiteness formula for commercial whites. The general agreement is that a sample is perceived as the whiter, the lighter, and the bluer it is. Thus, whiteness is characterised by high level of luminosity and finite saturation, with a blue hue [3]. The most widely used whiteness model in the paper industry, the CIE¹ whiteness, has been found to correlate well with visual estimation for many white samples having similar tint or fluorescence [4]. To prevent application of the whiteness formula to chromatic samples, the equation is only valid within given boundaries in the colour space.

The strive towards whiter paper has led to more bleaching and a substantial increase of the concentration of fluorescent whitening agents (FWA) and violet-blue shading dyes added in paper. FWA are dyes that absorb ultraviolet (UV) light and emit in the blue region of the spectrum, hence increasing the perceived whiteness by both increasing the lightness and the blueness of the paper. Excessive use of FWA and shading dyes has led to papers with a clear bluish tint and commercial copy papers can be measured outside the boundaries of the CIE whiteness equations [5].

Surface colour may change appearance depending on the illumination [6].

Colour is a visual sensation that depends on three interacting components: the light source, the object, and the observer. Due to the complexity of the human visual system, several colour appearance phenomena that cannot be physically measured influence the way an observer perceives colour. One effect is simultaneous contrast that causes a stimulus to shift in colour appearance when the background or adjacent colours are changed. This simultaneous contrast can affect the perceived whiteness of paper samples so that a pair of samples are ranked differently depending on the background and the shade of the other samples in the set to be evaluated [5]. In order to evaluate the perceived whiteness of a pair of paper samples with different tint, there is thus a need to model how the samples influence the appearance of each other.

What is interpreted by the brain through the eyes is the reflected light from the paper. Accurate models for the light scattering properties of paper products are essential tools for product development and development of the production pro-

1Commission Internationale de l’Eclairage

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Whiteness and Fluorescence in Paper

cess, by making it possible to design materials by means of modeling and prediction rather than full scale trial-and-error. Models including fluorescence are often based on Monte Carlo (MC) methods, especially in tissue applications [7, 8], but application to the reflectance from paper exists [9, 10]. However, for technological applications, simpler analytical models such as the Kubelka-Munk theory (KMT) [11, 12] are preferred [13, 14, 15]. Technological applications of these models rely on assumptions of how the model parameters are affected by the manufacturing process, thickness, composition, and other structural modifications or chemical interactions. The dependence of the scattering and absorption coefficients of the KMT on the paper structure has been studied thoroughly, as reviewed by Pauler [16] and Philips-Invernizzi [17].

For models including fluorescence, only a few results has been reported. Shake- speare [18] has shown that the light conversion efficiency of FWA, the quantum efficiency, was rather constant with FWA concentration. However, the results applied only to FWA added to identical pulp, and to measurements made on an opaque pad of samples. It is therefore of interest to study how the optical properties of FWA depend on the substrate composition.

KMT based models and most MC models treat a paper layer as a homogenous turbid medium with statistical scattering and absorption coefficients. These models are henceforth referred to as homogenous models. With these models, layer thickness variation and surface scattering can be included, to the cost of model complexity and computation time. Another great potential of MC models is their ability to handle anisotropic scattering, which has been shown to have a large impact on standardised reflectance measurements [19, 20, 21]. Homogenous models have been proved useful in predicting the reflectance of halftone prints on fluorescing paper substrate, both with KMT [22] and MC [23]. These applications are on the other hand restricted to different prints on the same paper, whose model parameters are determined experimentally. For paper product development, 3-dimensional particle level models are required to model the composite structure of real fluorescing papers, as suggested by Hainzl et al. [9]. However, the higher explanative power of these structure models as compared to homogeneous models is yet to be demonstrated, as far as fluorescence is concerned, and attempts to validate their predictable power easily end with far more parameters to be determined than available measurement inputs. To get full advantage of MC particle level simulations, there is thus a need for dedicated parameter estimation methods that are part of the model itself.

1.2 Overall Aim

This thesis aims at modelling perceived whiteness from the paper composition with two objectives: understanding the mechanisms and efficiency limitations of fluorescent whitening agents, and providing a tool for design and optimisation of the paper structure in terms of perceived whiteness.

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1.3 Scope

Predicting the perceived whiteness of paper includes psycho-physical modelling of perceived whiteness from measurable light reflectance properties and physical modelling of light scattering and fluorescence from the paper composition. These two modelling approaches are treated independently, but the results from the physical modelling can be used as input in the psycho-physical models, hence linking the paper structure to how white it appears to a group of real observers.

One goal is to provide improved models for perceived whiteness including simultaneous contrast effects. This will allow optimising the reflectance properties of a paper that is partially printed or compared to other papers. Light scattering models are in turn used to model the reflectance properties from the paper composition and structure.

This thesis focuses on uncoated single layer paper. Existing models are first tested and improvements are suggested and evaluated. For light scattering modelling, simpler models treating paper as a homogeneous turbid medium are used and the dependence of the model parameters on the paper composition is determined.

Of special interest is how the optical properties of FWA are affected by the paper composition, and how FWA at different concentrations affects spectral reflectance factor and whiteness. To prepare for future particle level simulation describing the composite and inhomogeneous structure of paper, a Monte Carlo simulation tool is implemented and parameter estimation methods are developed.

2 Theoretical background

This section gives a brief description of colour science, colour appearance models (with emphasis on whitness), and light scattering models . It provides the necessary background, definitions, and terminologies used throughout the thesis. A more thor- ough review of colour science is given by Wyszecki and Stiles [24]. Hunt [25] provides basic knowledge about colour measurement, and Fairchild [26] about colour appearance modelling. For an introduction on the optical properties of paper and paper whiteness, refer to Pauler [16], and for light scattering in paper to Rydefalk and Wedin [27], Philips-Invernizzi et al. [28], and Lehto [29].

2.1 Colour appearance modelling

It is tempting to say that a certain wavelength of light (or a certain object) has a certain colour and then treat colour only as a physical quantity. However, a surface colour may change appearance depending on the illumination. This is why any colour measurement must be performed and communicated with a known illumination. Colour is a visual sensation that depends on three interacting components: the light source, the object, and the observer. Due to the complexity of the human visual system, several colour appearance phenomena that cannot be physically measured

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influence the way an observer perceives colour. One particular effect of interest for this thesis is simultaneous contrast that causes a stimulus to shift in colour appearance when the background or adjacent colours are changed. The perveived colour of a stimulus does not only depend on the stimulus reflective properties and the illumination, but also on a potential proximal or induction field (the immediate environment with the stimulus), on the background (extending for about 10^ofrom the edge of the proximal field ) and on the surround (outside the background) [26].

The spectral distribution of the light reflected by a surface is interpreted as a colour by the brain through the three colour sensors in the eyes, the cones. Since the exact sensitivity of the cones is difficult to measure and may vary between individ- uals, the CIE defined in 1931 the standardised CIE XY Z color-matching functions (CMF) that represent the average human spectral response to light stimuli. Accord- ing to this standard, a colour is represented by its X, Y , and Z tristimulus values, obtained by integrating the reflected light, weighted by the respective CMF, over the wavelength range. Two different sets of CMF exists. The 1931 standard colorimetric observer , referred to as the 2ôobserver, and the 1964 supplementary standard colorimetric observer , which was defined using a visual field of 10ôinstead of the 1931 2ô visual field. A colour should therefore be reported together with the actual standard observer used. Since the colour depends on the spectral characteristics of the illumination, it is also reported in a given illumination, usually one of the CIE standard illuminants, such as A representing a tungsten filament lamp, or D65 representing daylight at a colour temperature of 6500K.

The CIE tristimulus values represent colours in the three-dimensional XY Z colour space. This colour space is not perceptually uniform, in the sense that Euclidian distance in XY Z do not map perceived colour differences. The CIE proposed in 1976 the CIE L^∗a^∗b^∗colour space (CIELAB), which is a non-linear transformation of the XY Z tristimulus values, and is approximately perceptually uniform. CIELAB makes use of a simple chromatic adaptation transform, modelling the ability of the human visual system to discount the colour of the light source in order to preserve the appearance of an object viewed in different illumination conditions. L^∗ repre- sents the lightness, while a^∗ and b^∗ represent the hue and chroma on a red-green axis (a^∗), and a yellow-blue axis (b^∗).

CIELAB is a well-established international standard that performs well as a colour appearance model (CAM) in many applications [26]. The major limitations of CIELAB reported in the literature are due to its simplified chromatic adaptation transform.

Moreover, CIELAB cannot predict luminance-level dependency or cognitive effects, such as discounting the illuminant, which is important in cross-media colour repro- duction. Neither does it provide correlates for the absolute appearance attributes of brightness and colourfulness. For applications restricted to reflective materials viewed in an average daylight illumination, the limitations discussed above are often not of concern. However CIELAB does not take into account induction field, background and surround dependency.

Of the other CAM proposed over the years, only the Hunt model [6] is capa- ble of directly accounting for simultaneous contrast and assimilation effects. Hunt suggested that the chromatic adaptation process is influenced by the local colours

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2.2 Perceived whiteness

of the induction field and background, and proposed an algorithm for calculating the adjusted white point. A simplified version of the Hunt model, CIECAM02, was introduced in 2003 [30, 26]. Since the simultaneous contrast prediction part showed poor correlation to visual assessment [6], the effect was not included in CIECAM02.

Wu and Wardman [31] proposed recently a modification of the Hunt model in which the white point is modified differently for Lightness than for Hue and Chroma.

2.2 Perceived whiteness

The CIE set up a subcommittee on whiteness in 1969 and recommended the CIE whiteness formula as an assessment method of white materials in 1986. This formula has been found to correlate with visual estimation for many white samples having similar tint or fluorescence [4]. CIE whiteness is given by

W_CIE= Y + 800(xⁿ−x) + 1700(yⁿ−y), (1) where x and y are the CIE chromaticity coordinates, and xⁿ and yⁿare the coordinates for the perfect reflecting diffuser at the given illumination. To prevent application of the whiteness formula to chromatic samples, the equation is only valid within given boundaries in the colour space,

−3 < T < 3, (2)

where T = 900(xⁿ−x) − 650(yn−y), for the 10^oobserver, and

40 < W_CIE< 5Y − 280. (3)

Since the approval of the CIE whiteness, the L^∗a^∗b^∗system has been introduced and is now used for routine colorimetry. Researchers assessing the whiteness by colorimetric methods usually want to see and evaluate whiteness directly in the colour system that they are used to. For this purpose, Ganz and Pauli [32] derived an ap- proximation of the CIE whiteness formula based on the L^∗a^∗b^∗ colour coordinates, given by

W_CIE≈2.41L^∗−4.45b^∗(1 − 0.009(L^∗−96)) − 141.4. (4) Thus, the CIE whiteness is linear in xyY and remains nearly linear in the L^∗a^∗b^∗ space. More recently new non-linear whiteness formulae based on the L^∗a^∗b^∗system have also been published [33, 4]. At the same time a new ISO standard introduced the concept of ”indoor whiteness” [34]. This standard stipulates the use of the CIE illuminant C , which gives a much lower relative UV content than the adjustments to the CIE illuminant D65 specified in the ISO 11475 ”outdoor whiteness” standard.

It is argued that the UV content of the illumination under such conditions is closer to that generally experienced in an indoor environment, where paper is normally sold, bought, and used [35].

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2.3 Light scattering and fluorescence

Light scattering refers to all physical processes that affect the direction of the light in a medium. Both absorption and scattering reduces the intensity of the light trav- elling in one direction through a medium. This intensity reduction is referred to as extinction. When light is absorbed by a molecule, it can be transformed into heat or re-emitted at another wavelength in fluorescence processes. Light scattering is caused by local variation of the refractive index within a heterogeneous medium and is accurately described by the Maxwell equations.

The Maxwell equations can only be solved exactly for a few simple geometries, and there is no general quantitative solution to the problem of multiple scattering from packed particles of varying size and shape. For those turbid media, radiative transfer theory is often used instead [36]. It describes the interaction of radiation with scattering media, with a scattering and an absorption coefficient, and a phase function defining the direction probability distribution of scattered light. The equation of radiative transfer was stated by Chandrasekhar [37]. This equations lacks a general analytical solution and numerical methods are required [38]. Only recently a general numerical solution for layered structures has been presented by Edstr ¨om [39]. Therefore, simplified models such as KMT or Monte Carlo methods have been used to model light scattering in paper.

Whenever the equations describing a physical problem can be written down, but the solution to these equations is intractable, it is appealing to turn to Monte-Carlo methods. By following the path of wave packets interacting with different components according to local physical rules, it is possible to calculate the average spatial- and angle resolved reflectance and transmittance. The method has been extensively used in medicine applications to model the interaction of light with tissues [40].

Carlsson et al. [41] introduced a three-dimensional model of the internal structure of paper including flattened cylindrical fibres, ellipsoidal pores, and fine particles located on the fibre surface that cause random anisotropic scattering . Hainzl et al.

[9] proposed an extended implementation that includes rough surface scattering, layer thickness variation, and fluorescence. This statistical description of the paper structure, using component distributions and size distributions, does not render the structure of the sheet, but it simulates the average optical response of the paper. An- other approach is to use physical models of the structure of the fibre network. Light scattering simulation models using such generated paper structures were suggested by Nilsen et al. [42] and Jensen [43] among others, and improved fibre web modelling has been published recently [44, 45].

Models including fluorescence are often based on Monte Carlo methods, especially in tissue applications [7, 8]. Monte Carlo has also been used to model fluorescence in paper [10, 9]. However, for technological applications, simpler analytical models such as the Kubelka-Munk theory (KMT) [11, 12] are preferred. Several extensions of the KMT theory have been proposed to include fluorescence in the KMT [13, 14, 46, 18], all requiring either a defined polychromatic illumination or a semi-infinite layer. More recently, Kokhanovsky [15, 47] presented a simple general analytic extension of the KMT to include fluorescence, which is tested in Paper III.

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Summary of the papers

3 Summary of the papers

The papers included in this thesis address both the modelling of perceived whiteness from measurable radiative properties of paper (Paper I-II), and the modelling of these radiative properties from the paper composition and structure (Paper III-V).

Paper I states the limitation of the CIE whiteness for commercial papers with pro- nounced blue tint and suggests a new whiteness formula that corresponds better to the perceived whiteness of such papers. Paper II shows that the perceived whiteness of a piece of paper is largely influenced by the shade of neighbouring colours, and that this influence can be predicted by recent colour appearance models. Paper III studies the influence of the composition of uncoated papers on the quantum efficiency of fluorescent whitening agents (FWA). Paper IV examines the saturation of the measured whiteness with increasing FWA concentration. In Paper V, a Monte Carlo simulation tool is implemented and parameter estimation methods are developed.

3.1 Paper I

This paper assesses the perceived whiteness of 45 commercial copy papers using ranking and magnitude estimation methods. Two published non-linear whiteness models are tested together with two models proposed by the authors.

The first set of twenty samples were evaluated by a total of 45 observers, 15 observers in each of the evaluations, made under three different overhead illumina- tions: 5000 K, 6500 K, and 6500 K with an additional UV lamp. The mean observer cross-correlation was low (R²min= 0.46), in line with previous research, but the median rankings in different illumination conditions were highly correlated (R²= 0.9).

This suggests two things. First, all the commercial copy papers used show a similar fluorescence. Hence, they all get whiter as the UV content of the illumination increases, and their internal ranking is not affected. Secondly, different groups of observers can agree on which paper is the whitest of two papers, different individual observers do not.

The second set of 25 samples was evaluated only under 6500 K illumination, alone and with the first set (all 45 samples together). In the first model proposed by the authors, WNEW, the maximum whiteness is set at a lower b^∗value than in the CIE whiteness formula (Figure 1). This model correlated well with ranking and scaling (R²> 0.7). The higher correlation obtained with this model indicates that the upper whiteness limit in the CIE whiteness could be extended. The second model, W_eCIE, keeps the CIE whiteness formula within the region of validity of the CIE whiteness, and applies a penalty function similar to the first model outside this region. In this case the maximum whiteness is determined by the CIE whiteness upper limit. The performance of this model was slightly inferior (rank correlation R² = 0.56 for the first set of samples and R² = 0.69 for the second set), but its main advantage is that all samples within the CIE whiteness limits are given the same whiteness values as with the CIE whiteness formula. The results showed that the introduction of

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Figure 1: Comparison of the CIE whiteness to WNEW for L^∗ = 95. The CIE whiteness is the white rectangular like area and is only defined in the region delimited by the whiteness inequality conditions. WNEWis the continuous surface.

a penalty function replacing the CIE whiteness limits is a promising way to handle white materials at the vicinity of the upper CIE whiteness limit.

3.2 Paper II

The purpose of this paper is to quantify and model the effect of simultaneous contrast on perceived whiteness. This effect causes a stimulus to shift in colour appearance when the background or adjacent colours are changed, but it has never been reported for whiteness. The perceived whiteness of white patches surrounded by induction fields of different shades was evaluated by asking observers to give a magnitude estimate of perceived whiteness of the patches in comparison to a white reference. The reflectance factor of the samples was measured in the actual 5000 K illumination with a spectroradiometer. The perceived whiteness of patches with identical measurable colour was highly dependent on the shade of the induction field, and the patch size used in the study did not significantly affect the perceived whiteness.

A colour appearance model, CIECAM02-m2 [31], was applied to predict the perceived whiteness of patches with 10 different surrounding colours (the induction field). The spectroradiometer measures the ”physical” colour of the white patch.

With CIECAM02-m2, the L^∗a^∗b^∗ values of the patch surrounded by an induction colour are computed as if the patch was seen on a neutral background. From these new L^∗a^∗b^∗values, the perceived whiteness is predicted with the CIE whiteness and

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3.3 Paper III

the whiteness models from Paper I. Yellowish induction fields make the white patch appear bluer, sometimes to such an extent that the patch was perceived less white than without the induction field. For this reason, the whiteness model developed in Paper I, WNEWperforms better than the CIE whiteness formula, since it penalises samples with a too bluish tint.

The combination of CIECAM02-m2 and W_NEW predicts much of the observed simultaneous contrast effect. However, the model performs better for dark induction fields than for light induction fields. The model rates patches surrounded by light blue and light yellow equally, whereas the observers clearly rate the patches with light yellow induction field as whiter than the patches with light blue induction field. A deeper analysis of the CIECAM02-m2 model indicates that the simultaneous contrast model used cannot accurately predict the change in hue for high lightness induction fields. A potential improvement for prediction of simultaneous contrast effects would be to base the calculations not only on the difference between the induction field and the background, but on the difference between the patch stimulus and the induction field.

3.3 Paper III

In this paper, the quantum efficiency of FWA:s are characterised in samples with different compositions. The aim is to test the Kokhanovsky extended Kubelka-Munk model including fluorescence [15], and map how its parameters are dependent on the paper composition in terms of FWA concentration, filler concentration, and fibre type. A kraft pulp and a sulphate pulp bleached at three different levels were used.

Two common FWA types, one disulpho and one tetrasulpho, were added to the pulp at different concentrations. A new method is introduced to determine the scattering coefficient, s(µ), the absorption coefficient, k(µ), and the quantum efficiency, Q(µ, λ) by optimisation of theses parameters to fit the measured Donaldson [48] radiance factor, D(µ, λ = 540nm) and D(µ, µ) at two basis weights. The Donaldson radiance factor is a discrete, illuminant independent, matrix representation of the bispectral radiance factor for each excitation wavelength and emission wavelength.

With increasing FWA concentration, the emission spectrum of the FWA, given by quantum efficiency at one excitation wavelength, is slightly shifted to longer wavelengths. The quantum efficiency dependency on fibre type, basis weight, and filler concentration is negligible. However, the scattering coefficient is dependent on the absorption coefficient, and hence the FWA concentration. This result is in line with previous observations made by several authors. On the other hand, it is also shown that for an opaque pad of samples, for which the whiteness is measured, the fluorescent radiance factor depends only on the ratio k/s. Thus, for opaque samples, s can be assumed to be independent of the FWA concentration and the fluorescence of paper with increasing FWA concentration is well predicted by an increase of k and constant Q.

For samples of finite thickness (or basis weight), simulation of the FWA concentration dependent fluorescence requires the use of FWA concentration dependent

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scattering. With this taken into account, the extended Kubelka-Munk model agrees well with the measured fluorescence from samples at different basis weights, with the basis weight as the only varying parameter.

3.4 Paper IV

This paper examines the dependence of the CIE whiteness of uncoated and unfilled papers on the FWA concentration. Since it is not known how much of the FWA is fixed in the paper, the analysis is made on the increasing absorption coefficient. As observed in Paper III, the emission spectrum of the FWA is moved towards longer wavelengths with increasing FWA concentration. As already known from several other studies, the absorption and emission band of the FWA overlap, and the FWA absorbs in the visible part of the electromagnetic spectrum.

By using the extended Kubelka-Munk model [15], hypothetic FWA:s with no emission spectrum shift and/or no absorption at wavelength above 400 nm are simulated to separate and quantify these effects on the whiteness value. It is shown that the shift of the emission spectrum with increasing concentration has a negligible effect on whiteness. The overlap of the absorption and emission bands of the FWA is the main cause of greening (a shift of the chromaticity towards green) and saturation of the fluorescence effect. With increasing FWA concentration, the positive effect of fluorescence is neutralised by the reduction of the reflectance factor in the violet-blue region of the spectrum induced by a significant absorption of the FWA in that region.

3.5 Paper V

This paper describes the development of a Monte Carlo simulation tool for light scattering and fluorescence in paper used in the continuation of this thesis. The aim is to model the composite structure of a paper layer on a particle level, as opposed to the model used in the precedent papers, which treats paper as a homogeneous turbid medium.

This paper describes the general modular structure of the Open Source simulation tool, Open PaperOpt. Selected physical models already implemented are presented. Simulating the scattering from a fibre web consisting of individual fibres and pores increases dramatically the number of model parameters and the model complexity. A method for the estimation of the model parameters is presented and discussed. The applicability of the simulation tool is demonstrated by modelling the effect of calendering on the optical properties of uncoated paper. The results indicate that a decrease in light scattering due to calendering can be explained by a change of pore size and shape, all else being equal.

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Contribution of the Thesis

4 Contribution of the Thesis

This thesis contributes to the area of colour appearance modelling and the area of light scattering in paper and print. The results are directly applicable to industrial applications for better instrumental measurement of whiteness and thereby optimising the use of FWA with the goal to improve the perceived whiteness. The overall contribution is to provide models to predict and optimise the paper composition to achieve the highest whiteness, as perceived by a group of real observers. High whiteness requires to take advantage of fluorescence. Hence, it is the combination of better understanding of the optical properties of FWA and of new insights on the human perception of whiteness that provides new knowledge applicable in the industry.

This section emphasizes the contribution and novelty of the papers included in the thesis and states the contribution of the author of the thesis to each paper.

4.1 Paper I

This paper proposes new whiteness equations that better predict the perceived whiteness of office papers. The equations are based on models proposed earlier by other authors. The proposed equations are refined to ensure continuity and definition in the whole colour space. The main contribution of this paper is to show that the blueness limit has been passed in today’s office papers. These papers are not given a CIE whiteness value, although they are perceived as white by most observers. The proposed non-linear equations give to these papers a whiteness value that correlates with their perceived whiteness. It is also shown that, although perceived whiteness depends to a large extent on individual preferences, the mean perceived whiteness of different groups of observers are similar.

This paper is the result of cooperation with Dr. Siv Lindberg and Dr. Staffan Rydefalk. The contribution of the author of this thesis was part of the experiment planning, development of the new whiteness equations, conduction of the perception studies, analysis and presentation of the results, and main part of the writing.

4.2 Paper II

This paper applies a colour appearance model for the first time to a special part of the colour space, namely white. It is experimentally shown that simultaneous contrast effects, known for coloured surfaces, also significantly influences the perception of whiteness. The model used, CIECAM02-m2, predicts much of the observed simultaneous contrast effect except for high lightness induction fields, for which a modification of the model is suggested. The findings of this paper indicate that two white papers will influence each other’s appearance when compared pair wise.

This paper is the result of cooperation with Dr. Siv Lindberg. The contribution of the author of this thesis was part of the problem statement, part of the experiment planning, conduction of the perception studies, analysis and presentation of

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the results, and main part of the writing.

4.3 Paper III

The main contribution of this paper lies in technological application of a relatively simple extended Kubelka-Munk model to predict the radiance factor of fluorescent papers from their composition. The results open for a determination of the model parameters as function of the FWA concentration and furnish. This is necessary in order to use the model for optimising fluorescence in the paper and textile industry. It is shown that the quantum efficiency of a FWA can be assumed to be constant with FWA concentration, pulp type, and filler concentration to predict the CIE whiteness of uncoated papers. For an opaque pad of paper samples, for which whiteness is usually measured, the scattering coefficient can also be assumed to be independent of the FWA concentration. For paper samples of finite thickness, this paper provides the first experimental verification of the Kokhanovsky Kubelka-Munk extended model for fluorescence.

This paper is the result of cooperation with Dr. Per Edstr ¨om and Dr. Mattias Andersson. The contribution of the author of this thesis was part of the problem statement, part of the experiment, modelling, main part in analysis and presentation of the results, and main part of the writing.

4.4 Paper IV

This paper points out the optical limitations of FWA in uncoated papers at high FWA concentrations. It is shown that the absorption by the FWA in the violet-blue region of the electromagnetic spectrum is the main cause of whiteness saturation and greening for uncoated papers. As already known, the emission spectrum of the FWA is moved red-ward with increasing FWA concentration, but this has negligible effect on whiteness. Whiteness saturation and greening are explained by the optical properties of the FWA, assuming constant quantum efficiency with increasing FWA concentration. This shows for the first time that no chemical interaction is required to produce a chromaticity shift and saturation. Together with Paper III, this paper indicates that the efficiency of FWA is mainly dependent on its ability to absorb light.

This paper is the result of cooperation with Dr. Per Edstr ¨om, Dr. Mattias Ander- sson, and MSc. Jussi Kinnunen. The contribution of the author of this thesis was to define the problem statement, modelling, main part in analysis and presentation of the results, and main part of the writing.

4.5 Paper V

This paper does not directly contribute to the understanding of whiteness or fluorescence, but it provides a tool for further research. The implemented Monte Carlo simulation tool allows simulating papers made of several layers of varying thick-

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Discussion and suggestions for further work

ness and refractive index. Particle level models were implemented in which single fibres and fillers can be simulated. In this paper, the models was applied to model the effect of calendering on the reflectance factor of non fluorescing, single layer papers. The results indicate that a decrease in light scattering due to calendering can be explained by a change of pore size and shape, all else being equal.

This paper is the result of cooperation with Dr. Per Edstr ¨om and a Master’s The- sis student in computer science, Mikael Lindquister, supervised by the other two authors. Mikael Lindquister developed the data format and database included in the simulation tool. The simulation tool itself is further a development of a previous model, Grace [9], to which the author of the thesis contributed earlier [49]. The contribution of the author of this thesis was part of the modelling, coding, analysis and presentation of the reported application, as well as main part of the writing.

5 Discussion and suggestions for further work

5.1 Whiteness perception

High paper whiteness is achieved by the use of FWA and dyes to enhance the lightness and blueness of paper. Lightness and blueness were early recognized as the main component of perceived whiteness, leading to the development of the CIE whiteness. Since the CIE whiteness became an ISO standard, paper producers strived to produce paper with the highest, easily measured, CIE whiteness. The CIE whiteness is however only defined in a limited region of the colour space given by Equa- tions 2 and 3. Paper I showed that many commercial copy papers were outside the region of validity of the CIE whiteness. They were too blue and should be given zero as CIE whiteness. Nonetheless, these papers are still perceived as white by most observers and the proposed non linear whiteness equations in Paper I predict well the perceived whiteness of all the samples. For a given L^∗, the CIE whiteness has its maximum at a given b^∗beyond which the sample is not assigned a whiteness value.

One of the proposed non-linear whiteness eqautions has its maximum at the same positions beyond which the whiteness starts to decrease, whereas better correlation between perceived whiteness ranking or rating with predicted whiteness was obtained with models allowing more blueness. This in accordance with the findings of Uchida [4], who stated that the blue limit in Equation 3 could be extended. Setting the position of the maximum whiteness is a difficult task because of large individual preferences. The observers tend to disagree in larger extent at the highest whiteness values, where the samples have a distinct bluish tint. Some rate the samples very low, penalizing the bluish tint, whereas others perceived the samples as very white.

No matter where the final maximum should be set, there is a clear benefit in using a continuous whiteness equation defined in the whole colour space. The equations are more complex than the original CIE whiteness, but since the calculations are performed by a computer, this should not be problem to use them as a new standard whiteness equation.

It is worth discussing here the individual preferences further. Paper I showed

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that the rankings from different groups of observers correlated, while the observer’s cross-correlation was low. Since the predicted whiteness correlated well with the median ranking and rating of the groups, this means that the measured whiteness is a better predictor of the perceived whiteness of a group, than the assessments of individual observers of the same group. In other words, as long as the goal is to predict what would be perceived in average by many observers, when one does not rank two papers in whiteness as the model, one is wrong and the model is right. The strong individual preferences will also influence how much instrumental whiteness increase is required to produce a detectable increase of perceived whiteness, i.e. the just noticeable difference (JND) in whiteness [50]. JND determined from repeated evaluations from a single observer are expected to be significantly lower than JND determined from single observations from a group of observers.

The ranking procedure used in Paper I, where all the samples are presented si- multaneously to the observer, corresponds to real life situations in which the whiteness of a paper is compared to other papers already on a table. Buyers and customers evaluate whiteness often by comparing two samples against each other. As Pa- per II shows, simultaneous contrast significantly influences the perception of whiteness. Paper II evaluated the perceived whiteness of patches surrounded by different colours. Light blue and light yellow backgrounds had the strongest impact on the the perceived whiteness. This suggests that papers with bluish tint will appear bluer when compared to achromatic or yellowish papers, than when compared to other bluish papers. This will be studied by pair wise evaluations where the samples are in contact with each other. The proposed improvement of the CIECAM02-m2 model will then be tested for induction fields of high lightness.

5.2 Light scattering and fluorescence modelling

In this thesis, the Kubelka-Munk based Kokhanovsky model was used to model the fluorescence of papers with different pulps, filler amount, and basis weight. De- spite the limitations of the conventional KM model, the Kokhanovsky model was shown to predict well the luminescent radiance factor of a paper at different basis weights. Testing the validity of the model requires producing samples of different basis weights, all else being equal. If an opaque pad of samples is taken as a sample of infinite thickness, assuming homogeneity in the sample, the structure of the sample may vary with basis weight. In Paper III, the scattering coefficient was lower at 40 g/m²than at larger basis weights.

The model can be used together with the parameter estimation method proposed in Paper III to determine s, k, and Q from measurements of Donaldson radiance factor of one single sheet and of an opaque pad. With the help of the model, the radiance factor of different papers can be explained by varying s, k, and Q. Paper IV uses the model in that way to quantify the effect of absorption of the FWA in the visible spectrum and of FWA concentration dependent Q on the radiance factor and whiteness.

The applicability of such a model in optimising the paper structure for the highest whiteness relies on ways to assess the model parameters as function of the structure.

Paper III investigates how these parameters vary with the structure. It is shown

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5.2 Light scattering and fluorescence modelling

that the change of Q with FWA concentration had a negligible effect on whiteness.

Furthermore, it is shown that the luminescent radiance factor is dependent on the ratio k/s for opaque samples. Hence, since whiteness is usually measured on an opaque pad of samples, it can be predicted at different FWA concentrations based on a change of absorption coefficient only. Since Q was shown to be rather independent of pulp, filler content, and even of the two FWA types used, a constant Q can be used in optimising the composition of plain paper.

A constant Q with different paper composition and FWA concentration means that the fluorescence efficiency only depends on the absorption of the FWA and on the scattering and absorption coefficients in the emission band. High whiteness requires of course high scattering and low absorption in the emission band of the FWA to fully take advantage of the luminesced light. The amount of luminescent light depends on the other hand only on the amount of light absorbed by the FWA, kFWA, in the absorption band of the FWA, when Q is constant. In the papers included in this thesis, kFWA was determined by measurement of samples with and without FWA.

This assumes additivity of the absorption properties of the FWA and of the furnish.

The results in paper III and IV suggest that this assumption is valid for the samples used in the respective studies. However, further studies could focus on different FWA application methods. For instance, if the FWA would lay on top of the fibres, the absorption coefficient should not be additive, and the FWA should be effective even on UV absorbing fibres, as long as the fibres do not absorb much in the emission band of the FWA. In order to use the model for prediction and optimisation, the non linear dependence of k_FWA on the FWA concentration needs to be studied further. This includes both adsorption properties of the FWA and chemical interaction with fibres and additives affecting the absorbing power of the FWA. High performance liquid chromatography (HPLC), which was applied on the samples of Paper IV, is a promising way to study the adsorption of FWA. Preliminary results indicate a rather linear dependence of the FWA amount fixed to the samples on the feed FWA amount.

Considering Q as independent of the pulp and additives in plain paper, high whiteness is obtained by maximising kFWA. However, Paper IV showed that the light absorption of the FWA in the visible spectrum explained most of the observed greening and whiteness saturation. Thus, increasing the FWA concentration does not automatically lead to higher whiteness. Further, chemical interactions with e.g.

salts may affect the absorption properties of the FWA.

In this thesis, the Kubelka-Munk based model proposed by Kokhanovsky was used and tested to predict and explain fluorescence. Paper III showed that the model predicts well the basis weight dependent fluorescent radiance factor, given that s and k are constant with basis weight. This is however generally not the case. This points out the difficulty of making a prediction model to be used for optimisation of the paper composition. Even testing the model with real samples is a difficult task. Nonetheless, the model predicts also the transmitted fluorescent radiance factor. This is needed for modelling layered constructions in ongoing studies.

Paper V presents a Monte Carlo simulation tool for light scattering and fluorescence in paper and prints. The main goal in this thesis is to put together a modular

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code for further work including spatial variation relevant for white-top mottle analysis, and allowing particle level simulations with a potential increased explanative power. As opposed to Kubelka-Munk based models, Monte Carlo models are angle- resolved. Since the reflectance is significantly anisotropic when the absorption is large [20], as in the absorption band of the FWA, this may affect the estimation of k_FWA. For particle level simulations, dedicated methods for parameter estimation need to be developed in order to tackle the increased model complexity and the increased number of model parameters, as pointed out in Paper V. Finally, this thesis was restricted to plain paper. The coming studies will include FWA in coating.

6 Conclusions

This thesis contributes to the area of colour appearance modelling and the area of light scattering in paper and print. The results are directly applicable in industrial applications to better measure instrumentally perceived whiteness and optimise the use of FWA. The CIE whiteness fails to predict the perceived whiteness of highly white and bluish samples. Instead, proposed whiteness equations defined in the whole colour space should be used. Because of large individual preferences, whiteness is not perceived equally by different observers. The proposed whiteness equations, applied on instrumental measurement, will perform better than a single real observation in predicting the perceived whiteness of a group of observer. When comparing different papers side by side, simultaneous contrast affects significantly the appearance of the papers, depending on the shade of the other papers. Thus, whiteness models neglecting simultaneous contrast effects should not be used to predict the relative whiteness of different papers seen together. These models predict only the perceived whiteness of a single paper on a gray background.

Extensions of the Kubelka-Munk theory including fluorescence explain much of the fluorescence of homogeneous plain paper in terms of paper composition, FWA concentration and basis weight. The quantum efficiency of two different FWA types was found to be rather equal and independent of the paper composition and FWA concentration. The fluorescence efficiency is essentially dependent only on the ability of the FWA to absorb light in its absorption band. However, absorption of the FWA in the violet-blue region of the electromagnetic spectrum explains most of the often observed greening and whiteness saturation at larger FWA concentrations.

These findings open for easier model parameter estimation, and for using the models to optimise the paper whiteness.

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Whiteness and Fluorescence in Paper: Perception and Optical Modelling