whiteness and fluorescence in paper - diva portal357563/... · 2010. 10. 18. · list of papers...

Whiteness and Fluorescence

in Paper

Perception and Optical Modelling

Ludovic Gustafsson Coppel

Supervisor: Prof. Per Edström, Mid Sweden UniversityExaminer: Prof. Håkan 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

MittuniversitetetDepartment of Natural Sciences, Engineering and Mathematics

ISBN 978-91-86073-97-8 SE-851 70 SundsvallISSN 1652-8948 SWEDEN

Akademisk avhandling som med tillstånd av Mittuniversitetet framlägges till of-fentlig granskning för avläggande av teknologie licentiatexamen tisdagen den 9 novem-ber 2010 i sal O111, Mittuniversitetet, Holmgatan 10, Sundsvall. Seminariet kommeratt hållas på engelska.

c©Ludovic Gustafsson Coppel, 2010

Printed by Kopieringen Mittuniversitetet, Sundsvall, Sweden, 2010

To Eliott, Elina, and Rebecka

Abstract

This thesis is about modelling and predicting the perceived whiteness of plain paperfrom the paper composition, including fluorescent whitening agents. This includespsycho-physical modelling of perceived whiteness from measurable light reflectanceproperties, and physical modelling of light scattering and fluorescence from the pa-per composition.

Existing models are first tested and improvements are suggested and evaluated.The standardised and widely used CIE whiteness equation is first tested on com-mercial office papers with visual evaluations by different panels of observers, andimproved models are validated. Simultaneous contrast effects, known to affect theappearance of coloured surfaces depending on the surrounding colour, are shown tosignificantly affect the perceived whiteness. A colour appearance model includingsimultaneous contrast effects (CIECAM02-m2), earlier tested on coloured surfaces,is successfully applied to perceived whiteness. A recently proposed extension ofthe Kubelka-Munk light scattering model including fluorescence for turbid media offinite thickness is successfully tested for the first time on real papers.

It is shown that the linear CIE whiteness equation fails to predict the perceivedwhiteness of highly white papers with distinct bluish tint. This equation is applica-ble only in a defined region of the colour space, a condition that is shown to be notfulfilled by many commercial office papers, although they appear white to most ob-servers. The proposed non-linear whiteness equations give to these papers a white-ness value that correlates with their perceived whiteness, while application of theCIE whiteness equation outside its region of validity overestimates perceived white-ness.

It is shown that the quantum efficiency of two different fluorescent whiteningagents (FWA) in plain paper is rather constant with FWA type, FWA concentra-tion, filler content, and fibre type. Hence, the fluorescence efficiency is essentiallydependent 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. Thisviolet-blue absorption tends to give a greener shade to the paper and explains mostof the observed greening and whiteness saturation at larger FWA concentrations. Ared-ward shift of the quantum efficiency is observed with increasing FWA concen-tration, but this is shown to have a negligible effect on the whiteness value.

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

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Acknowledgements

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

I want to thank my supervisor, Per Edström, for his confidence in me and supportthroughout this work. Together with my manager, Marie-Claude Béland, TorbjörnWidmark, and my earlier Master’s Thesis supervisor, Nils Pauler, Per was also veryactive in setting up this licentiate project, which later became a PhD project thanksto the PaperOpt project Per initiated.

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

I am grateful to Markku Hauta-Kasari for the good time and help I got duringmy stays at his group at the University of Eastern Finland in Joensuu. Special thanksgo 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 Lindström, Annika Kihlstedt, Caroline Cederströmand Maggan for help with lengthy evaluations in the perception lab and measure-ments. My co-authors, Siv Lindberg and Staffan Rydefalk for their expertise andsupport. Hjalmar Granberg for numerous discussions about light scattering.

My colleagues in Örnsköldsvik at the Digital Printing Center are thanked for thevery 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. MagnusNeuman with whom I have more and more discussions about light scattering andfluorescence.

This work was financially supported by the Swedish Governmental Agency forInnovation 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 inme, and my wife Anne and my children for all the good things that happened duringthese 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 Romannumerals:

Paper I Coppel L.G., Lindberg, S., and Rydefalk, S, Whiteness assessment of paper samples at thevicinity 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 perceivedwhiteness, 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 Edström, P., Quantum efficiency of fluorescent dyesin different furnishes, manuscript to be submitted to Applied Optics, (2010).

Paper IV Coppel L.G., Andersson, M., Edström, P., and Kinnunen, J., Limitations of the ef-ficiency of fluorescent whitening agents in uncoated paper, manuscript to be submitted toNordic Pulp and Paper Research Journal, (2010).

Paper V Coppel, L. G. , Edström, P. and Lindquister, M., Open source Monte Carlo simulationplatform 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 FWAamount, 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 imagequality, to appear in Proceedings of the 18th Color Imaging Conference, San Antonio,Texas, (November 2010).

Bohlin, E., Coppel, L.G., Andersson, C., Edström, P., Characterization and modelling of the effectof 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., Edström, P., Modelling of brightness decrease of coatedcartonboard as an effect of calendering - microroughness and effective refractive index aspects, toappear in Proceedings of the TAPPI 11th Advanced Coating Fundamentals Symposium(2010).

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Contents

Abstract i

List of Papers iii

1 Introduction 1

1.1 Background and Problem Motivation . . . . . . . . . . . . . . . . . . . 1

1.2 Overall Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theoretical background 3

2.1 Colour appearance modelling . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Perceived whiteness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Light scattering and fluorescence . . . . . . . . . . . . . . . . . . . . . . 6

3 Summary of the papers 7

3.1 Paper I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Paper II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 Paper III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Paper IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.5 Paper V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Contribution of the Thesis 11

4.1 Paper I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2 Paper II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.3 Paper III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.4 Paper IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.5 Paper V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Discussion and suggestions for further work 13

5.1 Whiteness perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Light scattering and fluorescence modelling . . . . . . . . . . . . . . . . 14

6 Conclusions 16

References 16

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Introduction

1 Introduction

1.1 Background and Problem Motivation

Whiteness of paper is a commercially important property, and a marketing goal forproduct segments such as office papers although the perfectly corresponding instru-mental measurement remains elusive [1]. According to Ganz [2], the assessment ofwhiteness depends on individual preference, the level and spectral power distribu-tion of the sample irradiation, the colour of the surround, and the acquired precon-ceptions in various trades. Despite the problems presented by the colorimetry offluorescent samples, most observers are able to arrange white samples of differentluminous reflectance, hue and saturation in a one-dimensional order according towhiteness, although little general agreement on whiteness can be reached. Regard-less of this disparity, attempts have been made to elaborate a standardised whitenessformula for commercial whites. The general agreement is that a sample is perceivedas the whiter, the lighter, and the bluer it is. Thus, whiteness is characterised byhigh level of luminosity and finite saturation, with a blue hue [3]. The most widelyused whiteness model in the paper industry, the CIE1 whiteness, has been foundto correlate well with visual estimation for many white samples having similar tintor fluorescence [4]. To prevent application of the whiteness formula to chromaticsamples, 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 in-crease of the concentration of fluorescent whitening agents (FWA) and violet-blueshading dyes added in paper. FWA are dyes that absorb ultraviolet (UV) light andemit in the blue region of the spectrum, hence increasing the perceived whiteness byboth increasing the lightness and the blueness of the paper. Excessive use of FWAand shading dyes has led to papers with a clear bluish tint and commercial copypapers 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: thelight source, the object, and the observer. Due to the complexity of the human visualsystem, several colour appearance phenomena that cannot be physically measuredinfluence the way an observer perceives colour. One effect is simultaneous contrastthat causes a stimulus to shift in colour appearance when the background or adjacentcolours are changed. This simultaneous contrast can affect the perceived whitenessof paper samples so that a pair of samples are ranked differently depending on thebackground and the shade of the other samples in the set to be evaluated [5]. Inorder to evaluate the perceived whiteness of a pair of paper samples with differenttint, there is thus a need to model how the samples influence the appearance of eachother.

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

1Commission Internationale de l’Eclairage

1

Whiteness and Fluorescence in Paper

cess, by making it possible to design materials by means of modeling and predictionrather than full scale trial-and-error. Models including fluorescence are often basedon Monte Carlo (MC) methods, especially in tissue applications [7, 8], but applicationto the reflectance from paper exists [9, 10]. However, for technological applications,simpler analytical models such as the Kubelka-Munk theory (KMT) [11, 12] are pre-ferred [13, 14, 15]. Technological applications of these models rely on assumptionsof how the model parameters are affected by the manufacturing process, thickness,composition, and other structural modifications or chemical interactions. The depen-dence of the scattering and absorption coefficients of the KMT on the paper structurehas 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 ef-ficiency, was rather constant with FWA concentration. However, the results appliedonly to FWA added to identical pulp, and to measurements made on an opaque padof samples. It is therefore of interest to study how the optical properties of FWAdepend on the substrate composition.

KMT based models and most MC models treat a paper layer as a homogenousturbid medium with statistical scattering and absorption coefficients. These modelsare henceforth referred to as homogenous models. With these models, layer thicknessvariation and surface scattering can be included, to the cost of model complexity andcomputation time. Another great potential of MC models is their ability to handleanisotropic scattering, which has been shown to have a large impact on standardisedreflectance measurements [19, 20, 21]. Homogenous models have been proved usefulin predicting the reflectance of halftone prints on fluorescing paper substrate, bothwith KMT [22] and MC [23]. These applications are on the other hand restricted todifferent prints on the same paper, whose model parameters are determined exper-imentally. For paper product development, 3-dimensional particle level models arerequired to model the composite structure of real fluorescing papers, as suggestedby Hainzl et al. [9]. However, the higher explanative power of these structure modelsas compared to homogeneous models is yet to be demonstrated, as far as fluores-cence is concerned, and attempts to validate their predictable power easily end withfar more parameters to be determined than available measurement inputs. To getfull advantage of MC particle level simulations, there is thus a need for dedicatedparameter estimation methods that are part of the model itself.

1.2 Overall Aim

This thesis aims at modelling perceived whiteness from the paper composition withtwo objectives: understanding the mechanisms and efficiency limitations of fluores-cent whitening agents, and providing a tool for design and optimisation of the paperstructure in terms of perceived whiteness.

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1.3 Scope

1.3 Scope

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

One goal is to provide improved models for perceived whiteness including si-multaneous contrast effects. This will allow optimising the reflectance properties ofa paper that is partially printed or compared to other papers. Light scattering mod-els are in turn used to model the reflectance properties from the paper compositionand structure.

This thesis focuses on uncoated single layer paper. Existing models are firsttested and improvements are suggested and evaluated. For light scattering mod-elling, simpler models treating paper as a homogeneous turbid medium are used andthe 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 papercomposition, and how FWA at different concentrations affects spectral reflectancefactor and whiteness. To prepare for future particle level simulation describing thecomposite and inhomogeneous structure of paper, a Monte Carlo simulation tool isimplemented 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 necessarybackground, definitions, and terminologies used throughout the thesis. A more thor-ough review of colour science is given by Wyszecki and Stiles [24]. Hunt [25] pro-vides basic knowledge about colour measurement, and Fairchild [26] about colourappearance modelling. For an introduction on the optical properties of paper andpaper whiteness, refer to Pauler [16], and for light scattering in paper to Rydefalkand 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 acertain colour and then treat colour only as a physical quantity. However, a sur-face colour may change appearance depending on the illumination. This is why anycolour measurement must be performed and communicated with a known illumina-tion. Colour is a visual sensation that depends on three interacting components: thelight source, the object, and the observer. Due to the complexity of the human visualsystem, 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 forthis thesis is simultaneous contrast that causes a stimulus to shift in colour appear-ance when the background or adjacent colours are changed. The perveived colourof a stimulus does not only depend on the stimulus reflective properties and theillumination, but also on a potential proximal or induction field (the immediate en-vironment with the stimulus), on the background (extending for about 10o from theedge 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 acolour by the brain through the three colour sensors in the eyes, the cones. Since theexact 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 thewavelength range. Two different sets of CMF exists. The 1931 standard colorimetricobserver , referred to as the 2o observer, and the 1964 supplementary standard colori-metric observer , which was defined using a visual field of 10o instead of the 1931 2o

visual field. A colour should therefore be reported together with the actual standardobserver used. Since the colour depends on the spectral characteristics of the illu-mination, it is also reported in a given illumination, usually one of the CIE standardilluminants, such as A representing a tungsten filament lamp, or D65 representingdaylight at a colour temperature of 6500K.

The CIE tristimulus values represent colours in the three-dimensional XY Z colourspace. This colour space is not perceptually uniform, in the sense that Euclidiandistance in XY Z do not map perceived colour differences. The CIE proposed in1976 the CIE L∗a∗b∗ colour space (CIELAB), which is a non-linear transformation ofthe XY Z tristimulus values, and is approximately perceptually uniform. CIELABmakes use of a simple chromatic adaptation transform, modelling the ability of thehuman visual system to discount the colour of the light source in order to preservethe 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-greenaxis (a∗), and a yellow-blue axis (b∗).

CIELAB is a well-established international standard that performs well as a colourappearance model (CAM) in many applications [26]. The major limitations of CIELABreported 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 attributesof brightness and colourfulness. For applications restricted to reflective materialsviewed in an average daylight illumination, the limitations discussed above are of-ten 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. Huntsuggested 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 calculatingthe adjusted white point. A simplified version of the Hunt model, CIECAM02, wasintroduced in 2003 [30, 26]. Since the simultaneous contrast prediction part showedpoor 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 whichthe 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 CIEwhiteness formula as an assessment method of white materials in 1986. This formulahas been found to correlate with visual estimation for many white samples havingsimilar tint or fluorescence [4]. CIE whiteness is given by

WCIE = Y + 800(xn − x) + 1700(yn − y), (1)

where x and y are the CIE chromaticity coordinates, and xn and yn are the coordi-nates for the perfect reflecting diffuser at the given illumination. To prevent applica-tion of the whiteness formula to chromatic samples, the equation is only valid withingiven boundaries in the colour space,

−3 < T < 3, (2)

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

40 < WCIE < 5Y − 280. (3)

Since the approval of the CIE whiteness, the L∗a∗b∗ system has been introducedand is now used for routine colorimetry. Researchers assessing the whiteness by col-orimetric methods usually want to see and evaluate whiteness directly in the coloursystem 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

WCIE ≈ 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∗ systemhave also been published [33, 4]. At the same time a new ISO standard introducedthe concept of ”indoor whiteness” [34]. This standard stipulates the use of the CIEilluminant C , which gives a much lower relative UV content than the adjustmentsto 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 tothat 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 lightin 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 asextinction. When light is absorbed by a molecule, it can be transformed into heator re-emitted at another wavelength in fluorescence processes. Light scattering iscaused by local variation of the refractive index within a heterogeneous mediumand 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 scatteringfrom packed particles of varying size and shape. For those turbid media, radiativetransfer theory is often used instead [36]. It describes the interaction of radiationwith scattering media, with a scattering and an absorption coefficient, and a phasefunction defining the direction probability distribution of scattered light. The equa-tion of radiative transfer was stated by Chandrasekhar [37]. This equations lacks ageneral analytical solution and numerical methods are required [38]. Only recentlya general numerical solution for layered structures has been presented by Edström[39]. Therefore, simplified models such as KMT or Monte Carlo methods have beenused to model light scattering in paper.

Whenever the equations describing a physical problem can be written down, butthe solution to these equations is intractable, it is appealing to turn to Monte-Carlomethods. By following the path of wave packets interacting with different compo-nents according to local physical rules, it is possible to calculate the average spatial-and angle resolved reflectance and transmittance. The method has been extensivelyused in medicine applications to model the interaction of light with tissues [40].Carlsson et al. [41] introduced a three-dimensional model of the internal structureof paper including flattened cylindrical fibres, ellipsoidal pores, and fine particleslocated 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 paperstructure, using component distributions and size distributions, does not render thestructure 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. Lightscattering simulation models using such generated paper structures were suggestedby Nilsen et al. [42] and Jensen [43] among others, and improved fibre web mod-elling has been published recently [44, 45].

Models including fluorescence are often based on Monte Carlo methods, espe-cially in tissue applications [7, 8]. Monte Carlo has also been used to model fluo-rescence in paper [10, 9]. However, for technological applications, simpler analyti-cal models such as the Kubelka-Munk theory (KMT) [11, 12] are preferred. Severalextensions of the KMT theory have been proposed to include fluorescence in theKMT [13, 14, 46, 18], all requiring either a defined polychromatic illumination or asemi-infinite layer. More recently, Kokhanovsky [15, 47] presented a simple generalanalytic 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 white-ness from measurable radiative properties of paper (Paper I-II), and the modellingof 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 tothe perceived whiteness of such papers. Paper II shows that the perceived whitenessof a piece of paper is largely influenced by the shade of neighbouring colours, andthat this influence can be predicted by recent colour appearance models. Paper IIIstudies the influence of the composition of uncoated papers on the quantum effi-ciency of fluorescent whitening agents (FWA). Paper IV examines the saturation ofthe measured whiteness with increasing FWA concentration. In Paper V, a MonteCarlo simulation tool is implemented and parameter estimation methods are devel-oped.

3.1 Paper I

This paper assesses the perceived whiteness of 45 commercial copy papers usingranking and magnitude estimation methods. Two published non-linear whitenessmodels 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 ob-servers 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 observercross-correlation was low (R2

min= 0.46), in line with previous research, but the me-

dian rankings in different illumination conditions were highly correlated (R2 = 0.9).This suggests two things. First, all the commercial copy papers used show a sim-ilar fluorescence. Hence, they all get whiter as the UV content of the illuminationincreases, and their internal ranking is not affected. Secondly, different groups of ob-servers can agree on which paper is the whitest of two papers, different individualobservers 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 bythe authors, WNEW, the maximum whiteness is set at a lower b

∗ value than in the CIEwhiteness formula (Figure 1). This model correlated well with ranking and scaling(R2 > 0.7). The higher correlation obtained with this model indicates that the upperwhiteness limit in the CIE whiteness could be extended. The second model, WeCIE,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 thiscase the maximum whiteness is determined by the CIE whiteness upper limit. Theperformance of this model was slightly inferior (rank correlation R2 = 0.56 for thefirst set of samples and R2 = 0.69 for the second set), but its main advantage isthat all samples within the CIE whiteness limits are given the same whiteness val-ues 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. WNEW is the continuous surface.

a penalty function replacing the CIE whiteness limits is a promising way to handlewhite 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 con-trast on perceived whiteness. This effect causes a stimulus to shift in colour ap-pearance when the background or adjacent colours are changed, but it has neverbeen reported for whiteness. The perceived whiteness of white patches surroundedby induction fields of different shades was evaluated by asking observers to give amagnitude estimate of perceived whiteness of the patches in comparison to a whitereference. The reflectance factor of the samples was measured in the actual 5000K illumination with a spectroradiometer. The perceived whiteness of patches withidentical measurable colour was highly dependent on the shade of the inductionfield, and the patch size used in the study did not significantly affect the perceivedwhiteness.

A colour appearance model, CIECAM02-m2 [31], was applied to predict the per-ceived whiteness of patches with 10 different surrounding colours (the inductionfield). The spectroradiometer measures the ”physical” colour of the white patch.With CIECAM02-m2, the L∗a∗b∗ values of the patch surrounded by an inductioncolour are computed as if the patch was seen on a neutral background. From thesenew 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 patchappear bluer, sometimes to such an extent that the patch was perceived less whitethan without the induction field. For this reason, the whiteness model developedin Paper I, WNEW performs better than the CIE whiteness formula, since it penalisessamples with a too bluish tint.

The combination of CIECAM02-m2 and WNEW predicts much of the observedsimultaneous contrast effect. However, the model performs better for dark induc-tion fields than for light induction fields. The model rates patches surrounded bylight blue and light yellow equally, whereas the observers clearly rate the patcheswith light yellow induction field as whiter than the patches with light blue inductionfield. A deeper analysis of the CIECAM02-m2 model indicates that the simultaneouscontrast model used cannot accurately predict the change in hue for high lightnessinduction fields. A potential improvement for prediction of simultaneous contrasteffects would be to base the calculations not only on the difference between the in-duction field and the background, but on the difference between the patch stimulusand the induction field.

3.3 Paper III

In this paper, the quantum efficiency of FWA:s are characterised in samples withdifferent compositions. The aim is to test the Kokhanovsky extended Kubelka-Munkmodel including fluorescence [15], and map how its parameters are dependent onthe paper composition in terms of FWA concentration, filler concentration, and fibretype. 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 pulpat different concentrations. A new method is introduced to determine the scatteringcoefficient, 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] radiancefactor, D(µ, λ = 540nm) and D(µ, µ) at two basis weights. The Donaldson radiancefactor is a discrete, illuminant independent, matrix representation of the bispectralradiance factor for each excitation wavelength and emission wavelength.

With increasing FWA concentration, the emission spectrum of the FWA, given byquantum efficiency at one excitation wavelength, is slightly shifted to longer wave-lengths. The quantum efficiency dependency on fibre type, basis weight, and fillerconcentration is negligible. However, the scattering coefficient is dependent on theabsorption coefficient, and hence the FWA concentration. This result is in line withprevious observations made by several authors. On the other hand, it is also shownthat for an opaque pad of samples, for which the whiteness is measured, the fluo-rescent radiance factor depends only on the ratio k/s. Thus, for opaque samples, scan be assumed to be independent of the FWA concentration and the fluorescence ofpaper with increasing FWA concentration is well predicted by an increase of k andconstant Q.

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

9


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

3.4 Paper IV

This paper examines the dependence of the CIE whiteness of uncoated and unfilledpapers on the FWA concentration. Since it is not known how much of the FWA isfixed in the paper, the analysis is made on the increasing absorption coefficient. Asobserved in Paper III, the emission spectrum of the FWA is moved towards longerwavelengths with increasing FWA concentration. As already known from severalother studies, the absorption and emission band of the FWA overlap, and the FWAabsorbs in the visible part of the electromagnetic spectrum.

By using the extended Kubelka-Munk model [15], hypothetic FWA:s with noemission spectrum shift and/or no absorption at wavelength above 400 nm are sim-ulated to separate and quantify these effects on the whiteness value. It is shown thatthe shift of the emission spectrum with increasing concentration has a negligible ef-fect on whiteness. The overlap of the absorption and emission bands of the FWA isthe main cause of greening (a shift of the chromaticity towards green) and saturationof the fluorescence effect. With increasing FWA concentration, the positive effect offluorescence is neutralised by the reduction of the reflectance factor in the violet-blueregion 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 lightscattering and fluorescence in paper used in the continuation of this thesis. The aimis to model the composite structure of a paper layer on a particle level, as opposedto the model used in the precedent papers, which treats paper as a homogeneousturbid medium.

This paper describes the general modular structure of the Open Source simula-tion tool, Open PaperOpt. Selected physical models already implemented are pre-sented. Simulating the scattering from a fibre web consisting of individual fibresand pores increases dramatically the number of model parameters and the modelcomplexity. A method for the estimation of the model parameters is presented anddiscussed. The applicability of the simulation tool is demonstrated by modelling theeffect of calendering on the optical properties of uncoated paper. The results indicatethat a decrease in light scattering due to calendering can be explained by a changeof 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 oflight scattering in paper and print. The results are directly applicable to industrialapplications for better instrumental measurement of whiteness and thereby optimis-ing the use of FWA with the goal to improve the perceived whiteness. The overallcontribution is to provide models to predict and optimise the paper composition toachieve the highest whiteness, as perceived by a group of real observers. High white-ness requires to take advantage of fluorescence. Hence, it is the combination of betterunderstanding of the optical properties of FWA and of new insights on the humanperception of whiteness that provides new knowledge applicable in the industry.

This section emphasizes the contribution and novelty of the papers included inthe 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 white-ness of office papers. The equations are based on models proposed earlier by otherauthors. The proposed equations are refined to ensure continuity and definition inthe whole colour space. The main contribution of this paper is to show that the blue-ness limit has been passed in today’s office papers. These papers are not given aCIE whiteness value, although they are perceived as white by most observers. Theproposed non-linear equations give to these papers a whiteness value that correlateswith their perceived whiteness. It is also shown that, although perceived whitenessdepends to a large extent on individual preferences, the mean perceived whitenessof different groups of observers are similar.

This paper is the result of cooperation with Dr. Siv Lindberg and Dr. StaffanRydefalk. The contribution of the author of this thesis was part of the experimentplanning, development of the new whiteness equations, conduction of the percep-tion 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 ofthe colour space, namely white. It is experimentally shown that simultaneous con-trast effects, known for coloured surfaces, also significantly influences the percep-tion of whiteness. The model used, CIECAM02-m2, predicts much of the observedsimultaneous contrast effect except for high lightness induction fields, for which amodification of the model is suggested. The findings of this paper indicate that twowhite papers will influence each other’s appearance when compared pair wise.

This paper is the result of cooperation with Dr. Siv Lindberg. The contributionof the author of this thesis was part of the problem statement, part of the experi-ment 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 relativelysimple extended Kubelka-Munk model to predict the radiance factor of fluorescentpapers from their composition. The results open for a determination of the modelparameters as function of the FWA concentration and furnish. This is necessary inorder to use the model for optimising fluorescence in the paper and textile indus-try. It is shown that the quantum efficiency of a FWA can be assumed to be constantwith FWA concentration, pulp type, and filler concentration to predict the CIE white-ness of uncoated papers. For an opaque pad of paper samples, for which whitenessis usually measured, the scattering coefficient can also be assumed to be indepen-dent of the FWA concentration. For paper samples of finite thickness, this paperprovides the first experimental verification of the Kokhanovsky Kubelka-Munk ex-tended model for fluorescence.

This paper is the result of cooperation with Dr. Per Edström and Dr. MattiasAndersson. The contribution of the author of this thesis was part of the problemstatement, part of the experiment, modelling, main part in analysis and presentationof 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 highFWA concentrations. It is shown that the absorption by the FWA in the violet-blueregion of the electromagnetic spectrum is the main cause of whiteness saturationand greening for uncoated papers. As already known, the emission spectrum of theFWA is moved red-ward with increasing FWA concentration, but this has negligibleeffect on whiteness. Whiteness saturation and greening are explained by the opticalproperties of the FWA, assuming constant quantum efficiency with increasing FWAconcentration. This shows for the first time that no chemical interaction is requiredto produce a chromaticity shift and saturation. Together with Paper III, this paperindicates that the efficiency of FWA is mainly dependent on its ability to absorb light.

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

4.5 Paper V

This paper does not directly contribute to the understanding of whiteness or fluo-rescence, but it provides a tool for further research. The implemented Monte Carlosimulation 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 singlefibres and fillers can be simulated. In this paper, the models was applied to modelthe effect of calendering on the reflectance factor of non fluorescing, single layer pa-pers. The results indicate that a decrease in light scattering due to calendering canbe explained by a change of pore size and shape, all else being equal.

This paper is the result of cooperation with Dr. Per Edström and a Master’s The-sis student in computer science, Mikael Lindquister, supervised by the other twoauthors. Mikael Lindquister developed the data format and database included inthe simulation tool. The simulation tool itself is further a development of a previousmodel, Grace [9], to which the author of the thesis contributed earlier [49]. The con-tribution of the author of this thesis was part of the modelling, coding, analysis andpresentation 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 light-ness and blueness of paper. Lightness and blueness were early recognized as themain component of perceived whiteness, leading to the development of the CIEwhiteness. Since the CIE whiteness became an ISO standard, paper producers strivedto produce paper with the highest, easily measured, CIE whiteness. The CIE white-ness 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 theregion of validity of the CIE whiteness. They were too blue and should be given zeroas CIE whiteness. Nonetheless, these papers are still perceived as white by most ob-servers and the proposed non linear whiteness equations in Paper I predict well theperceived whiteness of all the samples. For a given L∗, the CIE whiteness has itsmaximum 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 samepositions beyond which the whiteness starts to decrease, whereas better correlationbetween perceived whiteness ranking or rating with predicted whiteness was ob-tained with models allowing more blueness. This in accordance with the findings ofUchida [4], who stated that the blue limit in Equation 3 could be extended. Settingthe position of the maximum whiteness is a difficult task because of large individualpreferences. The observers tend to disagree in larger extent at the highest whitenessvalues, where the samples have a distinct bluish tint. Some rate the samples verylow, 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 usinga continuous whiteness equation defined in the whole colour space. The equationsare more complex than the original CIE whiteness, but since the calculations are per-formed by a computer, this should not be problem to use them as a new standardwhiteness 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’scross-correlation was low. Since the predicted whiteness correlated well with themedian ranking and rating of the groups, this means that the measured whitenessis a better predictor of the perceived whiteness of a group, than the assessments ofindividual observers of the same group. In other words, as long as the goal is topredict what would be perceived in average by many observers, when one does notrank two papers in whiteness as the model, one is wrong and the model is right. Thestrong individual preferences will also influence how much instrumental whitenessincrease is required to produce a detectable increase of perceived whiteness, i.e. thejust noticeable difference (JND) in whiteness [50]. JND determined from repeatedevaluations from a single observer are expected to be significantly lower than JNDdetermined 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 white-ness of a paper is compared to other papers already on a table. Buyers and customersevaluate whiteness often by comparing two samples against each other. As Pa-per II shows, simultaneous contrast significantly influences the perception of white-ness. Paper II evaluated the perceived whiteness of patches surrounded by differentcolours. Light blue and light yellow backgrounds had the strongest impact on thethe perceived whiteness. This suggests that papers with bluish tint will appear bluerwhen compared to achromatic or yellowish papers, than when compared to otherbluish papers. This will be studied by pair wise evaluations where the samples arein contact with each other. The proposed improvement of the CIECAM02-m2 modelwill 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 thefluorescence of papers with different pulps, filler amount, and basis weight. De-spite the limitations of the conventional KM model, the Kokhanovsky model wasshown to predict well the luminescent radiance factor of a paper at different basisweights. Testing the validity of the model requires producing samples of differentbasis weights, all else being equal. If an opaque pad of samples is taken as a sampleof infinite thickness, assuming homogeneity in the sample, the structure of the sam-ple may vary with basis weight. In Paper III, the scattering coefficient was lower at40 g/m2 than at larger basis weights.

The model can be used together with the parameter estimation method proposedin Paper III to determine s, k, and Q from measurements of Donaldson radiance fac-tor of one single sheet and of an opaque pad. With the help of the model, the radiancefactor of different papers can be explained by varying s, k, and Q. Paper IV uses themodel in that way to quantify the effect of absorption of the FWA in the visible spec-trum 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 highestwhiteness 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 theratio k/s for opaque samples. Hence, since whiteness is usually measured on anopaque pad of samples, it can be predicted at different FWA concentrations basedon a change of absorption coefficient only. Since Q was shown to be rather indepen-dent of pulp, filler content, and even of the two FWA types used, a constant Q canbe used in optimising the composition of plain paper.

A constant Q with different paper composition and FWA concentration meansthat the fluorescence efficiency only depends on the absorption of the FWA and onthe scattering and absorption coefficients in the emission band. High whiteness re-quires of course high scattering and low absorption in the emission band of the FWAto fully take advantage of the luminesced light. The amount of luminescent light de-pends on the other hand only on the amount of light absorbed by the FWA, kFWA, inthe absorption band of the FWA, when Q is constant. In the papers included in thisthesis, 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 samplesused in the respective studies. However, further studies could focus on differentFWA 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 effectiveeven on UV absorbing fibres, as long as the fibres do not absorb much in the emis-sion band of the FWA. In order to use the model for prediction and optimisation, thenon linear dependence of kFWA on the FWA concentration needs to be studied fur-ther. This includes both adsorption properties of the FWA and chemical interactionwith fibres and additives affecting the absorbing power of the FWA. High perfor-mance liquid chromatography (HPLC), which was applied on the samples of PaperIV, is a promising way to study the adsorption of FWA. Preliminary results indicatea rather linear dependence of the FWA amount fixed to the samples on the feed FWAamount.

Considering Q as independent of the pulp and additives in plain paper, highwhiteness is obtained by maximising kFWA. However, Paper IV showed that thelight absorption of the FWA in the visible spectrum explained most of the observedgreening and whiteness saturation. Thus, increasing the FWA concentration doesnot 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 wasused and tested to predict and explain fluorescence. Paper III showed that the modelpredicts well the basis weight dependent fluorescent radiance factor, given that sand k are constant with basis weight. This is however generally not the case. Thispoints out the difficulty of making a prediction model to be used for optimisationof the paper composition. Even testing the model with real samples is a difficulttask. Nonetheless, the model predicts also the transmitted fluorescent radiance fac-tor. This is needed for modelling layered constructions in ongoing studies.

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

15


code for further work including spatial variation relevant for white-top mottle anal-ysis, and allowing particle level simulations with a potential increased explanativepower. As opposed to Kubelka-Munk based models, Monte Carlo models are angle-resolved. Since the reflectance is significantly anisotropic when the absorption islarge [20], as in the absorption band of the FWA, this may affect the estimation ofkFWA. For particle level simulations, dedicated methods for parameter estimationneed to be developed in order to tackle the increased model complexity and the in-creased number of model parameters, as pointed out in Paper V. Finally, this thesiswas 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 oflight scattering in paper and print. The results are directly applicable in industrialapplications to better measure instrumentally perceived whiteness and optimise theuse of FWA. The CIE whiteness fails to predict the perceived whiteness of highlywhite and bluish samples. Instead, proposed whiteness equations defined in thewhole colour space should be used. Because of large individual preferences, white-ness is not perceived equally by different observers. The proposed whiteness equa-tions, applied on instrumental measurement, will perform better than a single realobservation in predicting the perceived whiteness of a group of observer. When com-paring different papers side by side, simultaneous contrast affects significantly theappearance of the papers, depending on the shade of the other papers. Thus, white-ness models neglecting simultaneous contrast effects should not be used to predictthe relative whiteness of different papers seen together. These models predict onlythe perceived whiteness of a single paper on a gray background.

Extensions of the Kubelka-Munk theory including fluorescence explain much ofthe fluorescence of homogeneous plain paper in terms of paper composition, FWAconcentration and basis weight. The quantum efficiency of two different FWA typeswas found to be rather equal and independent of the paper composition and FWAconcentration. The fluorescence efficiency is essentially dependent only on the abil-ity of the FWA to absorb light in its absorption band. However, absorption of theFWA in the violet-blue region of the electromagnetic spectrum explains most of theoften observed greening and whiteness saturation at larger FWA concentrations.These findings open for easier model parameter estimation, and for using the modelsto optimise the paper whiteness.

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