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U N I V E R S I T AT I S O U L U E N S I S

ACTAC

TECHNICA

OULU 2006

C 256

Erkki Alarousu

LOW COHERENCE

INTERFEROMETRY AND

OPTICAL COHERENCE

TOMOGRAPHY IN PAPER

MEASUREMENTS

FACULTY OF TECHNOLOGY,

DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING,

INFOTECH OULU,UNIVERSITY OF OULU

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A CTA UNIVE RS ITA T I S O

C Techn i c a 2 56

ERKKI ALAROUSU

LOW COHERENCE

INTERFEROMETRY AND O

COHERENCE TOMOGRAP

IN PAPER MEASUREMENT

Academic dissertation to be presented,

the Faculty of Technology of the Univ

public defence in Auditorium TS10

November 24th, 2006, at 12 noon

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Copyright 2006Acta Univ. Oul. C 256, 2006

Supervised by

Professor Risto Myllyl

Reviewed byProfessor Erik Vartiainen

Professor Dmitry Zimnyakov

ISBN 951-42-8213-2 (Paperback)

ISBN 951-42-8214-0 (PDF) http://herkules.oulu.fi/isbn9514ISSN 0355-3213 (Printed)

ISSN 1796-2226 (Online) http://herkules.oulu.fi/issn03

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Alarousu, Erkki, Low coherence interferometry and optical coheren

paper measurements

Faculty of Technology, University of Oulu, P.O.Box 4000, FI-90014 Univers

Department of Electrical and Information Engineering, Infotech Oulu, Univer

4500, FI-90014 University of Oulu, Finland

Acta Univ. Oul. C 256, 2006Oulu, Finland

Abstract

This thesis describes the application of Low Coherence Interferometry (LCI) an

Tomography (OCT) in paper measurements. The developed measurement syste

a profilometer and a tomographic imaging device, which makes the const

applicable in several paper measurement applications. The developed systemeasure the surface structure of paper.

Different grades of paper were selected to provide maximum variation in

results show that the developed system is capable of measuring grades of paper f

to highly coated photo printing paper.

To evaluate the developed system in surface characterization, the roughne

laboratory-made paper samples measured with the developed system and with

profilometer were compared. A linear correlation was found with roughness pa

Next, the surface quality of paper was evaluated using LCI, a Diffractive OGlossmeter (DOG), and a commercial glossmeter. The results show linear corre

gloss measured with the commercial glossmeter. The roughness Raand average

the DOG didn't give such a correlation, but a combination of these techn

properties of gloss and surface structure, which can be used to evaluate the loca

paper.

In the next study, determination of the filler content of paper using OC

measurement results show clear correspondence of the slope of the averaged log

envelope and the filler content.The last studies focus on 2D and 3D imaging of paper using OCT and beg

self-made wood fiber network. The visibility of the fibers was clear. Next, sev

matcing agents are studied by means of light transmittance and OCT measure

possible agent for enhancing the imaging depth of OCT in paper. Benzyl alcoh

the best possible combination of optical, evaporation, and sorption characteristi

2D and 3D visualizations of copy paper.

Keywords: filler, gloss, imaging, refractive index, roughness, wood fib

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Acknowledgements

The work reported in this thesis was carried out at the OptoelectronicTechniques Laboratory, Department of Electrical and Information EngTechnology, University of Oulu, during 2001-2006. I wish to exgratitude to Professor Risto Myllyl for supervising this work.

This research was done partly in collaboration with the DeparUniversity of Joensuu, and I wish to thank Professor Kai Peiponen andfor the fruitful discussions concerning paper III.

I wish to thank Professor Dmitry Zimnyakov and Professor E

reviewing this thesis. Thanks also to Mr. Keith Kosola for revising manuscript and Mr. Rauno Varonen for revising the English of the oriwish to thank the Infotech Oulu graduate school for their financial grants. Financial support from Seppo Synjkankaan tiedeedistmisti, Tauno Tnningin sti, and Kaupallisten ja Tetukisti is gratefully acknowledged.

I thank all my fellow workers at the Optoelectronics and Measu

Laboratory and in the Workshop of the Department, especially Mr. Vconsiderable mechanical work he did with the interferometer, DI Tuuassistance in most of the measurements and for the great discussionsfor developing the electronics for the measurement system, DI Tapiresearch and collaboration in paper V, and Dr. Jukka Hast for his suadvice in the early stage of this work

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List of terms, symbols, and abbreviati

2D Two-dimensional3D Three-dimensionalACF Autocorrelation functionAD Analog to digitalASCII American standard code for information interchangeBS Beam splitterDAQ Data acquisitionDOCT Doppler optical coherence tomography

DOG Diffractive optical element based glossmeterFD-OCT Fourier domain optical coherence tomographyGU Gloss unitLCI Low coherence interferometryLSQ Least squares

NA Numerical apertureOCDR Optical coherence domain reflectometry

OCT Optical coherence tomographyOLCR Optical low coherence reflectometryPC Personal computerPCC Precipitated calcium carbonatePCI Partial coherence interferometryPSD Power spectral density

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dg Geometric depth in the mediumEr Reference fieldEs Sample fieldEs Path length-resolved sample field densityf Frequencyf0 Center frequencyfd Doppler frequencyfl Focal length

i Number of data points in the transversal directionId Intensity of the photodetectorIr Intensity from the reference armIs Intensity from the sample armIsignal Signal carrying intensityk Specific absorption coefficientLc Coherence length

Lc,m Coherence length in a dispersive mediumlr Geometric length of the interferometer reference armLr Optical length of the interferometer reference armls Geometric length of the interferometer sample armLs Optical length of the interferometer sample armn Refractive indexn Number of effective reflecting layers

ng Group refractive indexR Path length-resolved diffuse reflectanceR0 Reflectance from a single sheetR Reflectance relative to a perfect diffuserRa Roughness averageRq Rms roughness averageRku Kurtosis; a distribution of spikes above and below the mRp Distance between the highest peak and the surface positiRsk Skewness; symmetry of depth distributionRt Distance between the highest peak and deepest pitRv Distance between the deepest pit and the average surfaceRz Average value of distance between the five highest peak

pits

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x Transversal coordinatey Transversal coordinateZk Surface position at transversal coordinate kZkl Surface position at transversal coordinates k and lZavg Average surface positionzr Rayleigh range

Correlation length0 Center wavelength of the light source Full width at half the maximum wavelength range of the Time delay Phase difference

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List of original papers

This thesis is a summary of the work published in the following six pap

I Alarousu E, Myllyl R, Gurov I & Hast J (2001) Optical CohereScattering Material for Industrial Applications. Proc. of SPIE 4595

II Alarousu E, Krehut L, Myllyl R & Hast J (2004) Optical cohdevice for paper characterization. Proc. of SPIE 5475: 48-55.

III Peiponen K E, Alarousu E, Juuti M, Silvennoinen R, OksmanPrykri T (2006) Diffractive optical element based glossmeter

interferometer in assessment of local surface quality of paper. Opt.IV Alarousu E, Prykri T, Hast J & Myllyl R (2004) Low Coherenc

Paper Surface and Near-surface Characterization. Proc. of ODIMA

V Fabritius T, Alarousu E, Prykri T, Hast J & Myllyl R (2006) Optically Cleared Paper by Optical Coherence Tomography. J36(2): 181-187. (DOI: 10.1070/QE2006v036n02ABEH013121)

VI Alarousu E, Krehut L, Prykri T & Myllyl R (2005) Study oncoherence tomography in measurements of paper properties. Mea1131-1137. (www.iop.org/journals/mst)

The research work described in this thesis was carried out duringOptoelectronics and Measurement Techniques Laboratory, DepartmenI f ti E i i d I f t h O l U i it f O l Fi l

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performed by the author and the paper was jointly written by the authoM. Juuti.

Paper IV deals with measuring the topography and roughness laboratory-made sheets of paper. The results measured using LCI results measured with a commercial optical profilometer. A 3D woodmeasured to show the capabilities of the OCT technique in paper imag

Paper V introduces the use of refractive index matching liquidimaging of paper. The author was responsible for the OCT measurem

was jointly written by the author and T. Fabritius.Paper VI gives on overview of the latest results and provides

capabilities of LCI in evaluating various kinds of paper surfaces, discuof measuring the filler content of paper, and finally shows the commercial paper measured using OCT.

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Contents

AbstractAcknowledgementsList of terms, symbols, and abbreviationsList of original papersContents1 Introduction .........................................................................................

1.1 Background and motivation of the work .....................................1.2 Contribution of the thesis .............................................................

1.3 Contents of this work....................................................................2 Paper structure and properties .............................................................

2.1 Geometry and dimensions of a single wood fiber ........................2.2 Paper geometry and composition.................................................2.3 Optical properties of paper ..........................................................

2.3.1 Interaction of light with paper ..............................................2.3.2 Models for simulating paper properties.................................

2.3.3 Paper gloss............................................................................2.4 Structural properties of the paper surface ....................................3 Low Coherence Interferometry and Optical Coherence Tomography .

3.1 Introduction to Low Coherence Interferometry...........................3.2 From Low Coherence Interferometry to Optical Coherence Tom3 3 Spatial resolution of an Optical Coherence Tomography imaging

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5.4 2D and 3D structural imaging of paper .......................................6 Discussion ..........................................................................................

6.1 Measurements...............................................................................6.2 Future research ............................................................................

7 Summary .............................................................................................ReferencesOriginal papers

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1

Introduction

1.1 Background and motivation of the wo

Optical Coherence Tomography (OCT) was first introduced in 1991 (HIt launched the development of the technique in various applicatiomedicine, and especially in opthalmology. OCT is based on low cohere(LCI), also known as White Light Interferometry (WLI), Optical CReflectometry (OCDR), Optical Low Coherence Reflectometry (O

Coherence Interferometry (PCI). The principle was first introduced byand later again by Fercher et al, Danielson et al., Youngwist et al., among others (Danielson et al. 1987, Fercher 1986, Fujimoto 200Masters 1999, Takada et al. 1987, Younguist et al. 1987, Kubotaimportant milestone in the development of these techniques was the

brightness semiconductor broadband light sources like superluminesdiodes (SLDs) in the early 1980s (Wang et al.1982).

As stated above, after the discovery of OCT in the early 1990s, tharea has been in medicine. Industrial applications of the OCT techniqfew, even though the first studies of its basis, LCI, dealt with testinguides (Hee et al.1995).

Because of the strong forest industry base in Finland, it was a cleathe capabilities of LCI and OCT in industrial applications There is a

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16

continued by Saarela et al.in 2000, when they did an experiment withflight in thermomechanical pulp (TMP) with a streak camera (Saarelawas measured for the first time in 2001, when Saarela et al.studied thflight to determine paper porosity (Saarela et al.2001). The first expelight interferometry were conducted in the Optoelectronics and MeasuLaboratory in the late 1970s. A commercially available white light inon a halogen light source was applied to plastic sheet thickness measur

The first experiment in which OCT was briefly applied to paper

published in 2000 (Fercher et al. 2000). In that experiment, a tunvoltage lamp was used as a light source in an OCT setup and two depi.e. a-scans, were presented from a sheet of white paper, but nothing mother group, excluding our group, has presented any results on measurOCT technique.

The combination of a non-invasive, non-contact measurement methsurface structure and internal structure of paper and simultaneous calcu

parameters was the ultimate goal of this work. The motivation camewhere there is a need for a fast and cost-effective way to study the Several conventional methods are separately used to determine the qua

1.2 Contribution of the thesis

The need to analyze and characterize paper is continually increasing inindustry. Current measurement methods tend to be either slow, labor-ior invasive. A combination of a profilometer and an imaging device wfor the researcher in the structural characterization of paper. Such a dof the strongest alternatives to conventional techniques, or it could rfuture as part of the standard methods used even in on-line quality cont

The industrial applications of the OCT technique are surprisingly

the case of paper, 2D and 3D imaging pose a great challenge becaucomplexity of paper, which unavoidably leads to optical complexicomplex photon migration in paper. The methods described in this theinternal structure of paper is visible to OCT when the sample is pretrits optical properties with appropriate agents.

The basic idea of this thesis is that a new industrial application i

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1.3 Contents of this work

In this thesis project, Low Coherence Interferometry and Optical Cohewere applied to measure paper properties like roughness and filler conthe 2D and 3D structure of paper, with refractive index matching aincrease the imaging depth of the developed OCT system, and withfree-space interferometer used as the core of the system.

Chapter 2 provides information about the structure and propertiefiber properties, paper geometry and composition, and finally optiphoton migration inside paper. This chapter also discusses different mthe structure of paper.

Chapter 3 presents the theory of Low Coherence InterferomCoherence Tomography in detail. The spatial resolution of the system

briefly.Chapter 4 introduces the measurement system in detail. The syst

four parts: (1) the interferometer, (2) an analog signal processing andata acquisition, and (4) digital signal and image processing. Ameasurement sequence, and communication between the parts are disgive an overview of how the measured signal is converted into the fina

The mesurements in chapter 5 begin with measurement of the paptopographies of various types of paper are presented. The roughnsamples are compared with values measured using a commercial op

This chapter provides an overview of the capabilites of the LCI techhighly different grades of paper.

Next, two methods, low coherence interferometry and diffractibased glossmetry (DOG), were utilized to give joint transversally loon surface roughness and gloss that helps papermakers in their researcof optimal paper surface quality, which is crucial for optimal ink

printing process. The commercial glossmeter was also used to give an

the gloss.Filler content evaluation is discussed next. Three samples with a difwere measured by using the slope of the LSQ-fitted line to determine th

The last part of chapter 5 introduces the capabilities of OCT in deter3D structure of paper. Beginning from single a-scans of paper, the image of a simulation network of paper followed by 2D images of

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2

Paper structure and properties

Paper is a stochastic network of fibers, but since the fibers are mu

thickness of a paper sheet, the network can be treated as planardimensional (Niskanen et al.1998). Fiber is one of the basic units frcomposed, and it is natural to begin the discussion of the structure individual fiber itself (Kolseth et al.1986).

2.1 Geometry and dimensions of a single wood

The dimensions of a wood fiber depend on the pulping process and rawrule of thumb is that the length of a softwood fiber is 100 times the fihardwood this ratio is in the order of 30-70. Additionally, the thicknessapproximately one tenth of the fibers diameter. However, the cross-sevary within one species, due to seasonal variations of earlywood and laalso due to age and the growing environment of the stem (Heikkurinen

The most important softwood species for the Finnish paper industry(Pinus) and spruce (Picea). These species are also the most commoworlds papermaking industry. The fibers of softwoods are cloEarlywood fibers have a thin wall and their cross-sectional shapeLatewood fibers have a thicker wall and a smaller lumen than earlywo

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Table 1.Typical cross-sectional dimensions of wood fibers (VTT Autom

Birch Eucalyptus Pine,

earlywood

Pine,

latewood

Spr

early

Fiber length

(mm)

1.1 1.0 2.9 2.9 2

Fiber diameter

(m)

22 16 35 20 3

Fiber wallthickness (m)

3 3 2.1 5.5 2

2.2 Paper geometry and composition

As stated earlier, paper can be considered a continuous three-dimen

solid material like fibers, fiber fragments, and possible fillers (Holmsthe same manner the pore structure inside paper forms a continuousnetwork of voids (Samuelsen et al.2001). Most of the fiber material i

plane of the paper. In machine-made papers, the fiber orientation is means more fibers are aligned closer to the machine direction than todirection (Niskanen et al.1998). In addition, most of the fibers lose tand collapse, leading to the flat shape of their cross-section (Carlsson e

The arrangement of the fibers in the z-direction can be layered ostructure will form if the fibers land on the wire one after another ansequence in the z-direction. In a felted structure, the sequence is n1998). Both cases are shown in Fig. 1.

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loosened (VTT Automation 2005). A common definition of fines ispasses through a 200 mesh is considered fines (Hiltunen 1999). In amount of fines is about 5-10% and in mechanical pulp, about 30% (HAn increase in fines content will improve surface smoothness and coefficient (Nesbakk et al.2001).

Most grades of paper also contain fillers, which are fine, white pigmfill in the spaces between the fibers and smooth the paper surface. Theevenness of formation, printability, opacity, dimensional stability, glo

but as a drawback, they reduce strength. Savings in manufacturintempting aspect of using fillers; filler material is cheaper than (Krogerus 1999). The most important fillers used in the papermakin(Al4Si4O10(OH)2), talc (Mg3Si4O10(OH)2), calcium carbonate (CaCO3)nH2O), precipitated calcium carbonate, titanium oxide (TiO2), and(Krogerus 1999, VTT Automation et al.2005).

Base paper forms the framework of paper. In most cases paper is co

to enhance its surface properties, like printability, appearance, and w(Aaltio 1969). The coating layer is also a complex structure; in adparticles, the coating material contains binders, thickeners, and addit2002, VTT Automation 2005). The main pigments used in coating pascarbonate, talc, and gypsum (VTT Automation 2005).

2.3 Optical properties of paper

Optical properties like opacity, brightness, transmittance, gloss, characterize the optical properties of paper (Borch 2002, Leskel 19981992). These optical properties are not discussed in detail in thiexception of gloss in section 2.3.3. Gloss and its transversal distributi

parameters compared in parallel with the surface structure measu

evaluate the surface quality of paper. In the case of LCI and OCT, features are light interaction and photon migration in paper. The cpropagation and estimation is discussed in section 2.3.1. Variationabsortion coefficients in paper are high due to the large differences in fillers and coating colors vary greatly in different grades of paper, and on light interaction in paper The models used to estimate these tw

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2.3.1 Interaction of light with paper

The interaction of light with paper is a complex process that depends ocomposition of the paper. Four types of interactions are presented in1999). Part of the light hitting the paper surface is reflected back

penetrates the sheet. After passing into paper, light can be transmittedback, or absorbed to form heat (Leskel 1998).

Fig. 2.

Interaction of light with paper.

It is practically impossible to obtain information about the actuphenomena in paper due to its complex structure and compositioncause multiple scattering events. Interferences in multiple scattering ar

either Maxwells equations or radiative transfer theory. The first chintractable because of the random nature of the paper structure and thsimplified solutions because it doesnt take into account theelectromagnetic fields, but only the superposition of scattered intensitie

Carlsson et al. have modeled light propagation through paper usimulations, but in their model the paper structure was highly simplfillers, and pores randomly appeared in the path along which the ph

(Carlsson et al. 1995). They also did experiments with time-of-fthrough paper and suggested that the path length of the tranapproximately ten times the thickness of a paper sheet (Carlssonsimplifications in the simulation models make estimation of light p

paper practically impossible, and for that reason most of the modconcentrate on giving exact information on the actual light scatter

Specularreflection

(gloss)Transmission Scattering

Absorpt(heat

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2.3.2 Models for simulating paper propertie

The Kubelka-Munk theory and its extensions are the most widely describing the optical properties of paper (Kubelka et al.1931, Kube1954). The theory was originally developed for paint films, but it w

paper. The basic idea of the model is the assumption that the interactilight and the material can be described using phenomenological paramscattering coefficient s and the specific absorption coefficient k of the from the reflectance values of R0and R, which are the reflectance and the reflectance relative to a perfectly reflecting diffuser, respe2004, Leskel 1998, Pauler 1986). A homogenous slab of even thicknoriginal model, where the material contains randomly distributed partiabsorb light, but it was later extended by Kubelka to multilayer strucKubelka 1948, Kubelka 1954, Leskel 1998). The Kubelka-Munk toptically thick materials in which multiple scattering occurs and transm

material becomes optically thick when half of the light is reflected andtransmitted. Grades of paper differ greatly in composition, so inrequirements of the Kubelka-Munk theory are not met and the theorywith caution (Pauler 1986). The fact is that no information on actuevents is obtained with the Kubelka-Munk theory. Another aspect oassumption that the material is homogenous or a stack of homogenofrom reality; spatial changes in paper properties are high due to the

added to enhance the paper properties. The Kubelka-Munk theoriaccount the scattering particle size, the refractive index, or the abs(Borch 2002).

Another model of paper was developed by Scallan and Borch (Scathis model paper is modeled as a stack of separate parallel layers th

bonded fibers. Light scattering is assumed to take place mainly inmedia interfaces. The parameters of the Scallan-Borch model are the n

reflecting layers n, layer transmittivity t, and layer reflectivity r. model is limited to sheets containing only fibers, but no additives, simplified (Borch 2002, Karppinen et al.2004, Leskel 1998).

The Scallan-Borch model was further modified by Leskel (Leskeltook into account additives like fillers and coatings in the paper model.

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(Leskel 1998). Coating color and surface smoothness are con

contributors to overall gloss. A higher coating weight usually givesgloss of coated paper is mainly affected by particle size, size distributiet al.2003). The effect of surface roughness on gloss is not so straigh

be two surfaces with different texture that have the same average rouggloss. The optical properties of surfaces and the orientation of pigcause this effect. Not only overall gloss, but also transversal gloss distappearance of paper. The distribution of coating color and local surfac

great effect on local gloss, i.e. the distribution of gloss over the measual.2000, Borch 2002).

2.4 Structural properties of the paper surfa

Surface properties like roughness are among the most important fa

quality of paper. Roughness refers to the uneven surface of paper especially important in printing papers, graphical boards, and many(Kajanto et al.1998). Surface properties must be selected according totissues, a rough, soft surface is usually desired. In printing papers, thrather smooth and homogenous to accept, retain, and present ink in ato the reader of the print (Bristow 1986).

Roughness can be divided into three categories: 1. macroroughn

microroughness (1-100 m), and 3. optical roughness (

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( )2

1

1 iq k avg

kR Z Z

i == ,

( )2

1 1

1 jiq kl avg

k l

R Z Zij = =

= .

They are obtained by squaring each height value in the data set and theroot of the mean. (Gadelmawla et al. 2002, Veeco Instruments Inc. 200

In addition to amplitude parameters Raand Rqand the parameters dof terms, symbols, and abbreviations, there are roughness parameters surface in the transversal direction. One of them is the autocorrelatiowhich describes the characteristics scale of surface height fluctuationdirection. It is a quantitive measure of the similarity between a la

unshifted version of the profile (Gadelmawla et al. 2002, VorbuWhitehouse 1994). A correlation length is used to describcharacteristics of the ACF. Its value defines the shortest distance in whto a certain fraction (Gademawlaet al. 2002). Information similar to function is provided by the power spectral density (PSD) functio

periodicity in the surface, a spike is seen in the PSD function in the cofrequency (Vorburger et al. 2002). The power spectral density fun

Fourier transform of the autocorrelation function (Bhushan et al. 1slope Sa and rms slope Sq parameters represent the slope of thassessment length (Gademawla et al. 2002). They are important paoptical point of view because they control light reflectance from the seffect on properties like gloss. There are a number of other parameterare not considered in detail in this thesis because only the main amwere discussed in the measurements.

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3

Low Coherence Interferometry and O

Coherence Tomography

3.1 Introduction to Low Coherence Interfero

Interference is the superposition of two or more waves, resulting in aWhen two light beams are combined, their fields add and producecoherence interferometry measures the field of an optical beam rathe(Fujimoto 2002). A simplified schematic of a low coherence interferFig. 3.

Lightsource Sa

Referencemirror

Lr = 2*n*lr

Ls = 2*n*ls

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The principle of low coherence interferometry can be analyzed in term

two-beam interference for partially coherent light. The light field fromcoherent but high spatially coherent light source is directed onto adivides the beam into a reference beam and a sample, i.e. a mAssuming that the sample in Fig. 3 is a perfectly reflecting mirror aeffects of light are ignored, the scalar complex functions ( )c/LtE ss represent the sample beam reflected from the specimen under mereference beam from the reference mirror, respectively. sL and rL ar

optical path lengths of the arms of the interferometer and c is the spassume that the photodetector collects all the light from the arms, the

dI can be written as

[ ][ ]++++= )t(E)t(E)t(E)t(E)(I rsrsd ,

where the angular brackets denote the time average over the integdetector and c/L= is the time delay corresponding the round-trip difference between the two arms, i.e. ( )rsrs lln2LLL == , and means the complex conjugate operation. 1n is the refractive index

rl are the geometric lengths of the arms. Because intensities Is( ) ( )++= tEtEI rrr , the resultant intensity becomes

( )( ) 2 Re

d s r s r mc

I I I I I V = + +

,

where the first two components sI and rI are the detected intensities bsample and the reference arm, respectively, and the last term gives thinterference fringes, which depends on the optical time delay set by reference mirror and which carries the information about the structure equation is termed as the generalized interference law for partially c

normalized mutual coherence function ( )mcV

in equation (6) gives tthe temporal and spatial characteristics of sE and rE match, and it can

( )( ) ( )rs

mcII

tEtEV

+=

.

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( ) ++=

0tcrsrsdf2cosVII2II)(I .

According to the Wiener-Khintcine teorem, the temporal coherence actually the Fourier transform of the power spectral density ( )fS owhich is fully characterized by its shape, its spectral bandwidwavelength (Akcay et al.2002, Gilgen et al.1989, Schmitt 1999, Wan

( ) ( ) ( )df2jexpfSV0

tc

= .

This relationship reveals that the shape and width of the emission spsource are important variables, because they influence the sensicoherence interferometer to the optical path difference of the samplearm. The equation (9) can be rewritten in a form that gives the intensi

of L

( ) LkcosLVII2II)L(I 0tcrsrsd ++= ,

where 00 /2k = is the average wave number and the relation 0 =to transform from the time domain to the path domain (Pan et al.1995

In the derivation of equation (11), the sample was treated as a pmirror that induces a time delay, but leaves the amplitude and coherbeam unchanged. In reality, this is an unrealistic situation. Actually,from the sample can be categorized into least backscattered light,undergoes only single scattering or very little scattering, and diffuse bthe light has scattered numerous times. In principle, least backscatteredcoherence and multiple scattered light loses it. If we take into accountinside the media, the total round-trip path length of the sample arm is

's0ss LLL += ,

where 0sL is the round-trip path length to the sample surface and'sL is

th l th i id th l i

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( ) rL

ss'sr

L

ss'sd )t(EdL)L,t(E)t(EdL)L,t(EI

0s0s

++

++=

where )L,t(E s's is the path length-resolved field density. Equation (14)

( ) ( )

++= LdLkcosLVLRII2II)L,L(I 0tcsrsrsrsd ,

where ( )sLR is the normalized path length-resolved diffuse renormalized derivative of the intensity depth distribution of the representing the fraction of power reflected from the layer located at the object. The signal-carrying interference term, i.e. the interfere

equation (15), can be expressed as a convolution

( ) ( ) ( )rssrsrssignal LLCLRII2L,LI = ,

where ( )rs LLC is the coherence function, i.e. the interferometer recase with a mirror in both arms multiplied by the cosine term. It is spread function PSF of the light source. denotes the convolution

conclusion, low coherence interferometry traces out variation in the preflectances defined by ( )sLR (Kulkarni et al. 1997, Pan et al. 199Schmitt 1999, Yung el al. 1999). In the case of a turbid media likdescription of the path length-resolved diffuse reflectance includes all in the time window defined by the coherence length of the light souruseful signal is masked by the diffuse component creating signal-which are formed by the out-of-focus light that scatters multiple times

the time delay set by the difference between the optical paths in thinterferometer (Schmitt et al. 1999). The effect of the diffuse compreduce it in paper measurements are discussed later in section 5.4.

The signal-carrying component is typically separated from dc boptical time delay between the arms by translating the reference m

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transimpedance amplifier, (2) a bandpass filter centered at fd, and

demodulator to extract the envelope of the interferometric signal (Hee As stated earlier, ( )rs LLC is the function which determines

response to path length-resolved reflectance variations. This means that the detector only when the reference arm distance matches the opreflective path through the sample within the coherence length of the is typically around 10 m. The coherence length is the parameter whresolving ability of backscattering or reflecting sites inside the samp

resolving ability is discussed in more detail in section 3.3.

3.2 From Low Coherence Interferometry to O

Coherence Tomography

Being a one-dimensional ranging technique, however, Low Coherenceits limitations. This is where Optical Coherence Tomography comes capacity to reconstruct cross-sectional images of an object from its prtomography is used whenever two-dimensional data is derived from aobject to construct a slice image of the object's internal structure.

parallel LCI scans are performed to generate the two-dimensional imsimilar to ultrasound B-mode imaging, except that infrared light acoustic waves are used, and the achieved axial resolution is up to

ranging from a few tens of micrometers to less than one micrometer. domain techniques, optical coherence tomography can be performewave light without the need for ultra-short pulse laser sources, which the laboratory environment (Ballif et al. 1997, Ettl et al. 1999, Ferc1999, Tearney et al.1997, Yoshimura et al.1995).

A typical measurement system consists of a Michelson interferomelow temporally coherent light source, such as a superluminescent d

object to be measured is placed in one arm of the interferometer. A memitted by the light source is reflected or scattered from the object wtimes, depending on the various optical properties of the differentobject. A longitudinal profile of reflectivity versus depth is obtainedreference mirror of the interferometer and synchronously recording th

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3.3 Spatial resolution of an Optical Coherence Tomimaging system

The spatial resolution of an OCT imaging system can be divided into resolution and the transverse resolution, which are independent fromaxial resolution depends on the characteristics of the light source resolution is determined by the optics of the imaging device. I

autocorrelation function of the OCT device has a Gaussian shape. emission spectrum of the light source, which in most cases has a resembles it. In the case of a Gaussian-shaped autocorrelation functlength cL is

=

20

c2ln2

L ,

where is the FWHM (full width at half maximum) wavelengthsource (Brezinski 1999). cL is a common value for the axial resocoherence length, the equation above is the most used. The definition a bit confusing. The coherence length in equation (18) is the round-trip

but it is also adopted as a definition for the longitudinal coherencactually twice as large as the value given by equation (18) (A

Baumgartner et al.1997, Baumgartner et al.1998, Fercher et al.1999,2002, Laubscher 2004, Tomlins 2005, Zhang et al.2001). In papers IIvalue for coherence length was adopted, but then it was mentioned ththe system is then described by dividing the coherence length by 2.Tvalid only in a vacuum. The usual case is that the sample is dispersiverefractive index of the sample material depends on the wavelensignificant when broadband sources are used (Sampson 2004). Thcoherence interferometry are optical distances. This means the geoderived by dividing the optical distance by the group refractive index n

=

d

dnnng .

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where gd is the geometric depth in the medium (Drexler et al.1998,

1998, Hitzenberger et al.1998, Hitzenberger et al.1999).As stated earlier, the transversal resolution is determined by the op

device and doesnt correlate with the axial resolution. The selection ooff between the transversal resolution and the imaging depth rangefocus. The transversal resolution x can be written as

= d

f4

x

l

,

where lf is the focal length of the focusing lens and d is the light belens aperture. It can be seen from equation (21) that a large numericala small spot size and a high resolution, but another aspect which haccount is that the imaging depth range, i.e. two times the Rayleigshorter. The Rayleigh range rz is

=4

xz

2

r .

The Rayleigh range gives the distance from the focal plane to the pobeam diameter has increased by a factor of 2 (Clements 2004, Drex2002). In most biological applications, the imaging depth should be

scale, but in the case of a paper sheet, there is no need for such high ima high NA can be used.

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4

Measurement system

A schematic of the free-space OCT setup used in the experiments is sh

experimental model is highly modifiable and can be used for sevpurposes: LCI, OCT, and DOCT. The main drawback of the devicemoderately skilled operator to change the procedure of the imaging sspace model is introduced in detail in the next subsections.

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procedure is described in subsection 4.3. Subsection 4.4 describes

signals are converted to images and gives an overview of the colorimages.

4.1 Interferometer

The schematic of the experimental measurement setup presented superluminescent diode (1Superlum Ltd. Model SLD-380-MP3-TOWLtd. Model SLD-380-HP2-TOW2-PD) as a low coherent light soemission power of 18.25 mW / 250 mW, the diode illuminates interferometer with a free-space configuration shown in Fig. 5.

Fig. 5.Free-space OCT measurement setup.

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According to equation (21), the size of the focused beam (defined by

the Gaussian beam) is then ~8.5m, which defines the transversal resothe arms, the reference arm, contains an axial scanner for prodmodulation, and it shifts the signal to a Doppler frequency that depenthe scanner. Scanning can be performed by either a piezoelectric instrumente P-783.ZL PZT element with an E-662.LR controller) o

Newport VP-25XA servo stage with an ESP300 controller). For linearity, a piezoelectric scanner is used in a closed loop mode. The ob

is placed in the other arm, the measuring arm, which contains steppstages (Newport DM11-25 actuators with an ESP300 controller) for sin transversal directions to achieve 2D and 3D images. All the stacontrolled and synchronized with each other via an RS232 or a GBIPand scattered beams are focused on the detection arm with a plano coGriot 06LXP007/076) and combined on the silicon photodetec13DSI005) to produce an interferometric signal containing informationstructure of the sample. This particular detector has a responsivity wavelength of 820 nm.

4.2 Analog signal processing and control u

After detection of the signal in the interferometer, the signal

transimpedance amplifier, which is designed and optimized especiallyby using appropriate, low-noise operational amplifiers. The architecstage transimpedance amplifier ensures a high signal-to-noise ratiostrong signal already at the output of the first stage without unnTransimpedance can be adjusted from 1 to 2 M by using an 8-posfront panel of the analog signal processing unit shown in Fig. 6.

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The task of this filter is to remove all the unnecessary noise coming

sources, vibrations, and the electronics itself. The frequency and bandsignal are closely related to the parameters of the light source and reference mirror. The signal of this output (OCT OUT) is used whenevamplitude and phase are needed.

The third ouput (DOCT OUT) is used when Doppler measuremeThis output stage also has an 8thorder Butterworth filter, but it also

bandwidth, which allows us to measure the velocity of flowing/moving

There is usually no need to record the phase of the interferometric smagnitude of the signal is recorded. That is also a practical way to refrequency and data storage space needed. The fourth output (ENVELOthe envelope of the fringe signal by using a demodulator that consistsa precision rectifier and a lowpass filter. With the rectifier sub-block, mVp-p can be rectified without significant distortions. The second sub-filter, is of the third order Butterworth type with the cutoff frequencremove the carrier frequency but leave the envelope undistorded.

The fifth output of the unit is the logarithmic envelope outputOUT), which amplifies the envelope signal logarithmically, which isdetecting signals with a very wide dynamic range. This was the output measurements made during this thesis project.

The internal sweep generator in the E-662.LR controller was flinearity and stability when the speed was increased. The bandpass of centered at 9.2 kHz, which corresponds to scanner speeds of 3.78 mm

with 822 nm and 832 nm light sources, respectively. An external swecontrol signal generator, was constructed to give a nice slope for trepetition rate, slope, and amplitude of this control signal can be adju

panel of the sweep generator unit. The control signal is then amplifiewhich drives the PZT element and follows the movement by means (Krehut 2003).

4.3 Data acquisition

Data acquisition is performed by using a National Instruments (NI PCIwith a 12 bit AD converter connected to the PCI bus of the PC The m

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controls the triggering of the depth scanning start pulse, which is g

generator of the PZT control unit.3.

The trigger pulse causes the sweep generator to give a sweep controller, which amplifies the control signal to the desired level othe same time the Labview program starts to acquire the signal of of the analog signal processing and control unit. Samples are taksampling intervals, so that the total amount of samples covers thrange of the PZT element.

4.

After the scanner has reached its maximum displacement, it is retuposition by the sweep generator and the sequence is started again fr

The acquired signals are saved in ASCII or binary format. The ASCII early measurements, but the binary format was adapted later because of saving. Thus there is a need to convert it to ASCII later for data proc

4.4 Digital signal and image processing

The process of signal to image conversion is quite a straightforwaramount of processing depends on the type of image formed. Three typused in this thesis project: topography, tomography, i.e. 2D images, anof these needs a slightly different procedure. All the processing was pMatlabTMprogram.

The first of these, the topography image, is the easiest one to form;save the whole measurement vectors in LabviewTM, but only the m

position originating from the surface of the sample, i.e. from the first aIt means that a matrix of these values is measured and then combinform a 3D matrix that gives us surface z-positions as a function of traThe set of position values is not very informative when we discussthus some processing of the data set is needed. MatlabTM has bu

combining a set of data points to form a surface. Every data pointmarked with a different color depending on its depth value. The mmaps are grayscale and false color maps. The grayscale map containsof gray levels from white to black. However, the human eye is noseparate a number of gray levels, which sometimes makes the

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The second option, the tomography image, gives a 2D slice ima

internal microstructure. The whole measurement vectors are now saveis no need for further processing of separate signals in Matlablogarithmic envelope output of the analog signal processing and conused., MatlabTMalso has its own function for slice images, called imatransforms the z-data to pixels. Each pixel has its own color selected used. The color depends on the pixels amplitude, i.e. the amplitudnumber of these vectors are placed in parallel so that they form a matr

Color map scaling and thresholding are the only actions done as poimages. The resolution of the 2D slice images is determined by the fthe coherence length of the light source.

The last option, a 3D image, gives the complete structure of thdimensions. This image type is the most complex to form and needs a to handle all the data. Also in this case the whole measurement vectthe logarithmic envelope output of the analog signal processing and coset has four values for each pixel; a position in three dimensions x, y, amplitude value at that particular position, and it can be stated as a vMatlabTM function called isosurface connects the adjacent points imatrix if their amplitude value crosses the specified value defined which in other words means the selection of a ROI (Region of Inteasily understood by taking a 3D image of a round tube as an exampimages of this tube, we have a number of parallel 2D images whishaped cross-section of the tube. Selection of a ROI is performed b

image data so that the cross-section is separated from the surrounding mof these cross-sections are connected by a surface, and we get a 3D object. An isocaps function in Matlab computes the isosurface end cavolume data, which in the case of a tube corresponds to the images tube. For 3D images, color map scaling and 3D smoothing are usuenhance image quality. The resolution of the 3D images is determinoptics and the coherence length of the light source.

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5

Low Coherence Interferometry and Optical

Tomography in paper measurement

This chapter presents the results of using Low Coherence InterferoCoherence Tomography for paper measurements. In section 5.1, LCI the surface structure of paper. Some of the roughness values are cvalues measured with the commercial profilometer. These measureme

papers II, V and VI. In section 5.2, the surface structure, roughness, local gloss of paper surfaces are discussed in parallel. A detailed analy

paper III. Section 5.3 gives an example of determining the filler c

using OCT, introduced originally in paper VI. Sections 5.4 and 5.5 imaging of paper, respectively. The results are collected from paperExcept for the measurement of the line profiles in Fig. 7 and 8 and thethe Superlum Ltd. model SLD-380-HP2-TOW2-PD was used in all the light source of the interferometer.

5.1 Surface analysis of paper

Several methods and standards are used to analyze the surface strmodern paper research and in-line quality control processes. Neverthemethods are non scientific; they all yield slightly different values and

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measure the roughness of paper samples: 1. the Bendtsen air-leak m

hard ring is pressed on top of the paper surface, and the ensuing air lthe ring is measured in unit time, and 2. the PPS method (Parker Pwhich uses a soft ring instead of a hard one and responds remarksurface imperfections.

However, air-leak methods are prone to failures, and paper sheetpenetrating through them which, naturally, affect the measurement reshappen with LCI. Moreover, LCI measurements do not require m

simultaneous imaging of the surface structure and measurement of rouAs a result, both time and money are saved.All the experiments presented in this section were measured using

in section 4.1. The surface position was determined by recording the mposition originating from the surface of the paper sample.

In the first experiment, two fine paper surfaces were measured toroughness parameters Ra and Rq. The first of these, code HW 75/25manufactured in a test paper machine by Metso Paper. It contains 75and 30% PCC (CaCO3 precipitated calcium carbonate) filler, andtypical commercial copy/printing paper whose exact constituents wstep increment between the transversal pixels was 1 m. The sumeasured from the samples are presented in Fig. 7 and 8 (Paper II).

Fig. 7.Surface line profile of fine HW 75/25 F30 fine paper.

-80

-60

-40

-200

20

40

0 100 200 300 400

Lateral position [m]

Depthposition[m]

20

40

[m]

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The surface roughness parameters Ra and Rqfor the line profiles in Fi

be calculated using equations (1) and (2). They give Ra=4.6 m and R75/25 F30 paper and Ra=5.5 m and Rq=6.9 m for copy paper. Whfigures above, there is a clear difference in surface line profiles. The the roughness values is yet only a scant micrometer, but the differencwhen the paper surface is physically touched. This is because, in theabove, we deal with local roughnesses, i.e. short transversal distancstructural changes in paper in the transverse directions. In the followin

measurements, a larger area was measured to give a more represensurface and a value for roughness.As stated earlier, LCI can be applied to various grades of paper to m

structural properties. In the next experiment, three widely disparate grmeasured to demostrate the capabilities of the LCI technique in visustructure of various types of paper and in calculating the correspondinaccording to equations (3) and (4). Fig. 9-11 show the results of thsizes of the images are 500 x 500 pixels with 5 m transversal stepyields 2.5 x 2.5 mm images. The smaller images in the top right cornex 0.5 mm images from the top left corner of the samples. Fig. 9 showa laboratory-made paper sheet. As seen, the sample containing long piPCC filler is quite rough; Ra=6.9 m. Furthermore, it is an uncosample with low gloss and high porosity. The visibility of individ

because no machining has been performed on the surface and no coabeen used to smooth it.

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The second sample, HW 75/25 F30 fine paper, is actually the same u

experiment presented in Fig. 7. The topography of this sample is scomparison with the laboratory-made paper, this sample is auncalendered. Other characteristics include moderate gloss and porosiof Ra=4.6 m. Manufactured in a test paper machine by Metso Paper, individual fibers is quite poor, due to manufacturing processes such addition of a fairly large amount of filler in the pulp.

Fig. 10.

Topography of an uncoated fine paper sheet. The size of the largmm and the smaller one is 0.5 x 0.5 mm.

The third sample, presented in Fig. 11, is also an uncalendered fine pawith high gloss and low porosity. It is highly coated to smooth theprinting and has a roughness of Ra=0.8 m. As in the case of HW 7visibility is more or less zero because of the coating on the surface. Thof filler in this sample were unknown.

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Fig. 11.

Topography of a coated fine paper sheet. The size of the large imand smaller one is 0.5 x 0.5 mm.

In the next experiment, five laboratory-made sheets of paper were m

The roughness parameters were calculated and the results were reference provided by the Technical Research Centre of Finland (VTcontains surface topografies and roughness parameters measured wAltisurf 500 optical profilometer. The size of the measurement beam osample is 1 m and it gives 10 nm depth resolution. The aim of this verify the results achieved by the device described in section 4.1.undergone a slightly different calendering method and filler properties

3 (VTT Processes 2003).

Table 3.Samples.

Sample Filler content C

A 0% un

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Fig. 12.

Topography of sample D.

x-position [mm]

y-position[mm]

0 0.5 1.0 1.5 2.0 2.5

0

0.5

1.0

1.5

2.0

2.5

y-position[mm]

0

0.5

1.0

1.5

2.0

2 5

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of them differ slightly and some of them are completely different.

differences can be the difference in the measured area. In the case of Lmm, and in the case of the Altisurf 500, it was 4 x 4 mm. Such an areour system.

Table 4.Roughness values for LCI and an Altisurf 500. The values are

Sample

A B C D

LCI

Ra 7.83 5.76 5.78 6.85Rq 9.92 7.44 7.41 9.32Rp 27.20 57.20 23.90 92.40Rv 58.30 74.10 75.70 64.30Rt 85.50 131.00 99.70 157.00Rsk -0.79 -1.04 -0.96 -0.26

Rku 3.99 5.47 4.69 8.65Rz 80.80 88.80 78.90 144.00

Altisurf

500

Ra 5.48 4.30 4.40 7.27Rq 7.36 5.86 6.46 10.72Rp 27.65 45.16 180.60 161.00

Rv 64.37 51.76 44.42 73.27Rt 92.02 96.92 225.00 234.20Rsk -1.69 -1.56 0.94 0.57Rku 7.27 7.31 38.02 13.65Rz 84.91 86.91 186.60 212.30

Table 5 gives us the linear correlation coefficients for all the roughnesbe seen that the parameters Ra and Rq have the highest correlation

correlate more or less badly. The differences in correlation can bedifferences in the focusing geometry of the measurement devices.

Table 5.Correlation coefficients for roughness values measured by L

500.

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for Rv. It is very likely that this focusing geometry lies at the root of

non-correlations, as well. As an example, take the parameter Rsk; if ththere are more peaks than pits on the paper surface, if the value is more pits. Consequently, because the Altisurf is better at detecting hig

pits, the Rsk value is in most cases positive compared with the valLCI. The same reason causes a larger value for Rkuwith the Altisurf, narrow peaks on the surface.

But, if we compare the results sample by sample by taking in

roughness parameters, we get interesting results. In table 6, thecoefficients are calculated for each sample.

Table 6.Correlation coefficient of all the roughness parameters measu

an Altisurf 500.

Sample

A B C D

CorrelationCoefficient

1.00 0.98 0.73 0.99

The above table reveals that although the correlation of single roubetween the samples is poor, excluding Raand Rq, the relationship of tthe same sample measured using LCI and the Altisurf 500 shows nivalue for sample C stays unknown.

5.2 Surface quality evaluation of paper using LCI a

Micro surface roughness and related gloss have been observed to be imin paper science (MacGregor 1994). In this section, two methointerferometry and diffractive optical element based glossmetry (DOGgive joint information on both surface roughness and gloss that heltheir research and development of optimal paper surface quality, woptimal ink absorption in the printing process.

Micro surface roughness of paper has an important role in thUnfortunately, commercial glossmeters do not provide information on

I th t i t LCI l d t th

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offset printing. Typically the filler used in SC paper is caoline.

supercalendered fine paper, was coated and different coating pigmentypically used. This paper product usually provides a glossy surfacequality. The supercalendering process tends to smooth the surface formation of mirror-like facets. As a rule of thumb, the smoother the

better the printing quality, which can be distinguished, e.g. in the reproThe samples were marked with needle holes so that the same area w

LCI and a DOG. The 3 x 3 mm areas were measured in 15 m transver

with both devices. In addition to LCI and a DOG, gloss was alsocommercial Zehntner ZGM 1020 Glossmeter with two angles of incidAll the gloss measurements were performed at the University of Joensu

Fig. 14 gives an example of one of the topographies measutopography of a Xerox copy paper sample. The corresponding gloss mPaper III, Fig. 6. A direct comparison of topography and gloss maps wthis study. Surprisingly, the roughest paper gives the lowest variation iis clearly seen if we compare the topographies and gloss maps of fin

paper samples. This effect is invisible if only the average values of rare used, and it leads to a conclusion that local variation in surfacimportant factor when surface quality is evaluated. All the topographcan be found in Paper III. The gloss and roughness values, with corrdeviations calculated for these samples, are shown in Table 7. Holes wcalculation of roughness values. Fig. 15 shows the normalized glosDOG and a Zehntner 1020 with two geometries as a function of roughn

2

3z

-position[m]

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Table 7.Averages and standard deviations of gloss measure with a

Zehnter 1020 (GU Gloss unit), and roughness measured using LCI (

Paper

sample

G G GU20 GU20 GU60 GU60

Fine 1.03 0.04 4.80 0.72 32.80 2.27

SC 1.08 0.03 2.56 0.15 13.10 0.86

Xerox 0.87 0.01 1.50 0.01 3.30 0.11

Fig. 15.

Normalized gloss G, GU 20, and GU 60 as a function of roughne

The Zehntner values GU20 and GU60 give nice negative linroughness; the correlation coefficients are -0.9994 and -0.9997, respeDOG value G, the correlation coefficient is only -0.61. It can be seenand Fine papers have large variations in local gloss measured with the

that the surface roughness of paper is not the only dominant factor affcant be used alone to predict gloss variation on the surface. Variatioindex due to the fillers and surface coatings can cause large differencecan be seen in Fig. 15, G seems to be less sensitive to roughness changGU60 Thats probably caused by the differences in measurement

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7 8

Ra [m]

Normalizedgloss[a.u.

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5.3 Filler content evaluation of paper

Fillers are added to paper mainly to improve printability, but their inendows other benefits. Thus, when machine-made paper started to rather than the number of sheets, as previously, cheap fillers camreasons for using fillers in printing paper include improving their opacto absorb printing ink, as well as surface smoothness and pleasantness,feels in ones hands. There are also drawbacks; fillers lower the s

properties of paper, for example (Aaltio 1969).Measuring the filler content of paper relies on the fact that the scat

pulp and paper change with the addition of filler. The filler actuabetween fibers, increasing the refractive index of the fibers surroundithe mismatch between the refractive indices of the different scatteringrefractive index of fiber is assumed to be 1.55, which is the index foindex of air is 1. Secondly, increasing the filler content makes

aggregate, which makes the fillers less effective in scattering lighConsequently, if the scattering coefficient of the investigated sampchanges in the filler content, the slope, i.e. the reflectivity versus depthsignal will show a corresponding change. In a highly scattering mescattering coefficient >> the absorption coefficient, and the scatteringevaluated using Lambert-Beers law, which gives the exponential dfunction of depth. When using this formala as a basis for measuring t

paper, one must choose the linear part of the signal where the detectedbe single-scattered. The maximum depth of determining the fillerapproach depends on the optical properties of the paper. This methodabsolute value for the filler content, and only relative changes can be m

To prove this assumption, three series of laboratory-made pameasured. These samples were made of unground pine pulp with a fi15% and 30%, respectively. Precipitated calcium carbonate (PCC) wamaterial, while Percol was employed as a retention chemical. In

content is defined by the contribution it makes to the dry weight of ttotal weight (Saarela 2003).

Two measurement sequences were performed for each sample. Firsrecorded in 2 m transversal step increments in the x-direction. A secperformed by moving the sample 0 5 mm in the y direction and agai

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be determined, the experiments do corroborate one fact: the effect o

obvious, indicating that the refractive index mismatches have indeed din smaller LSQ slope values.

Fig. 16.Example of slope determination using the LSQ line fitted into thethe logarithmic envelope output of the amplifier.

Table 8.Slopes of the LSQ lines.

0% filler

Sample no. (a.u.)

Slope (a.u.) 15% filler

Sample no. (a.u.)

Slope (a.u.) 30% f

Sample n

-8.09 -7.08 0/8

-8.05

15/3

-7.03

30/

-8.24 -6.45 0/10

-8.17

15/5

-6.47

30/

-8.20 -6.16 0/11-8.21

15/6-6.21

30/

-7.23 -6.43 0/12

-7.27

15/8

-6.43

30/

Average -7.93 -6.53

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 50 100

Optical depth [m]

Amplitud

e[V

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Fig. 17.

Normalized average value of slopes of the LSQ lines with stanfunction of filler content.

5.4 2D and 3D structural imaging of pape

As stated earlier, paper consists of a stochastic network of fibers, buare much longer than the thickness of the paper sheet, the network

planar and almost two-dimensional. This two-dimensional structurdetermine a number of parameters, and indirectly it even reveals sothree-dimensional characteristics. However, to improve paper qualityknow its three-dimensional porous structure (Niskanen 1998). Hitherstructure of paper has suffered from an absence of nondestructive, fast

measurement techniques for micro- and macrostructure imaging. Tradbased on ultrasonic, X-ray, magnetic resonance imaging (MRI), microscopy (SEM), and conventional light microscopy techniques (Het al.2001, Samuelsen et al.2001). A problem in many of these techthe measurement event itself; either the measurement is very slow and

Filler 0%

Filler 15%

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 10 20 3

Filler content [%]

Nor

m.slope[a.u.]

51

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paper, results in coherent multiple-scattering processes that degrade re

contrast (Bestemyanov et al.2004, Zakharov et al.2002, Zimnyakov eThis section introduces the use of OCT for 2D and 3D structural cimaging of paper. The first experimental measurement results of usingimaging of paper are introduced in Paper I. A 120 m thick paper samto give a view of the OCT a-scan of paper. Before these results, ointroduced few OCT a-scans of a sheet of paper in 2000. The rthickness was measured using a Lorenzen & Wettre Micrometer 51. Fiscans from a 120 m thick paper sample. The transversal step incremscans was 20 m.

Fig 18 A scans of paper

-50 0 50 100 150 2000

0.05

0.1

Depth [um]

Voltage[V]

-50 0 50 100 0

0.05

0.1

Depth [um

Voltage[V]

-50 0 50 100 150 2000

0.05

0.1

Depth [um]

Voltage[V]

-50 0 50 100 0

0.05

0.1

Depth [um

Voltage[V]

52

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other transversal position, the results would have been completely diff

cause for these peaks is random noise caused by speckle modulation and they can probably be made to disappear by averaging a sequence oobtained inside a small area in the zone of interest.

A paper sheet is a 3D network of fibers, fines such as fiber fragkinds of additives. Since this kind of network is optically very comp

begin with a controlled simulation network, with fibers as the only scexperiment, a 3D wood fiber network was constructed to simulate theA bleached pine pulp solution was spread on top of a smooth glassroom temperature for about 24 hours to obtain a completely dry saamount of data as small as possible, a fairly low sampling frequencydata was decimated before 3D transformation. Fig. 19 presents a 3D simulation network.

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The size of the image is 2 x 2 x 0.075* mm (*optical), and it was con

OCT slice images with 10 m transversal step increments. In additislightly smoothed using 3D Gaussian smoothing. A detailed data analformation was described in section 4.4. The image reveals that 3D ifiber network is possible even with a conventional OCT system withoof the sample. This is because the diameter of the fibers (30 m - 40the axial and transversal resolution of the system, and only fines like

present in addition to pure fibers. However, problematic with this kiand processing of 3D images using a regular PC is that the proceamount of time for processing, even after the slight decimation of thehas to be pointed out that this network is still quite different from a rwhich contains more densely packed fibers and variable additives. Bthat the first layers of the paper surface are visible to OCT if the plainscatterers. However, thats not a very common case in real commerciMost paper surfaces are calendered and coated, which changes the strone that is more complex for OCT imaging.

In the next experiments, typical copy paper was measured usresearch groups have corroborated the fact that OCT is incapablestructure of paper, such as copy paper, beneath the surface through maIt is partly true if no preparation is performed before the measurementcontains about 20-25% filler and its densely packed fibers and fines plight scattering. Nonetheless, there is a way to make the structure visible through measurements performed only on one side.

Several so-called clearing agents, i.e. refractive index matching agto paper to enhance its optical properties. One of the most applicablewhich fills the pores within the paper structure effectively andFurthermore, its refractive index, 1.54, is close to the refractive indexwhich is the main construction material of paper fibers. The effect oindex maching liquids on paper is discussed in Paper V. In this paper, the best possible liquid for this purpose and to demostrate experim

medical applications, the addition of an appropriate refractive indereduces the effective scattering coefficient and improves the probaphotons carrying important information about the inner structure of(Zimnyakov et al.2002).

The light transmittance measurements introduced in detail in Pape

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The effect of refractive index matching is demonstrated in Fig. 20, w

of the imaging depth was tested with dry paper and then by wetting the1-pentanol and benzyl alcohol. The reconstructed cross-section odemonstrates a case where the imaging depth of OCT is not adequatdepth of the OCT system was limited, the reconstructed cross-section pseems to be cut away (a). In reality, however, the signal will fade awaas in the sample representing 1-pentanol (b), where the bottom surfunclear. Although paper wetted with 1-pentanol transmits much m

paper, the imaging depth is still insufficient. A proper image of the shecan only be obtained with benzyl alcohol (c). Reflectance from thsurface of a paper sheet wet with benzyl alcohol was equal. Liabsorption, however, attenuates the reflected intensity from the bottom

Fig. 20.

Reconstructed 2D cross-sections of the paper samples: (a) dry pa(b) 1-pentanol and (c) benzyl alcohol. The pixel size of the images is 15 mtransversal and depth directions, respectively. The black arrow points tolimit of the measurement system used. The white dashed line depicts an esborder of the paper.

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measuring period. A droplet of benzyl alcohol was placed on both s

sheet, which was measured right after this liquid exposure. Fig. 21 pof this sample from the front side, while Fig. 22 shows an image from

Fig. 21.3D image of a copy paper sample (front). The size of the image ismm (*optical).

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The sizes of the images are 0.75 x 0.75 x 0.16* mm (*optical), and the

using 50 OCT slice images with 15 m transversal step increments. Hwas measured only from the front side and then flipped relative visualization program to show the back side. In addition, the imsmoothed using 3D Gaussian smoothing. A detailed data analysformation were described in section 4.4.

It is obvious that single fibers are not visible on the surface, duecontent, soft calendering, and the use of benzyl alcohol to fill surfa

Nonetheless, the back side of the sample can be made visible withoutthe sample. A Lorenzen & Wettre 51 standard thickness measurementhickness value of 102 m for the sample. Using OCT, the average othe sample was 158.8 m, corresponding to a physical thickness of 10

porosity of 50%.The experiments reported in this section suggest that optical coh

has the potential to become a new key method in paper characterizatiois a viable alternative to conventional methods and has the capacity to

information related to paper quality. In addition to surface characteristructure of paper can be imaged optically in a non-contact manner.complex process, optical imaging of the internal structure of pachallenge for research and development. The results presented here aris suggested that OCT can be applied to paper structure imaging. Foand 3D images of pulp and paper presented in Fig. 19-22 can be grusing a light source with a broader spectral bandwidth together wi

processing algorithms, such as the deconvolution method and Kalmaal. 2004, Izatt et al. 2002). As stated earlier, imaging paper necessiagent with a certain set of characteristics. Of particular interest herrefractive index, and chemical composition of the solution, because it and pits of the paper sheet without penetrating the fiber itself.

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6

Discussion

The need to analyze and characterize paper is continually increasing in

industry. Current measurement methods tend to be either slow, labor-ior invasive. The aim of this work was to explore the capabilites interferometry and optical coherence tomography in paper measuremidea was to provide a new method for on-line measurement of paper thnoticed at a very early stage that it was unrealistic to realize this idea wIt is a fact that unprepared paper presents such a challenge for opneither LCI nor OCT can give the desired result if the complete shefrom one side. There are several optical profilometers commercially

surprising that in the modern paper industry, old-fashioned methods asurface quality. Nevertheless, some of the methods are non-scientslightly different values for roughness, and they typically require a ndevices to cover all types of paper.

The combination of a profilometer and an imaging device construction used in this thesis project attractive. Commercial opticaaccurate devices for giving a nice topography of the sample and for

roughness parameters. This is where OCT comes in, by offeringreconstruct 2D and 3D images of papers internal structure. As stated straightforward process, and some preparation must be carriedmeasurement.

At the beginning of this project a multipurpose OCT device was co

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maps, and average gloss provides more information for the paper re

surface parameters and how they affect printability, for example.2D and 3D imaging of paper poses a great challenge for any imespecially for OCT, which has been used in medicine for years. Surpthere are very few industrial applications. Of course the optical comextremely high, which has slowed development of the OCT techniquapplication. But, as the experiments done in this thesis project indicatmake the paper structure visible to OCT.

6.1 Measurements

The first studies focused on surface analysis of paper. The surfaccorresponding roughness values of three different grades of paper pregive a nice overview of the capabilities of LCI in measuring severa

Grades of paper from rough base paper to smooth photo printing papeusing the same setup. Compared with conventional air-flow methods, wdevices are needed to cover all types of paper, LCI can offer a cost-eand in addition has less error sources present in the measurement event

The next set of samples was measured to compare roughness valuLCI with roughness values measured with a commercial profilocorrelation coefficient was acceptable only with roughness parametersother parameters correlated more or less badly. The noncorrelation w

caused by differences in the focusing geometry of the devices andmeasured area. This correlation coefficient was calculated by compavalues parameter by parameter. The interesting point was that when ala single sample from A to D measured using LCI and the Altisurf wecorrelation was found except in sample C. This revealed that even

parameters didnt correlate well, the relativity of the parameters insremained constant.

The surface roughness of paper has an important role in the glosinformation on both surface roughness and gloss could be measured atfrom the same areas of paper, it would help papermakers in tdevelopment of optimal paper surface quality, which is crucial for optin the printing process Three different paper samples were me

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Filler content evaluation of paper using OCT was discussed briefly

correspondence of the slope of the least squares (LSQ) fitted linlogarithmic fringe signal envelope was clear. A change from 0% to 30decreased the slope of the LSQ-fitted line by 22%. The correspondenclinear, but it is hard to speculate the sensitivity or linearity of the meaa limited number of samples. The effect is the well known optical clmedicine to enhance the imaging depth of OCT in living tissue, but it cto determine the filler content of paper. The refractive index of PCC, the refractive index of cellulose, n=1.55. The filler actually fills in tfibers, increasing the refractive index of the fibers surroundings amismatch between the refractive indices of the different scatteringcaused the slope, i.e. the reflectivity versus depth slope of the OCTcorresponding change. Secondly, increasing the filler content causes thaggregate, which makes the fillers less effective in scattering light (Le

partly effects the slope of the signal. The method is not capable of detvalue of the filler content without any calibration, but it is a useful tool

filler content in a manufacturing process.2D and 3D imaging of paper pose a great challenge for OCT. It has

thesis that imaging of unprepared commercial paper using OCT is imof measuring paper was introduced in section 5.4, where the simples

paper, i.e. a network of fibers, was constructed. The result was promthat a wood fiber network is visible to OCT. But, when the experimewith commercial paper, it was immediately clear that only the surfa

layers are visible in the image. Multiple scattering distorted the imagesscans showed a signal far beyond where the papers back surface woula clear effect of a far more complex structure than the measured simula

The idea of optical clearing of tissues in medicine was adapmeasurements. In the case of paper it is better to use the term refractiIt was found that certain alcohols had appropriate optical properties aenough to prevent damage to paper. The best agent was found to be benwas then used in the imaging of copy paper. However, commercial covery convenient grade of paper for the experiments because it is almknow its exact constituents. On the other hand, there is a practically usamples available for measurements and this grade of paper is widely its research interesting. The best possible sample would be a grade of

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6.2 Future research

Typically, a paper sheet is around 100 m thick, but the thickness depgrade of paper. It is obvious that it is not possible to reconstruct highlor 3D images of paper with a 15 m spatial resolution. A resolution m would highly enhance the capabilities of OCT in paper imagingresolution can be achieved, e.g. by using the Ti:Sapphire femtosecoavailable in the Optoelectronics and Measurement Techniques Laborat

of such a laser is the need for a stable and clean operating environlaboratory purposes, but the results that could possibly be achieved byresolution imaging system in paper imaging would greatly enhance thin this thesis.

Optimization of the speed of the imaging device was not the main pof this thesis project. More resources will be focused on that in the fuFD-OCT (Fourier Domain Optical Coherence Tomography) schsequence can be greatly speeded up to 30-50 2D slice images per smoving parts in the system (Endo et al.2005, Leitgeb et al.2004, Yata

Transversal resolution is limited by the focusing optics. There is aresolution and imaging depth. The smaller the focused spot in the samthe imaging depth if the transversal resolution is assumed to stay wrange. Fortunately a typical paper sheet is rather thin compared withmedicine, but still the setup could be enhanced by using a dynamicwhich ensures that the transversal resolution is constant in the depth di

The surface and near-surface structure of paper present challengincould be imaged using OCT. If the spatial resolution is enhanced to ththe coating color and printing layers could possibly be separated from tspatial distribution of the coating color, and spread and penetration

paper are important factors determining paper quality and printabilitassumed to be visible to OCT without any preparation of the paper.

Signal and image processing and post-processing were discussed

thesis. Techniques like iterative deconvolution can be used to enhancthe system without any changes to the imaging system itself (Paeswithout the knowledge of the phase of the fringe signal, the iteraalgorithm has been applied to enhance the resolution of OCT in paper et al 2005 Hast et al 2004) The Kalman filtering method can be

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7

Summary

In this thesis project, Low Coherence Interferometry and Optical Cohewere applied to paper measurements. The developed LCI/OCT systemmeasure the surface structure of paper. In the first experiments, differwere selected to give a maximum variation in surface structure. Thgrades of paper from a rough base paper to highly coated photo primeasured and roughness parameters calculated. The roughnesses Ra rto 6.9 m.

In the second experiment, a set of five samples was measureroughness values measured using LCI with roughness values

commercial Altisurf 500 profilometer. The linear correlation coefficionly with roughness parameters Ra and Rq: 0.97 and 0.98, respecti

parameters correlated more or less badly. This correlation coefficientcomparing the roughness values parameter by parameter. But, when ala single sample from A to D measured using LCI and the Altisurf wercorrelation coefficients ranging from 0.98 to 1 were found, except in scorresponding value was 0.73.

Next, the surface quality of paper was evaluated using LCI, a DOGglossmeter. The results show linear correlation between roughness ameasured with the commercial glossmeter. The linear correlation coeffroughness Raand Gloss GU20/ GU60 were -0.99/-0.99. The roughnegloss measured with the DOG didnt give a linear correlation coeffici

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found to have the best possible combination of optical, evapora

characteristics. Lastly, copy paper was exposed to benzyl alcohol anOCT. 3D visualizations of the sample were presented. The referthickness of the paper was measured with a Lorenzen & Wettre 51 measurement device, which gave a value of 102 m. The thickness c3D image obtained with OCT was 102.8 m when the porosity of the to have a value of 50%.

The combination of an optical profilometer and a tomographintroduced in this thesis makes the construction attractive. The res

surfaces of various grades of paper can be evaluated with a sincombined with a device that measures the local gloss of the surfacimportant parameters characterizing the surface properties of paper c

parallel to evaluate the surface quality, e.g. for printing. In addition structure beneath the surface is visible to the device. If only the vvisible without any paper preparation, the whole sheet can be imaresolution defined by the spectral characteristics and focusing optics

by using refractive index matching agents to fill the pores inside theintroduced in this thesis suggest that these methods and this device coa viable alternative to conventional methods in paper research and evecontrol.

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