46 INFOTECH OULU Annual Report 2006

OPTOELECTRONICS AND MEASUREMENT UNIT (OPME)

Professor Risto Myllylä and Professor Markku Moilanen, Optoelectronics and Measurement Techniques

Laboratory, Department of Electrical and Information Engineering, University of Oulu

Dr. Jouni Tornberg and Research Professor Harri Kopola, VTT, Oulu

Juha Kalliokoski, Research Director of the Measurement and Sensor Laboratory (Kajaani),

University of Oulu

risto.myllyla@ee.oulu.fi,markku.moilanen@ee.oulu.fi,

jouni.tornberg@vtt.fi, harri.kopola@vtt.fi, juha.kalliokoski@oulu.fi

http://www.infotech.oulu.fi/opme

Background and Mission

The Optoelectronics and Measurement Unit (OPME) com-

prises an intensive collaboration network of researchers at

the Optoelectronics and Measurement Techniques Labora-

tory at the University of Oulu, the Measurement and Sen-

sor Laboratory (MILA) in Kajaani and VTT. OPME

employs 37 postgraduate students and 41 other persons in

the research work. The group carries on high-level research

in optoelectronics and measurements techniques, wireless

instrumentation with particular emphasis on the practical

applications of these techniques, material techniques,

photonics integration and testing needed in advanced sen-

sors. A new technology field, printed intelligence, is inte-

grated to the Optoelectronics and Measurements Unit to

bring together professionals from all the above mentioned

fields into close collaboration to improve the know-how

and education possibilities in this area.

The practical research activities developed in the OPME

unit include, but are not limited to, measuring and model-

ing the propagation of light in turbulent and scattering me-

dia such as atmosphere, tissues, pulp and paper; electronics

testing techniques such as embedded testing and RF test-

ing; wireless sensors and sensor network for healthcare

applications; optical and micro- sensors produced with

MEMS technology, camera and machine vision in optical

measurement; advanced optical, ultrasonic, and microwave

techniques specially applying in wood processing; and print-

able manufacture techniques which integrate traditionally

optoelectronic devices to polymer platforms.

As a participant in the Infotech Oulu graduate school, the

OPME unit also arranges lecture series and graduate school

courses for graduate students. During 2006, the group or-

ganized the traditional 6th Infotech Oulu Workshop on Op-

toelectronic Devices and Instrumentation, concentrating this

time on ‘Printed electronics and optoelectronics’, In addi-

tion, six international lectures and courses were held in the

group. They were ‘The principles and applications of lin-

ear and nonlinear spectroscopy’ ‘Surface plasmon reso-

nance’, ‘Wireless sensor network technology for ubiquitous

healthcare applications’, ‘OCT system analysis and Monte

Carlo simulation’, ‘Laser diagnostics of highly scattering

media: evolution of optical properties’ and ‘Femtosecond

pulse propagation in scattering media’.

In 2006, there were 21 new projects opened in the OPME

unit, taking the total project number up to 42. The group

published 31 journal articles, 35 international conference

papers and 7 other scientific papers. Research collabora-

tion has been active with 16 domestic partners located in

Finnish universities and VTT, and 20 international partners

coming from universities and institutes in the USA, Canada,

Japan, Korea, China, Russia, the UK, Ireland, Poland, Ger-

many, and Sweden. The international visits for research

activities involved 15 foreign visits to the group and 11

research visits from the group aboard.

The Academy of Finland granted the OPME unit (together

with EMPART group) FiDiPro (Finland Distinguished Pro-

fessor Program) funding to hire professor Ghassan Jabbour

from Arizona State University, USA, to build up “The cen-

ter for printing of nano-to-mega photonics, electronics, and

bioinformatics”.

Scientific Progress

The following research examples illustrate the scientific

progress of the OPME unit. These research projects have

been carried out in active collaboration between both do-

mestic and international research institutions and industrial

partners.

Modeling of light transmission through sub-

wavelength apertures

Modern optical technologies, such as near-field scanning

optical microscopy, optical lithography, heat-assisted mag-

netic recording and optical data storage, demand efficient

near-field light sources that provide light confinement well

beyond the fundamental diffraction limit. In theory, the fun-

damental diffraction limit can be exceeded - in a brute force

fashion - by spatially limiting the extent of an incident

wavefront by imposing a very small transmitting aperture

in the path of an incident light beam. This approach, how-

ever, suffers from the extremely small light transmission

efficiency through the aperture. The main objective of this

project is to develop new ways to enhance light transmis-

sion through sub-wavelength apertures. During 2006, we

have studied, via finite difference time domain (FDTD)

modeling, fundamental characteristics of a single circular

hole in a thin metal film, light transmission through a sub-

wavelength aperture filled with a high index dielectric, light

coupling into surface plasmons by circular grooves, and

coupling of surface plasmons into apertures. The waveguide

chip in this project was fabricated from siloxane material,

which can be processed by UV-lithography to have a chan-

INFOTECH OULU Annual Report 2006 47

nel like waveguide structure. The sensitivity of the inte-

grated Young interferometer was studied and found to be in

the order of a 10-6 refractive index unit.

Evanescent wave fluorescence detection in

polymer waveguides

Polymer waveguides such as PC, PMMA, CA and PET can

be used to excite fluorescence molecules attached to the

surface of the waveguide. A small part of the light wave

which propagates in the waveguide can reach into the sur-

rounding medium and interact with that medium. This sen-

sitive method can be utilized in immunoreaction monitoring,

in which labelled antibodies in sample fluid are attached to

the antibodies on the sensor surface. The evanescent wave

excites the label fluorophores and the relaxation of the ex-

citation light can be seen as emitted light.

Electronics testing techniques

Miniaturization and new manufacturing technologies will

make testing of electronic products difficult in the future.

Access to certain locations in the device will be impossible

with traditional measurement techniques, or the access will

require new, possibly expensive, and time consuming meth-

ods.

One way to address these concerns is to incorporate mea-

surement capabilities into the product itself. In this, so-called

embedded testing, new test-specific circuit structures and

existing parts of the product are used together to perform

parameter, functional, interconnect and component self-tests

on different parts of the product.

Basic concepts for such embedded testing have been de-

veloped in the research group. Principles of component and

interconnect testing via analogue boundary scan test archi-

tecture have been developed, the frequency range of the

test bus has been extended to RF with the use of simple

integrated radio-frequency detectors, and the basic phenom-

ena related to the breaking-up of solder joints of ball grid

arrays have been studied.

Embedded testing forms one of the cornerstones upon which

the concurrently developed ideas on tests serving the entire

life-cycle of products from development to field repair are

laid. Since in this kind of scenario, test instruments and

measurement equipment are embedded as part of the prod-

uct, a method to remotely initiate and control the tests and

to access the measurement results is needed. Groundwork

leading towards standardized ways of performing such re-

mote testing and data telemetry is being carried out by the

on-going effort in the Wireless Test Standard (WTS)

workgroups. This work will provide the network protocols

and infrastructure for communication between the embed-

ded test enabling circuitry, software inside the device un-

der test, and the remote user.

Work has been carried out to develop a real-life product,

which has the above mentioned life-cycle test related ideas

built-in. The goal is that the product can be tested and diag-

nosed via a network (LAN/WLAN). A conventional refer-

ence product with the same functions but without the test

structures has been built to discover the impact of the struc-

tures on the performances of the product.

Optical measurements for steering a surgi-

cal, interactive robot in an MRI environment

The project is focusing on developing a tool for surgeons

to help during operations. The surgical robot will take bi-

opsy samples of the soft tissue or bone, and work on the

basis of MRI (Magnetic Resonance Imaging) and ordinary

camera information. The robot will be automatically navi-

gated, but also manual steering by the surgeon will be a

valuable characteristic. The purpose is to design miniatur-

ized optical angle and position sensors for measuring the

robot’s arm position accurately inside arm joints. Also a

three dimensional fiber optic joy stick for a robot wrist, to

be used for manual steering is one of the main research

topics. In addition, different common radio technologies

will be tested in an MRI environment. The whole robot

Magnetic field amplitude of a surface-plasmon-like field that

couples into a sub-wavelength slit in a metal film.

Principle of evanescent wave fluorescence detection in

polymer waveguides.

An MRI compatible robot and a 3-D optical joy stick. In the

left figure, the robot’s black arm is made of carbon fiber

material and others from aluminum or brass. The 3-D joy

stick, presented in the right figure, will be assembled in the

robot’s wrist.

48 INFOTECH OULU Annual Report 2006

system will be tested in the EMC-laboratory to ensure elec-

tromagnetic compatibility. All material must be MRI com-

patible and there cannot be any electronics in the robot’s

wrist.

Optical bio-sensing technology based on self-

mixing interferometry

Self-mixing Interferometry is a highly promising novel

method for detecting a change in the refractive index of

samples. One of its major applications is sensing molecular

interaction in biological samples. In our study, a self-mix-

ing interferometer (SMI) was chosen as the instrument for

measuring the refractive index in free–space. A GaN blue

laser diode was used as a light-emitting source. Compared

with traditional interferometric configurations, self-mixing

interferometry combined with the laser diode package has

the advantages of a compact setup and high sensitivity.

A self-mixing effect in a laser occurs when a part of the

emitted laser light enters back into the laser cavity, where it

interacts with the original laser light, causing fluctuations

of the laser power. These power fluctuations can be de-

tected using a photodetector placed on the opposite side of

the laser cavity from the primary light output. If the exter-

nal light is frequency-shifted and it is mixed coherently with

the original laser light, interference occurs and this can be

superimposed to many different measurement applications.

Generally, the self-mixing interferometer measures the su-

perposition of internal and external electric fields. This con-

figuration significantly reduces the size of the interferometer

compared to traditional configurations. In addition, the re-

duced number of optical components decreases the price of

the device. Moreover, this technique is accurate, and in a

certain range one λ/2 accuracy can be achieved. Finally, it

is simple to use because there is only one optical axis to

control. By far the smallest optical path length variation

detected with our measurement setup is 50 nm, correspond-

ing to a 10-6 change in the refractive index in the external

cavity.

Study of pulp samples by photoacoustic

methods

The paper industry needs to further develop in-line mea-

surement for process controlling of fiber and fines consis-

tencies. In the near infrared region, the wood fibers and

fines have weak optical absorption. Our former study

showed that the optical scattering ability of fibers is much

lower than that of fines. On the other hand, fibers cause

much stronger ultrasonic attenuation than fines. Therefore,

if there is a technique that can detect optical scattering and

acoustic attenuation simultaneously, it has a potential abil-

ity to determine fibers and fines contents in pulp suspen-

sion. A scattering photoacoustic technique developed in our

group is being applied to research of this issue.

In this technique, a high energy laser pulse with nanosec-

onds duration irradiates the studied pulp sample. A part of

the optical energy is absorbed by the sample, producing a

photoacoustic pulse in it. At same time, another part of the

optical energy is scattered to two absorbers contacted with

the sample, with a high absorption coefficient at the irra-

diation wavelength. Therefore, two sources are produced

at reception surfaces of the absorbers, respectively, in which

the intensities are strongly dependent upon the scattering

property of the sample. The ultrasonic waves emitted from

them can be detected by an ultrasonic transducer which

acoustically contacts with one absorber. Hence, the ampli-

tude ratio of received ultrasonic pulses emitted from two

absorbers has a close relationship with acoustic attenuation

of the pulp sample; this is because one pulse goes through

the sample while the other does not. Our preliminary re-

sults demonstrate that fibers and fines fractions can be dis-

tinguished when the consistency is lower than 1%. By

modifying the detection system, the current apparatus is

now being used to measure samples with higher consisten-

cies (up to 5%). Furthermore, we found that more informa-

tion of samples can be established, together with analyzing

the frequency spectra of laser induced ultrasonic signals

going through the pulp samples.

The electronics for a 3-D fiber optic joystick (left) and tem-

perature calibrated angle sensor (right). The electronics

for the 3-D joy stick comprised of two transmitter LEDs

(white) and two differential and one single channel PIN-

diodes (black). In the angle measurement, one LED is trans-

mitting light and two PIN-diodes are collecting reflected light

differentially.

Configuration of a self-mixing interferometer.

Optical path length variation in the external cavity of the

SMI.

Scattering photoacoustic signals (left) and laser ultrasonic

power spectra (right) obtained from TMP samples with dif-

ferent consistency.

INFOTECH OULU Annual Report 2006 49

Spectroscopic investigation of superficial

human and porcine skin layer using a tape

stripping technique

Tape stripping is a minimally invasive method to reveal in-

depth penetration profiles of substances topically applied

onto the surface of skin. Our research is focused on experi-

ments with porcine skin in vitro (ears of freshly slaugh-

tered pigs) and human skin in vivo (flexor forearm). The

tape stripping technique introduces consecutive removal of

micrometer-thick cell layers of the stratum corneum (SC)

from the same treated skin area using an adhesive tape. Pre-

requisite to the substance penetration profile is the recon-

struction of the removed SC by analyzing the amount of

corneocytes (cells of SC) stuck to each tape strip.

Such an analysis is carried out spectroscopically by mea-

suring light transmittance in visible and NIR spectral re-

gions (430 - 1050 nm) through the adhesive tape with

corneocytes. As we proved experimentally, there is a linear

dependence between the pseudoabsorption (equals to loga-

rithm of inverse transmittance) and the thickness of the

corneocytes on the tape strips for all wavelengths of the

investigated region. Dependence of the cumulative absor-

bance of the removed SC on the tape strip number can be

satisfactory fitted by exponential function. This relation-

ship allows evaluation of the share of the removed SC (in

%) without complete removal of this layer. All the obtained

results correlate well with those obtained on humans.

One of the frequently and widely used substances topically

administered onto the skin are various chemical agents (UV

filters) included into sunscreens for UV protection. The tape

stripping technique combined with spectroscopic analysis

make possible reconstruction of the in-depth profiles of such

substances. After tape stripping, strips with a UV filter on

them are put into pure ethanol causing dissolving of the

UV filter. An ultrasonic bath (for better stirring) and ensu-

ing centrifugation enable separation of corneocytes from

the dissolved chemical substance. Absorption of the UV

filter is measured within the UV range.

Research using optical coherence tomogra-

phy

Monte Carlo simulations of OCT images

Optical coherence tomography (OCT) is a modern non-in-

vasive technique utilizing principles of low-coherence in-

terferometry for visualization of the inner structure of

optically non-uniform objects. However, multiple scatter-

ing of light in the objects under study brings distortion to

the images causing requirements for the proper interpreta-

tion. A statistical Monte Carlo method for simulation of

light propagation in scattering media provides the ability

for analysis of formation of OCT-images from multilayer

samples.

A program code for the Monte Carlo simulation of 2D OCT

images of multilayer samples with various boundary shapes

was developed. OCT images of various skin and paper

samples were simulated. Such simulations allow one to sepa-

rate the contributions of low-order and multiple scattering

to the image. Such separation provides the information about

maximal depth of non-distorted imaging in the considered

objects which can be defined as the depth from which mul-

tiple scattering contribution starts to prevail in the image.

Paper structure analysis by OCT

The growth of the paper making industry has triggered a

continuous search for improved methods for diagnosing and

characterizing paper properties. Surface analysis, includ-

ing roughness, porosity and gloss estimation, is particularly

important in printing papers, because parameters like rough-

ness and surface porosity affect such optical properties of

paper as gloss and ink absorption. Current measurement

methods for structural analysis of paper tend to be slow,

labor-intensive, expensive or invasive.

Up-to-date OCT devices are fast and have a resolution which

competes with the conventional laboratory structural analy-

sis methods of paper. The advantage is the non-contact and

non-invasive approach. A resolution of 1µm can be reached

by the new Nd:YAG pumped Ti:Sapphire laser and Photo-

nic Crystal Fiber - PCF based OCT-system. Such a high

resolution enables the imaging of single fibers, coating paste

and color layers. Building such a system needs knowledge

of many areas like paper physics, photon migration in pa-

per, paper chemistry, signal processing and automation con-

trol of the imaging device, which means that the

collaboration between several scientists and research groups

has to be active. In addition to the imaging of paper, the

technique has been applied to fluid mechanics evaluation

by using the ability of the method to determine the velocity

profiles of flowing suspensions like pulp and coating paste

suspensions.

In-depth distribution of a chemical UV filter (utilized in sun-

screens) within porcine stratum corneum revealed using

the tape stripping technique.

Low-order (left) and multiple(right) scattering contributions

to the OCT image of paper.

Low-order (left) and multiple(right) scattering contributions

to the OCT image of skin.

50 INFOTECH OULU Annual Report 2006

OCT towards non-invasive glucose monitoring

Non-invasive glucose monitoring is a very demanding topic

in biomedical application. Because OCT is very suitable

for detecting changes in the refractive index distribution in

the sample and increasing glucose concentration will change

the refractive index of the background medium and/or scat-

tering particles, the glucose can be detected by analyzing

the slope value of the straight line fitted to the OCT signal

depth profile. The experiments were carried by adding glu-

cose in intralipid suspensions and by performing in-vivo

glucose tolerance tests with mice.

Pulp fiber research by STE

The increasing demand for high quality paper products re-

quires optimal and more efficient use of the available fiber

resources. Research and analytical tools for measuring the

properties of individual pulp fibers are essential for better

utilization of the available fiber resources. A wood fiber

can be approximated by its middle layer S2, which contains

the majority (80-95%) of the cell wall material. The angle

between the fibrillar direction and the fiber axis ϕ is termed

the microfibril angle (MFA). The MFA and the fiber’s cell

wall thickness (CWT) are closely related to the fiber’s me-

chanical properties, but difficult to measure due to the fiber’s

two-wall structure.

A new method based on spectroscopic transmission

ellipsometry (STE) was developed in the RESTE project,

which is capable of real-time measurement of a pulp fiber

oriented arbitrarily for the MFA ϕ and phase retardation ∆,

a parameter proportional to the CWT. The STE method

uses equipment that creates an image, which is insensitive

to the orientations of fibers, determined only by the fibers’

ϕ and ∆. A line spectral camera mounted in the equipment

captures a line image across the fiber sample and disperses

the line image into a spectral light intensity distribution,

from which an optimal estimation for ϕ and ∆ can be then

generated.

The STE method requires no sample pretreatment and per-

mits quantitative measurement of intact pulp fibers oriented

arbitrarily for the microfibril angle ϕ and retardation ∆, by

acquiring only one image from the fibers. As the method

exerts no limit to the measurement speed, the measurement

procedure is largely simplified. It is feasible to construct

equipment suitable for use not only in the laboratory but

also under on-line conditions based on the STE method for

measurement of ϕ and ∆.

Exploitation of the Results

Active interaction with industry ensures the rapid applica-

tion of research results. The work of the group paves the

way for the introduction of a new generation of optoelec-

tronic sensors and instruments based on micro-optics,

micromechanics and microelectronics, and polymer-based

manufacturing technology.

The research results of the group have been presented at

scientific conferences and published in professional jour-

nals. In projects funded by TEKES, the acquired knowl-

edge has been directly transferred to the participating

enterprises. Also commissioned research has been directly

reported to the enterprises concerned, and several senior

and junior researchers have taken up employment with them.

Future Goals

Attaining an acknowledged position as a top European re-

search unit in its own field forms a key objective for the

group. Another important goal involves becoming a central

cooperation partner for Finnish industry in the development

and application of measuring techniques and instrument

technology. In addition, the group advances the scientific

understanding of optoelectronics and measurement appli-

cations, and produces high-quality dissertations and publi-

cations. Moreover, the highly applicable, in-depth,

information produced by the group also makes a contribu-

tion to product development and the creation of new busi-

ness opportunities.

As a participant of the Infotech Oulu graduate school, the

group will continue effective education of graduate students.

The aim is to continue the traditional workshop series on

Optoelectronics Devices and Instrumentation. In addition,

internationally renowned scientists will continuously be in-

vited to give lectures to the students.

The scheme of TISSA-50 Ti:sapphire laser and photonic

crystal fiber based OCT system.

The OCT device and an OCT image from a mouse skin in

vivo during a glucose tolerance test.

Left: The STE equipment developed in the RESTE project.

Right: Micrograph of pine pulp fibers measured by the STE

equipment.

INFOTECH OULU Annual Report 2006 51

Personnel

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stnedutsetaudarg 73

srehto 51

latot 77

)%12TTV,%97.vinu(sraeynosrep 85

External Funding

ecruoS RUE

dnalniFfoymedacA 000943

noitacudEfoyrtsiniM 000161

sekeT 0006961

cilbupcitsemodrehto 0006301

etavirpcitsemod 000243

lanoitanretnirehto+UE 000028

latot 0004044

Doctoral Theses

Kinnunen M (2006) Comparison of optical coherence tomogra-

phy, the pulsed photoacoustic technique, and the time-of-flight

technique in glucose measurements in vitro. Acta Universitatis

Ouluensis C 249.

Alarousu E (2006) Low coherence interferometry and optical co-

herence tomography in paper measurement. Acta Universitatis

Ouluensis C256.

Sorvoja H (2006) Noninvasive blood pressure pulse detection and

blood pressure determination. Acta Universitatis Ouluensis C 259.

Sarén M-P (2006) Characterisation of properties of coniferous

wood tracheids by x-ray diffraction, laser scattering and micros-

copy. University of Helsinki, Department of Physics Science, Dis-

sertation 127.

Selected Publications

Olkkonen J, Kataja K & Howe D (2006) Light transmission

through a high index dielectric hole in a metal film surrounded by

surface corrugations. Optics Express 14: 11506-11511.

Olkkonen H, Pesola P, Olkkonen J & Zhou H (2006) Hilbert trans-

form assisted complex wavelet transform for neuroelectric signal

analysis. Journal of Neuroscience Methods 151(2): 106-113.

Yoshioka Y & Jabbour G (2006) Inkjet printing of oxidants for

patterning of nanometer-thick conducting polymer electrodes.

Advanced Materials 18: 1307-1312.

Williams E, Li J & Jabbour G (2006) Organic light-emitting di-

odes having exclusive near-infrared electrophosphorescence. Ap-

plied Physics Letters 89, 083506.

Williams E, Haavisto K, Li J & Jabbour G (2007) Excimer-based

white phosphorescent organic light emitting diodes with nearly

100% internal quantum efficiency. Advanced Materials 19: 197-

202.

Sorvoja H, Myllylä R, Kärjä-Koskenkari P, Koskenkari J, Lilja M

& Kesäniemi A (2006) Accuracy comparison of oscillometric and

electronic palpation blood pressure measuring methods using in-

tra-arterial method as a reference. Molecular and Quantum Acous-

tics 26: 235-260.

Reinvuo T, Hannula M, Sorvoja H, Alasaarela E & Myllylä R

(2006) Measurement of respiratory rate with high-resolution ac-

celerometer and emfit pressure sensor, SAS 2006 – IEEE Sensors

Applications Symposium, 7-9 February, Houston, USA, 192-195.

Kinnunen M, Myllylä R, Jokela T & Vainio S (2006) In-vitro

studies toward noninvasive glucose monitoring with optical co-

herence tomography. Applied Optics 45 (10): 2251-2260.

Fabritius T, Alarousu E, Prykäri T, Hast J & Myllylä R (2006)

Characterization of optically cleared paper by optical coherence

tomography. Quantum Electronics 36(2): 181-187.

Häkkinen J, Hannu J, Manninen J, Syri P & Moilanen M (2006)

Embedded high-frequency analogue boundary scan test solution

for a 900 MHz direct conversion I/Q transmitter. International

Mixed-Signals Testing Workshop, June 21-23, Edinburgh, Scot-

land, 105-109.

Saikkonen T & Moilanen M (2006) Component value calcula-

tions and characterizations for measurements in 1149.4 environ-

ment. International Mixed-Signals Testing Workshop, June 21-23,

Edinburgh, Scotland, 77-82.

Peiponen K, Alarousu E, Juuti M, Silvennoinen R, Oksman A,

Myllylä R & Prykäri T (2006) Diffractive optical element based

glossmeter and low coherence interferometer in assessment of lo-

cal surface quality of paper. Opt. Eng. 45(4), 043601.

Zhao Z, Törmänen M & Myllylä R (2006) Preliminary measure-

ment of fibres and fines in pulp suspensions by the scattering

photoacoustic technique. Measurement Science and Technology

17: 128-134.

Bykov A, Kirillin M & Priezzhev A (2006) Monte Carlo simula-

tion of signals from model biological tissues measured by an op-

tical coherence tomograph and an optical coherence doppler

tomograph. Optics and Spectroscopy 101(1): 33-39.

Kirillin M, Priezzhev A, Hast J & Myllylä R (2006) Monte Carlo

simulation of optical clearing of paper in optical coherence to-

mography. Quantum Electronics 36(2): 174-180.

Törmänen M, Niemi J, Löfqvist T & Myllylä R (2006) Pulp con-

sistency determined by a combination of optical and acoustical

measurement techniques. Measurement Science and Technology

17: 695-702.

Fabritius T & Myllylä R (2006) Investigation of swelling behaviour

in strongly scattering porous media using optical coherence to-

mography. Journal of Physics D: Applied Physics 39: 2609-2612.

Ye Chun (2006) Measurement of the microfibril angle and path

difference of intact pulp fibers by spectroscopic imaging

ellipsometer. Nordic Pulp and Paper Research Journal 21(4): 520-

526.

Ye Chun (2006) Low-loss tunable filter based on optical rotatory

dispersion. Applied Optics 45(6): 1162-1168.

Niskanen I, Räty J & Peiponen K-E (2006) A multifunction

spectrophtometer for measurement of optical properties of trans-

parent and turbid liquids. Measurement Science and Technology

17: N87-N91.

Sarén M-P & Serimaa R (2006) Determination of microfibril angle

distrubution by X-Ray diffraction. Wood Sci Technol 40: 445-

460.