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Int Chin J Dent 2004; 4: 45-50.

Polymerization behavior of a light-cured resin using the laser
speckle-correlation method
Tomomi Sato, DDS,a Masashi Miyazaki, DDS, PhD,a,b Akitomo Rikuta, DDS,PhD,a,b Hiroyasu Kurokawa, DDS, PhD,a,b and Toshiki Takamizawa, DDS, PhDaab

Department of Operative Dentistry, and Division of Biomaterial Science, Dental Research Center,
Nihon University School of Dentistry, Tokyo, Japan
Purpose: The aim of this study was to monitor the polymerization reaction of a light-cured resin using thelaser speckle-correlation method.
Materials and Methods: The experimental apparatus comprised semiconductor lasers, a speckle analyzer
and a computer-controlled data-acquisition system. The resin composite was condensed into a glass tube
and irradiated using a curing unit with a light intensity of 100 or 600 mW/cm2. The speckle patterns
obtained from the lateral surfaces and the underside of the specimen were monitored using a CCD camera
connected to a personal computer. A simplified correlation value was obtained for each pair of adjacent
speckle patterns. The images of the speckle patterns were analyzed using the phase-only correlation
method. The speckle contrast was then obtained as a function of the time that elapsed while measurements
were made.
Results: The speckle contrast decreased soon after light exposure commenced, then gradually increased
after irradiation. The changes in speckle contrast corresponded to changes in the polymerization reaction of
the light-cured resin. The speckle-contrast data obtained from the underside were lower than those fromthe lateral surfaces. This tendency was more pronounced when the specimen was irradiated with lower
intensity light.
Conclusion: This approach has the potential to record accurately the polymerization characteristics oflight-cured resins. (Int Chin J Dent 2004; 4: 45-50.)
Clinical Significance: These data suggest that the shrinkage characteristics of a light-cured resin wereinfluenced by the light intensity of the curing unit.
Key Words: laser speckle, light-cured resin, light irradiation, polymerization.

Introduction
The adhesive resin composites used in restorative dentistry undergo volumetric shrinkage during
polymerization. This can create marginal gaps that negatively influence the bonding and longevity of arestoration. Proper infiltration of the resin on the surface of an etched primed tooth and retention within thetooth are both required to minimize shrinkage, which might disrupt bonding.1 Various methods have been
investigated for monitoring the polymerization shrinkage of restorative materials.

For example, thespecimen can be studied using changes in the height of the mercury within a capillary column.2 Other
methods have utilized changes in the level of water in a pycnometer containing the material of interest.3-5
These systems have been modified in attempts to improve their accuracy, allowing them to monitor thepolymerization of light-cured resin composites.6-9 Dimensional strain in a material has also been measured
using non-volumetric dilatometric methods employing either a deflecting disk10-12 or the He-Ne
laser-scanning method.13
As the superficial outer layer of a restoration shrinks more quickly than the deeper layers, it has been

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Int Chin J Dent 2004; 4: 45-50.

suggested that the shrinkage vector of light-cured resin is oriented towards the irradiated surface.14
However, a finite elemental analysis has shown that the shrinkage vector is not significantly affected by theorientation of incoming light.15 Asymmetric stress can build up inside the resin restoration, resulting inasymmetric three-dimensional shrinkage. Therefore, the shrinkage vectors within the light-cured resin
might differ throughout the material.

Conversely, the information provided by volumetric-shrinkage

methods suggests that light-cured resin shrinks homogeneously. Hence, a new and independent method
was required to confirm the nature of the shrinkage process.
The laser speckle-correlation method is a simple non-contact non-destructive technique that does not
require special treatment of the specimen surface.15-18 The laser-speckle phenomenon occurs when anintense beam of coherent light is reflected on an object’s surface. Speckles are formed by the random
interference of coherent waves that are scattered from the rough surface. If the object has certain physicalcharacteristics, a three-dimensional diffraction field is observed with random fluctuations in intensity. The
relative intensity of bright and dark spots within the speckle pattern provides information about the material.
The contrast between the bright and dark spots can be recorded using a digital image. This forms the basisof speckle interferometry, a technique in which the speckle patterns of an object at two different status
points are compared in order to obtain the desired displacement information.19-22 In the case of light-cured
resins, the speckle patterns can reveal movement of the resin paste that has occurred during polymerization.
The polymerization reaction causes small fillers within the resin composite to be in a state of motion, which
increases as the reaction progresses.
The present study tested the hypothesis that the laser speckle-correlation method is a useful technique tostudy the reaction characteristics of light-cured resin polymerization. The main aim was to develop a laser
speckle-correlation method to determine the polymerization characteristics of a light-cured resin.

Materials and Methods
The resin composite Palfique Estelite (Shade A3, Lot No. J242, Tokuyama Dental, Tokyo, Japan) wasused in this study. The Optilux 401 curing unit (Demetron/Kerr, Danbury, CT, USA) was employed, andthe light intensity was adjusted to 100 or 600 mW/cm2, as measured using a dental radiometer (Model 100,Demetron/Kerr).
The test apparatus, including the laser-speckle equipment (Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan),is shown in Fig. 1. The experimental set-up comprised coherent light sources (MLXS-D13, Kikoh Giken
Co., Nishinomiya, Japan), charged couple device (CCD) cameras (CS8530-01, Tokyo Electronic Industry
Co., Tokyo, Japan) and a personal computer. The resin composite was placed in a glass mold (4.0 mm indiameter and 4.0 or 6.0 mm in height) on a glass slide (No. 5116, Muto Pure Chemicals Co., Tokyo, Japan).
The specimen was placed 20 cm from the laser source and the CCD cameras were placed 20 cm from thesample surface at an angle of 30° from each incident beam (Fig. 2). The spatial resolution of this system
was determined as 5 µm. Vibration interference was minimized by fixing the lasers to a block, and theCCD cameras and a sample stage were placed in a darkened chamber.

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1Fig. 1. Experimental equipment used in this study.
Fig. 2. Schematic diagram to show the laser speckle-contrast measurement system.

2The wavelengths of the beams from the semiconductor lasers were tuned to 635 and 670 nm with apower of 4 mW, and focused on the lateral surfaces and underside of the specimen, respectively. Different
wavelengths were used for the coherent light sources to avoid interference when detected by the CCD
cameras. The CCD cameras were adjusted until sharply focused speckle patterns were observed on thevideo monitor. The resin paste underwent light irradiation for 30 s, and the data were collected for 120 sfrom the start of the irradiation. During the polymerization reaction, the movement of the resin paste
produced dark and bright spots, which are referred to as speckle patterns. The speckle pattern recorded firstwas used as a reference, and was compared electronically with the speckle pattern obtained subsequently
from the resin surface during polymerization. As the native speckle patterns had a poor signal-to-noise
ratio, a Fourier phase-only correlation method23 was adopted to improve and enhance the quality of thepatterns. The images were then examined using an image analyzer connected to a personal computer.
Measurements were recorded at a constant temperature of 23 ± 1°C, a relative humidity of 50 ± 5% andwith illumination from only one small light source.

Results
The speckle contrasts for two light-cured resin samples of different thickness, which were polymerizedusing the light intensities indicated, are shown in Figs. 3-6 as a function of time. The overall magnitude of
the speckle contrasts decreased soon after light exposure commenced and remained at a low level during
irradiation.

After irradiation, the speckle contrasts gradually increased, particularly for the specimen

irradiated at a higher light intensity. Differences between the speckle contrasts obtained from the lateralsurfaces and underside of each specimen were observed for all specimens tested. A higher level of speckle
contrast was detected for the underside than for the lateral surfaces.
The differences in the speckle contrast obtained from the lateral surfaces and underside were more
pronounced when a lower light intensity was used. The speckle contrast decreased when light irradiationcommenced and remained at lower levels. After irradiation, the speckle contrast increased relatively slowly
for both lateral surfaces and undersides.

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Light intensity, 600 mW/cm2Int Chin J Dent 2004; 4: 45-50.

Light intensity, 100 mW/cm2

34Figs. 3, 4. Speckle-contrast measurements of the lateral surfaces and underside as a function oftime (mold diameter, 4 mm; mold height, 4mm).

Light intensity, 600 mW/cm2

Light intensity, 100 mW/cm2

56Figs. 5, 6. Speckle-contrast measurements of the lateral surfaces and underside as a function oftime (mold diameter, 4mm; mold height, 6mm).

Discussion
When the resin paste underwent light polymerization, the test area moved and the coherent wave fronts
were scattered, resulting in changes in the speckle patterns. Images of the speckle patterns were recordedon the CCD camera and stored in its memory. The digital absolute-value subtraction between the initial
image of the object and those taken during polymerization resulted in an image with speckle-patterndisplacement (Δd) that can be expressed as follows:

Δd = n λ/2sin αWhere, λ is the wavelength of the laser light, n is the number of fringes forming the interferogram and α isthe angle of inclination of the coherent wave front to the surface. The speckle patterns have a similar
appearance to moire fringes and represent contours of equal displacement, with each fringe resulting from adisplacement in the order of one-half of the wavelength of the laser light source used.19-22
The laser speckle-contrast measurement has the advantage that a small amount of sample can beanalyzed with great accuracy.16 The spatial resolution of the equipment employed in this study wasestimated as 5 µm, and this precision allows any movement of the resin composite during polymerization to48

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Int Chin J Dent 2004; 4: 45-50.

During this experiment, measurements were recorded 120 s from the start of lightirradiation; however, it is also possible to examine the polymerization reaction in greater detail over alonger period of time. A lower number for the speckle contrast indicates that the movement of the resin
composite occurred rapidly during polymerization. The majority of the polymerization shrinkage occurred
during and immediately after irradiation, which is reflected in the speckle contrasts measured during lightirradiation.
Since this method was first used to observe the polymerization reaction of light-cured materials, acommercially available resin known as Palfique Estelite has been introduced. The base resin of the paste isBis-GMA with an inorganic-filler content of 62 wt%. Comparisons among light-cured materials aregenerally inaccurate, even when the resin composites are produced by the same manufacturer. This isbecause their volumetric changes depend on many factors, including the loading, type and size of the filler.
Other factors that contribute to the volumetric changes are the monomer and initiating systems that
determine the structure of the hardened material. As most light-cured materials employ camphorquinone asa photoinitiator,24 an adequate intensity of visible light around the 470 nm wavelength is required toactivate polymerization. An appropriate wavelength is absorbed by the photosensitizer, which enters an
excited state and reacts with an amine-reducing agent to produce reactive free radicals.25 The transmissionof light through the material and the composition of the photoinitiator is known to influence the mechanicalproperties of light-cured materials.26 The intensity of light passing through a material is controlled by its
absorption and scattering properties.27

If a lower energy light is used, it could be difficult for thephotoinitiators to be excited in the deeper areas of the material. The data from this study show that the rateof speckle contrasts was slightly reduced at the underside of the specimen compared to that at the lateralsurface. This phenomenon was more pronounced when the specimens were irradiated with lower intensity
light.
The detailed behavior of samples with different measurement spots is shown graphically. The changes inspeckle contrasts obtained at the underside of the specimen were weaker than those from the lateralsurfaces located closer to the light source. The information provided by studies using volumetric-shrinkage
measurement is only equivalent in the case of homogenous shrinkage. Our data indicate that a symmetricmovement of the resin paste can be detected using the laser speckle-contrast approach.

Conclusion
We investigated the laser speckle-correlation method for characterizing the polymerization reaction of alight-cured resin. Changes in speckle patterns were converted into an intensity correlation, which provided
information on the status of the polymerization reaction. The method described here might offer analternative approach to determine the polymerization-shrinkage characteristics of light-cured resins withgreater accuracy and precision.
Acknowledgments
This work was supported in part by Grant from the Dental Research Center, Nihon University School of Dentistry,49

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Int Chin J Dent 2004; 4: 45-50.

Japan, 2003, Grant from the Ministry of Education, Culture, Sports, Science, and Technology of Japan to promotemulti-disciplinary research project, and Grant-in Aid for Encouragement of Young Scientists (B) 40287660 from theJapan Society for the Promotion of Science. The authors would like to thank Mr. Kouichi Kobayashi, Dr. Toshiki
Ooguro and Ms. Naoko Noguchi for their contributions to the experiments.

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Reprint request to:
Dr. Masashi Miyazaki
Department of Operative Dentistry, Nihon University School of Dentistry
1-8-13, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan
Fax: +81-3-3219-8347 E-mail: miyazaki-m@dent.nihon-u.ac.jp
Received March 4, 2004. Revised April 23, 2004. Accepted April 30, 2004.
Copyright ©2004 by the Editorial Council of the International Chinese Journal of Dentistry.

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