Application of the laser speckle-correlation method for determining the shrinkage vector of a light-cured resin.

The purpose of this study was to develop a method to monitor the free-shrinkage vectors of a light-cured resin using laser speckle contrast measurement. The laser speckle contrast measurement system comprised a semiconductor laser, a speckle analyzer, and a computer-controlled data acquisition system. A light-cured resin was condensed into a glass tube and irradiated using a curing unit, the light intensity of which was adjusted to either 100 or 600 mW/cm2. The speckle patterns obtained from the lateral surface of the specimen were monitored using a CCD camera connected to a computer. The speckle images were analyzed using a phase-only correlation algorithm, and movements of the pattern were traced. The speckle pattern revealed rapid movement almost immediately after light exposure was initiated, and the orientation of free-shrinkage vectors is dependent on light intensity. When the specimens were irradiated at the lower light intensity, the free shrinkage vectors were oriented towards the light source. On the other hand, when irradiated at the higher light intensity, the free-shrinkage vectors of these specimens tended to orient towards the bottom surface. The effectiveness of the employed method to determine free-shrinkage vectors is thus proven through the results of this study.