Photodeflection signal formation in photothermal measurements: comparison of the complex ray theory, the ray theory, the wave theory, and experimental results.

A comparison is made of three methods for modeling the interaction of a laser probe beam with the temperature field of a thermal wave. The three methods include: (1) a new method based on complex ray theory, which allows us to take into account the disturbance of the amplitude and phase of the electric field of the probe beam, (2) the ray deflection averaging theory of Aamodt and Murphy, and (3) the wave theory (WT) of Glazov and Muratikov. To carry out this comparison, it is necessary to reformulate the description of the photodeflection signal in either the WT or the ray deflection averaging theory. It is shown that the differences between calculated signals using the different theories are most pronounced when the radius of the probe beam is comparable with the length of the thermal wave in the region of their interaction. Predictions of the theories are compared with experimental results. A few parameters of the experimental setup are determined through multiparameter fitting of the theoretical curves to the experimental data. A least-squares procedure was chosen as a fitting method. The conclusion is that the calculation of the photodeflection signal in the framework of the complex ray theory is a more accurate approach than the ray deflection averaging theory or the wave one.

Measurement of the thermal diffusivity of dental filling materials using modified Angström's method.

OBJECTIVES: A new measuring technique for the determination of thermal diffusivity is proposed. Using this technique, the thermal properties of a few different dental filling materials were measured. METHODS: The proposed method for measurement of thermal diffusivity is based on the classical Angström's method. The method exploits the propagation of a plane thermal wave generated by a Peltier's device in a cylindrical sample along its axis. The thermal diffusivity of the sample is calculated from the phase difference between harmonic components of temperatures measured at sample surfaces, perpendicular to the direction of thermal wave propagation. The estimated accuracy of measurement is typically about 10% for samples with low thermal diffusivity. The proposed method was used for the determination of thermal diffusivities of Achatit Bichromatic, Charisma and Dentimet dental filling materials. RESULTS: The measured thermal diffusivities were: 0.295(0.020)x10(-6)m(2)s(-1) for Achatit Bichromatic, 0.321(0.015)x10(-6)m(2)s(-1) for Charisma and 1.70(0.12)x10(-6)m(2)s(-1) for Dentimet. The thermal conductivities of these materials were also estimated. The results were compared with values obtained from independent constant flux measurements with marble as a reference material. SIGNIFICANCE: There are no standard techniques for the determination of the thermal properties of dental filling materials. Moreover, it is difficult to find the thermal diffusivity and the thermal conductivity of many of them. The method proposed in this paper allows the simple and accurate measurement of thermal diffusivity. Thermal parameters of dental filling materials should be compatible with the parameters of human teeth. Lack of thermal compatibility can cause not only patient discomfort but also mechanical stresses leading to microcracks.