Thermal diffusivity of the sample is measured by the flash method under 1-dimensional thermal conduction. But in the case of measurement of the thin sample by ring laser irradiation or in the case of measurement of the sample larger than the laser diameter, 2-dimensional heat flux is generated, and 1-dimensional thermal conduction approximation is not appropriate. In this paper, thermal diffusivity analysis was carried out in consideration of heat radiation. Accordingly, cylindrical coordinate thermal conduction equation is analyzed in Laplace space and the method for thermal diffusivity analysis of the sample is investigated under 2-dimensional heat flux condition. It is necessary to analyze the thermal diffusivity and Biot number as with normal 1-dimensional analysis. 2 methods were investigated. One is the Direct method which treats these variables independent and the other is the Cooling time constant method which analyses under the condition that the theoretical cooling time constant and the measured cooling time constant are equal. It was confirmed that both methods were able to analyze the thermal diffusivity.

It has been confirmed that thermal conduction of optical thickness less than 0.1 is approximately composed of diffusive heat transfer and radiative heat transfer between the end faces. It was applied to single layer glass and the thermal diffusivity was measured at high temperature. Since the opaque film at both ends was made of a thin film whose heat capacity and thermal diffusiviry can be neglected, there was a temperature limit where evaporation and diffusion are active. In order to apply this method at a higher temperature, three layer material with an opaque plate on both ends was examined. This 3 layer material temperature equation was obtained in the Laplace space under the radiation loss. Analyzing method of the 2^nd layer thermal diffusivity was investigated. First method is a search of a root-mean-square deviation minimum in the Laplace space under the condition that the thermal diffusivity, diathermic coefficient and the Biot number are independent variable (Direct method). Second method is a method to search the minimum under the condition that the theoretical cooling time constant equals to the cooling time constant of the temperature data (Cooling time constant method). It was confirmed that it is possible to determine the thermal diffusivity by both methods.