The laser flash method for thermal diffusivity measurement is a standard method for opaque solid materials. It can also be used to measure liquid in a specially designed cell. The theoretical basis for the method is established based on pulse heating of one side of a thin opaque sample and measure the temperature response of the other side. In cases where the material is non-opaque or semi-transparent for the laser wavelength, existing theoretical models cannot be used directly. One way to overcome the problem is to coat a thin layer of graphite on the sample surface. The coating can absorb the laser energy and create a surface heating effect. However, coating may not be possible for special cases due to concerns of contamination of liquid samples. This paper reports the development of a theory that includes the transmission and absorption of the laser energy through out the sample thickness. The theory can be applied for samples with different absorption coefficient to obtain simultaneously thermal diffusivity and thermal conductivity of the sample. The original theory of the laser flash method becomes a limiting case of the current theory with an infinitely large absorption coefficient. The uncertainty analysis was performed and results indicated that that laser flash method can be used on non-opaque samples.

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