The rapid progress in development of new materials has also created more demands on their testing and characterization methods. In spite of the wide use of different plastics, there is a lack of methods to measure thermal diffusivity of polymers and other low-diffusivity materials. The diffusivity determination of oriented polymers is of particular interest: the drawing of a polymer foil orients the molecular chains, which strengthens the foil mechanically and also causes anisotropy to thermal conductivity making it higher parallel to the drawing direction than perpendicular to it. Anisotropy ratios as high as 100 have been reported [1]. The diffusivity through the foil could be measured by standard methods, like the flash method [2], but this way the diffusivity parallel to the foil surface can not be obtained. However, this can be measured by the optical beam deflection (OBD, mirage) technique [3]. Already the method has been applied to higher diffusivity samples than polymers, covering the range between 20 - 0.02 cm2/s [4, 5, 6, 7].