The in-plane thermal diffusivity of soft materials is critical information for the development of functional thermal materials in nano/microscale integrated devices. The laser induced local periodic heating and the imaging of the spatially distributed periodic temperature response is important technique to determine in-plane thermophysical properties.^[1] However, the method is highly affected by the 3D geometry of the sample and the environment of the surface such as the vacuum condition. To avoid this multidimensional effect to the thermal analysis, the frequency range of the periodic heating must be as high as possible, which is usually limited by the frame rate of the imaging system. In this study, the principle of heterodyne signal analysis was applied to thermal imaging to analyse the periodic heating response on the sample surface, which has a frequency much higher than the frame rate of the imaging system. (Fig. 1) The in-plane thermal diffusivity of thin polymer films was determined by phase analysis of the in-plane periodic temperature response induced by photo-thermal effects generated by irradiating with a near-infrared laser (l = 830 nm) at a micro-focus using an optical system with a wide frequency that exceeds the frame rate of InSb IR camera (sensitivity range 3 mm – 5 mm).