Strong influence of the surrounding gas in laser-spot lock-in thermography

In laser-spot lock-in thermography, the amplitude of the surface temperature decreases monotonically with the distance to the laser spot. This radial decrease is commonly used to measure the in-plane thermal diffusivity of the sample. However, in the case of thermal insulators, an unpredicted and abrupt slope change appears in the radial profiles. In this work, we first demonstrate that heat conduction to the air surrounding the sample is the cause of this unexpected behavior. Then, we take advantage of this slope change to retrieve simultaneously in-plane thermal diffusivity and conductivity of thermal insulators.