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The estimation of a thermal diffusivity map of a surface can be performed with an infrared camera by processing thermal images in response to a pulse of radiant energy. Indirectly, it is then possible to map other properties (humidity, density, etc.). One of the first requirements of this kind of device is to take into account the thermal measurement noise. When the thermal diffusivity varies by a small amount around a nominal value, a new method of sequential estimation based on an asymptotic expansion is proposed. The confidence interval of this method has been analytically compared with success, to the moment method and the method of logarithmic transformation. An experiment demonstrated that thickness variations of 50 μm can be detected on a plate 5 mm thick for a sample of polyvinyl chloride. © 1998 American Institute of Physics.