This work provides the experimental confirmation of a generalized theory for thermal diffusivity measurements of solid samples using the ‘‘mirage’’ technique presented by the authors in a previous paper [A. Salazar, A. Sanchez‐Lavega, and J. Fernandez, J. Appl. Phys. 6 5, 4150 (1989)]. Moreover the influence of different effects on such determinations has been analyzed, and the limits of validity of the method have been established. The most important influence relies on the finite nature of two parameters: the height of the probe beam above the sample surface h and the radius of the exciting beam a. Such effects in all solids are discussed in detail according to their bulk thermo‐optical properties and for two ways of measuring: ‘‘bouncing’’ (probe beam sent to the sample surface at a small angle) and ‘‘skimming’’ (probe beam parallel and grazing the surface). The first way of measuring gives reliable results while the second one must be managed with care. In any case, significant shifts in the thermal diffusivity measurement for nonzero values of h and a are specially observed when the thermal diffusivity of the sample is much lower than that of the deflecting medium.

A complete theoretical treatment of the ‘‘mirage’’ (optical beam deflection) technique has been performed for thermal diffusivity measurements in solids. Numerical calculations of the transverse deflection show there is a linear relation between the first noncentral zeros of the real part of this function and the inverse root frequency. The slope of this straight line is related to the thermal diffusivity through a factor (γ) which depends on the thermal and optical properties of the sample. We found γ=1 for transparent materials as well as for opaque thermally thin materials, whereas γ=1.44 for opaque thermally thick samples. We study further the experimental limits which make a sample to be considered as an opaque or transparent and as thermally thick or thin, and present a complete classification of solids according to their optical and thermal properties. The different behaviors observed may be related to the different geometry of thermal waves propagating through the media.

Photothermal radiometry has been widely used to measure the thermal diffusivity of bulk materials. In the case of thin plates and filaments, a one-dimensional heat propagation model including heat losses has been developed, predicting that the thermal diffusivity can be obtained by recording both the surface temperature amplitude and phase profile slopes (“slope method”). However, this method has given highly overestimated values of the thermal diffusivity of poor-conducting films and filaments. In this paper we analyze the effect of the experimental factors affecting the thermal diffusivity measurements of thin plates and filaments using infrared thermography, in order to establish the experimental conditions needed to obtain accurate and reliable values of the diffusivity of any kind of material using the slope method. We present the calculations of the surface temperature of thin isotropic and anisotropic plates heated by a modulated and tightly focused laser beam, showing that the slope method is also valid for this kind of pointlike heating. Special attention is paid to the effect of surface heat losses (convective and radiative) on the diffusivity measurements of small-dimension and poor-conducting materials. Lock-in thermography measurements performed in the best experimental conditions on a wide set of samples of different thermal properties (thin isotropic and anisotropic plates and filaments) confirm the validity of the slope method to measure accurately the thermal diffusivity of samples of these shapes.