An explicit theoretical model based on three-dimensional thermal conduction, thermoelasticity, and Fresnel diffraction integral is developed to describe the surface thermal lens (STL) with a continuous-wave (cw) modulated excitation. The surface displacements and STL amplitudes obtained with both top-hat and Gaussian beam excitations are theoretically computed and compared. A STL experiment is performed with the cw modulated top-hat beam excitation to investigate the dependences of the STL amplitude on the experimental parameters, namely, the detection distance, the modulation frequency, the radius of the excitation beam, etc. Good agreements between the experimental and theoretical results are obtained. The results indicate that high sensitivity of the STL technique can be achieved with the top-hat beam excitation and by optimizing the experimental configuration. (C) 2008 American Institute of Physics.