A nonuniform laser beam traversing a material sample undergoes thermal lensing, that is, distortion and defocusing due to thermally induced changes in refractive index and bulging of sample faces. Vector Kirchhoff diffraction theory is applied to provide a detailed picture of the transmitted beam properties in the presence of lensing. In particular, the time evolution of the focal properties and the intensity degradation are investigated. Four distinct characteristic time regimes are identified for rating solids: ultrasmall, small, transitional, and steady state. The calculations account for birefringence due to thermally induced stress, which is shown to play a major role in the alkali halides. The optical performance of transmitting materials in various time regimes is evaluated for Gaussian beams incident on thin circular samples. Detailed quantitative ratings are presented for a wide range of transmitting materials at 10.6 micrometers. (Author Modified Abstract).