This paper concerns the problem of describing and evaluating
thermal lensing phenomena that occur as a result of the absorption of
laser light in solid windows. The aberration function expansion method
is applied for deriving the two optical distortion coefficients x+ and xthat
characterize the degradation in light intensity at the Gaussian focus
of an initially diffraction-limited laser beam passing through a weakly
absorbing stress-birefringent window. In a pulsed mode of operation, the
concept of an effective optical distortion coefficient Xeff, which properly
combines the coefficients x+ and x- in terms of potential impact on focal
irradiances, then leads to the definition of a figure of merit for distortion.
The theory and calculations presented in this and earlier papers provide
simple analytical tools for predicting the optical performance of a windowmaterial
candidate in a specific system's environment.