The occurrence of filaments in fused silica irradiated by UV laser light is well described by the product of light Intensity by Length of propagation in the material. For a spatially gaussian peak, in the well-known treatment by Marburger et al, this product is predicted to depend upon input power and non linear index. At a wavelength of 355 or 351 nm, the compilation of our past and present measurements give a smaller critical intensity by length product, i.e. a higher non linear index, than previously measured.
These values of non linear parameters allow for the prediction of rear surface damage on thick windows. The predictions compare well with damage probability measurements. Even when the intensity is not high enough to generate filaments, self-focusing is still the cause of damage, due to the increase of output intensity and fluence.