Optical glasses have a higher absorption and temperature dependence than fused silica. So their temperature can be increased by high power lasers which results in a change of the spherical shape and refractive index. Different radii of curvature and refractive indices change the optical behavior of a scan lens and especially its focal length and working distance.
The thermal focus shift is often a problem for users of the S4LFT0163/126, which was designed for low power marking applications. Today some people also use mid or high power lasers even for applications like that, because of a fast laser development during the last few years.