What is Thermal Lens Effect?

When energy density is varied at the processing point, stable welding is hardly available even if optical power monitored is kept constant.
When energy density is varied at the processing point, optical power feedback cannot be an effective method for quality welding .
When sunlight is focused on black paper with a magnifier, focused beam burns the spot on it in a few seconds. It does not happen unless it is focued at the focal point, however. The same idea is applicable for Thermal Lens Effect.
It is not possible to control Thermal Lens Effect by means of the conventional optical power feedback system.