In real machining processes, it cannot generally be assumed that the laser beam reaches the workpiece with the exact properties that are produced when the beam is generated and shaped. As a rule, processing takes place in an atmosphere whose properties are characterized by the interplay of process-specific phenomena, e.g. flows of plasma or metal vapor, and influences by the process setup such as the type of shielding gas, its supply and extraction or blow-off measures. On its way from the processing optics to the interaction zone on the workpiece, the laser beam propagates through an atmosphere whose state in space and time is by no means homogeneous or constant.