The advent of high intensity low momentum surface heating devices is having a natural impact on the field of materials processing, particularly fusion welding.The laser and electron beam offer potential capability for precise control of energy and location which cannot be approached by older sources such as arcs and flames, and the fact that under certain operating conditions they transmit little or no thrust to the material being worked is an important advantage.However, the extreme intensity of these sources, expressed in dimensions of power per unit area, presents problems as well as advantages when addressed to the objective of local melting as in fusion welding.