An optical scan system welds or marks a part by directing an optical beam onto the part at a sufficient energy density level to weld or mark it. A method of controlling a pattern where the part is to be exposed to the beam at the sufficient energy density level includes disposing a waveguide between the part and an optical source of the optical scan system to prevent areas of the part that are not to be welded or marked from being exposed to the beam at the sufficient energy density level to weld or mark them, allowing the areas to be welded or marked to be exposed to the beam at the sufficient energy level. In an aspect, the waveguide prevents the beam from being reflected in an undesired direction. In an aspect, the waveguide redirects the beam from the areas of the part that are not to be welded or marked to areas that are to be welded or marked to concentrate the energy on the areas to be welded or marked. In an aspect, a dissipative waveguide dissipates energy of the beam in the areas of the part that are not to be welded or marked so that those areas are not exposed to the beam at the sufficient energy density level.