A method of additively manufacturing an object, the method comprising steps of: depositing a feedstock line along a print path, using a delivery guide, and wherein: the feedstock line has a feedstock-line length and an exterior surface, defining an interior volume of the feedstock line; the feedstock line comprises: elongate filaments, extending along at least a portion of the feedstock-line length; a resin, covering the elongate filaments; and at least one optical modifier, having an outer surface, covered by the resin; and at least the one optical modifier is interspersed among the elongate filaments; and directing electromagnetic radiation at the exterior surface of the feedstock line after the feedstock line is deposited by the delivery guide along the print path; and wherein the step of directing electromagnetic radiation at the exterior surface of the feedstock line after the feedstock line is deposited by the delivery guide along the print path causes the electromagnetic radiation to strike the outer surface of at least the one optical modifier, which in turn causes the electromagnetic radiation to irradiate, in the interior volume of the feedstock line, the resin that, due at least in part to the elongate filaments, is not directly accessible to the electromagnetic radiation, incident on the exterior surface of the feedstock line. depositing a feedstock line along a print path, using a delivery guide, and wherein: the feedstock line has a feedstock-line length and an exterior surface, defining an interior volume of the feedstock line; the feedstock line comprises: elongate filaments, extending along at least a portion of the feedstock-line length; a resin, covering the elongate filaments; and at least one optical modifier, having an outer surface, covered by the resin; and at least the one optical modifier is interspersed among the elongate filaments; and the feedstock line has a feedstock-line length and an exterior surface, defining an interior volume of the feedstock line; the feedstock line comprises: elongate filaments, extending along at least a portion of the feedstock-line length; a resin, covering the elongate filaments; and at least one optical modifier, having an outer surface, covered by the resin; and elongate filaments, extending along at least a portion of the feedstock-line length; a resin, covering the elongate filaments; and at least one optical modifier, having an outer surface, covered by the resin; and at least the one optical modifier is interspersed among the elongate filaments; and directing electromagnetic radiation at the exterior surface of the feedstock line after the feedstock line is deposited by the delivery guide along the print path; and wherein the step of directing electromagnetic radiation at the exterior surface of the feedstock line after the feedstock line is deposited by the delivery guide along the print path causes the electromagnetic radiation to strike the outer surface of at least the one optical modifier, which in turn causes the electromagnetic radiation to irradiate, in the interior volume of the feedstock line, the resin that, due at least in part to the elongate filaments, is not directly accessible to the electromagnetic radiation, incident on the exterior surface of the feedstock line.