An additive manufacturing system, the additive manufacturing system comprising: an additive manufacturing machine and a control system; wherein the additive manufacturing machine comprises: one or more irradiation devices (138, 140), the one or more irradiation devices (138, 140) respectively comprising: a beam source configured to emit an energy beam (142, 148), an optical assembly comprising one or more optical elements configured to focus the energy beam emitted by the beam source, a beam source sensor configured to determine a beam source sensor value from a source measurement beam representative of the energy beam prior to the energy beam passing through one or more optical elements of the optical assembly, and an optics sensor configured to determine an optics sensor value from an optics measurement beam representative of the energy beam downstream from the one or more optical elements of the optical assembly; and wherein the control system comprises: an irradiation control module configured to provide one or more control commands to the additive manufacturing machine based at least in part on the beam source sensor value and/or based at least in part on the optics sensor value; characterised in that, the one or more irradiation devices comprises: a source measurement beam splitter configured to split a portion of the energy beam emitted from the energy beam source to provide the source measurement beam to the beam source sensor; and an optics measurement beam splitter configured to split a portion of the energy beam downstream from the one or more optical elements of the optical assembly to provide the optics measurement beam to the optics sensor. an additive manufacturing machine and a control system; wherein the additive manufacturing machine comprises: one or more irradiation devices (138, 140), the one or more irradiation devices (138, 140) respectively comprising: a beam source configured to emit an energy beam (142, 148), an optical assembly comprising one or more optical elements configured to focus the energy beam emitted by the beam source, a beam source sensor configured to determine a beam source sensor value from a source measurement beam representative of the energy beam prior to the energy beam passing through one or more optical elements of the optical assembly, and an optics sensor configured to determine an optics sensor value from an optics measurement beam representative of the energy beam downstream from the one or more optical elements of the optical assembly; and wherein the control system comprises: an irradiation control module configured to provide one or more control commands to the additive manufacturing machine based at least in part on the beam source sensor value and/or based at least in part on the optics sensor value; characterised in that, the one or more irradiation devices comprises: a source measurement beam splitter configured to split a portion of the energy beam emitted from the energy beam source to provide the source measurement beam to the beam source sensor; and an optics measurement beam splitter configured to split a portion of the energy beam downstream from the one or more optical elements of the optical assembly to provide the optics measurement beam to the optics sensor. a beam source configured to emit an energy beam (142, 148), an optical assembly comprising one or more optical elements configured to focus the energy beam emitted by the beam source, a beam source sensor configured to determine a beam source sensor value from a source measurement beam representative of the energy beam prior to the energy beam passing through one or more optical elements of the optical assembly, and an optics sensor configured to determine an optics sensor value from an optics measurement beam representative of the energy beam downstream from the one or more optical elements of the optical assembly; and wherein the control system comprises: an irradiation control module configured to provide one or more control commands to the additive manufacturing machine based at least in part on the beam source sensor value and/or based at least in part on the optics sensor value; characterised in that, the one or more irradiation devices comprises: a source measurement beam splitter configured to split a portion of the energy beam emitted from the energy beam source to provide the source measurement beam to the beam source sensor; and an optics measurement beam splitter configured to split a portion of the energy beam downstream from the one or more optical elements of the optical assembly to provide the optics measurement beam to the optics sensor. a source measurement beam splitter configured to split a portion of the energy beam emitted from the energy beam source to provide the source measurement beam to the beam source sensor; and an optics measurement beam splitter configured to split a portion of the energy beam downstream from the one or more optical elements of the optical assembly to provide the optics measurement beam to the optics sensor.