Provided is a method of controlling multiple beams directed to a structure in a workpiece, the method comprising generating a first illumination beam with a first light source and a second illumination beam with a second light source, projecting the first and second illumination beams onto a separate illumination secondary mirror, reflecting the first and second illumination beams onto an illumination primary mirror, the reflected first and second illumination beams projected onto the structure at a first and second angle of incidence respectively, the reflected first and second illumination beams generating a first and second detection beams respectively. The separate illumination secondary mirror is positioned relative to the illumination primary mirror so as make the first angle of incidence substantially the same or close to a calculated optimum first angle of incidence and make the second angle of incidence substantially the same or close to a calculated optimum second angle of incidence. The first and second detection beams are diffracted off the structure at the corresponding angle of incidence to a detection primary mirror, reflected onto a separate secondary detection mirror and other optical components on the detection path, and onto spectroscopic detectors.