Irradiance control systems (“ICSs”) that control the irradiance of a beam of light are disclosed. ICSs include in a beam translator and a beam launch. The beam translator translates the beam substantially perpendicular to the propagating direction of the beam with a desired displacement so that the beam launch can remove a portion of the translated beam and the beam can be output with a desired irradiance. The beam launch attenuates the irradiance of the beam based on the amount by which the beam is translated. ISCs can be incorporated into fluorescent microscopy instruments to provide high-speed, fine-tune control over the irradiance of excitation beams.