A method includes directing a beam of radiation along an optical axis toward a workpiece support, measuring a spectrum of the beam at a first time to obtain a first profile, measuring the spectrum of the beam at a second time to obtain a second profile, determining a spectral difference between the two profiles, and adjusting a position of the workpiece support along the optical axis based on the difference. A different aspect involves an apparatus having a workpiece support, beam directing structure that directs a beam of radiation along an optical axis toward the workpiece support, spectrum measuring structure that measures a spectrum of the beam at first and second times to obtain respective first and second profiles, processing structure that determines a difference between the two profiles, and support adjusting structure that adjusts a position of the workpiece support along the optical axis based on the difference.