An alignment system having long term stability in illumination center wavelength is discussed. The alignment system includes a tunable radiation source and a feedback control system. The tunable radiation source includes a light source configured to provide a broadband radiation beam and a tunable multi-passband filter configured to filter the broadband radiation beam into narrow band radiation beam having a center wavelength value. The feedback control system is configured to measure the center wavelength value of the narrow band radiation beam and compare the measured center wavelength value with a desired center wavelength value. The feedback control system is further configured to generate a control signal based on the comparison in response to a difference being present between the measured center wavelength value and the desired center wavelength value and tune the tunable filter based on the control signal to eliminate or substantially reduce the difference.