A technique for testing the quality of optical components of a line-narrowing module is performed on-line. As the wavelength tends to drift from a desired value when the optical components of the line-narrowing module are heated due to absorption, one or more optical components is adjusted such as by rotation to tune the wavelength back to the desired wavelength using a feedback loop. The amount of adjustment of these optical components is monitored for indirectly measuring, and thereby testing, the absorption or quality of the optical components.