A novel method is described for spatial and wavelength resolved measurements of the reflectivity of inaccessible mirrors. The method is combined with laser treatment to restore the reflectivity of diagnostic mirrors after degradation by plasma deposition. Multiple mirrors, including an aluminum first surface reference mirror are mounted symmetrically in a vacuum chamber with a viewport. The mirrors are illuminated with a large area diffuse light source and imaged with a digital microscope. The chamber can be rotated to bring each mirror into view for comparison of the reflected intensity with the reference mirror. Optical filters can be introduced to restrict the observations to a specific wavelength band. The measurements are made before and after laser treatment to restore the reflectivity.