In a laser annealing system for workpieces such as semiconductor wafers, a pyrometer wavelength response band is established within a narrow window lying between the laser emission band and a fluorescence emission band from the optical components of the laser system, the pyrometer response band lying in a wavelength region at which the optical absorber layer on the workpiece has an optical absorption coefficient as great as or greater than the underlying workpiece. A multi-layer razor-edge interference filter having a 5-8 nm wavelength cut-off edge transition provides the cut-off of the laser emission at the bottom end of the pyrometer response band.