However, the interface between the window 36 and the polishing pad 18 is sealed, so that the slurry 38 cannot leak through to the platen 16. [0027] The window 36 should have at least some of the following properties: chemical resistance to the slurry or other materials used in the polishing process; good optical clarity (e.g., at least about 25% light transmission over the wavelength range of the