A belt (12) for polishing a workpiece (11) such as a semiconductor wafer in a chemical mechanical polishing system (10) includes an endless belt. The belt (12) has an aperture (40). The aperture is unobstructed, such as using no window. The system (10) includes a belt (12), a first roller (14), a second roller (13), a platen (25), a polishing head (18), a slurry dispenser (21), a conditioner (20), a monitor (28) and a controller (30). One or more apertures (40) may be provided through the belt (12). Preferably, three apertures (40) are evenly spaced apart along the length of the belt (12). The apertures are positioned in the belt (12) between the edges (19). The monitor (28) uses the aperture (40) to measure a property of the wafer (11). The monitor (28) comprises a light emitting and reception device for determining the type of film and/or film thickness of the surface of the wafer (11).