An apparatus for abrading a workpiece comprises a first plate assembly having a first surface for supporting a workpiece and a second plate assembly having a second surface for engaging the workpiece to abrade a portion thereof. The process is monitored by passing emitted light via a window, a thin film narrow band filter and a \u201cFabry-Perot\u201d etalon to a detector. This decoded data C is input to a first compensation section.