A polishing pad includes a guide plate, a compressible foam under layer disposed adjacent to a lower surface of the guide plate, and a plurality of polishing elements that extend in a first direction substantially normal to a plane defined by the guide plate and through the guide plate. The pad further includes an optical path along the first direction and which is defined by an aperture in the compressible foam under layer and the guide plate. The optical path includes a transparent window that extends above an upper surface of the guide plate but below tips of the polishing elements, the upper surface of the guide plate being opposite the lower surface thereof. An optional slurry distribution layer may be disposed on the upper surface of the guide plate, in which case the polishing elements extend through the slurry distribution layer and the transparent window extends beyond a top surface thereof.