A method and apparatus to generate an endpoint signal to control the polishing of thin films on a semiconductor wafer surface includes a through-bore in a polish pad assembly, a light source, a fiber optic cable, a light sensor, and a computer. The light source provides light within a predetermined bandwidth, the fiber optic cable propagates the light through the through-bore opening to illuminate the surface as the pad assembly orbits, and the light sensor receives reflected light from the surface through the fiber optic cable and generates reflected spectral data. The computer receives the reflected spectral data and calculates an endpoint signal by comparing the reflected spectral data with previously collected reference data. The comparison involves calculating an evaluation time based on the comparison, and calculating a difference time utilizing correlation to account for over polish/under polish. The endpoint is predicted utilizing the evaluation time and the difference time.