1. A computer implemented polishing method comprising: storing at least one normalized predetermined spectrum; polishing a substrate; measuring a sequence of current spectra from the substrate undergoing polishing with an in-situ optical monitoring system; normalizing each current spectrum in the sequence to generate a sequence of normalized current spectra, wherein normalizing includes determining a highest value and a lowest value within a range in the current spectrum and causing each normalized current spectrum of the sequence of normalized current spectra to have a peak-to-trough value equal to a peak-to-trough amplitude of the at least one normalized predetermined spectrum; and determining a polishing endpoint using the at least one normalized predetermined spectrum and the sequence of normalized current spectra.