1. A computer program product, tangibly stored on machine readable medium, the product comprising instructions operable to cause a processor to: select a reference spectrum, the reference spectrum representing a spectrum of white light reflected from a film of interest that has a thickness greater than a target thickness; obtain a current spectrum, the current spectrum being a spectrum of white light reflected from a film of interest that is on a substrate and that has a current thickness that is greater than the target thickness, the film of interest on the substrate being subject to a polishing step; and determine, for the substrate, whether an endpoint of the polishing step has been achieved based on particular spectrum based endpoint determination logic, determining that endpoint of the polishing step has been achieved being based on the current spectrum matching the reference spectrum, the reference spectrum being empirically selected for the particular spectrum based endpoint determination logic such that if polishing is halted at a time when the current spectrum is determined to match the reference spectrum then the thickness of the film of interest corresponds to the target thickness.