An optical metrology system collects data while scanning over the focal range. The data is evaluated to determine a peak intensity value from the data. In one embodiment, only data from one side of the peak value is used. The characteristic of the sample is determined based on the peak value. In one embodiment, monochromatic light is used. In another embodiment, polychromatic light is used and peak intensity values for a plurality of wavelengths are determine and combined to form a measured spectrum for the sample, which can then be used to determine the sample properties of interest. In one embodiment, the peak intensity is determined using curve fitting.