this invention relates to optical metrology tools that are used to evaluate small measurement areas on a semiconductor wafer , where the measurement area is surrounded by a material different from the measurement area . in one embodiment , a probe beam is scanned over the measurement area and the surrounding material as data is taken at multiple locations . a processor determines the characteristics of the measurement area by identifying an extremum value of the measurements which represents the center of the measurement area . in another embodiment , the processor determines the characteristics of the sample using a combination of light measured from within and without the measurement area . the measured data is treated as a combination of light from both regions and mathematically modeled to account for both the contribution of the light reflected from the measurement area and the light reflected from the surrounding material .