An optical metrology includes a library, a metrology tool and a library evolution tool. The library is generated to include a series of predicted measurements. Each predicted measurement is intended to match the measurements that a metrology device would record when analyzing a corresponding physical structure. The metrology tool compares its empirical measurements to the predicted measurements in the library. If a match is found, the metrology tool extracts a description of the corresponding physical structure from the library. The library evolution tool operates to improve the efficiency of the library. To make these improvements, the library evolution tool statistically analyzes the usage pattern of the library. Based on this analysis, the library evolution tool increases the resolution of commonly used portions of the library. The library evolution tool may also optionally reduce the resolution of less used portions of the library.