Accurate photoresist modeling parameters are required for correct lithographic simulations. In particular, three sets of data are required to model a typical non-chemically amplified resist: the refractive index as a function of wavelength and exposure, the exposure or ABC parameters (the Dill parameters), and the development parameters. This work focuses on an improved technique for the accurate extraction of development rate parameters for non-chemically amplified resists. Traditionally, the refractive index for photoresists is considered constant in simulations and other work. However, the refractive index of a photoresist varies as the chemical composition of the photoresist changes during exposure. This work presents a rigorous analysis technique for extracting development parameters from resist development rate data which accounts for this refractive index change. Development parameter measurements for a commercial resist are discussed. Comparisons of the various development rate models are made using this experimental data.