Optical properties (n and k) of the material films under measurement are commonly assumed invariant and fixed in scatterometry modeling. This assumption keeps the modeling simple by limiting the number of floating parameters in the model. Such scatterometry measurement has the potential to measure with high precision some of the profile parameters (critical dimension, sidewall angle). The question is: if the optical properties modeled as “fixed” are actually changing, would this modeling assumption impact the accuracy of reported geometrical parameters? Using the example of a resist profile measurement, we quantify the “bias” effect of unmodeled variation of optical properties on the accuracy of the reported geometry by utilizing a traditional fixed n and k model. With a second model, we float an additional optical parameter and lower the bias of the reported values, at the expense of slightly increased “noise” of the measurement (more floating parameters, less precision). Finally, we extend our multistack approach (previously introduced as an enabler to the product-driven material characterization methodology) to augment the spectral information and increase both precision and accuracy through the simultaneous modeling of multiple targets.