In this study, refractive index variation in injection-molded polymethylmethacrylate optical lenses under different levels of packing pressure was evaluated using a Shack-Hartmann wavefront sensor-based metrology system. The experimental results showed that optical polymer's refractive index exhibits a considerable amount of variation for the lenses molded under different packing pressures. Specifically, within the range investigated in this study, refractive index variation is more uniform when higher packing pressure was used but less so for lower packing pressure. Refractive index variation is believed to be the result of density variation, which itself is caused by shrinkage variation at different packing conditions. In addition, using density information in the plastic lenses predicted by finite element simulation, refractive index variation was numerically determined using the differential Lorentz-Lorenz equation and the results were in good agreement with the experiments conducted in this research. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 51-61, 2011; View this article online at wileyonlinelibrary.com. DOT 10.1002/adv.20211