The electric motor comprises an internal rotor and an external stator with stator windings. The filler material forms a window or a translucent insert in the deck. The method includes the steps of replacing the main spring with a replacement main spring having a higher spring constant and replacing the peak clipping spring with a replacement peak clipping spring having a higher spring constant. A downhole tool, such as a bridge plug, frac-plug, or packer, is also provided. The resulting images are combined according to mathematical prescriptions to produce a Just Noticeable Difference, JND value, indicative of a Spatial Standard Observer, SSO.