A semiconductor memory device permitting the data \u201c0\u201d and the data \u201c1\u201d to be arbitrarily written to a reference cell capacitor for generating a reference potential, having a non-volatile capacitor for storing the data to be written. Pressure from ciliary muscle contraction moves the optic anteriorly to focus the eye for near vision. To minimize the effects of thermal transients from the pulsed cleaning beam on measurement data, cleaning pulses can be timed to fall between data samples. The dummy plug has a design such that pressure blowout is prevented.