**5. Experiment**

Furthermore, to verify the simulation results, we characterized the adaptive lens at different applied voltages and temperatures [7]. The adaptive lens integrated with a pressure sensor and a temperature sensor, to compensate for the non-linear piezoelectric hysteresis and the fluid thermal expansion effect [7,26], was used in the characterization. Figure 12 shows the block diagram of the experimental setup used to characterize the adaptive lens.

A sinusoidal voltage was applied to the piezoelectric actuator as was assumed in the simulation. A voltage driver was used to limit the negative voltage to −40 V, and the positive voltage to 140 V. A sensor driver was used to measure the output from the temperature and pressure sensor. The adaptive lens was mounted on a heater, which was used to heat the adaptive lens to the required temperatures. The membrane deformation was measured using a profilometer connected to a confocal displacement sensor providing a resolution of 110 nm. During the characterization, the adaptive lens was actuated and the corresponding applied voltage, membrane deformation, sensor outputs were measured simultaneously. The characterization was repeated with the adaptive lens set to higher temperatures. The refractive power was subsequently calculated from the measured membrane surface and then defined as a function of both the internal fluid pressure and the temperature. The measurements show the refractive power varying from −16 m<sup>−</sup><sup>1</sup> to 17 m<sup>−</sup><sup>1</sup> at 25 ◦C, and from −15 m<sup>−</sup><sup>1</sup> to 28 m<sup>−</sup><sup>1</sup> at 75 ◦C.

The simulated and measured refractive power of the adaptive lens at 25 ◦C, 50 ◦C and 75 ◦C with different applied voltages are compared in Figure 13. The temperature drift of the refractive power in the positive direction is higher than that in the negative direction because of the superposing effects of the thermal expansion of the fluid, which contribute to positive drift, and the increased actuator deflections at higher temperatures, which contribute to both positive and negative drift [7,19]. Hence, the superposed effect causes a net positive drift.

**Figure 12.** Experimental setup to characterize the adaptive lens at applied voltages and higher temperatures.

**Figure 13.** Comparison of the measured and simulated results of refractive power.
