## 3 Lens Construction
### Monolithic Lens
However, droplet lenses face limitations in size, and their application in complex environments, such as vibration, is also restricted. 2. Spherical DEA  
Spherical dielectric elastomer-driven DETLs are driven by hemispherical or spherical DEAs that match the shape of the lens surface. Hemispherical DEAs typically use electrode materials with excellent light transmission, and zoom is achieved by varying the pressure inside the cavity when a voltage is applied. In contrast, spherical DEAs do not have electrodes coated in the centre of the light-through aperture, and zoom is achieved by adjusting the curvature of the light-through region of the lens when a voltage is applied. [PERSON] et al. [59] developed a DETL, shown in Figure 8a. It comprises a bent PDMS film sandwiched between a gold electrode and a sodium chloride solution. Upon voltage application, the film area expands, causing the central region to bulge, changing the lens's focal length. The magnification ratio can reach 122.9% at 900 V driving voltage. Dual-cavity lenses achieve a wide range of zoom through pressure changes. [PERSON] et al. [60] proposed a dual-cavity lens configuration, as shown in Figure 8b, consisting mainly of two liquid cavities with a transparent DEA sandwiched between them. In the initial state, the liquid pressure inside the two liquid cavities differs, making the two projections have different curvatures. After applying a voltage, the driver membrane will deform, thus changing the pressure distribution inside the liquid cavities and causing the lens's focal length to change. The lens can reach 400% of the variation ratio, and the response speed is tens of milliseconds. [PERSON] et al. [61; 62] optimised the chamber parameters to achieve a rise time of 75 ms at 154.81% of the variation ratio. [PERSON] et al. [63] established a computational model based on the structure. Theoretically, they analysed the effects of the chamber radius, shear modulus, permittivity, pre-stretching ratio, and the injected liquid's volume on the lenses' tuning performance. Parameters on the lens's tuning performance show that liquid volume adjustment can achieve positive and negative focal length switching, expanding the tuning dimension. [PERSON] et al. [64] proposed an active-passive membrane structure, as shown in Figure 8c, in which the lens consists of upper and lower acrylic DE membranes: one layer adopts VHB 4905, which has been pre-stretched by 100%, as the passive membrane, and the other layer adopts VHB 4910, which has been pre-stretched by 300%, as the active driving membrane. When a voltage was applied to the active membrane, a synergistic change in the curvature of the active and passive membranes was triggered, and the magnification ratio was up to 203%. [PERSON] et al. [65] developed a lens parameter-focus mapping model based on this configuration. [PERSON] et al. [66] prepared a spherical deficient DEA-driven focusable lens, as shown in Figure 8d, with a magnification ratio of up to 132.6%. The team then conducted detailed modelling of hemispherical and ball-deficient-shaped active-passive membrane structure lenses. It analysed the effects of different parameters on the lens performance, which enriched the theoretical basis for its structural design [67]. Spherical DEA-driven DETL features a more compact structure for the same light-through aperture. However, its response may take longer after applying voltage due to the liquid pressure under initial conditions. Additionally, there could be a liquid leakage issue during its preparation, which affects the lens's stability. Figure 8: Spherical actuator-driven DETL. (**a**) Hemispherical DEA-driven liquid single-cavity lens: (**a1**) top view of the lens object; (**a2**) lens zoom schematic, the solid and dashed lines are the shapes of the lens before and after the application of the voltage, respectively; (**a3**) lens zoom effect diagram, the upper and lower diagrams are the imaging images of the lens before and after the application of the voltage, respectively [59].