Electrically Tunable Lenses: A Review

Abstract

Optical lenses with electrically controllable focal length are of growing interest, in order to reduce the complexity, size, weight, response time and power consumption of conventional focusing/zooming systems, based on glass lenses displaced by motors. They might become especially relevant for diverse robotic and machine vision-based devices, including cameras not only for portable consumer electronics (e.g. smart phones) and advanced optical instrumentation (e.g. microscopes, endoscopes, etc.), but also for emerging applications like small/micro-payload drones and wearable virtual/augmented-reality systems. This paper reviews the most widely studied strategies to obtain such varifocal "smart lenses", which can electrically be tuned, either directly or via electro-mechanical or electro-thermal coupling. Only technologies that ensure controllable focusing of multi-chromatic light, with spatial continuity (i.e. continuous tunability) in wavefronts and focal lengths, as required for visible-range imaging, are considered. Both encapsulated fluid-based lenses and fully elastomeric lenses are reviewed, ranging from proof-of-concept prototypes to commercially available products. They are classified according to the focus-changing principles of operation, and they are described and compared in terms of advantages and drawbacks. This systematic overview should help to stimulate further developments in the field.

Keywords: deformable; elastomer; electrical; lens; liquid; silicone; soft; tunable.

FIGURE 1

Proposed classification of electrically tunable lenses operating in the visible range according to a modulation of either their refractive index or their shape (surface curvature). This classification intentionally covers only principles of operation that enable a controllable focusing of multi-chromatic light, with spatially continuous (i.e. continuously tunable) wavefronts and focal lengths.

FIGURE 2

Liquid crystal lens. (A) Schematic of a possible structure (taken as an example among various alternatives) and related principle of operation: an applied voltage creates a non-uniform electric field, which spatially varies the orientation of liquid crystals, causing a non-uniform refractive index and, so, a light focusing effect; (A′) Prototype sample, reproduced with permission from (Allen, 2014).

FIGURE 3

Electrically or electro-mechanically shaped meniscus lenses. Structures and principles of operation (left) and photos of examples (right), for different actuation strategies: (A) schematic of an electrowetting lens (example among various alternatives), where an applied electric field changes the contact angle of a conductive liquid droplet; (A′) commercial product by Corning Varioptic, adapted from (Varioptic, 2021); (B) schematic of a dielectrophoretic lens (example among various alternatives), where an applied electric field changes the contact angle of an insulating liquid droplet; (B′) prototype sample, reproduced with permission from (Almoallem and Jiang, 2017); (C) schematic of an electro-mechanically controlled meniscus lens, where the curvature of a meniscus is changed by an electrically induced deformation of an annular DE actuator; (C′) prototype sample, reproduced with permission from (Rasti et al., 2015b).

FIGURE 4

Electro-mechanically shaped encapsulated-fluid lenses. Structures and principles of operation (left) and photos of examples (right), for different actuation strategies: (A) schematic of a lens based on piezoelectric bending actuators, which translate a piston that displace the fluid; (A′) prototype sample, reproduced with permission from (Hasan et al., 2017); (B) schematic of a lens based on an annular DE actuator, which radially compresses the central lens; (B′) prototype sample, reproduced with permission from (Maffli et al., 2015); (C) schematic of a lens based on a transparent DE actuator embedded on the surface, which increases its curvature; (C′) prototype sample, reproduced with permission from (Shian et al., 2013).

FIGURE 5

Hydraulically shaped lenses. Structures and principles of operation (left) and photos of examples (right), for different actuation strategies: (A) schematic of a lens pressurized by an external pump; (A′) prototype sample, reproduced with permission from (Agarwal et al., 2004); (B) schematic of a lens pressurized by an electromagnetic actuator; (B′) commercial product by Optotune, adapted from (Optotune, 2021); (C) schematic of a lens depressurized by a toroidal DE actuator; (C′) prototype sample, reproduced with permission from (Wei et al., 2014); (D) schematic of a lens pressurized by a piezoelectric bending actuator; (D′) prototype sample, reproduced with permission from (Schneider et al., 2008); (E) schematic of a lens pressurized by an electrostatic zipping actuator; (E′) prototype sample, reproduced with permission from (Hartmann et al., 2020); (F) schematic of a lens pressurized by an electro-thermal expansion of the lens fluid; (F′) prototype sample, reproduced with permission from (Ashtiani and Jiang, 2013); (G) schematic of a lens meniscus deformed by an electro-thermal contraction of a polymer; (G′) prototype sample, reproduced with permission from (Dong et al., 2006).

FIGURE 6

Electrically deformed elastomeric lens, consisting of an electrically sensitive PVC gel. (A) Schematic of a possible structure (example among various alternatives) and related principle of operation: an applied voltage creates a non-uniform electric field, which deforms the lens-shaped block of gel, changing its curvature. (A′) Prototype sample, reproduced with permission from (Bae et al., 2017).

FIGURE 7

Electro-mechanically shaped elastomeric lenses. Structures and principles of operation (left) and photos of examples (right), for different actuation strategies: (A) schematic of a lens squeezed by an external plunger ring; (A′) shape-memory-alloy-actuated prototype sample, reproduced with permission from (Choi et al., 2009); (B) schematic of a lens pulled by radial extenders; (B′) servo-motors-actuated prototype sample, reproduced with permission from (Liebetraut et al., 2013); (C) schematic of a lens operated by an electrostatic actuator, consisting of stiff annular electrodes that squeeze a soft membrane; (C′) array of prototype samples, reproduced with permission from (Wang et al., 2017); (D) schematic of a lens operated by piezoelectric bending actuators, which deform a thin glass membrane acting on the lens; (D′) commercial product by poLight, adapted from (Polight, 2021); (E) schematic of a lens operated by a transparent DE actuator, which forms the whole lens and increases its own curvature by buckling (note: unidirectional buckling should be facilitated by an initial asymmetry); (E′) prototype sample, reproduced with permission from (Son et al., 2012). (F) schematic of a lens radially compressed by an annular DE actuator; (F′) prototype sample, reproduced with permission from (Ghilardi et al., 2019); (G) schematic of a lens made of and radially stretched by transparent gel electrodes of a DE actuator; (G′) prototype sample, reproduced with permission from (Liu et al., 2020).

FIGURE 8

Electro-thermally shaped elastomeric lens: (A) Structure (example among various alternatives) and related principle of operation: an electro-thermal expansion of the polymeric lens increases its curvature; (A′) prototype sample, reproduced with permission from (Lee et al., 2007).