Focus-tunable Lens Research Articles

Flexible Calibration Method of Electronically Focus-Tunable Lenses

The calibration of camera systems employing zoom lenses raises several challenges, as changes in lens control settings introduce various imaging conditions, especially different degrees of defocus. This article presents a Fourier-based defocus calibration method to calibrate a camera attached with an electronically focus-tunable lens (ETL). Different from the traditional method which requires changing the scale of the calibration object for sharp images, the proposed method is more concerned with the frequency characteristics of the captured images than the gray distributions. According to the invariance of the fundamental frequencies before and after defocusing the images, coordinates of feature points can be extracted accurately, even in different degrees of defocus. Therefore, one normal size target can also be qualified for zoom system calibration. Then polynomial approximation is used to account for the variation of the intrinsic and distortion parameters across the zoom range. Extensive simulations and experiments were implemented to demonstrate the proposed idea's robustness against defocus, flexibility, and accuracy.

Soft Tunable Lenses Based on Zipping Electroactive Polymer Actuators.

Compact and entirely soft optics with tunable and adaptive properties drive the development of life‐like soft robotic systems. Yet, existing approaches are either slow, require rigid components, or use high operating voltages of several kilovolts. Here, soft focus‐tunable lenses are introduced, which operate at practical voltages, cover a high range of adjustable focal lengths, and feature response times in the milliseconds range. The nature‐inspired design comprises a liquid‐filled elastomeric lens membrane, which is inflated by zipping electroactive polymers to tune the focal length. An analytic description of the tunable lens supports optimized designs and accurate prediction of the lens characteristics. Focal length changes between 22 and 550 mm (numerical aperture 0.14–0.005) within 260 ms, equal in performance to human eyes, are demonstrated for a lens with 3 mm aperture radius, while applying voltages below 500 V. The presented model, design rules, and fabrication methods address central challenges of soft electrostatic actuators and optical systems, and pave the way toward autonomous bio‐inspired robots and machines.

Performance enhancement of DFBL based near‐infrared CH4 telemetry system using a focus tunable lens

AbstractA methane telemetry system for 1653 nm DFB laser based on TDLAS‐WMS technology is developed in this article. The focus tunable lens is used as the collimating system of the telemetry device to solve the problem that the telemetry device cannot be dynamically adjusted under different detection environments. Experimental results show that the root mean square error was 7.6205 and the theoretical limit of detection (LoD) was 1.473 parts per million (ppm) while the optimal integration time reaches 30 s. The near‐infrared CH4 telemetry system is suitable for different detection environments of natural gas leakage and has good detection performance and stable and reliable operation.

Tunable-focus liquid lens actuated by a novel piezoelectric motor

In this paper, a tunable-focus liquid lens driven by a novel piezoelectric motor is proposed for the adaptive application. Compressing the liquid chamber, the curvature of the lens can be changed by increasing the liquid pressure. This mechanism requires the motor to provide a vertical force to deform the lens surface, whose curvature can be decreased and thereby increasing its focus. As a key part of the tunable-focus liquid lens, a novel piezoelectric motor with the compact structure is emphatically developed in this paper. The operation process of the motor is discussed in detail, whose geometrical parameters are calculated by the finite element simulations. And the motor prototype is then fabricated and tested by the experimental platform. The testing results indicate that the motor can operate steadily and continuously, whose maximum linear velocity can reach 0.065 mm/s under the frequency of 11.80 kHz and voltage of 400 [Formula: see text]. The measurement shows that the proposed lens driven by the piezoelectric motor can zoom ranged from 9.6 mm to 17.9 mm, which is suitable for adaptive eyeglass application. Compared with other liquid lens, the prototype with a compact structure, easy and low cost fabrication process can provide high-precision adjustment within a certain range. The presented device exhibits well zooming characteristic and stability in the experiment, which also realizes the successful application of piezoelectric motor in the liquid-lens. It shows greatly potential in the adaptive eyeglasses, and may be employed as the mobile system in the near future.

Large area liquid crystal lenses for correction of presbyopia.

Presbyopia is the failure of the eye lens to accommodate. The widely used presbyopia correction method involves wearing bi/trifocal or progressive glasses, which limits the field of view due to division of lens area into sections of different optical power. A large aperture focus tunable liquid crystal lens has the potential to correct human eye accommodation failure and provide a wide field of view. In this paper, we present characterization and demonstration of a segmented phase profile liquid crystal lens, which has the characteristics of a large area (diameter: 20 mm), being flat and thin (<2 mm), and having continuous focus tunability (1.5 D to 0 D), fast response time (<500 ms), low operating voltage (<5 V), and on-axis diffraction-limited performance (for a 5mm aperture). Considering all these properties, our lens provides performance details of an approach for presbyopia correction. We have tested the minimum resolution and visual acuity of 20 subjects using the designed lens and compared the results with a reference glass lens of the same optical power.

Monolithic focus-tunable lens technology enabled by disk-type dielectric-elastomer actuators

We propose a monolithic focus-tunable lens structure based on the dielectric-elastomer actuator (DEA) technology. In our focus-tunable lens, a soft lens and radial in-plane actuator mimicking the ocular focal-tuning mechanism are constructed in a single body of an optimized dielectric-elastomer film. We provide device fabrication methods including elastomer synthesis, structure formation, and packaging process steps. Performance test measurements show 93% focal tunability and 7 ms response time under static and dynamic electrical driving conditions, respectively. These performance characteristics are substantially enhanced from the previous polylithic DEA tunable lens by a factor 1.4 for the focal tunability and a factor 9.4 for the dynamic tuning-speed limit. Therefore, we obtain greatly enhanced focal tuning control in a remarkably simple and compact device structure.

50X five-group inner-focus zoom lens design with focus tunable lens using Gaussian brackets and lens modules.

This study presents a 50X five-group zoom lens design with two moving groups and one focus tunable lens (FTL) by applying Gaussian brackets and lens modules. After qualitative analysis of the paraxial properties, the initial structure is obtained by solving the power equations consisting of five groups by numerical analysis method. The optimized nearly 60X lens has a focal length of 5.1-300mm and f-number of 1.8-5.0 in wide and tele positions, with a total length of 160mm. The lens achieves a maximum FOV of 63° at short focal length. The tolerance analysis indicates that the design is suitable for mass production. The method eases the difficulty in initial structure design of the zoom lens with high zoom ratio, improves its optical design efficiency, and can be applied to the fields such as customs security cameras, military detecting devices, machine vision and industrial inspection, etc.

Origin of oblique optical axis of electrically tunable focusing lenses arising from initial anisotropic molecular tilts under a symmetric electric field. I

The optical axes of electrically tunable liquid crystal (LC) lenses are usually tilted, and the corresponding asymmetric wavefront aberrations hinder the lenses from being diffraction-limited. Looking back at the literature studies since 1979, researchers used different approaches to compensate the tilting of the optical axis for achieving a perfect lens, or they utilized the intrinsically asymmetric wavefront aberration as a physical-planar free-form optics. However, the physics behind axis tilting has not been discussed yet, and the origin of the oblique optical axes of LC lenses in terms of anisotropic properties of molecules requires an investigation. In this paper, we study the origin of the asymmetrical tilting of optical axes of LC lenses. We found that the initial anisotropic molecular tilts (the so-called pretilt angle) result in the dielectric torque difference even under a rotationally symmetric electric field. Moreover, it is discovered that the rotational symmetry of the wavefront can be broken by generating uneven tilt angles of the LC molecules even though the electric potential is rotationally symmetric. Numerical results are presented and discussed to illustrate the mechanism.

Speckle reduced holographic displays using tomographic synthesis.

Despite significant merit of depth representation, holographic displays have a considerable limitation: speckle. Here, we present speckle reduced holographic displays using an engineered light source with angle diversity for speckle reduction. The level of angle diversity is optimized with consideration of resolution, speckle contrast, and depth of field. To extend the depth of field sacrificed for speckle reduction, we apply tomographic synthesis, exploiting synchronization of a local illumination module and a tunable-focus lens. We implement a benchtop prototype to verify the proposed method, which reduces the speckle contrast averagely by 37.8% while preserving resolution and 4.0 diopter depth of field.

High-speed coherent diffraction imaging by varying curvature of illumination with a focus tunable lens.

A high-speed coherent diffraction imaging method is proposed by varying the curvature of illumination with a focus tunable lens. The imaging setup is free of conventional mechanical translation and takes only milliseconds to refocus by changing the electric signal applied on the lens. It is more compact and also an inexpensive alternative to coherent diffraction imaging with computerized translational stages. A detector that is kept at a fixed distance from the sample records diffraction patterns each time the spherical wavefront illuminations on the sample is changed with a control current. The complex wavefront of the object is then quantitatively recovered from the diffraction intensity measurements using an iterative phase retrieval algorithm. The feasibility of the proposed method is experimentally verified using various samples. Extremely short response time of the focus tunable lens makes the proposed method highly suitable for applications that requires high speed imaging.

Volumetric Head-Mounted Display With Locally Adaptive Focal Blocks.

A commercial head-mounted display (HMD) for virtual reality (VR) presents three-dimensional imagery with a fixed focal distance. The VR HMD with a fixed focus can cause visual discomfort to an observer. In this article, we propose a novel design of a compact VR HMD supporting near-correct focus cues over a wide depth of field (from 18 cm to optical infinity). The proposed HMD consists of a low-resolution binary backlight, a liquid crystal display panel, and focus-tunable lenses. In the proposed system, the backlight locally illuminates the display panel that is floated by the focus-tunable lens at a specific distance. The illumination moment and the focus-tunable lens' focal power are synchronized to generate focal blocks at the desired distances. The distance of each focal block is determined by depth information of three-dimensional imagery to provide near-correct focus cues. We evaluate the focus cue fidelity of the proposed system considering the fill factor and resolution of the backlight. Finally, we verify the display performance with experimental results.

Measurement of a flow-velocity profile using a laser Doppler velocimetry coupled with a focus tunable lens

This paper reports on the measurement of a flow-velocity profile using a laser Doppler velocimetry (LDV) system having a focus tunable lens (FTL). In the system, the FTL is installed in the transmitting optics of the LDV; therefore, it can measure the flow velocity profile by changing the measurement position without any mechanical scanning system. To demonstrate the concept of the technique, the velocity profile measurement of Poiseuille flow was conducted, and the measured velocity profile showed good agreement with the theoretical value.

Quasi-simultaneous multi-focus imaging using a lock-in pixel image sensor and TAG lens.

In this paper, a quasi-simultaneous multi-focus imaging technique named simulfocus imaging is reported. This technique was developed for measuring an entire object distributed in the depth direction beyond the depth of field (DOF) with high resolution in a single shot. Simulfocus imaging can acquire multiple focal planes in one shot by synchronizing a tunable acoustic gradient index (TAG) lens and a lock-in pixel image sensor. The TAG lens is a tunable-focus lens whose focal position can be changed at a high speed of several tens to several hundreds of kilohertz. The lock-in pixel image sensor is a special image sensor that can execute multiple exposures at an arbitrary timing during a single shooting. The sensor includes a number of photoelectron storage units in each pixel, and the units where the photoelectrons generated by each exposure are stored can be freely selected. Since an image can be acquired for a single storage unit, and the lock-in pixel image sensor has a number of storage units, the lock-in pixel image sensor can acquire multiple images in one shot. By assigning a specific exposure timing to each unit and synchronizing the exposure timing with the focus fluctuation of the TAG lens, it is possible to simultaneously acquire images in different focal planes. To evaluate the system, we conducted experiments to show the effectiveness of simulfocus imaging in microscope and telescope configurations. From the experimental results, it was confirmed that simulfocus was effective in both configurations.

Uniaxial three-dimensional phase-shifting profilometry using a dual-telecentric structured light system in micro-scale devices

This paper presents a novel method for uniaxial microscopic three-dimensional (3D) profilometry using a structured light system with dual-telecentric lenses in micro-scale devices. Specifically, a telecentric lens can produce an orthographic view of an object and provide the exact size of objects in the x and y directions. An electronically focus-tunable lens attached to the projector rapidly and precisely changes the focal plane of the projected structured patterns, a camera captures the structured patterns from the same perspective and a computational framework analyzes the relationship between the captured structure images and the drive current of the electronically focus-tunable lens to obtain depth information for each pixel. By replacing the normal lens with telecentric lenses, the proposed method dodges perspective error and makes precision measurement easier. We developed a shadow-free coaxial 3D shape measurement system with dual-telecentric lenses that achieved a spatial resolution of approximately 5.86 µm and 4.08 µm root-mean-square error with a depth range of 1200 µm.

Broadband tunable focusing lenses by acoustic coding metasurfaces

Acoustic tunable focusing lens has important applications in acoustic fast imaging and three-dimensional displays. Previously developed metasurface lenses suffer from narrow bandwidth and fixed geometric configuration, thereby limiting their further practical applications. Here, we design an acoustic coding metasurface (ACM) lens to achieve a broadband tunable focusing in air. This simple ACM lens only includes two coding bits: a space-folding structure is designed to act as the bit ‘1’ and an air unit with the same size is used as the bit ‘0’. By adjusting the coding sequence of the ACM, the focus location of the ACM lens can be modified efficiently and be steered in the whole working plane. Moreover, the focusing behaviors of the ACM lens are well performed in a large frequency range of ∼3.1 to ∼4.6 kHz. Our ACM lens may be beneficial to applications that traditionally need different and complicated structures for tunable acoustic focusing.

Autofocusing method for a digital fringe projection system with dual projectors.

This paper presents a novel technique to achieve autofocusing for a three-dimensional (3D) profilometry system with dual projectors. The proposed system uses a camera that is attached with an electronically focus-tunable lens (ETL) that allows dynamic change of camera's focal plane such that the camera can focus on the object; the camera captures fringe patterns projected by each projector to establish corresponding points between two projectors, and two pre-calibrated projectors form triangulation for 3D reconstruction. We pre-calibrate the relationship between the depth and the current being used for each focal plane, perform a 3D shape measurement with an unknown focus level, and calculate the desired current value based on the initial 3D result. We developed a prototype system that can automatically focus on an object positioned between 450 mm to 850 mm.

Hysteresis characterization of an electrically focus-tunable lens

Electrically focus-tunable lenses are a type of lens whose focal distance can be varied and controlled electronically and fast. These advantages are making them suitable for many optical applications and research experiments in fields like visual optics, optical metrology, optical information processing, nonlinear optical-properties measurements, among others. For most applications, the focal distance must be known with precision and reliability, becoming an important issue that the focal distance programmed on the tunable lens corresponds to the truly obtained focal distance. However, recently, a research article has presented evidence that these tunable lenses have hysteresis. In this work, we confirmed the hysteresis of three tunable lenses, determined how predictable is the focal distance programmed under different schemes, corroborated the existence of an optical-power time drift, and proposed two schemes to be able to predict the optical-power outcome with reliability. A case of study is presented to highlight the importance of the characterization.

Electrically modulated varifocal metalens combined with twisted nematic liquid crystals.

Focus-tunable lenses are indispensable to optical systems. This paper proposes an electrically modulated varifocal metalens combined with twisted nematic liquid crystals. In our design, a metalens is employed to focus on different points depending on the polarization state of incident light. We demonstrated that the varifocal metalens has a sub-millisecond response time. Furthermore, the numerical aperture of both the first and second focal points can be customized to achieve a wide range of 0.2-0.7. Moreover, the full width at half maximum approached the diffraction limit at multiple focal points. Because of the advantages of our proposed electrically modulated metalens, it has the potential for application in optical technology and biomedical science, both of which require high image quality and a rapid response time.

Bulk ZnSe and CdS two-photon absorption measurement with an F-scan nonlinear absorption spectrometer

Using an electrically focus tunable lens (EFTL) in an F-scan setup and a tunable femtosecond-pulse laser (Mai Tai HP), we were able to measure the degenerated two-photon absorption coefficient (in transmission) of CdS and ZnSe in an extended range of wavelengths (690-1040 nm), with a 5 nm resolution. The process of measuring takes less than 30 minutes. We compared our results with theoretical approaches for the dispersion relations of the nonlinear properties of semiconductors and found excellent agreement with the experimental results. We also compare our results with those reported in the literature. We derive the nonlinear refraction using a Kramers-Kronig relation and compare it with the values reported in the literature. The system has no moving parts, is highly compact, and is fully automated.

Illuminated Focus: Vision Augmentation using Spatial Defocusing via Focal Sweep Eyeglasses and High-Speed Projector.

Aiming at realizing novel vision augmentation experiences, this paper proposes the IlluminatedFocus technique, which spatially defocuses real-world appearances regardless of the distance from the user's eyes to observed real objects. With the proposed technique, a part of a real object in an image appears blurred, while the fine details of the other part at the same distance remain visible. We apply Electrically Focus-Tunable Lenses (ETL) as eyeglasses and a synchronized high-speed projector as illumination for a real scene. We periodically modulate the focal lengths of the glasses (focal sweep) at more than 60 Hz so that a wearer cannot perceive the modulation. A part of the scene to appear focused is illuminated by the projector when it is in focus of the user's eyes, while another part to appear blurred is illuminated when it is out of the focus. As the basis of our spatial focus control, we build mathematical models to predict the range of distance from the ETL within which real objects become blurred on the retina of a user. Based on the blur range, we discuss a design guideline for effective illumination timing and focal sweep range. We also model the apparent size of a real scene altered by the focal length modulation. This leads to an undesirable visible seam between focused and blurred areas. We solve this unique problem by gradually blending the two areas. Finally, we demonstrate the feasibility of our proposal by implementing various vision augmentation applications.