Varifocal Lenses Research Articles

Surface profiling of object using varifocal lens with image contrast

The measurement of automated and fast focusing on the improvement of speed and accuracy is an important issue in industrial inspection and biomedical microscopy. In this study, we developed a novel optical imaging system in which varifocal lenses are used to characterize the 3D surface topography of samples. The performance of the proposed system was evaluated using twelve focusing algorithms. Experimental results demonstrate the feasibility and effectiveness of the proposed system in the near real-time construction of multi-focus fusion images with large depth of field from which to derive 3D surface profiles.

Varifocal zoom imaging with large area focal length adjustable metalenses

Varifocal lenses are essential components of dynamic optical systems with applications in photography, mixed reality, and microscopy. Metasurface optics has strong potential for creating tunable flat optics. Existing tunable metalenses, however, typically require microelectromechanical actuators, which cannot be scaled to large area devices, or rely on high voltages to stretch a flexible substrate and achieve a sufficient tuning range. Here, we build a 1 cm aperture varifocal metalens system at 1550 nm wavelength inspired by an Alvarez lens, fabricated using high-throughput stepper photolithography. We demonstrate a nonlinear change in focal length by minimally actuating two cubic phase metasurfaces laterally, with focusing efficiency as high as 57% and a wide focal length change of more than 6 cm (> 200%). We also test a lens design at visible wavelength and conduct varifocal zoom imaging with a demonstrated 4x zoom capability without any other optical elements in the imaging path.

MEMS-tunable dielectric metasurface lens

Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-μm movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 μm or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

Dielectric-elastomer-based fabrication method for varifocal microlens array.

We report on a method to fabricate a varifocal microlens array that employs a dielectric elastomer (DE) sandwiched between two electrodes as the lens material. The microlens array is patterned on the electrode plates, and when the electrodes are subjected to a controllable operating voltage, the DE material is "squeezed" by the Maxwell force to deform the lens array pattern, thus resulting in curvature deformation yielding a tunable lens profile. The tunable focal length performance ranges from 950 mm to infinity. When compared with liquid-filled lenses, solid-based varifocal lenses are more robust to thermal expansion, gravity, and vibrational motion. Our approach can be utilized in applications such as machine vision systems.

Pseudo-random masks for angular alignment.

We present an alignment technique that exploits angular correlations by employing a pair of masks, which encode in an angular format pseudo-random sequences. The angular correlator generates peaked irradiance distributions on-axis, provided that the elements of the pair are aligned. Otherwise, the on-axis irradiance distribution decreases to a minimum value. Since the proposed angular correlator is independent of the lateral magnification, it is useful for testing the performance of varifocal lenses. A merit function describes the tolerance to focus errors associated with the location of a small size detector. We use linearly polarized films for showing that the technique also works well with broad band light.

Diffractive array optics tuned by rotation

In this work, we apply the Alvarez–Lohmann principle for varifocal lenses to diffractive off-axis elements tuned by rotation. Two methods to combine multiple elements into arrays are presented. Further, we show that inverse phase sections result from a 2π ambiguity of the rotation. Quantization techniques are applied to eliminate these unwanted sections. As proof of concept, a retrofocus lens using a tunable diffractive lens array is presented.

Numerical simulations on the focus-shift multiplexing technique for self-referential holographic data storage

For increasing the data density of holographic data storage (HDS), combining more than two multiplexing techniques is effective. This is also true in self-referential holographic data storage (SR-HDS) that enables holographic recording purely with a single beam. In this paper, a focus-shift multiplexing technique is applied to \(xy\)-shift multiplexed SR-HDS, the feasibility of which has been shown in our previous work. The focus-shift multiplexing technique enables the multiplexing of datapages by slightly changing the focal length of the objective lens. However, the required focus-shift distance for multiplexing and the implementation method of the focus-shift have not been clarified. First, the focus-shift selectivity is investigated by the numerical simulations. In the case where the focus-shift multiplexing technique is applied to \(xy\)-shift multiplexed SR-HDS, the inter-page crosstalk properties are clarified to decide the recording layout that can achieve a low-crosstalk readout. Second, the technique of displaying an additional phase pattern onto the spatial light modulator (SLM) is introduced, which is a focus-shift method without any special optical components, such as varifocal lenses. Finally, we investigate the relationship between the accuracy of the focus-shift and the parameters of SLM.

Factors influencing survival in elderly dialysis patients

Introduction.– Poor vision and inappropriate visual aids increases falls risk. We undertook a prospective audit to evaluate the vision and use of visual aids of patients admitted to an elderly care ward and patients’ use of optometric services. Methods.– Fifty consecutive patients with MMSE>24/30 were assessed. A questionnaire regarding existing glasses, pre-existing ophthalmic conditions and use of optometric services was administered. Visual acuity (VA) testingwas undertaken using amodified 3m Snellen chart. The audit standard of vision was 6/12 as recommended by the joint British Optometrists and British Geriatrics Society Guidance. Results.– Eighty-eight percent of patients had VA worse than 6/12 in one eye and 54% in both eyes. Sixteen percent had known irreversible visual impairment. Fourteen percent had 6/60 or no vision in at least one eye with no known diagnosis. Ninety-eight percent wore glasses of which 93% were prescribed. Seventy-one percent wore bifocal or varifocal lenses with 28% wearing single vision lenses. Ninety-three percent had brought their glasses into hospital. Of these, 11%were inpoor condition. Age of glasses ranged from 1 day to 10years old. Duration since last optician review ranged from 3days to never. Thirty-eight percent were unable to attend an optician, mainly through ill health and transport unavailability. Only 1 of these patients was known to domiciliary optometric services. In total, 82% of patients reported a fall within the last 12months. Key conclusion.– Despite guidance suggesting that visual screening is a part of falls assessment, 1/3 of our surveyed patients were unable to attend optometric screening. To investigate further we plan to assess the impact of domiciliary optometric assessment in frail, community dwelling older people.

Visual impairment in elderly in-patients: A prospective survey

Introduction.– Poor vision and inappropriate visual aids increases falls risk. We undertook a prospective audit to evaluate the vision and use of visual aids of patients admitted to an elderly care ward and patients’ use of optometric services. Methods.– Fifty consecutive patients with MMSE>24/30 were assessed. A questionnaire regarding existing glasses, pre-existing ophthalmic conditions and use of optometric services was administered. Visual acuity (VA) testingwas undertaken using amodified 3m Snellen chart. The audit standard of vision was 6/12 as recommended by the joint British Optometrists and British Geriatrics Society Guidance. Results.– Eighty-eight percent of patients had VA worse than 6/12 in one eye and 54% in both eyes. Sixteen percent had known irreversible visual impairment. Fourteen percent had 6/60 or no vision in at least one eye with no known diagnosis. Ninety-eight percent wore glasses of which 93% were prescribed. Seventy-one percent wore bifocal or varifocal lenses with 28% wearing single vision lenses. Ninety-three percent had brought their glasses into hospital. Of these, 11%were inpoor condition. Age of glasses ranged from 1 day to 10years old. Duration since last optician review ranged from 3days to never. Thirty-eight percent were unable to attend an optician, mainly through ill health and transport unavailability. Only 1 of these patients was known to domiciliary optometric services. In total, 82% of patients reported a fall within the last 12months. Key conclusion.– Despite guidance suggesting that visual screening is a part of falls assessment, 1/3 of our surveyed patients were unable to attend optometric screening. To investigate further we plan to assess the impact of domiciliary optometric assessment in frail, community dwelling older people.

Focal length measurement of a varifocal liquid lens with capacitance detection

This paper reports on a detailed deformation model of varifocal liquid lenses fabricated by Parylene-on-liquid-deposition (POLD), which can be applied to measure and adjust the focal length of such lenses without using extra sensors or sensing mechanisms. The lens was fabricated by encapsulating a liquid between a transparent electrode and a polymer film that was covered with a metal electrode. When voltage is applied to the two electrodes, the lens deforms due to the electrostatic force, and its focal length and the capacitance between the two electrodes change simultaneously. This characteristic enables the focal length of the lens to be adjusted and detected by measuring the capacitance change. The focal length of the fabricated varifocal liquid lens changed from 153.6 to 82.6 mm by applying 150-V. The focal length change of the liquid lens was calculated from the change in its capacitance. Finally, to confirm the efficiency of this varifocal liquid lens, we fabricated a confocal distance sensor using the lens for laser scanning and demonstrated that this system can be used to measure distances of 94–140 mm with an average error of 0.83 mm and a standard deviation of 0.77 mm.

A transparent and stretchable graphene-based actuator for tactile display

A tactile display is an important tool to help humans interact with machines by using touch. In this paper, we present a transparent and stretchable graphene-based actuator for advanced tactile displays. The proposed actuator is composed of transparent and compliant graphene electrodes and a dielectric elastomer substrate. Since the electrode is coated onto the appointed region of the substrate layer by layer, only the area of the dielectric elastomer substrate with electrodes bumps up in response to the input voltage, which consequently produces actuation. The actuator is proven to be operable while preserving its electrical and mechanical properties even under 25% stretching. Also, the simple fabrication of the proposed actuator is cost-effective and can easily be extended to multiple arrays. The actuator is expected to be applicable to various applications including tactile displays, vari-focal lenses etc.

Adaptive optical probe design for optical coherence tomography and microscopy using tunable optics

We present a tunable, adaptive optical imaging probe for multimodal imaging such as optical coherence tomography and microscopy. The probe is compatible with forward-looking scanning laser imaging devices such as an endoscope. The lens configuration includes a tunable iris and two varifocal lenses, both driven by microelectrofluidics, as well as several conventional fixed focus lenses. The modulation transfer function and spot size in the focal plane is evaluated, and we show using optical simulations that there are three possible imaging modes with different transverse resolutions and focal depths.

Fast response varifocal lenses using KTa1−xNbxO3 crystals and a simulation method with electrostrictive calculations

We fabricated cylindrical varifocal lenses with fast responses by using the strong Kerr effect of KTa(1-x)Nb(x)O(3) (KTN) single crystals. We observed focus shifts of up to 87 mm with the assistance of a 250 mm focal length lens, which corresponds to a focus shift from infinity to 720 mm by the KTN lens itself. The response time was as fast as 1 μs. We also present a simulation method for calculating refractive index distributions in KTN single crystals, which is essential when designing the lens. The method is characterized by the strain contribution, which has not conventionally been typical of electro-optic simulations. We used this method to explain the refractive index modulations that are characteristic of the varifocal lenses.

Chapter 4 - Adaptive Lens

Publisher Summary Adaptive or varifocal lenses with variable focusing powers have attracted much attention because of their wide applications in vision care, consumer electronics such as digital cameras, aberration correction, optical interconnects, and three-dimensional biomedical imaging. The focal power of an optical system is usually changed by nonlinear movement of lenses with spherical or cylindrical powers along the optical axis and such a system consists of several lens components. A lot of efforts have been devoted to the research in adaptive lens. Recent advances in this field have been reviewed in this chapter. For liquid crystal lens, refractive and diffractive LC lenses based on discrete electrodes, hole-patterned electrodes, modal control electrode, hybrid alignment, and polymer/LC materials have been discussed in detail. For liquid lenses, lenses based on mechanic pressure, electrowetting effect, dielectric effect, and hydrogel and thermal effect have been reviewed. Examples of applications in vision care, aberration compensation, zoom lens, and optical tweezers have been discussed.

Varifocal liquid lenses with integrated actuator, high focusing power and low operating voltage fabricated on 200 mm wafers

We developed an innovative type of varifocal liquid lens actuated by electrostatic parallel plates. The 3mm diameter lens is made of a polymer membrane that encapsulates a high permittivity liquid in a cavity on top of a glass wafer. Annular electrodes situated below the membrane and on the glass wafer form the electrostatic parallel plates actuator. Applying a voltage between the electrodes reduces the gap and pushes the liquid towards the center of the lens changing the curvature of the membrane. Compared to previous liquid lenses, significant outline improvement (<6×6×0.7 mm) and supply voltage reduction (<25 V ) is demonstrated. Wave front measurements indicate an optical power change of 8 m−1 at 22V RMS that can be further improved. The lenses were fabricated on 200 mm wafers using standard microelectronics processes that make our solution a promising small outline, low voltage and low cost candidate for auto-focus devices in camera phones.

Improved optical performance of the Radon-Wigner display for one-dimensional signals

An improved optical implementation of the Radon-Wigner display for one-dimensional signals based on fractional Fourier transform is proposed. All of the fractional Fourier transform orders with the correct magnification can be achieved simultaneously by the suggested new setup with high precision, which consists of only two optical elements, a cemented linear varifocal lenses and a curvy image plane. This setup can offer practical applications for analyzing and processing of stationary or non-stationary signals.

Reduced accommodative function in dyskinetic cerebral palsy: a novel management strategy

A9-year-old boy with dyskinetic cerebral palsy secondary to neonatal encephalopathy is described. He presented with blurring of near vision which had begun to impact on his school work. Objective assessment of accommodation showed that very little was present, although convergence was almost normal. The near-vision symptoms were completely removed and reading dramatically improved with the provision of varifocal spectacles. Varifocal lenses provide an optimal correction for far, intermediate (i.e. for computer screens), and near distances (i.e. for reading). Managing this type of patient with varifocal spectacles has not been previously reported. It is clearly very important to prescribe an optimal spectacle correction to provide clear vision to optimize learning.

Curves and surfaces in the context of optometry. Part 1: Curves*

This paper introduces the differential geom-etry of curves in Euclidean 3-space, the motiva-tion being the writer’s belief that, despite their fundamental importance, curves are inadequate-ly treated in optometric educational programs. Curvature and torsion are defined along a curve. Two numerical examples are presented. The fundamental theorem of curves is stated. The relationship of the geometry of varifocal lenses and curves known as involutes are discussed. A brief treatment of the theory of contact is given with suggestions of applications in contact between spectacle lenses and frames, contact lenses and corneas (including orthokeratology), intra-ocular lenses and structures in the eye, and spectacle frames and the face.

Verification of the vertex powers of varifocal rigid contact lenses

Introduction: Increasingly aspherical optics are used in contact lens designs to produce a varifocal effect to correct presbyopia. Whilst no current ISO standard describes a method to verify these products, the vertex power needs to be verified by the manufacturer and optometrist. Visionix claim that the VC2001 instrument is able to resolve the vertex power of complex contact lenses and that it has been adopted by contact lens manufacturers as their “gold standard” method of measure. Practitioners rely on the focimeter for contact lens verification and accept that it has poorer resolution. This raises the question of whether practitioners can verify the vertex powers of varifocal contact lenses to a sufficient sensitivity. The purpose of this study is to determine whether a focimeter can differentiate between various vertex powers incorporating differing degrees of asphericity that are used in contact lens designs. Methods: A range of varifocal contact lenses were used with single vision contact lenses as controls. Three types of focimeters were used: a manual focimeter, a projector focimeter and a computerised focimeter (standard and contact lens modes). The VC2001 assessed the vertex powers over the optical zone and was used as a reference for the focimeters. The measures from the VC2001 and the focimeters were analysed by discrepancy analysis. Results: All discrepancy measures fell outside two standard deviations for either the varifocal contact lenses or the single vision contact lenses when the focimeters were compared to the VC2001. This raises the issue of whether the focimeters are measuring the same parameter as the VC2001. Only the analysis of the repeated measurements from the manual focimeter for single vision contact lenses demonstrated an acceptable measure of precision (±0.21, 2S.D.) with a very small degree of bias (mean difference −0.024, ±0.02 [2S.E.]). Conclusion: These results suggest that the manual focimeter can measure the vertex powers for single vision contact lenses to an acceptable degree of precision. The vertex powers of varifocal lenses cannot be verified using standard practice equipment. The VC2001 has poor precision for power measurements and may not be measuring the same parameter as the focimeters and therefore questions the validity of the VC2001 as the “gold standard” for vertex power measurement.

Ocular spherical aberration and theoretical through-focus modulation transfer functions calculated for eyes fitted with two types of varifocal presbyopic contact lens

Through focus modulation transfer functions (MTFs) are compared for six different cases: an aberration free eye, an eye suffering from typical amounts of undercorrected spherical aberration, and each of such eyes wearing either a Hydrocurve II or PS45 varifocal contact lens. It is assumed that the measured power profiles of the varifocal lenses, which are typical of centre distance and centre-near designs, can be approximated to primary spherical aberration, that the lenses are well centred, and that the spherical aberrations of each lens and eye are additive. It is found that in general, with a 4mm pupil, reasonable modulation transfer is maintained over a greater focal range (by about 1 dioptre) with the varifocal-wearing eyes. This implies that useful vision will be given over a greater range of object distances, as required by the presbyope. However, the maximal modulation transfer with a varifocal is degraded compared with that of the eye alone. Since the amount and sign of ocular spherical aberration may vary with the individual, it would be expected that any particular design of varifocal would be more successful with some wearers than with others.