Focal Length Changes Research Articles

Temperature effects on dielectric liquid lenses

The thermal stability of dielectric liquid lenses is studied by measuring the focal length at different temperatures. Two types of liquids lenses are investigated: Type-I (SL-5267/glycerol) and Type-II (glycerol/ BK7 matching liquid). A threshold-like behavior is found. Below the threshold temperature, the focal length is temperature insensitive. Above the threshold, the focal length changes exponentially with the temperature. Both refractive index and surface profile are responsible for the focal length change, although the former decreases linearly with the temperature. The threshold temperature of Type-I and Type-II liquid lens are 60°C and 40°C, respectively. Type-I lens shows a good temperature stability in a wide range. Moreover, the lens can recover to its original state even though it is operated at a high temperature.

Continuous zoom calibration by tracking salient points in endoscopic video.

Many image-based systems for aiding the surgeon during minimally invasive surgery require the endoscopic camera to be calibrated at all times. This article proposes a method for accomplishing this goal whenever the camera has optical zoom and the focal length changes during the procedure. Our solution for online calibration builds on recent developments in tracking salient points using differential image alignment, is well suited for continuous operation, and makes no assumptions about the camera motion or scene rigidity. Experimental validation using both a phantom model and in vivo data shows that the method enables accurate estimation of focal length when the zoom varies, avoiding the need to explicitly recalibrate during surgery. To the best of our knowledge this the first work proposing a practical solution for online zoom calibration in the operation room.

Approximate Correction of Length Distortion for Direct Georeferencing in Map Projection Frame

Many geometric distortions, such as earth curvature distortion and length distortion, exist in the map projection frame. Therefore, in aerial photogrammetry, if direct georeferencing is performed in the map projection frame, the crucial work becomes compensating for the effect of the various geometric distortions. This letter mainly focuses on length distortion and proposes two new correction approaches: the changing image coordinates method and the changing object coordinates method. The experimental results show that the changing object coordinates method is less influenced by terrain fluctuation, and its correction accuracy is therefore commonly higher than the changing image coordinates method as well as two existing approaches (i.e., the changing flight height method and the changing focal length method).

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.

Simple-structured capillary-force-dominated tunable-focus liquid lens based on the higher-order-harmonic resonance of a piezoelectric ring transducer

This paper proposes a tunable-focus liquid lens implemented with a simple cylindrical container structure and liquid as the lens material. The cylindrical container was constructed using a Pb [Zr(0.52)Ti(0.48)]O(3) (PZT) ring transducer and a polydimethylsiloxane membrane that was attached to a flat side of the transducer. The free surface of the liquid in the cylindrical container can be driven as a static-like convex lens with different curvatures because the higher-order harmonic resonance of the PZT transducer was electrically controlled. Based on a capillary-force-dominant design, the activated liquid lens maintained surface curvature in an arbitrary orientation without a gravitational effect. Profiles of the liquid lenses were characterized with the driving voltages of the transducer ranging from 12 to 60 V peak-to-peak (Vpp) at a resonant frequency of 460 kHz. The temperature effects on the lenses caused by the continuous operation of the transducer were measured. Images showed the various curvatures of the lenses with a range of actuation voltages. A change in focal length of eight times (5.72 to 46.03 cm) was demonstrated within the 10 Vpp variation of the driving voltage. For the characterized liquid lenses, the distortion was less than 2%, and the modulation transfer function reached 63 line pairs per mm (lp/mm) using ZEMAX analysis.

Variable-Focus Fluid Lens Using an Electromagnetic Actuator

A variable-focus uid lens controlled by an electromagnetic actuator and its characteristics are presented in this paper. The repulsive force between an NdFeB magnet and a solenoid through electromagnetic induction causes de ections in the elastomeric membranes integrated in the lens resulting in a focal-length change. The controllability of the focal length of the fabricated lens with an aperture size of 2 mm was approximately from in nity to 15 mm (with input current of 100 mA). The actuator including the solenoid was carefully designed to avoid thermal crosstalk of the lens uid and magnet from the electromagnetic induction of the solenoid. The settling time to form a convex shape of the lens membrane was approximately 2.5 ms at an input current of 20 mA.

Improving the Accuracy of Depth Estimation in Binocular Vision for Robotic Applications

Binocular vision or stereo vision for extraction of three-dimensional information from stereo images has been widely used in many applications like robot navigation, recovering the three-dimensional structure of a scene, and optical inspection systems. More recently, the majority of research in binocular vision has focused on the establishment of stereo matching. However, to date, there has been relatively little research conducted on the effect of computational models of binocular vision with variable focal length of lens. In this paper, a modified computational model of binocular vision is presented to develop a new depth estimation algorithm with no effect of changes in focal length. This method provides an obvious advantage in accuracy of depth estimation by reducing the effect of changing the lens focal length. The experimental results show that the proposed depth estimation method in binocular vision provides better accuracy than conventional method. Finally, we apply the new depth estimation method to a stereo-vision-based automatic docking system for a mobile robot to verify its accuracy.

Method of designing astigmatic compensation cavity for mode-locked laser based on propagation circle

The cavities of ultrashort pulsed lasers are mostly based on folded resonators with multi-mirrors. Astigmatism is an important issue to affect the performance of the mode-locked laser. An effective method of astigmatic ally compensating a continuous-wave passively mode-locked laser is presented in this paper. This method, in which the resonator propagation circle graphic theory is used, is easy and intuitive to seek the optimal location of the semiconductor saturable absorber mirror (SESAM), where the astigmatism can be compensated. Theoretical results show that the tangential and the sagittal spot size at the SESAM are equal and that the astigmatism can be compensated, when an SESAM is located at the tangential and the sagittal propagation circle intersection. The mode-locked resonator is insensitive to external perturbation including the vibration and the change of the thermal lens focal length, which leads to the instability of the mode locking operation. The antijamming ability of the resonator is outstanding. The experimental results indicate that the mode-locked laser works in a stabilized continuous-wave mode locking state and operates extremely steadily, when the SESAM is located at the tangential and the sagittal propagation circle intersection. The experimental results of the mode locking lasers show good agreement with the theoretical studies.

Silica final lens performance in laser fusion facilities: HiPER and LIFE

Nowadays, the projects LIFE (Laser Inertial Fusion Energy) in USA and HiPER (High Power Laser Energy Research) in Europe are the most advanced ones to demonstrate laser fusion energy viability. One of the main points of concern to properly achieve ignition is the performance of the final optics (lenses) under the severe irradiation conditions that take place in fusion facilities. In this paper, we calculate the radiation fluxes and doses as well as the radiation-induced temperature enhancement and colour centre formation in final lenses assuming realistic geometrical configurations for HiPER and LIFE. On these bases, the mechanical stresses generated by the established temperature gradients are evaluated showing that from a mechanical point of view lenses only fulfil specifications if ions resulting from the imploding target are mitigated. The absorption coefficient of the lenses is calculated during reactor startup and steady-state operation. The obtained results reveal the necessity of new solutions to tackle ignition problems during the startup process for HiPER. Finally, we evaluate the effect of temperature gradients on focal length changes and lens surface deformations. In summary, we discuss the capabilities and weak points of silica lenses and propose alternatives to overcome predictable problems.

Cylindrical panoramic mosaicing from a pipeline video through MRF based optimization

Stratum structure detection is a fundamental problem in geological engineering. One of the most commonly employed detection technologies is to shoot videos of a borehole using a forward moving camera. Using this technology, the problem of stratum structure detection is transformed into the problem of constructing a panoramic image from a low quality video. In this paper, we propose a novel method for creating a panoramic image of a borehole from a video sequence without the need of camera calibration and tracking. To stitch together pixels of neighboring image frames, our camera model is designed with a focal length changing feature, along with a small rotational freedom in the two-dimensional image space. Our camera model assumes that target objects lie on a cylindrical wall and that the camera moves forward along the central axis of the cylindrical wall. Based on these two assumptions, our method robustly resolves these two degrees-of-freedoms in our camera model through KLT feature tracking. Since the quality of the result video is affected by possible illumination overflow, camera lens blurring, and low video resolution, we introduce a cost function for eliminating seams between stitching strips. Our cost function is designed based on Markov Random Field and optimized using a belief propagation algorithm. Using our method, we can automatically construct a panorama image with good resolution, smoothness, and continuousness both in the texture and illumination space. Experiment results show that our method could efficiently generate panoramas for long video sequences with satisfying visual quality.

A Retrospective Photometric Study of 82 Published Reports of Mastopexy and Breast Reduction

A Retrospective Photometric Study of 82 Published Reports of Mastopexy and Breast Reduction

Adaptive properties of a liquid crystal cell with a microlens-profiled aligning surface

An array of electrically controlled adaptive microlenses produced as a liquid crystal cell with microlens-profiled aligning surface is presented. Measurements of microlens’ adaptivity are quantitatively explained by a proposed theoretical model. Details of microlens array production and characterization are presented along with the achievable electrically controlled focal length changes for nematic liquid crystals 5CB and ZLI-4801. Effects related to liquid crystal reorientation on the curved surface of the spherical lenses are discussed.

Estimation of Focal Length Variations of a 100-m Radio Telescope’s Main Reflector by Laser Scanner Measurements

Due to gravitation, the main reflector of a radio telescope underlies a deformation that causes a change in focal length depending on the variations of the elevation angle of the telescope. To estimate these gravity dependent deformations of the main reflector of the 100-m radio telescope at Effelsberg, Germany, this study proposes a measurement concept based on a laser scanner being mounted upside down on the subreflector. The measurements that have been performed at seven different elevations between 90 and 7.5° are used to estimate the focal length variation of the main reflector parameterized by a rotational paraboloid. To guarantee reliability of the adjustment, this study uses an orthogonal distance regression (ODR) rather than a classical least squares adjustment in a Gauss-Helmert model and formulates the independence of the focal length estimation from the absolute position and orientation of the main reflector in space as a requirement for a reliable adjustment approach. This investigation attests that the ODR has superior reliability with regard to this criterion. A three-step adjustment procedure based on an alteration of the ODR and several outlier eliminations is used to determine the variations of the focal length due to gravitation. The estimated focal length decreases by a maximum of 12.6 mm when tilting the reflector from 90 to 7.5° elevation angle. The postfit discrepancies between the best-fit paraboloid and the reflector’s surface are Gaussian distributed within the accuracy of the measurements. This face supports the assumption of a homologous deformation of the main reflector.

Antioxidants Prevention of Diabetic Damage in the Organ Culture Bovine Lenses

Oxidative stress represents a mechanism which could lead to diabetic cataract. We exposed bovine lenses in culture conditions for two weeks to high glucose concentration (450 mg%) and investigated the damage to the lens and possible protection by special antioxidants - N-acetyl-L-cysteine (NAC) and the zinc complex of desferrioxamine (DFO), a selective chelator for iron. We monitored the optical quality of the lenses and the oxidation of the epithelium with dichlorofluorescein (DCF) assay, as well as the changes in lens proteins profile by 2D gel electrophoresis. Under high glucose changes in lens focal length, increased oxidation, and changes in lens crystalline were observed. NAC and Zn- DFO nearly completely protected the lenses; DFO showed only partial protection. The results demonstrated that antioxidants should be considered as treatment modality protecting the lens from high glucose damage. It is proposed that a combination of NAC and Zn/DFO could prove highly efficient.

A Ray Transfer Matrix Model for an Elastomeric Lens

A Ray Transfer Matrix (RTM) of a variable-focus elastomeric fluidic lens is explored and modeled in this paper. A HeNe (543.45nm wavelength) laser is incident on the tunable lens and the effective focal length changes are explored based on this model. Results show that there are two possible focal lengths and that focal lengths are independent of the elastomer thickness.

Design of polarization-insensitive multi-electrode GRIN lens with a blue-phase liquid crystal

We report the design and simulation results of an adaptive GRIN lens based on multi-electrode addressed blue phase liquid crystal. A high dielectric constant layer helps to smoothen out the horizontal electric field and reduce the operating voltage. Such a GRIN lens is insensitive to polarization while keeping parabolic phase profile as the focal length changes.

Tunable solid-body elastomer lenses with electromagnetic actuation

We present novel biconvex solid-body elastomer (polydimethylsiloxane) lenses, which can be tuned in focal length by using magnetic or mechanical actuation. The focal length change is induced by applying radial elastic strain and is investigated for different initial radii of curvature of the lenses and different actuation designs. In all cases, a linear correlation between induced strain and focal length tuning, in the range of about 10% (approximately 3 mm), is found. These results compare favorably with finite element simulations.

Fabrication of Focus-Variable Fluidic Microlens Using Single Casting

An elastomeric microfluidic lens of variable focal length was fabricated by single casting from a microfabricated mold. The lens is a poly(dimethylsiloxane) (PDMS) microfluidic chamber covered with a thin membrane. By introducing fluid inside, the pressure varied in the microfluidic chamber, which produced a shift in the microlens focal plane resulting in a change in the back focal length of the lens. Hot pressing was used to fabricate the microfluidic chamber and membrane in a single body. The fabrication processes are simple, avoid handling difficulties of the thin membrane, and provide uniform thickness of the membrane, which essentially affect lens performance. The optical properties of the lens such as lens shape, back focal length, and numerical aperture were calculated and measured at various lens geometries and applied pressures. The back focal length of the lens changed approximately from 30 to 5 mm. The relationships of the optical properties, namely, back focal length and numerical aperture, with applied pressure were studied and verified using optical image tests. The variable focal length of the microfluidic lens is critical to increase the efficiency of light detection in optical or biophotonic applications.

Simulation and measurement of acoustic field by HIFU transducer

Objective In the treatment of tumor with high intensity focused ultrasound (HIFU),the acoustic pressure distribution determines the safety and reliability of HIFU.It is necessary to predict the acoustic field produced by HIFU transducer.Methods To analyze the acoustic pressure distribution of HIFU generated by different excitation frequency and variable incentive power,comparison was made between the acoustic pressure simulated by finite difference time domain (FDTD) method and the ones measured by experiments.Results The resultsshowed that the pressureat focalpoint increasedwiththe increasing of excitation power,while the change in the focal length was negligible.The focal point moved towards the transducer with the frequency increasing until the maximum sound pressure was gained at the resonant frequency (RF).Conclusion The simulation results are consistent with the experiment results,indicating that the acoustic fields can be predicted by simulation. Key words: Finite difference time domain method; High intensity focused ultrasound transducer; Acoustic field

Picosecond laser-induced color centers in glass optics

It is known that ionizing radiation (x ray, gamma rays, and electrons) can induce numerous changes in the physical properties of glasses. The most obvious effect is visible coloration, which is caused by the accumulation of color centers (defects) in the irradiated volume. The application of such induced color centers has prompted a renewed interest since they can be generated and bleached reversibly. We present results on laser-induced color centers in different types of glasses using picosecond laser pulses. By applying different focusing optics the influence of changing focal length and energy flux density on the volume coloring effect has been analyzed. The changes in the optical transmission compared to the unperturbed state are significant.