Focal Length Research Articles

Mammalian lures monitored with time-lapse cameras increase detection of pythons and other snakes

Background Enhancing detection of cryptic snakes is critical for the development of conservation and management strategies; yet, finding methods that provide adequate detection remains challenging. Issues with detecting snakes can be particularly problematic for some species, like the invasive Burmese python (Python bivittatus) in the Florida Everglades. Methods Using multiple survey methods, we predicted that our ability to detect pythons, larger snakes and all other snakes would be enhanced with the use of live mammalian lures (domesticated rabbits; Oryctolagus cuniculus). Specifically, we used visual surveys, python detection dogs, and time-lapse game cameras to determine if domesticated rabbits were an effective lure. Results Time-lapse game cameras detected almost 40 times more snakes (n = 375, treatment = 245, control = 130) than visual surveys (n = 10). We recorded 21 independent detections of pythons at treatment pens (with lures) and one detection at a control pen (without lures). In addition, we found larger snakes, and all other snakes were 165% and 74% more likely to be detected at treatment pens compared to control pens, respectively. Time-lapse cameras detected almost 40 times more snakes than visual surveys; we did not detect any pythons with python detection dogs. Conclusions Our study presents compelling evidence that the detection of snakes is improved by coupling live mammalian lures with time-lapse game cameras. Although the identification of smaller snake species was limited, this was due to pixel resolution, which could be improved by changing the camera focal length. For larger snakes with individually distinctive patterns, this method could potentially be used to identify unique individuals and thus allow researchers to estimate population dynamics.

Investigation of an Active Focusing Planar Piezoelectric Ultrasonic Transducer.

Ultrasonic focusing transducers have broad prospects in advanced ultrasonic non-destructive testing fields. However, conventional focusing methods that use acoustic concave lenses can disrupt the acoustic impedance matching condition, thereby adversely affecting the sensitivity of the transducers. In this paper, an active focusing planar ultrasonic transducer is designed and presented to achieve a focusing effect with a higher sensitivity. An electrode pattern consisting of multiple concentric rings is designed, which is inspired by the structure of Fresnel Zone Plates (FZP). The structural parameters are optimized using finite element simulation methods. A prototype of the transducer is manufactured with electrode patterns made of conductive silver paste using silk screen-printing technology. Conventional focusing transducers using an acoustic lens and an FZP baffle are also manufactured, and their focusing performances are comparatively tested. The experimental results show that our novel transducer has a focal length of 16 mm and a center frequency of 1.16 MHz, and that the sensitivity is improved by 23.3% compared with the conventional focusing transducers. This research provides a new approach for the design of focusing transducers.

Comparison on performances of continuous and pulsed operation in anti-misalignment laser of Er:YAG and Er:LuAG crystals

The thermal focal lengths, absorption efficiencies, and laser performances of Er:YAG and Er:LuAG crystals are systematically demonstrated within an anti-misalignment double corner-cube-prisms resonator. The maximum continuous-wave output power of 5.12 W at 1645.39 nm and 6.43 W at 1650.33 nm are obtained, with slope efficiencies of 16.29 % and 36.78 %. In pulsed operation, Er:YAG yielded a maximum pulse energy of 5.1 mJ, and Er:LuAG delivered 4.7 mJ, both at the pulse repetition rate of 200 Hz. The beam qualities, represented by M2 factors, are measured at 1.52 for Er:YAG and 1.44 for Er:LuAG.

Investigation on the effect of laser surface texturing on the wettability of pure zinc substrate

Zinc finds a prominent metallic alternative for existing metallic bio-implant materials. Wettability significantly alters the corrosion resistance and cell adhesion of bio-metallic materials. The wettability of the metallic materials can be engineered by laser surface texturing (LST). In this work, nanosecond laser surface texturing experiments were conducted on the pure zinc substrate by varying the laser scan speed and texture pitch. Laser scan speeds were varied from 10 to 50 mm/s, and the texture pitch varied from 100 to 300 µm. A nanosecond 28 W Nd:YAG laser system was used with 1064 nm wavelength, repetition rate of 5 kHz, focal length of 234 mm, and pulse width of 2 ms. The white light interferometer (WLI) was utilized to explore the surface roughness of the textured samples, and the scanning electron microscope (SEM) was used to investigate the surface morphology. The results revealed that LST on pure zinc follows the Weinzel model which enhances the hydrophobic nature of its surface. Surface roughness plays a significant role in improving the hydrophobic nature of the LST zinc substrates.

Proficient Calibration Methodologies for Fixed Photogrammetric Monitoring Systems

This work focuses on investigating the accuracy of 3D reconstructions from fixed stereo-photogrammetric monitoring systems through different camera calibration procedures. New reliable and effective calibration methodologies that require minimal effort and resources are presented. A full-format camera equipped with fixed 50 and 85 mm focal length optics is considered, but the methodologies are general and can be applied to other systems. Four different calibration strategies are considered: (i) full-field calibration (FF); (ii) multi-image on-the-job calibration (MI); (iii) point cloud-based calibration (PC); and (iv) self (on-the-job) calibration (SC). To evaluate the calibration strategies and assess their actual performance and practicality, two test sites are used. The full-field calibration, while very reliable, demands significant effort if it needs to be repeated. The multi-image strategy emerges as a favourable compromise, offering good results with minimal effort for its realisation. The point cloud-based method stands out as the optimal choice, balancing ease of implementation with quality results; however, it requires a reference 3D point cloud model. On-the-job calibration with monitoring images is the simplest but least reliable option, prone to uncertainty and potential inaccuracies, and should hence be avoided. Ultimately, prioritising result reliability over absolute accuracy is paramount in continuous monitoring systems.

Stretchable Dual-Axis Terahertz Bifocal Metalens with Flexibly Polarization-Dependent Focal Position and Direction.

Varifocal lenses are essential components in any optical system, while traditional lenses suffer from bulky volume, fixed focal position, and limited working spectra. As well-arranged subwavelength structures, metalenses overcome the abovementioned obstacles and exhibit merits of ultrathin thickness, flexible focal length, and multifocus. The electromagnetic responses of metasurfaces, including metalens, rely on the phase distributions of phase-shifting elements. The steerable focal direction is investigated to obtain the combinations of focusing and anomalous refraction phase distribution. To fully explore the flexibility of focal length and direction, seven designs of double layers of terahertz (THz) bifocal metalenses are proposed and investigated in this study. They exhibit dependent and independent relationships of tunable focal length and direction with flexible tuning mechanisms. Along with polarization multiplexing, two different focuses can be obtained when the incident waves are x-linear and y-linear polarization states, respectively. The simulation results agreed well with the theoretical predictions. These designs provide a new method to modulate the focal position precisely with promising applications in wireless communication, imaging, and on-chip optical integration systems.

Continuous Optical Zoom Compound Eye Imaging Using Alvarez Lenses Actuated by Dielectric Elastomers.

The compound eye is a natural multi-aperture optical imaging system. In this paper, a continuous optical zoom compound eye imaging system based on Alvarez lenses is proposed. The main optical imaging part of the proposed system consists of a curved Alvarez lens array (CALA) and two Alvarez lenses. The movement of the CALA and two Alvarez lenses perpendicular to the optical axis is realized by the actuation of the dielectric elastomers (DEs). By adjusting the focal length of the CALA and the two Alvarez lenses, the proposed system can realize continuous zoom imaging without any mechanical movement vertically to the optical axis. The experimental results show that the paraxial magnification of the target can range from ∼0.30× to ∼0.9×. The overall dimensions of the optical imaging part are 54 mm × 36 mm ×60 mm (L × W × H). The response time is 180 ms. The imaging resolution can reach up to 50 lp/mm during the optical zoom process. The proposed continuous optical zoom compound eye imaging system has potential applications in various fields, including large field of view imaging, medical diagnostics, machine vision, and distance detection.

Harmonic Detection Methods and Optimization Strategies for Charger in Power Grid

The charger’s operation in the power grid will cause harmonic pollution, affecting the service life of the equipment and the quality of the power grid. The harmonic characteristics and detection during the charging process are studied. The simulation model of charger is established first using MATLAB, the results of the experiment show that odd harmonics are mainly generated during charging, and the distortion rate of the current harmonics remains between 0.231 and 0.443 during the charging process. A new wavelet neural network is proposed to detect the harmonic method. Experimental results indicate that, compared with previous detection methods, this network has better detection efficiency and accuracy for harmonics. But detection result of this network is unstable and does not even converge. A new adaptive bat algorithm with improved focusing distance is presented here to optimize the wavelet neural network, which effectively solves the defect of being sensitive to the node number of hidden layer, resulting in higher detection accuracy, faster detection speed, and stable convergence. The detection error range is reduced from −0.03 to 0.02, and the number of iterations is reduced to around 1000.

Microsphere lens array embedded microfluidic chip for SERS detection with simultaneous enhancement of sensitivity and stability

Surface enhanced Raman spectroscopy (SERS) utilizes the fingerprint features of molecular vibrations to identify and detect substances. However, in traditional single focus excitation scenarios, its signal collection efficiency of the objective is restricted. Furthermore, the uneven distribution of samples on the SERS substrate would result in poor signal stability, while the excitation power is limited to avoid sample damage. SERS detection system always requires precise adjustment of focal length and spot size, making it difficult for point-of-care testing applications. Here, we proposed a SERS microfluidic chip with barium titanate microspheres array (BTMA) embedded using vacuum self-assembled hot-pressing method for SERS detection with simultaneous enhancement of sensitivity and stability. Due to photonic nano-jets and directional antenna effects, high index microspheres are perfect micro-lens for effective light focusing and signal collecting. The BTMA can not only disperse excitation beam into an array of focal points covering the target uniformly with very low signal fluctuation, but enlarge the power threshold for higher signal intensity. We conducted a proof-of-principle experiment on chip for the detection of bacteria with immuno-magnetic tags and immuno-SERS tags. Together with magnetic and ultrasonic operations, the target bacteria in the flow were evenly congregated on the focal plane of BTMA. It demonstrated a limit of detection of 5 cells/mL, excellent signal reproducibility (error∼4.84%), and excellent position tolerance of 500 μm in X–Y plane (error∼5.375%). It can be seen that BTMA-SERS microfluidic chip can effectively solve the contradiction between sensitivity and stability in SERS detection.

Design of elemental-image-predistortion and splicing seamless integral near-eye display based on aspherical microlens arrays

The near-eye display (NED) has long been a challenging and intriguing research focus in the fields of augmented reality (AR) and virtual reality (VR). In this paper, we propose a design method of elemental-image (EI) predistortion and splicing seamless integral NED based on microlens arrays (MLAs), which combines rigorous optical design with image preprocessing. The construction of MLA is based on sub-channel optimization and splicing, enabling the realization of both VR-NED and AR-NED. The calibration of each sub-channel in NED is conducted using a perfect lens that possesses an effective focal length equivalent to that of the human eye. To achieve a high-precision seamless integral NED, the precise location and acquisition of initial EIs are accomplished through chief ray tracing and image clipping. The predistortion EI array (EIA) is further obtained by combining chief ray tracing and radial basis function (RBF) image warping techniques. A seamless MLA-based integral AR-NED with a full field of view (FOV) of 60° is realized and verified. A prototype of a seamless MLA-based integral VR-NED is further implemented by integrating multiple discrete single-focal predistortion EIAs. The effectiveness of the proposed method has been validated through illumination and luminance simulations as well as experiments.

Simple Method for Calcium Determination in Crude Oil Emulsion Using Digital Image Colorimetry

A portable device for digital image colorimetry (DIC) is reported for the determination of Ca in crude oil emulsions. ASTM D4807 was used for sample preparation for the isolation of saline species containing Ca. DIC was evaluated based on the complexation reactions with Ca using calcein, murexide, and calcon. Partial least squares (PLS) provided the best determination coefficients (higher than 0.990) for all complexing reagents. Significant interference of Mg was observed for the determination of Ca using calcein. PLS models with the lowest root mean square error of cross validation (RMSECV) values were obtained for murexide and calcon for a concentration of 0.250 mmol L−1 at pH of 13. The best conditions for camera focal length were 11 and 2 mm for murexide and calcon. For light intensity, the optimized conditions for murexide and calcon was 180 lux. Stabilities of color complexes over the time were 30 and 15 min for murexide and calcon, respectively. DIC/murexide provided the lowest root mean square error of prediction (RMSEP) and a linear response was observed from 0.5 to 10 mg L−1 Ca and the relative standard deviation (RSD) was lower than 10%. Significant differences were not observed between the results obtained by DIC/murexide and those obtained by microwave-assisted wet digestion (MAWD) with determination by inductively coupled plasma-optical emission spectrometry (ICP-OES). A limit of quantification of 0.50 mg kg−1 Ca were obtained using DIC/murexide. This approach is an alternative and useful method for the determination of Ca in crude oil samples, with advantages of miniaturization, low cost, and ease of operation.

Hierarchical Inverse Distance Transformer for Enhanced Localization in Dense Crowds

Achieving precise individual localization within densely crowded scenes poses a significant challenge due to the intricate interplay of occlusions and varying density patterns. Traditional methods for crowd localization often rely on convolutional neural networks (CNNs) to generate density maps. However, these approaches are prone to inaccuracies stemming from the extensive overlaps inherent in dense populations. To overcome this challenge, our study introduces the Hierarchical Inverse Distance Transformer (HIDT), a novel framework that harnesses the multi-scale global receptive fields of Pyramid Vision Transformers. By adapting to the multi-scale characteristics of crowds, HIDT significantly enhances the accuracy of individual localization. Incorporating Focal Inverse Distance techniques, HIDT adeptly addresses issues related to scale variation and dense overlaps, prioritizing local small-scale features within the broader contextual understanding of the scene. Rigorous evaluation on standardized benchmarks has unequivocally validated the superiority of our approach. HIDT exhibits outstanding performance across various datasets. Notably, on the JHU-Crowd++ dataset, our method demonstrates significant improvements over the baseline, with MAE and MSE metrics decreasing from 66.6 and 253.6 to 59.1 and 243.5, respectively. Similarly, on the UCF-QNRF dataset, performance metrics increase from 89.0 and 153.5 to 83.6 and 138.7, highlighting the effectiveness and versatility of our approach.

Analytical study on the upward laser beam propagation in the turbulent atmosphere

The upward laser beam propagation in the turbulent atmosphere is studied analytically, where the nonlinear self-focusing, the atmospheric turbulence, and the atmospheric extinction effects are all taken into account. The analytical propagation expression of intensity is derived. The initial beam defocusing and the optimal beam power are two methods to improve the beam quality at the target. The analytical expressions of the focal length considering the initial beam defocusing and the corresponding target intensity are derived. Moreover, the analytical expressions of optimal beam power and the corresponding target intensity are also derived, and the target intensity is independent of the atmospheric extinction. In particular, the criterion for which method (the initial beam defocusing or the optimal beam power) to use is derived to optimize the target beam quality. The choice of the two methods depends on the laser beam parameters and the turbulent atmosphere parameters.

Graded flat lens with negative index for silicon photonics

We have realized a graded flat lens with negative effective index operating at telecom wavelength for Silicon Photonics. We report on its design, fabrication, and experimental characterization. This photonic device behaves as a convergent lens; it has been designed so as to be fabricated on a Silicon on Insulator platform and to operate at 1.55 μm. It consists of a graded photonic crystal whose filling factor has been varied so as to fit the profile of index of the graded index lens. Since it operates in the second band of Photonic Crystals, the effective index is negative. It has been characterized in two steps: (i) first, the output beam has been scanned by a set of waveguides and a CCD camera. (ii) Second, the focusing has been investigated by Scanning Near-field Optical Microscopy. This characterization validates the design: the focal length is about 5 μm (≃ 3λ), while the width of the focal spot is 0.87 μm (0.53 λ). This compact photonic device is at the wavelength scale (thickness = 4.4λ). Our design, based on Graded Index Optics, may be adapted to design new devices perfectly suited for Photonics applications, such as a short taper or a mode converter.

Inertial focusing of small particles in oscillatory channel flows

This study leverages the immersed boundary-lattice Boltzmann method to investigate the inertial focusing dynamics of a small neutrally buoyant particle in oscillatory channel flows driven by two distinct pressure gradient (PG) waveforms. Through an in-depth analysis of particle behavior, this research delineates the effects of PG waveforms, particle Reynolds number (Rep), and Womersley number (Wo) on the inertial focusing processes, and contrasts these findings with conventional Poiseuille flow-induced inertial focusing. Our results reveal that oscillatory flows induce more intricate focusing patterns than those observed in Poiseuille flow. The streamwise PG oscillation alters the particle focusing positions and introduces minor lateral oscillations in the post-focusing phase. The square PG waveform positions particles closer to the channel wall compared to Poiseuille flow, whereas the sine PG waveform positions particles closer to the channel center. Moreover, an increase in Rep nudges the focusing positions towards the channel center, while a higher Wo pushes them towards the walls. Notably, at Wo ≥ 5, the flow's oscillatory effect dominates over its inertial effect (i.e. Rep) on particle focusing. The study also underscores the capability of oscillatory flows to reduce the required channel length for particle focusing, albeit at the cost of increased focusing time. Contrary to Poiseuille flow, where the focusing length shortens with an increased Rep, oscillatory flow demonstrates an inverse correlation, with an increase in Rep extending the focusing length. Furthermore, while a higher Rep is beneficial in Poiseuille flow, it adversely affects the focusing effect in oscillatory flow. The focusing efficiency peaks at Wo = 3.5 for both waveforms, with the sine PG waveform offering superior efficiency due to the lower lift forces generated. Consequently, for optimal focusing in similar conditions, it is recommended to set Rep and Wo to 0.075 and 3.5, respectively.

Multi-trap optical tweezers based on kinoform Silver Mean lenses

In this paper, we present the design and implementation of multi-trap optical tweezers based on new quadrifocal kinoform lenses. The phase distribution of these diffractive lenses is characterized by the Silver Mean sequence. The focusing properties of the resulting aperiodic DOEs coined Kinoform Silver Mean Lenses (KSMLs) are numerically examined. It is shown that, under monochromatic illumination, a KSML drives most of the incoming light into four single foci whose focal lengths are related to the Silver ratio. In this way, a KSML improves the diffraction efficiency of binary Fresnel Silver Mean Zone Plates. Through experimental results, the simultaneous trapping of particles in the four focal planes and their three-dimensional manipulation is demonstrated.

A planar compound eye based microsystem for high precision 3D perception

Three-dimensional (3D) panoramic vision system plays a fundamental role in the biological perception of external information, and naturally becomes a key system for embodied intelligence to interact with the outside world. A binocular vision system with rotating eyeball has long baseline, large volume and weak sensitivity to motion. A compound eye system has small volume, high sensitivity to motion but poor precision. Here, a planar compound eye microsystem for high precision 3D perception is proposed by combining semiconductor manufacturing process and biological compound eye structure. Using a semiconductor planar image sensor as the sensing unit, a space-coded planar sub-eye array is designed and its sub field of view (FOV) is dynamically mapped to the image sensor. It solves the problem that a traditional vision system cannot simultaneously accommodate wide FOV with long focal length and high sensitivity to motion with high resolution. The parallax among different sub-eyes enables the system to accurately perceive and dynamically track the 3D position of the target in the range of 10 m and within the FOV of 120 ° in a single compound eye. This system is of great significance in the fields of intelligent robot and intelligent perception.

Eagle-Eye Inspired Meta-Device for Phase Imaging.

The dual-focus vision observed in eagles' eyes is an intriguing phenomenon captivates scientists since a long time. Inspired by this natural occurrence, the authors' research introduces a novel bifocal meta-device incorporating a polarized camera capable of simultaneously capturing images for two different polarizations with slightly different focal distances. This innovative approach facilitates the concurrent acquisition of underfocused and overfocused images in a single snapshot, enabling the effective extraction of quantitative phase information from the object using the transport of intensity equation. Experimental demonstrations showcase the application of quantitative phase imaging to artificial objects and human embryonic kidney cells, particularly emphasizing the meta-device's relevance in dynamic scenarios such as laser-induced ablation in human embryonic kidney cells. Moreover, it provides a solution for the quantification during the dynamic process at the cellular level. Notably, the proposed eagle-eye inspired meta-device for phase imaging (EIMPI), due to its simplicity and compact nature, holds promise for significant applications in fields such as endoscopy and headsets, where a lightweight and compact setup is essential.

Self-Calibration for Star Sensors.

Aiming to address the chicken-and-egg problem in star identification and the intrinsic parameter determination processes of on-orbit star sensors, this study proposes an on-orbit self-calibration method for star sensors that does not depend on star identification. First, the self-calibration equations of a star sensor are derived based on the invariance of the interstar angle of a star pair between image frames, without any requirements for the true value of the interstar angle of the star pair. Then, a constant constraint of the optical path from the star spot to the center of the star sensor optical system is defined to reduce the biased estimation in self-calibration. Finally, a scaled nonlinear least square method is developed to solve the self-calibration equations, thus accelerating iteration convergence. Our simulation and analysis results show that the bias of the focal length estimation in on-orbit self-calibration with a constraint is two orders of magnitude smaller than that in on-orbit self-calibration without a constraint. In addition, it is shown that convergence can be achieved in 10 iterations when the scaled nonlinear least square method is used to solve the self-calibration equations. The calibrated intrinsic parameters obtained by the proposed method can be directly used in traditional star map identification methods.

Testing of spherical mirrors with extremely large R numbers using a quasi-autocollimation method

Spherical mirrors with large R numbers have extreme advantages in optical systems with large apertures and long focus lengths for improving imaging resolution and field of view. Their manufacturing and testing accuracy directly affects the final wavefront aberration of the systems. However, due to the influence of airflow disturbance in the long optical path, testing large R number spherical mirrors with high precision is difficult for the direct testing method. In this paper, a quasi-autocollimation test (QACT) method is proposed based on the consistency and reverse compensation between sphere and paraboloid with large R numbers. In QACT, the length of the testing path is reduced to half, and the test sensitivity of the surface figure error is doubled. According to the Kolmogorov turbulence theory, the suppressed degree of QACT to airflow disturbance is quantitatively evaluated, and QACT can suppress the random error in test to 35%. A comparative experiment between direct test and QACT method is carried out on a ∅100 spherical mirror with 8-meter radius of curvature. Airflow disturbance in the testing environment is calculated by mathematical fitting and the repeated accuracy is improved from 4 nm to 1.4 nm in a single test using the QACT method, which is consistent with theoretical analysis. The research results will provide a new strategy for ultra-precision and efficiency tests of large R number spherical mirrors.