Short Focal Length Research Articles

P‐81: Electrically Tunable GRIN Liquid‐Crystal Lenticular Lens Array with Short Focal Length

An electrically tunable GRIN liquid crystal (LC) lenticular lens array with short focal length is proposed. By using the nematic LC material with large birefringence, the LC lenticular lens array possesses ideal lens‐like phase profile and the shortest focal length of ∼1.72mm when the driving voltage is 6V.

Spatial acuity-sensitivity trade-off in the principal eyes of a jumping spider: possible adaptations to a 'blended' lifestyle.

Jumping spiders (Salticidae) are diurnal visual predators known for elaborate, vision-mediated behaviour achieved through the coordinated work of four pairs of camera-type eyes. One pair ('principal' eyes) is responsible for colour and high spatial acuity vision, while three pairs ('secondary' eyes) are mostly responsible for motion detection. Based on its unusual capacity to visually discriminate specific prey in very low, but also under bright light settings, we investigated the structure of the principal and one pair of secondary eyes (antero-lateral eyes) of Cyrba algerina to determine how these eyes achieve the sensitivity, while maintaining spatial acuity, needed to sustain behaviour in low light. Compared to salticids that live in bright light, the principal eyes of C. algerina have a short focal length, and wide contiguous twin rhabdomeres that support optical pooling, overall favouring sensitivity (0.39 μm2), but without fully compromising acuity (12.4 arc min). The antero-lateral eye retinae have large receptors surrounded by pigment granules, providing effective shielding from scattered light. These adaptations may be beneficial for a xeric salticid species with a 'blended' lifestyle: generally living and hunting under stones in the dark, but sometimes venturing above them, in dramatically different light conditions.

A dual-functionality metalens to shape a circularly polarized optical vortex or a second-order cylindrical vector beam

Using e-beam lithography and reactive ion beam etching of a 120 nm amorphous silicon film, we fabricate a 30-µm spiral metalens that has a very short focal length that is equal to the incident 633 nm wavelength (numerical aperture about 1) and is composed of 16-sectored subwavelength binary gratings with a 220 nm period. Its unique feature is that the metalens generates a left-hand circularly polarized optical vortex with topological charge 2 when illuminated by a left-hand circularly polarized light but generates a cylindrical second-order vector beam when illuminated by linearly polarized light. Both for linearly and circularly polarized light, an inverse energy flow occurs near the strong focus of the metalens. Transverse near-focus intensity distributions measured with a scanning near-field optical microscope are found to be in qualitative agreement with those obtained via the FDTD-aided numerical simulation. Thus, the near-focus reverse energy flow is experimentally confirmed to occur.

Design of a zoom telescope optical system with a large aperture, long focal length, and wide field of view via a catadioptric switching solution

This study presents a catadioptric switching solution for a zoom telescope optical system to achieve a large aperture, long focal length, and wide field of view. The proposed optical system combines a Newton-type main system with a collimator group and a switching zoom group. Furthermore, clear imaging of long and short focal lengths under different operating ranges and temperatures is realized using a moving collimator group. For the field of view at both long and short focal lengths, the modulation transfer function exceeds 0.5 at the Nyquist frequency, 80% of the energy is concentrated in a diameter of 0.025 mm (i.e., less than two pixels), and the maximal distortion is 0.962%. The presented switching-zoom system exhibits high imaging quality for each focal length as well as a simple structure and stable performance.

Short focal length lens array based on liquid crystal material with large Δ<italic>n</italic>

Short focal length lens array based on liquid crystal material with large Δ<italic>n</italic>

Single-Image Super Resolution Using Convolutional Neural Network

Increasing threats to U.S. national security satellite constellations have resulted in an increased interest in constellation resilience and satellite redundancy. CubeSats have contributed to commercial, scientific and government applications in remote sensing, communications, navigation and research and have the potential to enhance satellite constellation resilience. However, the inherent size, weight and power limitations of CubeSats enforce constraints on imaging hardware; the small lenses and short focal lengths result in imagery with low spatial resolution. Low resolution limits the utility of CubeSat images for military planning purposes and national intelligence applications. This paper implements a super-resolution deep learning architecture and proposes potential applications to CubeSat imagery.

Fish-Eye Image Distortion Correction Based on Adaptive Partition Fitting

The acquisition of images with a fish-eye lens can cause serious image distortion because of the short focal length of the lens. As a result, it is difficult to use the obtained image information. To make use of the effective information in the image, these distorted images must first be corrected into the perspective of projection images in accordance with the human eye’s observation abilities. To solve this problem, this study presents an adaptive classification fitting method for fish-eye image correction. The degree of distortion in the image is represented by the difference value of the distances from the distorted point and undistorted point to the center of the image. The target points selected in the image are classified by the difference value. In the areas classified by different distortion differences, different parameter curves were used for fitting and correction. The algorithm was verified through experiments. The results showed that this method has a substantial correction effect on fish-eye images taken by different fish-eye lenses.

Study on ITO Fresnel Pattern Liquid Crystal Lens

Everyone is getting old and the presbyopia arise. Conventional presbyopia correction lenses include convex lenses and progressive lenses, each of which has advantages and disadvantages. The convex lens is a short focal length lens, which has the advantage of being able to see immediately when looking at the close, but has the disadvantage that the lens is thick, heavy, and the glasses have to be removed when looking at the distance. Progressive Lens is a multifocal lens that can focus from near to far. It is relatively thin compared to convex lenses, but requires an adaptation period of about 1 to 2 months. To solve this problem, we developed light, thin, and variable refractive index ITO Fresnel pattern lenses using liquid crystals.[1] ITO Fresnel patterns were formed by photo lithography, and the voltages were applied differently for each pattern to drive the liquid crystals, and the focal length was changed as the applied voltage changed. The diopter (D) of the lens has changed over 9V, and showed the maximum diopter of 1 at 13V.[1] Li, Guoqiang, et al. "Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications." Proceedings of the National Academy of Sciences 103 (16), pp.6100-6104 (2006)

Design of hybrid refractive-diffractive infrared dual-band zoom optical system

In this paper, the influence models of different diffraction elements on diffraction efficiency were established, and the diffraction efficiency among single diffraction element, harmonic diffraction element and double diffraction element was compared. The advantages of using double diffraction elements in infrared optical system were analyzed. The average diffraction efficiency of different material combinations was calculated. Based on this, a hybrid infrared dual-band and dual-field optical system suitable for airborne platform was designed. The resolution of the large field of view was 1.5 m@16 km. The long and the short focal length were 960 mm and 480 mm respectively. The zoom function was realized by switching the mirror to ensure the optical axis stability. The simulation results show that the MTF curves are smooth and close to the diffraction limit under the large temperature difference of −40 - +60 ℃. The RMS radius is within the radius of airy spots, and the minimum characteristic size of the binary diffraction surface is 6.9 μm. The design results meet the engineering requirements.

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.

Optical Aberrations of Guinea Pig Eyes.

PurposeThe guinea pig is widely used in studies of refractive error development and myopia which often involve experimental optical manipulations. The study described here investigated the optical quality of the guinea pig eye, for which there are limited data, despite its fundamental importance to understanding visually guided eye growth.MethodsThe ocular aberrations of eight adolescent New Zealand pigmented guinea pigs (6–11 weeks old) were measured after cycloplegia using a custom-built Shack–Hartmann aberrometer and fit with a Zernike polynomial function to the 10th order (65 terms). The optical quality of their eyes was assessed in terms of individual Zernike coefficients, and data were further analyzed to derive root-mean-square (RMS) wavefront errors, modulation transfer functions (MTFs), point spread functions (PSFs), Strehl ratios, and depth of focus. A 4-mm pupil was used in all computations. The derived data are compared with equivalent data from normal young adult human eyes.ResultsThe guinea pigs exhibited low hyperopia and a small amount of positive spherical aberration, with other aberration terms decreasing with increasing order. Their average depth of focus, estimated from through-focus modulation, was 3.75 diopters. The RMS wavefront error of the guinea pig eye was found to be larger than that of the human eye for the same pupil size, reflecting a higher degree of aberrations, although the PSF (area) on the retina was smaller and sharper due to its shorter focal length. The radial average best-focus MTF derived for the guinea pig eye showed good performance at very low spatial frequencies, with a steeper decline with increasing frequency than for the human eye, dropping below 0.3 at 9 cpd. When converted to linear units (cycles/mm), the guinea pig eye had a higher spatial frequency cutoff and a slight contrast advantage for low spatial frequencies compared to the human eye.ConclusionsThe optical quality of the guinea pig eye is far superior to their reported behavioral visual acuity. This implies a neuroanatomical limit to their vision, which contrasts with the close match of optical and neural limits to spatial resolution in human eyes. The significance for eye growth regulation of the relative optical advantages exhibited by guinea pig eyes, when optical quality is expressed in linear rather than angular retinal units, warrants further consideration.

Radial-shearing interferometric imaging with Theon-Kepler bifocal telescope.

A method of radial-shearing interferometric imaging based on the Theon-Kepler bifocal telescope is proposed. The Theon-Kepler bifocal telescope system consists of two identical Theon photon sieves with bifocal spots. The short focal length of the first photon sieve coincides with the long focal length of the second photon sieve. At the same time, the first focal length coincides with the second short focal length. This setup naturally constitutes two sets of 4f systems. When a plane wave is incident on the bifocal telescope, two plane waves of different sizes are emitted. These two beams undergo radial-shearing interference. The common-path setup of this new type of radial-shearing interferometer is simple, with low requirements for the experimental environment and strong anti-interference ability.

Imaging system with brightness amplification for a metal-nanopowder-combustion study

This work discusses an optical system with brightness amplification—a laser monitor, as well as the system's application for real-time imaging of the surface of metal nanopowders during high-temperature combustion. The advantage of the laser monitor is its combination of microscopic magnification, laser backlighting, and narrow-band filtering, which, together with high-speed video recording, makes it possible to visualize the nanopowder surface through the intense background lighting produced by a high-temperature burning sample. We used two laser-monitor schemes with short and long focal lengths to study the dynamics of the combustion process at different spatial resolutions. For compounds whose combustion is accompanied by intense scattering of the combustion products, we recommend using the laser monitor with increased monitoring distance via a mirror-imaging scheme. This proposed technique allows real-time monitoring of the high-temperature-combustion processes accompanied by intensive lighting and product scattering at a distance of 50 cm from the optical system. Both systems allow quantitative characterization of the combustion process by registering the average output of the brightness amplifier together with the overall brightness of glowing. The combustion of nanoAl + nanoFe and nanoAl + nanoFe + microAl powder mixtures was visualized using a laser monitor for the first time and compared with the combustion of aluminum nanopowder without additives.

Flat-Field and Colour Correction for the Raspberry Pi Camera Module

The Raspberry Pi camera module is widely used in open source hardware projects as a low cost camera sensor. However, when the stock lens is removed and replaced with other custom optics the sensor will return a non-uniform background and colour response which hampers the use of this excellent and popular image sensor. This effect is found to be due to the sensor's optical design as well as due to built-in corrections in the GPU firmware, which is optimised for a short focal length lens. In this work we characterise and correct the vignetting and colour crosstalk found in the Raspberry Pi camera module v2, presenting two measures that greatly improve the quality of images using custom optics. First, we use a custom "lens shading table" to correct for vignetting of the image, which can be done in real time in the camera's existing processing pipeline (i.e. the camera's low-latency preview is corrected). The second correction is a colour unmixing matrix, which enables us to reverse the loss in saturation at the edge of the image, though this requires post-processing of the image. With both of these corrections in place, it is possible to obtain uniformly colour-corrected images, at the expense of slightly increased noise at the edges of the image.

Miniature Broadband NIR Spectrometer Based on FR4 Electromagnetic Scanning Micro-Grating.

This paper presents a miniaturized, broadband near-infrared (NIR) spectrometer with a flame-retardant 4 (FR4)-based scanning micrograte. A 90° off-axis parabolic mirror and a crossed Czerny–Turner structure were used for creating an astigmatism-free optical system design. The optical system of the spectrometer consists of a 90° off-axis parabolic mirror, an FR4-based scanning micrograte, and a two-color indium gallium arsenide (InGaAs) diode with a crossed Czerny–Turner structure optical design. We used a wide exit slit and an off-axis parabolic mirror with a short focal length to improve the signal-to-noise ratio (SNR) of the full spectrum. We enabled a miniaturized design for the spectrometer by utilizing a novel FR4 micrograte for spectral dispersion and spatial scanning. The spectrometer can detect the full near-infrared spectrum while only using a two-color InGaAs diode, and thus, the grating scanning angle of this spectrometer is small when compared to a dual-detector-based spectrometer. In addition, the angle signal can be obtained through an angle sensor, which is integrated into the scanning micrograte. The real-time angle signal is used to form a closed-loop control over the scanning micrograte and calibrate the spectral signal. Finally, a series of tests was performed. The experimental results showed that the spectrometer has a working wavelength range of 800–2500 nm. The resolution is 10 nm at a wavelength range of 800–1650 nm and 15 nm at a wavelength range of 1650–2500 nm. Similarly, the stability of these two wavelength ranges is better than ±1 nm and ±2 nm, respectively. The spectrometer’s volume is 80 × 75 × 65 mm3 and its weight is 0.5 kg. The maximum spectral fluctuation does not exceed 1.5% and the signal-to-noise ratio is 284 after only one instance of averaging.

Short focal length tunable liquid crystal lenticular lens array based on fringe field effect

AbstractA liquid crystal (LC) lenticular lens array based on fringe field effect is proposed. The gradient refractive index (GRIN) profile can be generated in the LC layer because of the fringe field between the strip‐shaped electrodes and the bottom electrode. The proposed LC lenticular lens array possesses ideal lens‐like phase profile and shortest focal length (1.199 mm) when the driving voltage is 5.4 V. The focal length can be tuned with millisecond response time by changing the driving voltage of the proposed LC lenticular lens array. The rise time τrise and decay time τdecay of the proposed LC lenticular lens array are 162 and 94 ms, respectively.

Vehicle Detection and Ranging Using Two Different Focal Length Cameras

Vehicle detection is a crucial task for autonomous driving and demands high accuracy and real-time speed. Considering that the current deep learning object detection model size is too large to be deployed on the vehicle, this paper introduces the lightweight network to modify the feature extraction layer of YOLOv3 and improve the remaining convolution structure, and the improved Lightweight YOLO network reduces the number of network parameters to a quarter. Then, the license plate is detected to calculate the actual vehicle width and the distance between the vehicles is estimated by the width. This paper proposes a detection and ranging fusion method based on two different focal length cameras to solve the problem of difficult detection and low accuracy caused by a small license plate when the distance is far away. The experimental results show that the average precision and recall of the Lightweight YOLO trained on the self-built dataset is 4.43% and 3.54% lower than YOLOv3, respectively, but the computing speed of the network decreases 49 ms per frame. The road experiments in different scenes also show that the long and short focal length camera fusion ranging method dramatically improves the accuracy and stability of ranging. The mean error of ranging results is less than 4%, and the range of stable ranging can reach 100 m. The proposed method can realize real-time vehicle detection and ranging on the on-board embedded platform Jetson Xavier, which satisfies the requirements of automatic driving environment perception.

Design and Fabrication of Microscale, Thin-Film Silicon Solid Immersion Lenses for Mid-Infrared Application.

Lens-based optical microscopes cannot resolve the sub-wavelength objects overpass diffraction limit. Recently, research on super-resolution imaging has been conducted to overcome this limitation in visible wavelength using solid immersion lenses. However, IR imaging, which is useful for chemical imaging, bio-imaging, and thermal imaging, has not been studied much in optical super-resolution by solid immersion lens owing to material limitations. Herein, we present the design and fabrication schemes of microscale silicon solid immersion lenses (µ-SIL) based on thin-film geometry for mid-infrared (MIR) applications. Compared with geometrical optics, a rigorous finite-difference time-domain (FDTD) calculation of proposed silicon microlenses at MIR wavelengths shows that the outstanding short focal lengths result in enhanced magnification, which allows resolving objects beyond the diffraction limit. In addition, the theoretical analyses evaluate the influences of various structural parameters, such as radius of curvature (RoC), refractive index, and substrate thickness, in µ-SIL. In particular, the high refractive index of µ-SIL is beneficial to implement the outstanding near-field focusing, which corresponds to a high numerical aperture. On the basis of this theoretical background, novel methods are developed for the fabrication of a printable, thin-film silicon microlens array and its integration with a specimen substrate. From the result, we provide a physical understanding of near-field focusing phenomena and offer a promising tool for super-resolution far-field imaging in the MIR range.

Design of microscope eyepiece with high magnification and wide field of view based on freeform surface

With the great improvement of microscope property, the eyepiece of microscope is required to have a larger field of view, high magnification, and better imaging quality. Because of the external aperture stop and short focal length, the design difficulty of microscope eyepiece lies in how to correct the distortion and other off-axis aberrations caused by the large field of view, and how to obtain an eye relief that meets the requirements for human observation is also important. The main aberrations of the eyepiece optical system, especially the effect of the optical system with distortion on imaging is analyzed in this paper. The freeform surface is applied to the distortion correction of the optical system of the microscope eyepiece. A wide-angle microscope eyepiece with field of vision of 60° and a magnification of 25× is described in this paper. The full-field distortion is less than 5%. The microscope eyepiece adopts a five-piece three-element structure, in which the freeform surface lens is made of plastic material and is symmetrical about the <i>XOZ</i> plane and <i>YOZ</i> plane. A high-performance microscope eyepiece design with simple structure, easy processing, and low cost is realized.

变焦鱼眼镜头系统设计

The zoom fish-eye lens had the characteristics of much larger field-of-view angle, much larger relative aperture, and much larger anti-far ratio. In the work described in this paper, the initial structure of a fish-eye lens with fixed focal length was firstly designed using the theory of a plane symmetric optical system. And then the initial structure components of the fish-eye lens were divided into two groups, the former-group and the rear-group, and then the entire zoom fish-eye lens system was optimized using Gaussian optical theory. Finally, a zoom fish-eye lens system with good imaging quality was obtained. This fish-eye lens system had a field-of-view angle of 180° with an 8 mm short focal length and a field-of-view angle of 90° with a 16 mm long focal length. Its relative aperture was 1/3.5. The design results show that the modulation transfer function of this zoom lens system at different focal lengths is no less than 0.45 when the spatial frequency is 50 lp/mm. This zoom fish-eye photographic objective lens had higher imaging quality than other zoom fish-eye lenses.