Compact Imaging System Research Articles

Varifocal liquid lens driven by a conical dielectric elastomer actuator

A varifocal lens is an important part of optical systems with applications in biomedicine, photography, smartphones, and virtual reality. In this paper, we propose and demonstrate a varifocal liquid lens driven by a conical dielectric elastomer actuator. When the conical dielectric elastomer is subjected to an actuation voltage, the conical dielectric elastomer works as an out-plane actuator and makes the surface curvature of the liquid droplet increase; then the focal length of the proposed varifocal liquid lens changes. The overall dimensions of the proposed varifocal liquid lens are 9.4 mm in diameter and 12.5 mm in height. The focal length tuning range is 15.07mm∼9.50mm when the actuation voltage increases from 0 kV to 5.0 kV. The focal power variation of the proposed varifocal liquid lens is 35.5 D. The rise and fall times of the proposed varifocal liquid lens are 215 ms and 293 ms, respectively. The ability of the proposed liquid lens to focus on objects at different distances without any moving parts is demonstrated. The compact varifocal liquid lens driven by the conical dielectric elastomer actuator in the current study has the potential to be used in various compact imaging systems in the future.

Development and commissioning of a compact Cosmic Ray Muon imaging prototype

Due to the muon tomography’s capability of imaging high Z materials, some potential applications have been reported on inspecting smuggled nuclear materials in customs. A compact Cosmic Ray Muons (CRM) imaging prototype, Lanzhou University Muon Imaging System (LUMIS), is comprehensively introduced in this paper including the structure design, assembly, data acquisition and analysis, detector performance test, and material imaging commissioning etc. Cast triangular prism plastic scintillators (PS) were coupled with Si-PMs for sensitive detector components in system. LUMIS’s experimental results show that the detection efficiency of an individual detector layer is about 98%, the position resolution for vertical incident muons is 2.5 mm and the angle resolution is 8.73 mrad given a separation distance of 40.5 cm. Moreover, the image reconstruction software was developed based on the Point of Closest Approach (PoCA) to detect lead bricks as our target. The reconstructed images indicate that the profile of the lead bricks in the image is highly consistent with the target. Subsequently, the capability of LUMIS to distinguish different materials, such as Pb, Cu, Fe, and Al, was investigated as well. The lower limit of response time for rapidly alarming high-Z materials is also given and discussed. The successful development and commissioning of the LUMIS prototype have provided a new solution option in technology and craftsmanship for developing compact CRM imaging systems that can be used in many applications.

Noise-robust phase retrieval by optics path modulation with adaptive feedback

We propose a compact computational imaging system with adaptive weighted feedback amplitude-phase retrieval (awfAPR) algorithm, in which the traditional axial linear scanning method is replaced by periodic optical path modulation (OPM) strategy. The introduced adaptive feedback scenario can significantly suppress noise contamination. Specifically, OPM is achieved by the rotation of gradient thicknesses of the transparent optical medium. Compared with multi-distance/height scanning (MDPR), all system components in the OPM system are fixed at discrete lateral positions. Besides, according to the auxiliary of corresponding auto-focusing criterion and sub-pixel registration algorithms, the OPM system is also self-calibration and independent of the accuracy of rotation and thickness parameter. Furthermore, experimental results show that the adaptive weighted feedback strategy remarkably improves the stability and robustness toward the noise. OPM has flexible adaptability to optical path modulation method. The proposed optical system configuration has minimal hardware requirements and is anticipated to expand dynamic real-time imaging.

High-throughput and controllable manufacturing of liquid crystal polymer planar microlens array for compact fingerprint imaging.

The microlens array (MLA) with a small geometric footprint and unique performances, is the key enabler to push the development of photonic devices toward miniaturization, multi-function and large-scale integration. However, the realization of 100% fill-factor (FF) MLAs with high controllability and its mass manufacturing without complex steps has always been a difficult issue. Here, we propose an efficient, highly flexible and low-cost manufacturing approach for MLAs with a high FF via snapshot polarization patterning. The digitalized linear polarization pattern was distributed across the photo-alignment layer with both high efficiency and accuracy, enabling large-area liquid crystal MLA with parameter controllability from element to element. The MLA manufacturing process does not involve developing, etching and deposition steps and is suitable for industry up-scaling. We further proposed a novel compact compound-eye imaging system for biometrics with the obtained MLAs. The 100% FF MLA enables high light utilization efficiency and low background crosstalk, yielding compact biometrics indentation with high recognition accuracy. The realization of such planar optics would lead to a plethora of different miniaturized multiaperture imaging systems in the future.

Development of a Compact Photoacoustic Tomography Imaging System with Dual Single-Element Transducers for Image Enhancement

ObjectiveThis paper proposes a new photoacoustic computed tomography (PACT) imaging system employing dual ultrasonic transducers with different frequencies. When imaging complex biological tissues, photoacoustic (PA) signals with multiple frequencies are produced simultaneously; however, due to the limited bandwidth of a single-frequency transducer, the received PA signals with specific frequencies may be missing, leading to a low imaging quality.MethodsIn contrast to our previous work, the proposed system has a compact volume as well as specific selection of the detection center frequency of the transducer, which can provide a comprehensive range for the detection of PA signals. In this study, a series of numerical simulation and phantom experiments were performed to validate the efficacy of the developed PACT system.ResultsThe images generated by our system combined the advantages of both high resolution and ideal brightness/contrast.ConclusionThe interchangeability of transducers with different frequencies provides potential for clinical deployment under the circumstance where a single frequency transducer cannot perform well.

Holographic 3D particle reconstruction using a one-stage network.

Volumetric reconstruction of a three-dimensional (3D) particle field with high resolution and low latency is an ambitious and valuable task. As a compact and high-throughput imaging system, digital holography (DH) encodes the 3D information of a particle volume into a two-dimensional (2D) interference pattern. In this work, we propose a one-stage network (OSNet) for 3D particle volumetric reconstruction. Specifically, by a single feed-forward process, OSNet can retrieve the 3D coordinates of the particles directly from the holograms without high-fidelity image reconstruction at each depth slice. Evaluation results from both synthetic and experimental data confirm the feasibility and robustness of our method under different particle concentrations and noise levels in terms of detection rate and position accuracy, with improved processing speed. The additional applications of 3D particle tracking are also investigated, facilitating the analysis of the dynamic displacements and motions for micro-objects or cells. It can be further extended to various types of computational imaging problems sharing similar traits.

Flexible fabrication of Fresnel micro-lens array by off-spindle-axis diamond turning and precision glass molding

Fresnel micro-lens arrays with close-to-wavelength features have significant advantages in minimization of optical systems. However, in the fabrication of a Fresnel micro-lens array by conventional diamond turning, the discontinuous features of the surface profile cause interference between the tool flank face and the finished surface. In this manuscript, a novel off-spindle-axis diamond turning method is proposed to fabricate Fresnel micro-lens array mold insert and a toolpath generation algorithm is developed. The generation of each lenslet is treated as an independent turning operation, and the whole fabrication of a Fresnel micro-lens array is completed by a succession of repetitive turning operations. With this approach, the interference between the tool and the finished surface is avoided, and the form accuracy and surface integrity are improved for each lenslet. Then, the Fresnel micro-lens array mold insert is further utilized in precision glass molding to replicate the micro-lens structures onto transparent polymer substrates. The experimental results indicate that both the diamond-turned and molded Fresnel micro-lens arrays are achieved with homogeneous quality. As an application, a compact imaging system based on the molded Fresnel micro-lens array is demonstrated. The proposed machining method in this study can be employed in the fabrication of Fresnel micro-lens arrays and other micro-optics with discontinuous profiles with high accuracy and machining efficiency.

Ultrasmall compact CMOS imaging system for bioluminescence reporter-based live gene expression analysis.

.Significance: Gene expression analysis is an important fundamental area of biomedical research. However, live gene expression imaging has proven challenging due to constraints in conventional optical devices and fluorescent reporters.Aim: Our aim is to develop smaller, more cost-effective, and versatile imaging capabilities compared with conventional devices. Bioluminescence reporter-based gene expression analysis was targeted due to its advantages over fluorescence-based imaging.Approach: We created a small compact imaging system using micro-CMOS image sensors (). The model had an improved pixel design and a patterned absorption filter array to detect the low light intensity of bioluminescence.Results: The device demonstrated lower dark current, lower temporal noise, and higher sensitivity compared with previous designs. The filter array enabled us to subtract dark current drift and attain a clearer light signal. These improvements allowed us to measure bioluminescence reporter-based gene expression in living mammalian cells.Conclusion: Using our system for bioluminescence imaging in the future, the device can be implanted in vivo for simultaneous gene expression imaging, behavioral analysis, and optogenetic modulation.

FlocARAZI: An In‐Situ, Image‐Based Profiling Instrument for Sizing Solid and Flocculated Suspended Sediment

AbstractAn inexpensive and compact underwater digital camera imaging system was developed to collect in situ high resolution images of flocculated suspended sediment at depths of up to 60 meters. The camera has a field of view of 3.7 × 2.8 mm and can resolve particles down to 5 . Depending on the degree of flocculation, the system is capable of accurately sizing particles to concentrations up to 500 mg/L. The system is fast enough to allow for profiling whereby size distributions of suspended particles and flocs can be provided at multiple verticals within the water column over a relatively short amount of time (approximately 15 min for a profile of 15 m). Using output from image processing routines, methods are introduced to estimate the mass suspended sediment concentration (SSC) from the images and to separate identified particles into sand and mud floc populations. The combination of these two methods allows for the size and concentration estimates of each fraction independently. The camera and image analysis methods are used in both the laboratory and the Mississippi River for development and testing. Output from both settings are presented in this study.

Fluorescence hyperspectral imaging system for analysis and visualization of oil sample composition and thickness.

In this paper, a compact fluorescence hyperspectral imaging system based on a prism-grating-prism (PGP) structure is designed. Its spectrometer spectral range is 400-1000 nm with a spectral resolution of 2.5 nm, and its weight is less than 1.7 kg. The PGP imaging spectrometer combines the technical advantages of prism and grating, by not only using six lenses for imaging and collimation to realize the dual telecentres of object and image but also having a "straight cylinder" structure, which makes the installation and adjustment simple, compact, and stable. By the push-broom method, we obtained the three-dimensional cubic data of different oil products. By normalization processing, minimum noise separation transformation processing, visualization processing, and support vector machine classification processing of different oil fluorescence hyperspectral data, we demonstrate that the fluorescence hyperspectral imaging system can identify different kinds of oil and recognize the oil film thickness. The fluorescence hyperspectral imaging system can be used in oil spill detection, resource exploration, natural disaster monitoring, environmental pollution assessment, and many other fields.

Tunable lens using dielectric elastomer sandwiched by transparent conductive liquid.

We propose and demonstrate a compact tunable lens with high transmittance using a dielectric elastomer sandwiched by transparent conductive liquid. The transparent conductive liquid not only serves as the refractive material of the tunable lens but also works as the compliant electrode of the dielectric elastomer. The overall dimensions of the proposed tunable lens are 16 mm in diameter and 10 mm in height, and the optical transmittance is as high as 92.2% at 380-760 nm. The focal power variation of the tunable lens is -23.71D at an actuation voltage of 3.0 kV. The rise and fall times are 60 ms and 185 ms, respectively. The fabrication process of the tunable lens is free of the deposition of opaque compliant electrodes. Such a tunable lens promises a potential solution in various compact imaging systems.

Organic glass scintillator bars with dual-ended readout

Organic glass scintillator (OGS) bars (6 × 6 × 50 mm3) were melt-cast and characterized with dual-ended readout by silicon photomultiplier arrays. The results were compared to trans-stilbene bars of the same size. Energy resolution was measured via a 137Cs Compton-coincidence experiment. Time resolution was determined via a 22Na coincidence-timing experiment. Depth-of-interaction resolution was measured by moving a gamma-ray fan beam along the major axis of the bars. Pulse shape discrimination capability and neutron light output were determined via a time-of-flight measurement of a 252Cf source. The OGS bars exhibited better energy resolution at 478 keV (10.3±0.5% vs. 11.2±0.5%), better time resolution from 200–341 keVee (270ps vs. 340ps FWHM), slightly poorer depth-of-interaction resolution, poorer PSD performance, and higher neutron light output than the stilbene bars. The low cost, ease of manufacturing, brightness, and excellent time resolution make OGS a strong candidate for use in compact radiation imaging systems.

Technical Note: Compact thermoacoustic imaging system based on a low-cost and miniaturized microwave generator for invivo biomedical imaging.

Most of existing thermoacoustic imaging (TAI) studies generally utilized a linear modulator-based high peak power magnetron generator (MG) for efficient TA signal excitation. However, a linear modulator-based MG is bulky and expensive. Here we present a low-cost and compact thermoacoustic imaging (TAI) system based on a miniaturized MG. The MG is based on solid-state modulator and operates at 3.05±0.025GHz, with a peak power of up to 60kW and adjustable pulse duration from 70 to 600ns. The dimensions and weight of this MG are 350×210×70mm3 (Width×Length×Height) and 7.5kg, respectively. The spatial resolution of the miniaturized MG-based TAI system is determined to be from 0.3 to 1.4mm using controlled phantom experiments. The system is further evaluated using invivo experiments where the finger joints and vasculature in the forearm and opisthenar of human participants are successfully imaged. This study suggests that the miniaturized MG based TAI systems can be used for invivo joint and vascular imaging with multiscale resolutions (0.3-1.4mm).

Isotropic topological second-order spatial differentiator operating in transmission mode.

Differentiation has widespread applications, particularly in image processing for edge detection. Significant advances have been made in using nanophotonic structures and metamaterials to perform such operations. In particular, a recent work demonstrated a topological differentiator in which the transfer function exhibited a topological charge, making the differentiation operation robust to variations in operating conditions. The demonstrated topological differentiator, however, operates in reflection mode at off-normal incidence and is difficult to integrate into compact imaging systems. In this work, we design a topological differentiator that operates isotropically in transmission mode at normal incidence. The device exhibits an optical transfer function with a symmetry-protected topological charge of ±2 and performs second-order differentiation. Our work points to the potential of harnessing topological concepts for optical computing applications.

Curved Sensors for compact and high-performance imaging systems

Curved sensors enable a substantial simplification of optical systems and significant improvement of the image quality. Both lead to the reduction of the mass-volume budget of the instruments, simplify the manufacturing and shrink the integration time, resulting in a drop of the consolidated cost. The field curvature is an optical aberration describing the light going through an imager and producing an image on a portion of a sphere. This aberration is present in every single optical system and particularly in wide-angle imaging. Current solutions applied to correct the images imply the complexification of the instruments by using field flatteners, ie. more optics of complex shape, thus increasing the mass-volume and cost budget. The use of curved sensors helps overcoming these issues. It corrects the field curvature directly at the focal surface. For 10 years, the CNRS-LAM demonstrated, through theoretical studies and prototyping, that the use of curved sensors allows to remove 30 to 50 percent of the lenses count and drastically releases constraints in optical systems. I am currently leading a parametric study regarding the contribution of curved sensors for UV imaging system. This study is done through an imager aiming at observing aurora with a wide field of view (more than 60 degrees) in 130-138nm bandwidth with a signal to noise ratio higher than 10. The first main constraint in this project is to get a high transmissive system in FUV bandwidth. To address this problematic, the number of lenses must be reduced. However, reducing the number of lenses means reducing the number of degrees of freedom to correct the aberrations of such a wide field system. To maintain high optical performance, I have considered the use of curved and freeform sensors to correct the field curvature and astigmatism directly in focal surface. In this talk I will present the parametric study I have been leading on the design of this FUV imager. I will quantify the benefits of using curved and freeform sensors compared to flat sensor to keep high performance whilst reducing the number of lenses from 6 to 3 to increase the throughput by a factor 3.

Proposal of a compact grating-based phase contrast imaging system using an x-ray polycapillary lens

As the grating-based phase contrast imaging has drawn much attention in various scientific fields, there is an increasing demand for a compact imaging system with high efficiency in practical applications. In this paper, a compact imaging configuration by replacing the source grating in a conventional imaging system with a robust, easily-fabricated, and cost-effective x-ray polycapillary lens is proposed and its feasibility is examined using a full-scale Monte Carlo simulation. A typical x-ray spectrum for diagnostic mammography is adopted in the simulation. Under this spectrum, a new polycapillary lens is designed and optimized to meet the requirement of the imaging system. The phantom consists of polyethylene, cortical bone, and adipose tissue spheres of different sizes. An accurate phase image of the phantom is successfully reconstructed using an eight-step phase-stepping method. Excellent agreement of the phase shift and attenuation of the phantom is obtained between the simulation and the theoretical prediction. Compared with a conventional imaging system using a source grating at the same parameters, the imaging efficiency of our proposed system is improved 1.7 times, while its background visibility is reduced from 44.4% to 12.4%. Our results suggest the possibility of developing such a bench-top system for flexible phase contrast imaging applications.

Imaging Properties of Large Field-of-View Quadratic Metalenses and Their Applications to Fingerprint Detection

Dielectric metasurfaces, extremely thin nanostructured dielectric surfaces, hold promise to replace conventional refractive optics, such as lenses, due to their high performance and compactness. However, designing large field-of-view (FOV) metalenses, which are of particular importance when imaging relatively big objects at short distances, remains one of the most critical challenges. Recently, metalenses implementing a quadratic phase profile have been put forward to solve this problem with a single element, but despite their theoretical ability to provide 180° FOV, imaging over very large FOV has not been demonstrated yet. In this work, we provide an in-depth analysis of the imaging properties of quadratic metalenses and, in particular, show that due to their intrinsic barrel distortion or fish-eye effect, there is a fundamental trade-off between the FOV achievable in a given imaging configuration and the optical resolution of the metalens and/or the detector resolution. To illustrate how to harness the full potential of quadratic metalenses, we apply these considerations to the fingerprint detection problem and demonstrate experimentally the full imaging of a 5 mm fingerprint with features of the order of 100 μm, with a metalens ten times smaller in size and located at a distance of only 2.5 mm away from the object. This constitutes the most compact imaging system reported so far for fingerprint detection.

Freeform optics for imaging

In the last 10 years, freeform optics has enabled compact and high-performance imaging systems. This article begins with a brief history of freeform optics, focusing on imaging systems, including marketplace emergence. The development of this technology is motivated by the clear opportunity to enable science across a wide range of applications, spanning from extreme ultraviolet lithography to space optics. Next, we define freeform optics and discuss concurrent engineering that brings together design, fabrication, testing, and assembly into one process. We then lay out the foundations of the aberration theory for freeform optics and emerging design methodologies. We describe fabrication methods, emphasizing deterministic computer numerical control grinding, polishing, and diamond machining. Next, we consider mid-spatial frequency errors that inherently result from freeform fabrication techniques. We realize that metrologies of freeform optics are simultaneously sparse in their existence but diverse in their potential. Thus, we focus on metrology techniques demonstrated for the measurement of freeform optics. We conclude this review with an outlook on the future of freeform optics.

Freeform optical surface design in an off-axis reflective imaging system by a double seed curve extension algorithm.

A double seed curve extension (DSCE) method is proposed to design a freeform surface directly in an off-axis reflective imaging system. Compared with the basic seed curve extension (SCE) method, the DSCE can effectively reduce the error of freeform surface construction and improve the imaging quality of the off-axis reflective imaging system. In addition, the method can be employed to design an off-axis reflective imaging system consisting of multiple freeform surfaces with several virtual image points set in advance. In order to verify the DSCE method, three examples are given. One is the off-axis freeform one-mirror system, one is a compact off-axis three-mirror imaging system with two freeform surfaces, and the other is an off-axis reflective system with three freeform surfaces. The modulation transfer function (MTF) of the one-mirror system is greater than 0.9 at 20 lp/mm, which is close to the diffraction limit. The average of the sagittal and tangential MTFs of the second system designed by the SCE and DSCE methods are 0.26 and 0.74 at spatial frequency of 20 lp/mm, respectively. And the MTF of the last system designed by the DSCE method is greater than 0.9 at 20 lp/mm, which is better than that of the SCE method.

Compact dual optical path single-pixel imaging system (Invited)

Compact dual optical path single-pixel imaging system (Invited)