Chromatic Aberration Correction Research Articles

Shaping of Electron Beams Using Sculpted Thin Films.

Electron beam shaping by sculpted thin films relies on electron–matter interactions and the wave nature of electrons. It can be used to study physical phenomena of special electron beams and to develop technological applications in electron microscopy that offer new and improved measurement techniques and increased resolution in different imaging modes. In this Perspective, we review recent applications of sculpted thin films for electron orbital angular momentum sorting, improvements in phase contrast transmission electron microscopy, and aberration correction. For the latter, we also present new results of our work toward correction of the spherical aberration of Lorentz scanning transmission electron microscopes and suggest a method to correct chromatic aberration using thin films. This review provides practical insight for researchers in the field and motivates future progress in electron microscopy.

Broadband achromatic metalens for linearly polarized light from 450 to 800 nm.

Metalens is a planar optical component that uses nanostructures with a thickness on the order of the wavelength to manipulate the wavefront of the incident light. A key problem, especially in color imaging and display applications, is the correction of chromatic aberration, which is an inherent effect caused by the dispersion of periodic lattices and resonance modes. However, the current achromatic metalenses either use the PB phase method that is only valid for circularly polarized light or nanostructures with complex cross sections that are difficult to manufacture. Here, we designed a broadband achromatic metalens for linearly polarized light from 450 to 800 nm. Rectangular titanium dioxide nanofins of various lengths and widths were applied to modulate the phase and dispersion of the incident light. The metalens can fulfill three target phases simultaneously by using an optimization method. The designed metalens has a stable focus from 450 to 800 nm with an average focusing efficiency of 64%. It can be potentially applied in microscopes, lithography machines, sensors, and displays.

Lateral chromatic aberration correction method and lens design feedback for dynamic optical tilt

A method of generating lateral chromatic aberration correction factor (CACF) during lens development is presented. This CACF will be able to feedback to lens designer whether the design will work when tilt Optical Image Stabilization (OIS) compensation is needed in real application. CACF will be used to correct any chromatic aberration caused by the OIS tilt during image capture. No post-processing or minimum effort is required to correct any significant chromatic aberration. The output image from the OIS tilt will not show color fringes artifact. It also simulates and predict the allowable tilt angle during design stage, thus reduce the amount of cost and resources to build prototype to characterize the camera

32‐4: Student Paper: Chromatic Aberration Correction Enabled by Broadband Cholesteric Liquid‐Crystal Lens for Pancake Virtual‐Reality Optics

A broadband cholesteric liquid crystal (CLC) lens is fabricated and then integrated with a refractive lens, forming an ultracompact catadioptric pancake lens that can be employed as a virtual reality (VR) viewing optic with reduced chromatic aberrations. The demonstrated VR lens is a strong contender for next‐generation VR displays.

Achromatic and athermal design using cost-effective glass selection and aberration-corrected point matching on a glass map.

This paper presents a new method for determining a pair of cost-effective optical materials and powers to achromatize and athermalize a lens system, by introducing the material selection index including price level (MSIP). To obtain a relatively low-cost material effectively, we propose, to the best of our knowledge, a novel glass map including the price levels of the glasses. The materials with a small MSIP were selected on the glass map to correct chromatic aberration and thermal defocus easily, along with providing a cost-effective design. Although there are many material combinations, we can rationally identify the best selection of glasses by checking the MSIP index. Finally, the matching of the aberration-corrected point to an available material by redistributing the optical powers yields a good achromatic and athermal design. Using this aberration-corrected point-matching method, we efficiently obtained an achromatic and athermal system with cost-effective material selection, over the specified temperature and waveband ranges.

Broadband achromatic metasurfaces for sub-diffraction focusing in the visible.

Conventional achromatic optical systems are matured to achieve effective chromatic aberration correction and diffraction-limited resolution by the multiple bulky lenses. The emergence of the super-oscillation phenomenon provides an effective method for non-invasive far-field super-resolution imaging. Nevertheless, most super-oscillatory lenses are significantly restricted by the chromatic aberration due to the reliance on delicate interference; on the other hand, most achromatic lenses cannot break the diffraction limit. In this article, a single-layer broadband achromatic metasurface comprising sub-wavelength anisotropic nanostructures has been proposed to achieve sub-diffraction focusing with a focal length of f=60 µm and a diameter of 20 µm in the visible ranging from 400 nm to 700 nm, which are capable of generating sub-diffraction focal spots under the left-handed circularly polarized incident light with arbitrary wavelength in the working bandwidth at the same focal plane. This method may find promising potentials in various applications such as super-resolution color imaging, light field cameras, and machine vision.

Vertical Pupil in a Lemon Shark

The lemon shark (Negaprion brevirostris) (Fig A) is mainly piscivorous, coastally distributed, and mostly found in the Western Hemisphere. Its startling vertical slit pupils (Fig B) are generally found in predators, with horizontal pupils being found in prey species. There are many theories regarding the value and purpose of such pupils including correction of chromatic aberration, response to various levels of illumination, improvement of astigmatic depth of field, and multifocality. Lemon shark eyes have a visual streak of concentrated photoreceptors, including both rods and cones in the horizontal meridian, although its functional interaction with the pupil is not understood.

DRAM Bandwidth Optimal Perspective Transform Engine

Perspective transform (or Homography) is commonly used algorithms in ADAS and Automated Driving System. Perspective transform is used in multiple use-cases e.g. viewpoint change, fisheye lens distortion correction, chromatic aberration correction, stereo image pair rectification, This algorithm needs high external DRAM memory bandwidth due to inherent scaling, resulting in nonaligned two dimensional memory burst accesses, resulting in large degradation in system performance and latencies. In this paper, we propose a novel perspective transform engine to reduce external memory DRAM bandwidth to alleviate this problem. The proposed solution consists of multiple regions slicing of input video frame with block size tuned for each region. The paper also gives an algorithm for finding optimal region boundaries with corresponding block size tuned for each region. The proposed solution enables average BW reduction of 67% compared to traditional implementation and achieves clock up-to 720 MHz with output pixel throughput of 1 cycle/pixel in 16nm FinFET process node.

Blind Correction of Lateral Chromatic Aberration in Raw Bayer Data

Chromatic aberration is an error that occurs in color images due to the fact that camera lenses refract the light of different wavelengths in different angles. The common approach today to correct the error is to use a lookup table for each camera-lens combination, e.g., as in Adobe PhotoShop Lightroom or DxO Optics Pro. In this paper, we propose a method that corrects the chromatic aberration error without any priot knowledge of the camera-lens combination, and does the correction already on the bayer data, i.e., before the Raw image data is interpolated to an RGB image. We evaluate our method in comparison to DxO Optics Pro, a state-of-the-art tool based on lookup tables, using 25 test images and the variance of the color differences (VCD) metric. The results show that our blind method has a similar error correction performance as DxO Optics Pro, but without prior knowledge of the camera-lens setup.

Lattice contrast in the core-loss EFTEM signal of graphene

The realization of chromatic aberration correction enables energy-filtered transmission electron microscopy (EFTEM) at atomic resolution even for large energy windows. Previous works have demonstrated lattice contrast from ionization-edge signals such as the L2,3 edges of silicon or titanium. However, the direct interpretation as chemical information was found to be hampered by contributions from elastic contrast with dynamic scattering, especially for thick samples. Here we demonstrate that even for thin samples with light atoms, the interpretation of the ionization-edge signal is complicated by inversions from bright-atom to dark-atom contrast. Our EFTEM experiments for graphene show lattice contrast in the carbon K-edge signal, and we find bright-atom and dark-atom contrast for different defoci.

40‐4: Cost‐Efficient Polymer Flat Lens for Chromatic Aberration Correction in Virtual Reality Displays

An optical chromatic aberration correction method for virtual reality displays is demonstrated. With a cost‐efficient ultra‐broadband liquid crystal flat lens based on the Pancharatnam‐Berry phase, the proposed method can reduce the lateral color breakup by more than 10 times. Both simulation and experimental results are presented and discussed.

Multi-camera Panoramic Stitching with Real-time Chromatic Aberration Correction

Due to uneven lighting conditions and differences between camera lenses, in actual engineering applications, there is a difference in brightness of the image collected by the camera, which will cause obvious gaps in video stitching and poor stitching effect of panoramic pictures. This paper designs and implements an algorithm based on the gray statistics of overlapping areas to achieve global correction, and combined with the fade-in and fade-out image fusion algorithm to make the splicing transition natural. The experimental results show that in the Visual Studio 2015 platform environment, real-time stitching of 8-channel cameras has a fast algorithm speed and can achieve a stitching speed of 25 frames per second. The video is smooth and the picture quality is good without obvious color difference.

Inverse-designed achromatic flat lens enabling imaging across the visible and near-infrared with diameter > 3 mm and NA = 0.3

It is generally thought that correcting chromatic aberrations in imaging requires multiple surfaces. Here, we show that by allowing the phase in the image plane of a flat lens to be a free parameter, it is possible to correct chromatic aberrations over a large continuous bandwidth with a single diffractive surface. In contrast to conventional lens design, we utilize inverse design, where the phase in the focal plane is treated as a free parameter. This approach attains a phase-only (lossless) pupil function, which can be implemented as a multi-level diffractive flat lens that achieves achromatic focusing and imaging. In particular, we experimentally demonstrate imaging using a single flat lens of diameter > 3 mm and focal length = 5 mm (NA = 0.3, f/1.59) that is achromatic from λ = 450 nm (blue) to 1 μm (NIR). This simultaneous achievement of large size, NA, and broad operating bandwidth has not been demonstrated in a flat lens before. We experimentally characterized the point-spread functions, off-axis aberrations, and broadband imaging performance of the lens.

Doublet metalens design for high numerical aperture and simultaneous correction of chromatic and monochromatic aberrations.

Metalens is one of the most prominent applications among metasurfaces since it gives possibilities to replace the conventional lenses for compactness and multi-functionalities. Recently, many studies have been demonstrated to overcome the aberrations of the metalenses for high performance practical applications. Previous studies have used the methods that control the dispersion of meta-atoms for correcting chromatic aberrations and use doublet platform for correcting monochromatic aberrations. Despite these studies and the large demands for simultaneous correction of the aberrations in high numerical aperture metalens, the simultaneous correction has not been demonstrated yet. In this paper, we report the doublet metalens design with high numerical aperture which corrects longitudinal chromatic aberration and four monochromatic aberrations including spherical aberration, coma, astigmatism, and field curvature simultaneously for the three primary visible colors. Based on the novel doublet platform, the multi-wavelength targeted correction lens and geometric phase lens with color filtering functionality are utilized. Our doublet metalens has numerical apertures of 0.33, 0.38, and 0.47 for 445 nm, 532 nm, and 660 nm, respectively. The back focal length of our doublet metalens remains nearly 360 µm for target wavelengths and incident angles up to 30 degrees.

Color Fourier ptychographic microscopy based on symmetrical illumination and wavelength multiplexing

Fourier ptychographic microscopy (FPM), by using a LED array as its light source and each LED being lit up sequentially, can recover a high resolution (HR) and large field of view image from a sequence of low resolution (LR) images. It is difficult to obtain a perfect high resolution color image by a monochrome camera because of non-overlapping focal planes of different color channels resulting from the residual chromatic aberration of the microscopic objective. In this paper, a kind of method, based on symmetrical LED illumination and wavelength multiplexing, is applied in a FPM system to collect LR images and recover HR color images. Compared with single LED illumination, symmetrical illumination has higher defocus tolerance. It is proven by simulated and experimental results that higher-quality HR color images can be obtained based on wavelength multiplexing and symmetrical illumination by a monochrome camera when defocus or chromatic aberration exist. It is demonstrated that symmetrical illumination is helpful in chromatic aberration correction in color FPM imaging.

Saturation-Value Total Variation model for chromatic aberration correction

<p style='text-indent:20px;'>Chromatic aberration generally occurs in the regions of sharp edges in captured digital color images. The main aim of this paper is to propose and develop a novel Saturation-Value Total Variation (SVTV) model for chromatic aberration correction. In the proposed optimization model, there are three terms for the correction purpose. The SVTV regularization term is to model the target color image in HSV color space instead of RGB color space, and to avoid oscillations in the recovering process. In correction process, the gradient matching terms based on the green component are used to govern both red and blue components, and the intensity terms are employed to fit red, green and blue data components. The existence of the minimizer of the optimization model is analyzed and an efficient optimization algorithm is also developed for solving the resulting variational problem. Experimental results are presented to illustrate the effectiveness of the proposed model and to show that the correction results are better than those by using the other testing methods.

Optical performance evaluation and chromatic aberration correction of a focus tunable lens used for 3D microscopy.

Recently, the development of motion-free 3D microscopy utilizing focus tunable lenses (FTL) has been rapid. However, the downgrade of optical performance due to FTL and its gravity effect are rarely discussed in detail. Also, color dispersion is usually maintained purely depending on the FTL material without further correction. In this manuscript, we provide a quantitative evaluation of the impact of FTL on the optical performance of the microscope. The evaluation is based on both optical ray tracing simulations and lab experiments. In addition, we derive the first order conditions to correct axial color aberration of FTL through its entire power tuning range. Secondary spectrum correction is also possible and an apochromatic motion-free 3D microscope with 2 additional doublets is demonstrated. This study will serve a guidance in utilizing FTL as a motion-free element for 3D microscopy.

Practical Chromatic Aberration Correction in Virtual Reality Displays Enabled by Cost‐Effective Ultra‐Broadband Liquid Crystal Polymer Lenses

AbstractChromatic aberration (CA) is a critical issue in immersive virtual reality displays. The current digital compensation method can reduce the CA by pre‐processing the image contents but at the cost of extra memory space, processing time, and power consumption. Moreover, it is not possible to digitally compensate the CA within each color channel. Here, a practical optical approach is presented to correct the CAs, including the sub‐channel ones, in the virtual reality system, whose functionality is verified by both ray‐tracing analysis and experimental results. In this device, a compact ultra‐broadband Pancharatnam‐Berry phase lens (PBL) with complex axial molecular structure is fabricated and then hybridized with a refractive Fresnel lens. Due to their opposite chromatic dispersion, the system's CA is significantly reduced. To eliminate ghost image from the zero‐order leakage of PBL, a broadband circular polarizer is implemented to block the stray light. As a result, clear image within the entire 100° field‐of‐view is achieved. The proposed large‐size cost‐efficient broadband wide‐view flat optics can practically benefit not only virtual reality displays but also general imaging systems for practical applications and scientific research.

Full-field neutron microscopy based on refractive optics

Placing a compound refractive lens (CRL) as an objective in a neutron beam generates new possibilities for 2D and 3D nondestructive mapping of the structure, strain and magnetic domains within extended objects. A condenser setup is introduced that allows correction for the lateral chromatic aberration. More generally, for full-field microscopy the loss in performance caused by the chromatic aberration can be more than offset by introducing arrays of CRLs and exploiting the fact that the field of view can be much larger than the physical aperture of the CRL. Comments are made on the manufacture of such devices. The potential use is illustrated by comparisons between state-of-the-art instrumentation and suggested approaches for bright-field microscopy, small-angle neutron scattering microscopy, grain mapping and mapping of stresses. Options are discussed for depth-resolved imaging inspired by confocal light microscopy. Finally, experimental demonstrations are given of some of the basic properties of neutron full-field imaging for a single CRL.

Multiwavelength Digital Holographic Imaging and Phase Unwrapping of Protozoa Using Custom Fiji Plug-ins

Multiwavelength digital holographic microscopy (DHM) has been used to improve phase reconstructions of digital holograms by reducing 2π phase ambiguities. However, most samples used as test images have been solid or adhered to a surface, making it easy to determine focal planes and correct for chromatic aberration. In this study we apply 3-wavelength off-axis DHM to swimming protozoa containing distinct spectral features such as chlorophyll and carotenoids. We reconstruct the holograms into amplitude and phase images using the angular spectrum method. Methods for noise subtraction, chromatic aberration correction, and image registration are presented for both amplitude and phase. Approaches to phase unwrapping are evaluated and compared to expected results from simulated holograms. The algorithms used are implemented in plug-ins using the open source Fiji platform and are available for use, significantly expanding the open-source software available for DHM.