Spatial Transfer Function Research Articles

Optical directional differential operation enabled visual chirality detection.

Directional differential operation can extract the changes of directional information from complex signals, and plays an important role in target recognition and texture image processing. Here, we propose an optical directional differential operation based on large cross-polarization rotation, and realize the visual detection of chiral enantiomers. By using cross-polarization rotation in a specified direction, we design a corresponding directional spatial spectral transfer function whose transmission efficiency increases as the incident angle approaches the Brewster angle. The differential direction can be adjusted by changing the initial polarization state, and can be used to detect the concentration of chiral solutions. Finally, we apply the directional differential operation to achieve the visual detection of chiral enantiomers.

Reflection-type orientation-switchable optical differentiator exemplified by the BK7-MoS2 interface.

Optical differential operation is an ultra-fast edge information extraction technology that enables identifying the image features, whereas current optical differentiators mainly operate along one or two differential orientations and is incompetent for switching the differential orientation. A reflection-type orientation-switchable optical differentiator is proposed and the BK7-MoS2 interface is used as an example to analyze the edge detection performance. Theory suggests that there are dual modulation methods, one is tuning the incident polarization angle with a Brewster incident angle, and the other one is altering the incident angle with a nearly vertical incident polarization angle. The reason is that the term rp/rs tan α ≪ 1 is an ignored small quantity under these two situations and the spatial spectral transfer function is closely related to the scale coefficients Cx and Cy, whose magnitudes are both the incident polarization angle and incident angle dependent. Our optical differentiator can be combined with other tuning means since some two-dimensional materials are responsive to external stimuli, such as electrical or magnetic fields and defect engineering, these findings have potential applications in optical sensing, machine vision, and biomedical imaging.

A new synthetic correlation electron cyclotron emission diagnostic for validating nonlinear gyrokinetic simulations of electron temperature turbulence.

To validate nonlinear gyrokinetic simulations of electron temperature turbulence, the experimental correlation electron cyclotron emission (CECE) measurements are to be compared using a synthetic CECE diagnostic, which generates modeled CECE measurement quantities by implementing realistic measurement parameters (e.g., spatial and wavenumber resolutions, radial location, etc.) to nonlinear gyrokinetic simulations. In this work, we calculate the radial and vertical spatial and wavenumber transfer functions, which are defined by the electron cyclotron emission emissivity radial profile and vertical probing antenna pattern, respectively. These transfer functions are applied to nonlinear gyrokinetic simulations of electron temperature turbulence using the continuum gyrokinetic code. A simultaneous comparison of the experimental electron temperature turbulence power spectrum and root-mean-square (RMS) level, as well as the radial correlation length with the new synthetic CECE diagnostic at a core location ρ ∼ 0.75 in an L-mode DIII-D tokamak plasma, is presented. The preliminary result shows that the synthetic CECE output underestimates the RMS level by ∼42% and overestimates the radial correlation length by ∼40%.

Pixel-super-resolved lens-free quantitative phase microscopy with partially coherent illumination

Lens-free on-chip microscopy (LFOCM) has been widely utilized in digital pathology, drug screening, point-of-care testing (POCT), and quantitative phase imaging (QPI) due to its high throughput imaging capability and compactness. Initially, coherent laser sources were used in LFOCM to generate interference fringes to reconstruct the intensity and phase information of an object. The use of partially coherent light-emitting diodes (LEDs) in LFOCM offers a more portable and cost-effective alternative to conventional coherent illumination sources. However, the coherence-gating effect from a relatively low degree of coherence may cause a blur of high-frequency information in holograms, leading to an inaccurate object recovery. Thus, we present a pixel-super-resolved lens-free quantitative phase microscopy (PSR-LFQPM) with partially coherent illumination, which not only compensates for the impact of low coherence without increasing the volume of the system but also suppresses the theoretical Nyquist-Shannon sampling resolution limit imposed by the sensor pixel size (0.9 μm). Based on the partially coherent imaging model, we integrate the spatial coherence transfer function (SCTF) obtained from the pre-calibrated LED source distribution during the iteration process to obtain an accurate high-resolution recovery. Applying PSR-LFQPM to image living HeLa cells in vitro, we achieve real-time dynamic high-throughput QPI performance (half-pitch resolution of 780 nm with a 1.41-fold improvement compared to results without considering the effect of coherence) across a wide FOV (19.53 mm2). The proposed method provides a compact, low-cost, and high-throughput lens-free on-chip microscopy system for biomedical and POCT applications.

P‐49: Angular Dependent Point Spread Function Analysis for Virtual Reality Head Mounted Displays

We demonstrate a point spread function method for measuring the angular dependence of the spatial resolution of a virtual reality head mounted display. The angular dependence of spatial resolution of the imaging detector was characterized to remove the detector's contribution at each angular location measured on the VR HMD. This method allows measuring the twodimensional spatial resolution and modulation transfer function.

Theoretical proposal for actively manipulating optical differential operation in the quasi-PT symmetric structure

Optical differential operation is an important scheme for image edge detection due to its advantages of high efficiency, real time and low consumption. In this paper, the actively manipulating optical differential operation is proposed in a quasi-PT-symmetric structure containing graphene, and applied to realize edge imaging with tunable contrast. It is found that there are abrupt changes of the in-plane shift and transverse shift in the photonic spin Hall effect at the Fermi energy of 0.4 eV, where the quasi-exceptional points exist according to the degeneracy analysis of scattering matrix eigenvalues. Based on the photonic spin Hall effect near the quasi-exceptional points, we design the optical differential operation whose spatial transfer function is actively manipulated by changing the external electric field to adjust the Fermi energy of graphene. Finally, the edge imaging with different contrast can also be achieved when the optical differential operation is applied to image processing. The actively manipulating optical differential operation opens up new possibilities in microscopic imaging and smart driving.

Overcoming the challenges of performing metasurface-based general spatial transfer functions

The concept of performing mathematical operations with metasurfaces has been suggested by Silva et al (Science 343, 160 (2014)). However, their proposed structure in implementing any transfer function (corresponding to any mathematical operator) for various input signals faces limitations. To tackle this issue, in this study, four different scenarios are proposed on their metasurface-based structure to generalize in a way that can implement each spatial transfer function. To evaluate the performance of the presented scenarios, seven different transfer functions are simulated to encompass a wide range of mathematical operators in the spatial domain. The implementations are based on the Fourier approach. Simulation results based on the finite element method closely match the desired values. From the results of this study, it can be seen that the third and fourth scenarios provide better accuracy. For example, when the fifth transfer function is performed by the basic structure and the fourth scenario, the normalized root mean square error, decreases from the value of 0.235 to the value of 0.0348, respectively. Furthermore, a tunable structure is achievable using the third scenario to produce different operators on the same structure. The realization of these scenarios is possible by using nanostructure-based metasurfaces.

Experimental evaluation of bilateral Ambisonics-based binaural room transfer function synthesis with application to personal sound zones

This study experimentally evaluates the bilateral Ambisonics method for synthesizing binaural room transfer functions (BRTFs) and explores its application in generating personal sound zones (PSZs) around listeners’ ears. Bilateral Ambisonics is proposed for improving the spatial sound reproduction accuracy at a limited Ambisonics order, by shifting the origin of Ambisonics representation from the head center to the two ears. While numerical simulations have demonstrated its superiority over traditional Ambisonics, little attention has been given to validating its usability for reproduction in realistic environments with loudspeakers. Experiments are conducted wherein the BRTFs of a dummy head listener are first synthesized by combining the spatial room transfer functions (SRTFs) measured with a spherical Ambisonics microphone array at the ear position and the corresponding head-related transfer functions (HRTFs), and then compared with the actual in situ measured ones. Furthermore, the quality of the synthesized BRTFs is examined in the context of an in-house PSZ system, by objectively evaluating the performance of PSZ filters designed with such BRTFs in terms of acoustic isolation metrics. The application of synthesized BRTFs to PSZ systems circumvents the need for in-situ BRTF measurements, making it highly feasible to deploy such systems while achieving high acoustic isolation.

Expanded field of view frequency-selective incoherent holography by using a triple-beam setup.

We propose a new, to the best of our knowledge, method of incoherent optical frequency selection called three-pack frequency-selective incoherent holography. Compressed holography is reconstructed using phase shift intercepts and spatial transfer function convolution in the form of separation without loss of magnification or resolution. The frequency-selective reconstruction process removes the conjugate and DC terms along with the interception of the object wave. This work attempts three-dimensional reconstruction and selected-frequency phase extraction of axial slices in submicron steps, and the experimental results show the potential of the proposed method in areas such as compressed holography, extended field of view, and slice tomography.

In vivo chromatic and spatial tuning of foveolar retinal ganglion cells in Macaca fascicularis.

The primate fovea is specialized for high acuity chromatic vision, with the highest density of cone photoreceptors and a disproportionately large representation in visual cortex. The unique visual properties conferred by the fovea are conveyed to the brain by retinal ganglion cells, the somas of which lie at the margin of the foveal pit. Microelectrode recordings of these centermost retinal ganglion cells have been challenging due to the fragility of the fovea in the excised retina. Here we overcome this challenge by combining high resolution fluorescence adaptive optics ophthalmoscopy with calcium imaging to optically record functional responses of foveal retinal ganglion cells in the living eye. We use this approach to study the chromatic responses and spatial transfer functions of retinal ganglion cells using spatially uniform fields modulated in different directions in color space and monochromatic drifting gratings. We recorded from over 350 cells across three Macaca fascicularis primates over a time period of weeks to months. We find that the majority of the L vs. M cone opponent cells serving the most central foveolar cones have spatial transfer functions that peak at high spatial frequencies (20-40 c/deg), reflecting strong surround inhibition that sacrifices sensitivity at low spatial frequencies but preserves the transmission of fine detail in the retinal image. In addition, we fit to the drifting grating data a detailed model of how ganglion cell responses draw on the cone mosaic to derive receptive field properties of L vs. M cone opponent cells at the very center of the foveola. The fits are consistent with the hypothesis that foveal midget ganglion cells are specialized to preserve information at the resolution of the cone mosaic. By characterizing the functional properties of retinal ganglion cells in vivo through adaptive optics, we characterize the response characteristics of these cells in situ.

Performance evaluation of two interventional fluoroscope suites for cardiovascular imaging.

Interventional cardiology involves catheter‐based treatment of heart disease, generally through fluoroscopically guided interventional procedures. Patients can be subject to considerable radiation dose due to prolonged fluoroscopy time and radiographic exposure, and therefore efforts to minimize patient dose should always be undertaken. Developing standardized, effective quality control programs for these systems is a difficult task owing to cross‐vendor differences and automated control of imaging protocols. Furthermore, analyses of radiation dose should be performed in the context of its associated effects on image quality.The aim of the study is to investigate radiation dose and image quality in two fluoroscopic systems used for interventional cardiology procedures. Image quality was assessed in terms of spatial resolution and modulation transfer function, signal‐to‐noise and contrast‐to‐noise ratios, and spatial–temporal resolution of fluoroscopy and cineradiography images with phantoms simulating various patient thicknesses under routine cardiology protocols. The entrance air kerma (or air kerma rate) was measured and used to estimate entrance surface dose (or dose rate) in the phantoms.

High-resolution elliptical Kirkpatrick-Baez microscope for implosion higher-mode instability diagnosis.

High-resolution X-ray imaging diagnosis is a critical method for measuring Rayleigh-Taylor instability growth and hot spot interface morphology in inertial confinement fusion experiments. In this study, we develop a quasi-monochromatic elliptical Kirkpatrick-Baez microscope based on aberration theory, breaking the aberration limit of conventional Kirkpatrick-Baez microscopes. The microscope was characterized in the laboratory for spatial resolution performance and modulation transfer function before being implemented in cavity experiments at the SG-III prototype laser facility. The results demonstrate that the edge-based method achieves a spatial resolution of <2 µm in the central field of view and modulation of 800 lp/mm spatial frequency of >20%.

Concept validation of a high dynamic range point-diffraction interferometer for wavefront sensing in adaptive optics.

The direct detection and imaging of exoplanets requires the use of high-contrast adaptive optics (AO). In these systems quasi-static aberrations need to be highly corrected and calibrated. To achieve this, the pupil-modulated point-diffraction interferometer (m-PDI) was presented in an earlier paper. This present paper focuses on m-PDI concept validation through three experiments. First, the instrument's accuracy and dynamic range are characterized by measuring the spatial transfer function at all spatial frequencies and at different amplitudes. Then, using visible monochromatic light, an AO control loop is closed on the system's systematic bias to test for precision and completeness. In a central section of the pupil with 72% of the total radius, the residual error is 7.7 nm rms. Finally, the control loop is run using polychromatic light with a spectral FWHM of 77 nm around the R-band. The control loop shows no drop in performance with respect to the monochromatic case, reaching a final Strehl ratio larger than 0.7.

Analysis by Systemic Approach of Magnetic Dipole Source Location Performances by Using an IoT Software Gradiometer Head

Based on the development of numerous commercial cheap or low noise magnetometers, some publications feature systems of magnetic anomaly detection. They are meanly based on matrix inversion gradients and are built with multi-magnetometer single head in order to derive spatially the sensed magnetic field. Meanwhile, these examples often suffer from a lack of true performance analysis, based on a &#x201C;systemic approach.&#x201D; As an example, they take account partially of the main sensor, mechanical or conditioning electronic characteristics. In that way, we propose a theoretical modeling of 1D and 2D magnetic head detection capabilities that could be extended to 3D. We show that the distance of detection could be quantified with the help of three main parameters: the baseline, the sensor mismatch in terms of sensitivity (V/T) or misalignment, and a defined signal-to-noise ratio of the measurement (<inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula>). With a reasonable built system (sensitivity disparity of 0.6 &#x0025;, misalignment lower than some (&#x00B0;) and <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> of 120 dB), a magnetic dipole could be localized and quantified, from <inline-formula> <tex-math notation="LaTeX">$\times1$ </tex-math></inline-formula> up to <inline-formula> <tex-math notation="LaTeX">$\times20$ </tex-math></inline-formula> gradiometer base line with a relative error lower than 30 &#x0025;, and from <inline-formula> <tex-math notation="LaTeX">$\times3$ </tex-math></inline-formula> up to <inline-formula> <tex-math notation="LaTeX">$\times50$ </tex-math></inline-formula> with a <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> of 160 dB, as an example. Notice that the highest distance of detection is mainly bounded by the <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> value and the intrinsic head spatial transfer function. At a close distance, we have enlarged significantly the surface of the source localization, by using optimization algorithms when classical gradiometric method appears non-functional. To exemplify this systemic approach, we have built and characterized a setup. It is based on the IoT principles, where acquired data are pushed over-the-air through a gateway. Finally, they are post-processed in quasi-real time with a laptop.

Theoretical and Experimental Research on Spatial Performances of the Long-slit Streak Tube

Abstract The streak tubes are widely used in National Ignition Facility (NIF), Inertial Confinement Fusion (ICF), and streak tube imaging lidar (STIL) as radiation or imaging detectors. The spatial resolution and effective photocathode area of the streak tube are strongly dependent on its operating and geometry parameters (electron optical structure and applied voltage). Studies about this dependence do not cover the full range of the parameters. In this paper, 3-D models are developed in Computer Simulation Technology Particle Studio (CST-PS) to comprehensively calculate the spatial resolution for various parameters. Monte Carlo Sampling method (M-C method) and spatial modulation transfer function method (SMTF) are employed in our simulation. Simulated results of the optimized spatial resolution are validated by the experimental data. Finally, the radii of the photocathode (R c) and phosphor screen (R s) are optimized. Geometry parameters of R c=60 mm and R s=80 mm are proposed to optimize the streak tube performances. Simulation and experimental results show that the spatial resolution and effective photocathode area of this streak tube are expected to reach 16 lp/mm and 30 mm-length while the voltage between cathode and grid (U cg) is 150 V.

Tunable optical differential operation based on the cross-polarization effect at the optical interface

To achieve optical differential operation based on the cross-polarization effect at the optical interface, one just needs an optical interface composed of two uniform media with different refractive indices. When certain conditions are satisfied, the reflection co-efficient of the light field at the interface conforms to the form of the spatial spectrum transfer function required by the spatial differentiation, the spatial analog operation can be achieved with a single interface. In this paper, based on the optical differentiation of Brewster effect, we propose a tunable optical differentiation based on the cross-polarization effect at the optical interface. We theoretically derive the tunable optical differentiation and then conduct an experiment to demonstrate theoretical results. It is found that the differentiator can achieve the tunable optical differentiation by adjusting the polarization of output beam. While getting the clear edge of the object, we can also observe the imaging of the middle part to different degrees, which realizes the multi-degree of freedom imaging for the measured target. This provides a potential way to develop devices more suitable for microscopic imaging and target detection.

Reconstruction of Microscopic Thermal Fields from Oversampled Infrared Images in Laser-Based Powder Bed Fusion.

This article elucidates the need to consider the inherent spatial transfer function (blur), of any thermographic instrument used to measure thermal fields. Infrared thermographic data were acquired from a modified, commercial, laser-based powder bed fusion printer. A validated methodology was used to correct for spatial transfer function errors in the measured thermal fields. The methodology was found to make a difference of 40% to the measured signal levels and a 174 °C difference to the calculated effective temperature. The spatial gradients in the processed thermal fields were found to increase significantly. These corrections make a significant difference to the accuracy of validation data for process and microstructure modeling. We demonstrate the need for consideration of image blur when quantifying the thermal fields in laser-based powder bed fusion in this work.

Analogue Optical Spatiotemporal Differentiator

AbstractAs a significant part in optical computation, analogue optical spatial/temporal differentiators could play a great role in all‐optical signal processing, feature extraction, and optical storage. In the past few years, they have experienced rapid development but mostly for a single function. For many scenarios, optical computation with integrated functionalities is highly desired. In this work, an analogue optical spatiotemporal differentiator utilizing all‐dielectric multilayer is proposed. The film stack is specifically engineered to satisfy the requirements for both spatial and temporal transfer functions of the ideal differentiator. The time‐space performance of the device is numerically examined from the output field profile of the classical Gaussian beam or pulse. A wavelength‐scale high resolution for edge detection is attained using the integrated differentiator. This work may pave a potential way to establish ultracompact, high‐bandwidth, and real‐time optical multiple functional computation systems.

An undergraduate experiment to illustrate spatial transfer function concepts in Fourier optics

Fourier analysis is a key tool in physics and engineering in the broadest sense and is particularly used in the field of optics to design and study the properties of advanced imaging systems. Fourier optics as a distinct topic is usually taught in final-year undergraduate or first-year postgraduate studies, and a wide range of teaching approaches have been used to describe and explain its key concepts. Concerning practical work to accompany classroom lectures, the simplest laboratory experiments are based on studying how the properties of some diffracting aperture (pupil) are related to the corresponding Fraunhofer diffraction pattern, but this approach usually does not consider in detail the concepts of the optical transfer function and spatial frequencies. We here describe a simple experimental setup that fills this particular gap and that illustrates the spatial frequency-domain response of a simple optical system using incoherent light. The Modulation Transfer Function is directly measured for several different pupil geometries and shows very good agreement with theoretical calculations. This experiment clearly demonstrates spatial transfer function concepts in Fourier optics, complementing and extending other studies of Fourier transforms in physics that may consider similar ideas in a time and frequency signal-processing context.

Cone-beam CT dose and imaging performance evaluation with a modular, multipurpose phantom.

A modular phantom for dosimetry and imaging performance evaluation in cone-beam computed tomography (CBCT) is reported, providing a tool for quantitative technical assessment that can be adapted to a broad variety of CBCT imaging configurations and clinical applications. The phantom presents a set of modules that can be ordered in various configurations suitable to a particular CBCT system. Modules include slabs containing a uniform medium, low-contrast inserts, line-spread features, and disk features suitable to measurement of image uniformity, noise, noise-power spectrum (NPS), contrast, contrast-to-noise ratio (CNR), Hounsfield (HU) accuracy, linearity, spatial resolution modulation transfer function (MTF), and magnitude of cone-beam artifact. Automated software recognizes the phantom configuration in DICOM images and provides structured reporting of such test measures. In any modular configuration, the phantom permits measurement of air kerma in central and peripheral locations with an air ionization chamber (e.g., Farmer chamber). The utility and adaptability of the phantom were demonstrated across a spectrum of CBCT systems, including scanners for orthopaedic imaging (Carestream OnSight 3D, Rochester NY), breast imaging (Doheny prototype, UC Davis), image-guided surgery (IGS, Medtronic O-arm, Littleton MA), angiography (Siemens Artis Zeego, Forcheim Germany), and image-guided radiation therapy (IGRT, Elekta Synergy XVI, Stockholm Sweden). The phantom provided a consistent platform for quantitative assessment of dose and imaging performance compatible with a broad spectrum of CBCT systems. The purpose of the survey was not to obtain head-to-head performance comparison of systems designed for such distinct clinical applications. Rather, the survey demonstrated the suitability of the phantom to a broad spectrum of systems in a manner that provides characterization pertinent to disparate applications and imaging tasks. For example: the orthopaedic CBCT system (pertinent clinical tasks relating to high-resolution bone imaging) was shown to achieve MTF consistent with imaging of high-contrast trabecular bone structures (i.e., the MTF reduced to 10% at spatial frequency, =1.2mm-1 ); the breast system (even higher-resolution imaging of microcalcifications) exhibited =2.2mm-1 ; the IGS system (tasks including both bone and soft-tissue contrast resolution) provided =0.9mm-1 and soft-tissue =1.64; the angiography system (soft-tissue body interventions) demonstrated =1.2 in soft tissues approximating liver lesions; and the IGRT system (pertinent tasks emphasizing HU linearity and image uniformity) showed linear response with HU values ( =1), with a cupping artifact ( =5.8%) due to x-ray scatter. The phantom provides an adaptable, quantitative basis for CBCT dosimetry and imaging performance evaluation suitable to a broad variety of CBCT systems. The dosimetry and image quality metrics are consistent with up-to-date methods for rigorous, quantitative, physics testing and should be suitable to emerging standards for CBCT quality assurance.