Pupil Filters Research Articles

Development and Testing of Artificial Intelligence-Based Mobile Application to Achieve Cataract Backlog-Free Status in Uttar Pradesh, India

BackgroundUttar Pradesh (UP), the most populous state in India, has about 36 million people aged 50 years or older, spread across more than 100,000 villages. Among them, an estimated 3.5 million suffer from visual impairments, including blindness due to untreated cataracts. To achieve cataract backlog-free status, UP is required to screen this population at the community level and provide treatment to those suffering from cataracts. We envisioned an AI-powered primary screening app utilizing eye images, deployable to frontline health workers for community-level screening. This paper outlines insights gained from developing the AI mobile app “Roshni” for cataract screening. MethodThe AI-based cataract classification model was developed using 13,633 eye images and finalized after three stages of experiments, detecting cataracts in images focused on the eye, iris, and pupil. Overall, 155 experiments were conducted using multiple deep learning algorithms, including ResNet50, ResNet101, YOLOv5, EfficientNetV2, and InceptionV3. We established a minimum threshold of 90 % specificity and sensitivity to ensure the algorithm’s suitability for field use. ResultsThe cataract detection model for eye-focused images achieved 51.9 % sensitivity and 87.6 % specificity, while the model for iris-focused images, using a good/bad iris filter, achieved 52.4 % sensitivity and 93.3 % specificity. The classification model for segmented-pupil images, employing a good/bad pupil filter with UNet-based semantic segmentation model and EfficientNetV2, yielded 96 % sensitivity and 97 % specificity. Field testing with 302 beneficiaries (604 images) showed an overall sensitivity of 86.6 %, specificity of 93.3 %, positive predictive value of 58.4 %, and negative predictive value of 98.5 %. ConclusionThis paper details the development of an AI mobile app designed to facilitate community screening for cataracts by frontline health workers.

Improved design method for suppressing optical focal spots and experimental verification

Pupil filtering is an effective method to modulate the 3D point spread function distribution of an optical system. Based on vector Debye-Wolf diffraction integral and a new fusion optimization algorithm, an improved method is described for efficiently designing the most widely-used concentric multi-belt pupil filters. The main advantage of this method is that it actively selects the minimum belt width and the total belt number for facilitating low-cost batch processing. Pupil filters for visible and near-infrared bands have been separately designed for improving the spatial resolution of optical imaging and ultrafast laser processing. The method has been experimentally demonstrated for suppressing the optical focal spot. The proposed method can be readily modified for designing more sophisticated multi-level pupil filters for practical applications in high-resolution imaging and fine laser micromachining.

Eigenmode high-visibility imaging in the far-field ghost imaging system with discrete chaotic light

The eigenmode sampling theorem of the single-frequency chaotic light in far-field ghost imaging (GI) scheme is theoretically studied. In the GI scheme, the chaotic source not only plays the role of illumination, but also acts as a pupil filter, determining the unequal-weighted eigenmode sampling. Here, we have theoretically and experimentally studied a filtering tool to achieve equal-weighted eigenmode sampling, thereby producing ghost images of higher visibility than traditional methods. The presented work can be considered as a step towards meeting the visibility requirement of multiphoton imaging system with classical light.

Fast simulation and design of the fiber probe with a fiber-based pupil filter for optical coherence tomography using the eigenmode expansion approach.

Fiber probes for optical coherence tomography (OCT) recently employ a short section of step-index multimode fiber (SIMMF) to generate output beams with extended depth of focus (DOF). As the focusing region of the output beam is generally close to the probe end, it is not feasible to adopt the methods for bulk-optics with spatial pupil filters to the fiber probes with fiber-based filters. On the other hand, the applicable method of the beam propagation method (BPM) to the fiber probes is computationally inefficient to perform parameter scan and exhaustive search optimization. In this paper, we propose the method which analyzes the non-Gaussian beams from the fiber probes with fiber-based filters using the eigenmode expansion (EME) method. Furthermore, we confirm the power of this method in designing fiber-based filters with increased DOF gain and uniformly focusing by introducing more and higher-order fiber modes. These results using the EME method are in good agreement with that by the BPM, while the latter takes 1-2 orders more computation time. With higher-order fiber modes involved, a novel probe design with increased DOF gain and suppressed sidelobe is proposed. Our findings reveal that the fiber probes based on SIMMFs are able to achieve about four times DOF gain at maximum with uniformly focusing under acceptable modal dispersion. The EME method enables fast and accurate simulation of fiber probes based on SIMMFs, which is important in the design of high-performance fiber-based micro-imaging systems for biomedical applications.

Design of pupil filter for scalable generation of uniform foci array

In this paper, the original focus under tightly focusing radially polarized Bessel–Gaussian beam is split into a foci array with a specially designed pupil filter modulation. With this method, foci in the array are precisely controlled at accurate positions, exhibiting almost identical intensities and weak side lobes. Furthermore, a compound modulation through superposition of several filters is theoretically investigated, and a method for scalable generation of uniform foci array on the basis of their subarrays is proposed, which greatly reduced the design complexity of the large-scale foci array. With this method, various kinds of uniform foci arrays with large amount of foci and attractive distributions are well modulated, which may find potential applications in multifocal multiphoton microscopy and parallel laser fabrication.

Superresolution technology based on a heterodyne detection system

Diffractive superresolution elements (DSEs) placed on a pupil plane can generate a diffractive main lobe whose width is smaller than that of an Airy disk, allowing for the realization of superresolution technology based on pupil filtering. However, the energy of the main lobe decreases dramatically with the decreasing of main lobe width, namely, the implementation of this superresolution technology is at the cost of effective signal power. This restricts greatly the development of this technology. In order to solve this problem, this study suggests the use of a heterodyne detection system (HDS) with this technology. The resolution characteristics of the HDS are analyzed through theoretical deduction. According to research results, HDS has the same longitudinal resolution and twice as high transverse resolution as a direct detection system (DDS). More significantly, the theoretical analyses show that HDS can increase detection sensitivity significantly compared with DDS. Hence, the proposed method makes it possible to detect extremely faint signals using this superresolution technology. In addition, because HDS lowers the requirements on main lobe energy due to its high sensitivity, the design of DSE can achieve a smaller width of main lobe, which can further improve the resolution of the superresolution technology based on pupil filtering.

Uniform focusing with an extended depth range and increased working distance for optical coherence tomography by an ultrathin monolith fiber probe.

It is difficult to maintain high transverse resolution over an increased depth range using miniature probes for optical coherence tomography (OCT) due to the rapid divergence of light and the space limitation. To solve this problem, we introduce a fiber-based filter in the proposed probe to manipulate its output beam. Significant mode interference (MI) is exploited to enhance the depth of focus (DOF), and the mode phase difference is tuned to achieve a uniform axial intensity within the DOF. The magnified MI field instead of the diffracted one is adopted as the final pupil filter in the probe to increase its working distance (WD). The probe is fabricated with a diameter of 125 µm and a total length of 2.6 mm for its distal fiber optics. Compared to the conventional probe with similar minimal lateral resolution of better than 4.4 µm, the proposed probe achieves two times that of the DOF gain and 1.7 times that of the WD. Improvements in performance of the probe are demonstrated by OCT imaging using a fresh lemon and human skin. With merits of enhanced imaging quality and easy fabrication, the proposed probe poses great potential for important applications, especially for endoscopic imaging of human internal organs in vivo.

Single-shot phase retrieval with complex diversity.

The concept of complex diversity is introduced that adequately accounts for special considerations in the design of the system and the reconstruction algorithm for single-shot phase retrieval techniques. Complex-number pupil filters containing both amplitude and phase values are extracted by numerical propagation from a computer-generated hologram design, which generates multiple images in a single acquisition. The reconstruction is performed by a Fourier iterative algorithm modified with an area restriction to avoid noise amplification. Numerical simulations show that the complex diversity technique estimates extrinsic Kolmogorov aberration better than conventional single-shot techniques for a distant point object. Experiments show that sensorless adaptive optics correction is achieved using the complex diversity technique.

Adaptive optics scanning laser ophthalmoscopy with two sources

Adaptive confocal laser ophthalmoscope with the high-resolution and dynamic imaging ability has been widely applied in specific biomedical and clinical medical fields. In order to apply the noncircular pupil filter and other pupil modulation technology without influence in wavefront detection, the system needs two light sources for imaging and aberration correction respectively. This paper has designed an adaptive optics scanning laser ophthalmoscopy with two sources, and then analyzed the differences of the aberration of the two light sources. Then, the aberration correction and high-resolution imaging ability of the system have been verified, and the brightness, contrast and resolution of the image have been significantly improved after close-loop. Finally, we have studied the feasibility of realizing the dark field imaging by semi-circular pupil and obtained the bright and dark field images of the artificial eye.

Heterodyne confocal microscopy using symmetrical shifted-focus phase filters.

We propose a heterodyne confocal microscopy technique that uses the symmetry properties of conjugate phase-pupil filters to shift the axial intensity-response curves of two confocal systems symmetrically with respect to the geometric focal plane. A monotonic position-measuring curve is obtained by subtracting the output signals of the two confocal systems collected synchronously, while eliminating the influence of environmental noise. The system's three-dimensional tomographic ability can be improved by optimizing the pupil filter. Theoretical analyses and experiments indicate that the proposed method improves the axial and transverse resolutions of differential confocal microscopy and simplifies system installation and integration.

Wavelength-dispersed optical-line scanning super-resolution interferometry for fast measurement of precision surface

A new precision surface measurement technology which is based on wavelength-dispersed optical-line scanning interferometry and is endowed with both vertical super-resolution and lateral super-resolution is proposed. The light from a broadband light source is dispersed by a blazed grating to be an optical line scanning the surface to realize fast three dimensional (3D) surface measurement. Vertical super-resolution of less than 1 nm is obtained by the means of optical interferometry. By employing an optical amplitude pupil filter, the lateral resolution in one direction is achieved to be less than 0.5 μm. And by expanding the interferometric beam before being detected by a linear array CCD, the lateral resolution in the other direction can be less than 0.8 μm. The wavelength of the light reflected from each measured point on the surface is unvariable and the measurement results can be wavelength-traceable exactly.

Numerical modeling of two-photon focal modulation microscopy with a sinusoidal phase filter.

A spatiotemporal phase modulator (STPM) is theoretically investigated using the vectorial diffraction theory. The STPM is equivalent to a time-dependent phase-only pupil filter that alternates between a homogeneous filter and a stripe-shaped filter with a sinusoidal phase distribution. It is found that two-photon focal modulation microscopy (TPFMM) using this STPM can significantly suppress the background contribution from out-of-focus ballistic excitation and achieve almost the same resolution as two-photon microscopy. The modulation depth is also evaluated and a compromise exists between the signal-to-background ratio and signal-to-noise ratio. The theoretical investigations provide important insights into future implementations of TPFMM and its potential to further extend the penetration depth of nonlinear microscopy in imaging multiple-scattering biological tissues.

Adaptive optics scanning system with two-zone phase filters

ABSTRACTAn adaptive confocal system employing pupil filters and two light sources has been designed, providing improved transverse resolution. Coherent transfer functions are studied to show the transfer capability at different frequencies and the limitations of the filters in obtaining clear improvements in the focal plane. Then, intensity point spread functions are analysed to reveal the dependence of the resolution on the size of the pinhole and the inner radius of the filter. An experiment has been carried out using a liquid crystal phase modulator and an artificial eye to validate the simulation results. It proves that the system with two light sources can correct the aberrations, with the basic effect of the phase modulation being to decrease the low spatial frequencies and increase the high spatial frequencies. Improvement of the transverse resolution can be achieved by sacrificing light intensity and contrast.

Effects of pupil filter patterns in line-scan focal modulation microscopy.

Line-scan focal modulation microscopy (LSFMM) is an emerging imaging technique that affords high imaging speed and good optical sectioning at the same time. We present a systematic investigation into optimal design of the pupil filter for LSFMM in an attempt to achieve the best performance in terms of spatial resolutions, optical sectioning, and modulation depth. Scalar diffraction theory was used to compute light propagation and distribution in the system and theoretical predictions on system performance, which were then compared with experimental results.

Resolution enhancement of pump–probe microscope with an inverse-annular filter

Optical pump–probe microscopy can provide images by detecting changes in probe light intensity induced by stimulated emission, photoinduced absorbance change, or photothermal-induced refractive index change in either transmission or reflection mode. Photothermal microscopy, which is one type of optical pump–probe microscopy, has intrinsically super resolution capability due to the bilinear dependence of signal intensity of pump and probe. We introduce new techniques for further resolution enhancement and fast imaging in photothermal microscope. First, we introduce a new pupil filter, an inverse-annular pupil filter in a pump–probe photothermal microscope, which provides resolution enhancement in three dimensions. The resolutions are proved to be improved in lateral and axial directions by imaging experiment using 20-nm gold nanoparticles. The improvement in X (perpendicular to the common pump and probe polarization direction), Y (parallel to the polarization direction), and Z (axial direction) are by 15 ± 6, 8 ± 8, and 21 ± 2% from the resolution without a pupil filter. The resolution enhancement is even better than the calculation using vector field, which predicts the corresponding enhancement of 11, 8, and 6%. The discussion is made to explain the unexpected results. We also demonstrate the photothermal imaging of thick biological samples (cells from rabbit intestine and kidney) stained with hematoxylin and eosin dye with the inverse-annular filter. Second, a fast, high-sensitivity photothermal microscope is developed by implementing a spatially segmented balanced detection scheme into a laser scanning microscope using a Galvano mirror. We confirm a 4.9 times improvement in signal-to-noise ratio in the spatially segmented balanced detection compared with that of conventional detection. The system demonstrates simultaneous bi-modal photothermal and confocal fluorescence imaging of transgenic mouse brain tissue with a pixel dwell time of 20 µs. The fluorescence image visualizes neurons expressing yellow fluorescence proteins, while the photothermal signal detected endogenous chromophores in the mouse brain, allowing 3D visualization of the distribution of various features such as blood cells and fine structures most probably due to lipids. This imaging modality was constructed using compact and cost-effective laser diodes, and will thus be widely useful in the life and medical sciences. Third, we have made further resolution improvement of high-sensitivity laser scanning photothermal microscopy by applying non-linear detection. By this, the new method has super resolution with 61 and 42% enhancement from the diffraction limit values of the probe and pump wavelengths, respectively, by a second-order non-linear scheme and a high-frame rate in a laser scanning microscope. The maximum resolution is determined to be 160 nm in the second-order non-linear detection mode and 270 nm in the linear detection mode by the PT signal of GNPs. The pixel rate and frame rate for 300 × 300 pixel image are 50 µs and 4.5 s, respectively. The pixel and frame rate are shorter than the rates, those are 1 ms and 100 s, using the piezo-driven stage system.

二维多项式位相光瞳滤波实现超分辨望远成像

二维多项式位相光瞳滤波实现超分辨望远成像

Super-resolution pupil filtering for visual performance enhancement using adaptive optics

Ocular aberration correction can significantly improve visual function of the human eye. However, even under ideal aberration correction conditions, pupil diffraction restricts the resolution of retinal images. Pupil filtering is a simple super-resolution (SR) method that can overcome this diffraction barrier. In this study, a 145-element piezoelectric deformable mirror was used as a pupil phase filter because of its programmability and high fitting accuracy. Continuous phase-only filters were designed based on Zernike polynomial series and fitted through closed-loop adaptive optics. SR results were validated using double-pass point spread function images. Contrast sensitivity was further assessed to verify the SR effect on visual function. An F-test was conducted for nested models to statistically compare different CSFs. These results indicated CSFs for the proposed SR filter were significantly higher than the diffraction correction (p < 0.05). As such, the proposed filter design could provide useful guidance for supernormal vision optical correction of the human eye.

Image formation in image scanning microscopy, including the case of two-photon excitation.

The effect of combining the image scanning microscopy (ISM) technique with two-photon fluorescence microscopy is analyzed. The effective spatial frequency cutoff can be doubled, as compared with conventional two-photon fluorescence microscopy, and the magnitude of the optical transfer function near the cutoff of conventional two-photon microscopy is increased by orders of magnitude. For the two-photon case, it is found that the optimum pixel reassignment factor in ISM is not equal to one half, as is often assumed in single-photon fluoresence image scanning microscopy, because the excitation and detection point spread functions are different. The optimum reassignment factor depends on the noise level, and in general the useful cutoff spatial frequency is about 1.8 times that for conventional two-photon microscopy. The effect of altering the reassignment factor in single-photon fluorescence ISM with a Stokes shift is also investigated. Illumination using pupil filters, such as by a Bessel beam, is considered. Using a ring detector array is found to result in good imaging behavior, exhibiting a sharpening of the point spread function by a factor of 1.7 compared with conventional fluorescence. Image formation in ISM can be considered in a four-dimensional spatial frequency space, giving new insight into the imaging properties. This approach is related to phase space representations such as the Wigner distribution function and the ambiguity function. A noniterative algorithm for image restoration is proposed.

Planar binary-phase lens for super-oscillatory optical hollow needles

Optical hollow beams are suitable for materials processing, optical micromanipulation, microscopy, and optical lithography. However, conventional optical hollow beams are diffraction-limited. The generation of sub-wavelength optical hollow beams using a high numerical aperture objective lens and pupil filters has been theoretically proposed. Although sub-diffraction hollow spot has been reported, nondiffracting hollow beams of sub-diffraction transverse dimensions have not yet been experimentally demonstrated. Here, a planar lens based on binary-phase modulation is proposed to overcome these constraints. The lens has an ultra-long focal length of 300λ. An azimuthally polarized optical hollow needle is experimentally demonstrated with a super-oscillatory transverse size (less than 0.38λ/NA) of 0.34λ to 0.42λ, where λ is the working wavelength and NA is the lens numerical aperture, and a large depth of focus of 6.5λ. For a sub-diffraction transverse size of 0.34λ to 0.52λ, the nondiffracting propagation distance of the proposed optical hollow needle is greater than 10λ. Numerical simulation also reveals a good penetrability of the proposed optical hollow needle at an air-water interface, where the needle propagates through water with a doubled propagation distance and without loss of its super-oscillatory property. The proposed lens is suitable for nanofabrication, optical nanomanipulation, super-resolution imaging, and nanolithography applications.

Achromatic super-oscillatory lenses with sub-wavelength focusing

Lenses are crucial to light-enabled technologies. Conventional lenses have been perfected to achieve near-diffraction-limited resolution and minimal chromatic aberrations. However, such lenses are bulky and cannot focus light into a hotspot smaller than a half-wavelength of light. Pupil filters, initially suggested by Toraldo di Francia, can overcome the resolution constraints of conventional lenses but are not intrinsically chromatically corrected. Here we report single-element planar lenses that not only deliver sub-wavelength focusing, thus beating the diffraction limit of conventional refractive lenses, but also focus light of different colors into the same hotspot. Using the principle of super-oscillations, we designed and fabricated a range of binary dielectric and metallic lenses for visible and infrared parts of the spectrum that are manufactured on silicon wafers, silica substrates and optical fiber tips. Such low-cost, compact lenses could be useful in mobile devices, data storage, surveillance, robotics, space applications, imaging, manufacturing with light and spatially resolved nonlinear microscopies.