Locally adaptive super-resolution through spatially variant interpolation.

Systems that do not meet the requirements of the sampling theorem produce images corrupted by aliasing. Higher resolution images are attainable by unfolding aliased spatial frequencies. Multiple-image super-resolution has seen much attention in the literature though with no clear optimum algorithm for many real-world applications. We propose a method of multiframe super-resolution using a set of convolutional sinc kernels, tailored to the specific shifts between images, capable of resolving up to the diffraction limit. We demonstrate our method for the case of global shifts before we treat a pixel-level super-resolution.

Estimating axial resolution with diffraction theory.

Depth estimation is a classic machine vision and image processing problem aiming at mapping the distances of objects from the camera. The accuracy of this depth map depends on the axial resolution achieved by the system, which is usually estimated using geometrical optics theory. This paper proposes a novel formula using diffraction theory. A comparison with the geometrical approach for estimating the axial resolution is provided, and results for several simulations are discussed.

Imaging system to assess objectively the optical density of the macular pigment in vivo.

This paper presents an optical system called MacPI, which implements a two-color reflectance technique in combination with various hardware and software tools to assess objectively the macular pigment (MP) optical density in vivo. The system consists of a bespoke optical design, a control architecture, driver electronics, a collection of image-processing techniques, and a graphical user interface. The deficiencies of the technique employed and the solutions implemented in the MacPI system to confront those inherent frailties are presented. An overview of the effective interpretation of the acquired data and the techniques employed by MacPI in the acquisition of that data is discussed. The result of a comparison trial with an alternative device is also presented. We suggest that appropriate design of the hardware and an efficient interpretation of the acquired data should produce a system capable of consistent, accurate, and rapid measurements, while retaining the distinction of ease of use, portability, comfort for the subject, and a design that is economic to produce. Its versatility should allow both for a clinical screening application and for further investigation and establishment of the physiological role of the MP in a laboratory-based environment.

Comparison of optical vortex detection methods for use with a Shack-Hartmann wavefront sensor.

In this paper we compare experimentally two methods of detecting optical vortices from Shack-Hartmann wavefront sensor (SHWFS) data, the vortex potential and the contour sum methods. The experimental setup uses a spatial light modulator (SLM) to generate turbulent fields with vortices. In the experiment, many fields are generated and detected by a SHWFS, and data is analysed by the two vortex detection methods. We conclude that the vortex potential method is more successful in locating vortices in these fields.

Subjective evaluation of intraocular lenses by visual acuity measurement using adaptive optics.

We present a new method for subjectively evaluating intraocular lenses (IOLs) without implantation surgery. To illustrate the method, three types of single-piece IOL (equispherical monofocal, rotational symmetric aspheric monofocal, and diffractive bifocal) were assembled into a model eye and evaluated using an ocular adaptive optics system by a single subject. To separate the spherical aberration of the crystalline lens, the subject's corneal topography and wavefront aberrations were measured and modeled. Three levels of Zernike spherical aberration were generated and superposed on the IOLs and the subject's eye. The corrected distance visual acuity was measured by psychophysical visual procedure.

Illumination correction of retinal images using Laplace interpolation.

Retinal images are frequently corrupted by unwanted variations in intensity that occur due to general imperfections in the image acquisition process. This inhomogeneous illumination across the retina can limit the useful information accessible within the acquired image. Specifically, this can lead to serious difficulties when performing image processing tasks requiring quantitative analysis of features present on the retina. Given that the spatial frequency content of the shading profile often overlaps with that of retinal features, retrospectively correcting for inhomogeneous illumination while maintaining the radiometric fidelity of the real data can be challenging. This paper describes a simple method for obtaining an estimate of the illumination profile in retinal images, with the particular goal of minimizing its influence upon features of interest. This is achieved by making use of Laplace interpolation and a multiplicative image formation model.

Intraocular lens implantation position sensitivity as a function of refractive error.

PURPOSE: To quantify the change in ocular aberrations due to intraocular lens (IOL) implantation position errors for pseudo-phakic eyes with different refractive errors. METHODS: The theoretical part of this work was based on ray-tracing in emmetropic and myopic eye models. The possible misalignments i.e. the decentration, tilt and axial translation, of the IOLs were statistically combined together and analysed using Monte-Carlo simulations. Spherical IOLs with a 3 and 5 mm pupil and an aspheric IOL with a 5 mm pupil were analysed as a function of refractive error. In the experimental part of the work, we built an IOL optical test bench including a model eye. The white light discrete point spread functions of misaligned IOLs were recorded by a CCD and we compared the change of the spread function as a result of misalignments for two spherical IOLs with different optical powers. RESULTS: The Monte-Carlo simulations showed that the average root-mean-square spot size at the retinal plane decreased with increasing myopic refractive error, i.e. lower power IOL. The experiments showed that a lower optical power spherical IOL had a less distributed point spread function than a higher optical power IOL, which supported the results of the simulation. CONCLUSION: Regarding IOLs designed for myopic patients, low power IOLs (for high myopes) were shown to be less sensitive to the misalignment than high power ones. Aspheric IOLs were more sensitive to position errors than spherical IOLs under the same conditions.

Spin-to-orbital angular momentum conversion in focusing, scattering, and imaging systems.

We present a general theory of spin-to-orbital angular momentum (AM) conversion of light in focusing, scattering, and imaging optical systems. Our theory employs universal geometric transformations of non-paraxial optical fields in such systems and allows for direct calculation and comparison of the AM conversion efficiency in different physical settings. Observations of the AM conversions using local intensity distributions and far-field polarimetric measurements are discussed.

Experimental detection of optical vortices with a Shack-Hartmann wavefront sensor.

Laboratory experiments are carried out to detect optical vortices in conditions typical of those experienced when a laser beam is propagated through the atmosphere. A Spatial Light Modulator (SLM) is used to mimic atmospheric turbulence and a Shack-Hartmann wavefront sensor is utilised to measure the slopes of the wavefront surface. A matched filter algorithm determines the positions of the Shack-Hartmann spot centroids more robustly than a centroiding algorithm. The slope discrepancy is then obtained by taking the slopes measured by the wavefront sensor away from the slopes calculated from a least squares reconstruction of the phase. The slope discrepancy field is used as an input to the branch point potential method to find if a vortex is present, and if so to give its position and sign. The use of the slope discrepancy technique greatly improves the detection rate of the branch point potential method. This work shows the first time the branch point potential method has been used to detect optical vortices in an experimental setup.

Far-field polarization-based sensitivity to sub-resolution displacements of a sub-resolution scatterer in tightly focused fields.

We present a system built to perform measurements of scattering-angle-resolved polarization state distributions across the exit pupil of a high numerical aperture collector lens. These distributions contain information about the three-dimensional electromagnetic field that results from the interaction of a tightly focused field and a sub-resolution scatterer. Experimental evidence proving that the system allows for high polarization-dependent sensitivity to sub-resolution displacements of a sub-resolution scatterer is provided together with the corresponding numerical results.

Role of ocular aberrations in photopic spatial summation in the fovea.

An adaptive optics vision simulator was used to measure the role of ocular aberrations in the photopic foveal spatial summation curve for three young subjects. After the correction of the ocular aberrations over a 6 mm pupil, the increment luminance threshold was lowered for small stimuli and the estimated area of complete summation (Ricco's area) was reduced by a factor of 2.6 on average. These results show that the image spread due to natural ocular aberrations partly accounts for the spatial summation observed in the fovea in the photopic light regime.

Optical nanoprobing via spin-orbit interaction of light.

We show, both theoretically and experimentally, that high-numerical-aperture (NA) optical microscopy is accompanied by strong spin-orbit interaction of light, which translates fine information about the specimen to the polarization degrees of freedom of light. An 80 nm gold nanoparticle scattering the light in the focus of a high-NA objective generates angular momentum conversion, which is seen as a nonuniform polarization distribution at the exit pupil. We demonstrate remarkable sensitivity of the effect to the position of the nanoparticle: Its subwavelength displacement produces the giant spin-Hall effect, i.e., macroseparation of spins in the outgoing light. This brings forth a far-field optical nanoprobing technique based on the spin-orbit interaction of light.

Measured double-pass intensity point-spread function after adaptive optics correction of ocular aberrations.

The double-pass intensity point-spread function was recorded in four subjects using a monochromatic source emitting at 543 nm, through a 6.7-mm diameter pupil i) at the fovea after adaptive optics correction of the ocular aberrations, ii) at the fovea without adaptive optics correction, and iii) at 2 degrees of eccentricity with adaptive optics correction. The half-width at half-maximum of the double-pass point-spread function was narrower after correction of the ocular aberrations. At 2 degrees of eccentricity this width was larger than at the fovea. The minimum widths were about 1.1 arcmin in dark pigmented eyes and 1.6 arcmin in light pigmented eyes. These values are 6 to 9 times larger than the width expected from diffraction alone.

Correction of ocular and atmospheric wavefronts: a comparison of the performance of various deformable mirrors.

The main applications of adaptive optics are the correction of the effects of atmospheric turbulence on ground-based telescopes and the correction of ocular aberrations in retinal imaging and visual simulation. The requirements for the wavefront corrector, usually a deformable mirror, will depend on the statistics of the aberrations to be corrected; here we compare the spatial statistics of wavefront aberrations expected in these two applications. We also use measured influence functions and numerical simulations to compare the performance of eight commercially available deformable mirrors for these tasks. The performance is studied as a function of the size of the optical pupil relative to the actuated area of the mirrors and as a function of the number of modes corrected. In the ocular case it is found that, with the exception of segmented mirrors, the performance is greatly enhanced by having a ring of actuators outside the optical pupil, as this improves the correction of the pupil edge. The effect is much smaller in the case of Kolmogorov wavefronts. It is also found that a high Strehl ratio can be obtained in the ocular case with a relatively low number of actuators if the stroke is sufficient. Increasing the number of actuators has more importance in the Kolmogorov case, even for the relatively weak turbulence considered here.

Adaptive optics for ophthalmic applications using a pyramid wavefront sensor.

A new adaptive optics system for the eye using a pyramid wavefront sensor interfaced in closed-loop with a piezoelectric deformable mirror is presented. Sensing parameters such as CCD integration time, pupil sampling and beam steering amplitude are tested on the bench and in vivo on several volunteers to optimize real-time optical correction. The system allows closed-loop operation at a frame rate of 55 Hz and reduces ocular aberration up to lambda/5 residual RMS over a 6 mm pupil. Aberration correction and mirror control stability clearly increase when smaller beam steering amplitudes synonymous of higher wavefront sensing sensitivity are used. This result suggests that using pyramid wavefront sensors can improve the performance of adaptive-optics system for ophthalmic applications.

Adaptive optics enhanced simultaneous en-face optical coherence tomography and scanning laser ophthalmoscopy.

A novel combination of adaptive optics with a simultaneous en-face OCT/SLO system for high resolution imaging of the in-vivo human retina is presented. Pairs of retinal images are shown and performance of the system is evaluated with and without dynamic wavefront correction. The adaptive optics closed loop system operates at a frame rate of 9 Hz and incorporates a Shack-Hartmann wavefront sensor based on a highly sensitive Andor camera and a 37 actuator OKO membrane deformable mirror to correct for ocular aberrations. The system produces C-scan pairs of images at a frame rate of 2 Hz. The correction of aberrations produced by the adaptive optics closed-loop system increased the signal-to-noise ratio in images obtained from volunteer eyes by up to 6 dB in the OCT channel and up to 9 dB in the SLO channel. A slight improvement in the lateral resolution was also obtained, from 6.5 mum before to 5 mum after closing the adaptive optics loop.

Comparative analysis of deformable mirrors for ocular adaptive optics.

We have evaluated the ability of three commercially available deformable mirrors to compensate the aberrations of the eye using a model for aberrations developed by Thibos, Bradley and Hong. The mirrors evaluated were a 37 actuator membrane mirror and 19 actuator piezo mirror (OKO Technologies) and a 35 actuator bimorph mirror (AOptix Inc). For each mirror, Zernike polynomials and typical ocular aberrated wavefronts were fitted with the mirror modes measured using a Twyman-Green interferometer. The bimorph mirror showed the lowest root mean square error, although the 19 actuator piezo device showed promise if extended to more actuators. The methodology can be used to evaluate new deformable mirrors as they become available.

Extended object wavefront sensing based on the correlation spectrum phase.

In this paper we investigate the performance of a Fourier based algorithm for fast subpixel shift determination of two mutually shifted images subjected to noise. The algorithm will be used for Shack-Hartmann based adaptive optics correction of images of an extended object subjected to dynamical atmospheric fluctuations. The performance of the algorithm is investigated both analytically and by Monte Carlo simulations. Good agreement is achieved in relation to how the precision of the shift estimate depends on image parameters such as contrast, photon counts and readout noise, as well as the dependence on sampling format, zero-padding and field of view. Compared to the conventional method for extended object wavefront sensing, a reduction of the computational cost is gained at a marginal expense of precision.

Benefit of higher closed-loop bandwidths in ocular adaptive optics.

We present an ocular adaptive optics system with a wavefront sampling rate of 240 Hz and maximum recorded closed-loop bandwidth close to 25 Hz, but with typical performances around 10 Hz. The measured bandwidth depended on the specific system configuration and the particular subject tested. An analysis of the system performance as a function of achieved bandwidth showed consistently higher Strehl ratios for higher closed-loop bandwidths. This may be attributed to a combination of limitations on the available technology and the dynamics of ocular aberrations. We observed dynamic behaviour with a maximum frequency content around 30 Hz.

Measurement of the retinal arteriolar response to a hyperoxic provocation in nonsmokers and smokers, using a high-resolution confocal scanning laser ophthalmoscope.

We used a high-resolution confocal scanning laser ophthalmoscope to measure the magnitude of change in retinal arteriolar diameters in response to oxygen breathing in young, healthy nonsmokers and smokers. Image sequences were obtained before and during oxygen breathing. Image sequences were desinusoided, registered, and averaged, before vessel diameters were measured using a sliding linear regression filter. Arteriole diameters were observed to constrict during the first 5 min. of oxygen breathing, plateau, and remain stable while hyperoxia was maintained, returning to baseline at the end of the hyperoxic period. Blood flow to the temporal retina was found to be higher than to the nasal retina (p=0.008). The percentage constriction of vessels did not vary across retinal quadrants (p=0.372, analysis of variance) and did not depend on vessel size (p=0.538). Baseline diameters were unaffected by acute cigarette smoking. The magnitude of vasoconstriction was diminished in smokers compared to nonsmokers (p=0.017), while acute smoking did not influence the percentage constriction attained by the vessels (p=0.621). Using a high-resolution imaging technique allowed us to measure reactivity to a high degree of accuracy and to assess it in vessels of smaller caliber than were previously studied.